Tag: tube

Description and Schematic

SG-HM2 is a modular He-Ne laser power supply based on IC-HI1 with some minor enhancements. The first version is for laser tubes up to approximately 1 mW (2 mW with trivial modifications) but it should be straightforward to go to 5 mW or even higher power tubes by replacing the SG-HM2 HV Module (HVM2-1) with one with a higher voltage and current rating, along with a higher power MOSFET and minor component value changes to the Control Module (suggestions below). I have added an adjustment for tube current, a current limiting resistor and Zener to protect against output short circuits, an enable input (ground to turn on), a bleeder resistor to virtually eliminate the shock hazard after the power supply is turned off, and power and status LEDs.

• Get the schematic for SG-HM2 (1 mW version) in PDF format: [download id=”5610″]

Modifying SG-HM2 for Higher Power He-Ne Laser Tubes

The following are guidelines for modifying SG-HM2 to drive various power He-Ne lasers. The PCB layout below with two versions of the HV Module should accommodate He-Ne laser tubes up to 10 mW. All assume input of around 12 V though a higher power system can generally run lower power lasers at reduced input voltage. If operation at rated power on another input voltage is desired, the number of turns on the inverter transformer can be adjusted accordingly. As noted above, the 1 mW HV Module (HVM2-1) should run tubes up to about 2 mW, though increasing the µF values of some of the HV capacitors may be desirable to reduce ripple at the higher tube current. Minor changes may also be needed in the components on the SG-HM2 Control Module including using a higher power MOSFET for Q1 and reducing the values of R7 and/or R8 for the higher tube current. Or, just populate the Control Module with Q1 being an IRF644, R7 being 150 ohms, and R8 being 750 ohms for compatibility with all the HV modules. For that matter, the HVM2-5 PCB HV Module should be usable with lower power lasers.

   Laser Power       1 mW         2 mW         5 mW         10 mW
-----------------------------------------------------------------------
   Voltage           1200 V       1500 V       2300 V       3500 V
   Current           2-4mA        3-5mA        5-7mA        5-7mA

 SG-HM2 HV Module:

   PCB Version       HVM2-1       HVM2-1       HVM2-5       HVM2-5

   T101
    Core (DxH)       18x11 mm     18x11 mm     26x16 mm     26/16 mm
    Primary          9T,#28       9T,#28       9T,#26       9T,#26
    Secondary        450T,#40     450T,#40     600T,#40     900T,#40
     Res. (Est)      60 ohms      60 ohms      (90 ohms)    (120 ohms)

   D101-106          2kV          2kV          3kV          5kV

   C101-104          1nF,3kV      2nF,3kV      2nF,6kV      2nF,6kV
   C105              47pF,3kV     47pF,3kV     100pF,6kV    100pF,6kV
   C106              3nF,10kV     5nF,10kV     6nF,15kV     6nF,15kV

   R102              10K,1/2W     10K,1/2W     10K,1W       10K,1W
   R103              200M,10kV    200M,10kV    200M,15kV    200M,15kV 
   R106-107 (total)  10M          10M          15M          20M

 SG-HM2 Control Module:

   Q1              IRF630       IRF630       IRF640       IRF644
   R7              300          250          150          150
   R8              500          250          100          100

SG-HM2 Inverter Transformer

The inverter transformer for HVM2-1 is wound on a ferrite pot core with a small air-gap (about 0.005″). It is 18 mm in diameter by 11 mm high. While specified to use a 9 turn primary and 450 turn secondary, these values can be adjusted somewhat to handle various input and output requirements. Don’t go much lower on the primary as this may result in core saturation. The 9/450 transformer should be fine for 1 to 2 mW He-Ne laser tubes running on 8 to 15v DC input. With 9/300, it will operate on about 12 to 20v DC. Increasing the number of secondary turns (e.g., 9/600) may result in operation on a slightly lower input voltage, but probably not by much. The 9/450 transformer may even run He-Ne laser tubes larger than 2 mW but I haven’t yet tested this since I haven’t built a prototype of HVM2-5 as yet.

It doesn’t matter very much whether the primary (P) is wound first or the secondary (S) is wound first though the former appears to work slightly better, running the tube at about 8v DC input instead of 9v DC input for the same 9/450 transformer. P over S is slightly easier to wind since the primary doesn’t get in the way and increase the lumpiness of the secondary layers. However, with S over P, insulation is somewhat less critical since the HV lead is out away from anything else. With the P over S, additional insulation is needed between them. Also, since the primary coil is larger diameter, it will have more resistance and there will be greater inter-winding capacitance (though probably not significant). The secondary should be constructed as multiple layers of about 50 or 60 turns each, with insulating tape between layers. Each should be wound in as close to a single layer as possible with alternating layers staggered to prevent arc-over. This doesn’t have to be perfect but try to go gradually from one side to the other to keep wires at high relative potential away from each other. Make sure the HV output leads (particularly the one away from the dot) are well insulated as they exit the transformer. And, as noted, if the primary is over the secondary, there must be high voltage insulation between them. The peak output voltage when the MOSFET turns off (the flyback pulse) may be more than 5 times higher than what would be expected from the DC input voltage and the turns-ratio alone – several kV and this *will* try to find a path to ground! There are more detailed transformer construction instructions in the next section.

Note that this transformer is slightly larger physically than the one from IC-HI1. This is for two reasons: (1) It is easier to wind with more space and a larger wire size for the secondary, and (2) continuous operation should be possible with 2 mW laser tubes, which might have been marginal with the original transformer used in IC-HI1. A by-product of the larger core is that its 9 turn primary should be roughly equivalent to the 12 turn primary of the smaller core in terms of inductance and core saturation limitations.

Interestingly, a similar transformer found in a different commercial power supply, had no insulating tape anywhere. It would appear that with very precise machine-wound HV secondary, done first, the voltage is distributed so uniformly that this is unnecessary.

I’ve now built and tested several transformers in IC-HI1, removing the original transformer and installing socket pins so either the original or an adapter board can be plugged in. This setup is then equivalent to SG-HM2 with the HVM2-1 HV Module. The minimum input voltage values that follow are when driving a 0.5 mW He-Ne laser tube:

           Turns        Pot Core   Vin (VDC)
   ID   P/S    Order    (DxH mm)   Min  Max           Comments
 ------------------------------------------------------------------------------
   1* 12/600  S over P    14x8     7.5  15   Original IC-HI1 transformer
   2  12/350  S over P    18x11    14   22   First prototype, described above
   3   9/350  S over P    18x11    11   18   #2 with 3 P T added out-of-phase
   4   9/425  P over S    18x11     9   16
   5   9/450  P over S    18x11     9   16
   6   9/450  S over P    18x11     8   15
   7  12/500  P over S    26x16     8   15

*The number of turns on the original (#1) is not really known exactly and may be lower or higher by up to 25 percent based on the measured secondary resistance (45 ohms) and estimated wire size (somewhere between #38 and #40. (Even with the larger wire, the amount of bobbin area taken up by the wire is less than 50 percent so it should fit even with many layers of insulating tape. The transformer is Epoxy impregnated and likely to be impossible to disassemble into any form that can be analyzed!)

All of these transformers will drive He-Ne laser tubes of up to at least 2.5mW using the equivalent of the HVM2-1 HV Module which is part of IC-HI1. Even with the 2.5mW tube, the minimum operating voltage was only about 0.5v higher than for the 0.5mW tube. There is a good chance they would drive even larger He-Ne laser tubes (though possibly at a slightly higher input voltage) but I don’t dare try using the existing HV circuitry as it might not survive for long. I suspect that transformers #4, #5, and #6 would run on an input voltage of less than 8v DC but the salvaged cores I am using have a larger air-gap than might be optimal and I don’t have anything to reduce it without heavy losses. They attempt to start the tube at around 6v DC but are unable to maintain it and flicker rapidly. (#2 and #3, which use the same style core, would also benefit somewhat.) Operation using #1 and #5 is virtually identical, with the original running at perhaps 0.5v DC less input. I expect they would be even more identical if the air-gap on #5 were smaller, and #6 with its smaller air-gap does indeed run at the lower input voltage. I haven’t actually confirmed that anything blows up above the maximum voltages listed above, which were arbitrarily chosen. But I am guessing that bad things might happen at some point. 🙂

I have also constructed a transformer which will need to be used with HVM2-5: 12/1200, P over S, on a 30×19 pot core. I will also construct a 9/900. S over P, on a 30×19 pot core (or on a 26×16 if I can find one). Testing of these will have to await an HVM2-5 prototype.

SG-HM2 Transformer Construction

Here are details on construction of the inverter transformer for SG-HM2. With all parts and tools on hand, it takes about an hour start to finish. Only a small portion of this time is in the actual winding (at least if a coil winding machine is used). Most of the time is spent in adding the insulation tape and terminating the leads. After constructing a few of these, it does go quicker. 🙂

Step-by-step instructions are provided for the HVM2-1 transformer. The changes needed for HVM2-5 are summarized at the end of this section. Some sort of coil winding machine is almost essential as #40 wire is extremely thin and easy to break. (Anything larger than #40 will not fit on the bobbin.) It doesn’t have to be fancy. Mine is probably 50 years old of the type that is (used to be?) advertised in the back of electronics magazines. However, a couple of spindles – one that is fixed or free to rotate for the wire supply and the other which can be turned for the coil being wound – are really all that are needed. Don’t use any sort of powered approach though (unless you have a *real* professional coil winder!) as it is all too easy to break the wire if there is no tactile feedback to detect snags.

1. Parts required for T101 of HVM2-1:
   • 18×11 mm (1811) ferrite pot core with a small air-gap (no more than 0.005″) or no air-gap, and a single section bobbin. These are available from several manufacturers but surplus or salvaged cores may be easier to obtain. Radio Shack used to have a “ferrite kit” which included a variety of sizes of cores (only 1 each though so you’d have to buy two kits and there were no bobbins!). I doubt the kit still exists though.
   • Approximately 1.5 feet of #28 magnet wire for the primary (9 turns wound first) and approximately 60 feet of #40 magnet wire for the secondary (450 turns wound on top of the primary). I found both these size wire in various solenoids and relays I’ve discombobulated. 🙂 Wire sizes aren’t critical but these are known to fit and the #40 can be handled with a reasonable chance of not breaking.
   • Sleeving to protect the primary wires where they leave transformer. I used approximately 2″ of insulation (each lead) from the individual wires in some 25 pair phone cable.
   • Wirewrap wire or other thin insulated wire to terminate the secondary wires where they leave the transformer.
   • Insulating tape. 1 mil Mylar or similar is desirable. However, I’ve found that thin clear (non-reinforced) packing tape does an adequate job, though it probably doesn’t have as much dielectric strength as real insulating tape so additional layers are required. It will also likely not stand up to overheating too well. Electrical tape is way too thick and would prevent enough turns from fitting.
   • A piece of Perf. board with holes on 0.1″ centers, 0.8″x0.8″. There should be 7 rows of holes each way so that one hole lines up in the center.
   • A Nylon 4-40 screw and nut to fasten the transformer to the board.
   • Four (4) machined-type IC socket pins or something similar to use as terminals.
2. Wind the primary:
   • Slip a piece of sleeving over the start of the primary wire and position the sleeving so it extends about 1/2 turn inside the bobbin on the left side.
   • Wrap exactly 9 turns of this wire clockwise around the bobbin, left to right. The wires should enter and exit on the same angular position (slot) of the bobbin on opposite sides.
   • Slip another piece of sleeving over the wire end exiting the bobbin so that it too is about 1/2 turn inside the bobbin.
   • Wrap 1.5 to 2 turns of tape tightly over the primary winding to secure and insulate it.
3. Wind the secondary:
   • Strip 1/8″ or so from the end of a 2″ piece of wire-wrap wire and solder the start of the wire for the secondary winding to it. Make sure the insulation on the fine magnet wire has been removed – usually just heating it while soldering will do this. Leave an inch or so of the magnet wire extending from the connection so that continuity can be confirmed with a multimeter, then snip it off. Install this in the opposite slot of the bobbin also on the left side with about 1/4″ of insulation inside the bobbin against the side and separated from the primary. Leave a little slack in the fine secondary wire so that slight motion won’t break it. Add a small piece of tape to protect and insulate this connection.
   • Using your coil winding machine (you do have one, correct?), build up the secondary in layers of about 50 to 75 turns in a counter-clockwise direction (bobbin being rotated clockwise). A single layer of wire won’t fit in the 1/8″ or so available (in the 18×11 mm core bobbin) so there will have to be some overlap. But, do this several times across the layer so that any given wire won’t be next to one with a much different voltage. In other words, wind a few turns and back up so that there will in essence be multiple sub-windings of 5 or 10 turns, repeated several times across the layer. Keep the wire at least 1/32″ away from either edge of the bobbin.
   • After each full layer or wire, add just over 1 layer of insulating tape making sure it covers the entire width of the bobbin. There should be just enough overlap to assure there is at least 1 layer of insulation but not much more as excessive tape will end up taking up too much space.The entire 450 turn winding will then require 6 to 9 full layers. Add another layer of insulating tape over the last winding layer leaving the wire end exposed.
   • Terminate the end of the secondary winding with another piece of thin wire by soldering as above. Confirm continuity with a multimeter. For the 450 turn secondary, the resistance should be about 60 ohms. Add a piece of thicker sleeving over this at the HV end if space is available. Else, use some bits of tape to insulate the wirewrap wire lead from the core and exposed inner layers that it may come near as it exits out the side of the bobbin. Add another layer of tape to secure the lead in place.
   • Add several more layers of insulating tape to complete the bobbin assembly.
4. Prepare the mounting board:
   • Widen the center hole to 7/64″ to accommodate a 4-40 nylon screw.
   • Widen the holes at the 4 corners of the board to accept the 4 IC socket pins (if used) as a press-fit or glue them in place with 5 minute Epoxy or SuperGlue.
5. Final assembly:
   • Install the ferrite pot core halves to the bobbin taking care not to crunch any of the wires. Orient it so that the primary and secondary leads are conveniently located with respect to the 4 pins, e.g., primary start: bottom left; primary end: top left, secondary start: bottom right; and primary end: top right.
   • Use the nylon 4-40 screw and nut to *gently* secure the transformer to the mounting board. The head of the 4-40 screw should be underneath the board. Don’t over-tighten or it may crack the core, especially if it has an air-gap in the middle.
   • Carefully remove the insulation from the ends of the wires. The secondary wires will still be fragile even with the wirewrap wire terminations. For the magnet wire, the easiest way to remove the insulation is to burn it off with a match or hot soldering iron and then clean with fine sandpaper.
   • Push the wires into their respective socket pins. (The wirewrap wires are too thin to be secure but they will make adequate contact for testing.)
   • Use a multimeter to confirm continuity of the primary (close to 0 ohms) and secondary (about 50 to 75 ohms).
6. Testing:
   • Install the transformer in you HV Module. Attach a He-Ne laser tube and ballast resistor.
   • Power up on an variable DC power supply and check for reliable starting and stable operation. Adjust the core gap if needed. A smaller gap may result in more operating power available at a given input voltage. A larger gap will result in attempts to start on a lower input voltage. Somewhere around 0.005″ is probably a good compromise.
   • After testing the transformer (and adjusting the core gap if needed), use some adhesive to secure the pot core sections and to protect the transformer leads. Solder the leads into the socket pins.

The final result is shown on an adapter below:

The instructions for winding the HVM2-5 transformer are similar except for the dimensions, wire sizes and lengths, and number of turns for the primary and secondary:

• Differences in parts list for T501 of HVM2-5 compared to T101 of HVM2-1:
  • 26×16 mm (2616) ferrite pot core with a small air-gap (no more than 0.005″) or no air-gap, and a single section bobbin.
  • Approximately 2.0 feet of #26 magnet wire for the primary (12 turns wound first) and approximately 75 to 120 feet of #40 magnet wire for the secondary (600 or 900 turns wound on top of the primary).
  • A piece of Perf. board with holes on 0.1″ centers, 1.0″x1.0″. There should be 9 rows of holes each way so that one hole lines up in the center.
  • A Nylon 10-32 screw and nut to fasten the transformer to the board.

Since the peak voltage on the HVM2-5 secondary may be 2 to 3 times higher than for HVM2-1, extra insulation and clearances will be required on the secondary.

SG-HM2 Printed Circuit Board Layout

A printed circuit board layout is also available. The Control Module is 2″x1.2″. The HV Modules are 3.6″x1.2″ and 4.5″x1.8″ for the 1 mW (HVM2-1) and 5 mW (HVM2-5), respectively. The Control and HV Modules are connected by a 2 pin cable for transformer drive and a 3 pin cable for current sensing from the laser tube. The two boards can easily be merged if desired.

The layout of the 3 PCBs may be viewed as a GIF file (draft quality) as below:

.

A complete PCB artwork package for SG-HM2 (all PCBs on one sheet) may be downloaded in standard (full resolution 1:1) Gerber PCB format (zipped) as [download id=”5612″]

The Gerber files include the component side copper, soldermask, and silkscreen; solder side copper and soldermask, and drill control artwork. The original printed circuit board CAD files and netlist (in Tango PCB format) are provided so that the circuit layout can be modified or imported to another system if desired. The text file ‘sghm2.doc’ (in sghm2grb.zip) describes the file contents in more detail.

Note: The netlist does NOT include wiring for the HVM2-5 HV Module. Also, part numbers on the HVM2-5 PCB actually begin with a “5” instead of a “1” since Tango PCB will not allow duplicate part numbers on the same layout.

Effects of Magnetic Fields on He-Ne Laser Operation

If you open the case on a higher power (and longer) He-Ne laser head or one that is designed with an emphasis on precision and stability, you may find a series of magnets or electromagnetic coils in various locations in close proximity to the He-Ne tube. They may be distributed along its length or bunched at one end; with alternating or opposing N and S poles, or a coaxial arrangement; and of various sizes, styles, and strengths.

Magnets may be incorporated in He-Ne lasers for several reasons including the suppression of IR spectral lines to improve efficiency (such as it is!) and to boost power at visible wavelengths, to control its polarization, and to split the optical frequency into two closely spaced components. There are no doubt other uses as well.

The basic mechanism for the interaction of emitted light and magnetic fields is something called the ‘Zeeman Effect’ or ‘Zeeman Splitting’. The following brief description is from the “CRC Handbook of Chemistry and Physics”:

“The splitting of a spectrum line into several symmetrically disposed components, which occurs when the source of light is placed in a strong magnetic field. The components are polarized, the directions of polarization and the appearance of the effect depending on the direction from which the source is viewed relative to the lines of force.”

Magnetic fields may affect the behaviour of He-Ne tubes in several ways:

• He-Ne tubes with long discharge paths will tend to amplify the (generally unwanted) IR wavelengths (probably the one at 3.39µm which is one of the strongest, if not the strongest of all lines) at the expense of the visible ones. The purpose of these magnets is to suppress spectral lines that do not contribute to the desired lasing wavelength (usually the visible red 632.8nm for these long tubes). As a result of the Zeeman Effect, if a gas radiates in a magnetic field, most of its spectral lines are split into 2 or sometimes more components. The magnitude of the separation depends on the strength of the magnetic field and as a result, if the field is also non-uniform, the spectral lines are broadened as well because light emitted at different locations will see an unequal magnetic field. These ‘fuzzed out’ lines cannot participate in stimulated emission as efficiently as nice narrow lines and therefore will not drain the upper energy states for use by the desired lines. The magnitude of the Zeeman splitting effect is also wavelength dependent and therefore can be used to control the gain of selected spectral lines (long ones are apparently affected more than short ones on a percentage basis).The Doppler-broadened gain bandwidth of neon is inversely related to wavelength. At 632.8nm (red) it is around 1.5 to 1.6 GHz; at 3,391nm (the troublesome IR line), it is only around 310MHz. A magnetic field that varies spatially along the tube will split and move the gain curves at all wavelengths equally by varying amounts depending on position. However, a, say, 100 or 200MHz split and shift of the gain curve for the 632.8nm red transition won’t have much effect, but it will effectively disrupt lasing for the 3,391nm IR transition.Without the use of magnets, the very strong neon IR line at 3.39µm would compete with (and possibly dominate over) the desired visible line (at 632.8nm) stealing power from the discharge that would otherwise contribute to simulated emission at 632.8 nm. However, the IR isn’t wanted (and therefore will not be amplified since the mirrors are not particularly reflective at IR wavelengths anyhow). Since the 3.39nm wavelength is more than 5 times longer than the 632.8 nm red line, it is affected to a much greater extent by the magnetic field and overall gain and power output at 632.8nm may be increased dramatically (25 percent or more). The magnets may be required to obtain any (visible) output beam at all with some He-Ne tubes (though this is not common).

  The typical higher power Spectra-Physics He-Ne laser will have relatively low strength magnets (e.g., like those used to stick notes to your fridge) placed at every available location along the exposed bore along the sides of the L-shaped resonator frame. They will alternate N and S poles pointing toward the bore. Interestingly, on some high mileage tubes, brown crud (which might be material sputtered off the anode) may collect inside the bore – but only at locations of one field polarity (N or S, whichever would tend to deflect a positive ion stream into the wall). The crud itself doesn’t really affect anything but is an indication of long use. And on average, tubes with a lot of brown crud may be harder to start, and require a higher voltage to run, and have lower output power.

  I do not know how to determine if and when such magnets are needed for long high power He-Ne tubes where they are not part of an existing laser head. My guess is that the original or intended positions, orientations, and strengths, of the magnets were determined experimentally by trial and error or from a recipe passed down from generation to generation, and not through the use of some unusually complex convoluted obscure theory. 🙂

  The only thing I can suggest other than contacting the manufacturer (like any manufacturer now cares about and supports He-Ne lasers at all!) is to very carefully experiment with placing magnets of various sizes and strengths at strategic locations (or a half dozen such locations) to determine if beam power at the desired wavelength is affected. Just take care to avoid smashing your flesh or the He-Ne tube when playing with powerful magnets. Though the magnets used in large-frame He-Ne lasers with exposed bores aren’t particularly powerful, to produce the same effective field strength at the central bore of an internal mirror He-Ne tube may require somewhat stronger ones, though even these needn’t be the flesh squashing variety. And, magnets that are very strong may affect other characteristics of the laser including polarization, and starting and running voltage. Enclosing the He-Ne tube in a protective rigid sleeve (e.g., PVC or aluminium) would reduce the risk of the latter disaster, at least. 🙂 If there is going to be any significant improvement, almost any arrangement of 1 or 2 magnets should show some effect.

  There may be an immediate effect when adding or moving a magnet. However, to really determine the overall improvement in (visible) output power and any reduction in the variation of output power with mode sweep, the laser should be allowed to go through several mode sweep cycles for 3.39 µm. These will be about 5.4 times the length of the mode sweep for 632.8 nm.

  CAUTION: For soft-seal laser tubes in less than excellent health (i.e., which may have gas contamination), changing the magnet configuration near the cathode may result in a slow decline in output power (over several hours) which may or may not recover. I have only observed this behaviour with a single REO one-Brewster tube, but there seems to be no other explanation for the slow decline to about half the original power, and then subsequent slow recovery with extended run time after the magnets were removed entirely. Possibly simply leaving the magnets in the new configuration would have eventually resulted in power recovery, but at the time the trend was not encouraging.

  (From: Lynn Strickland (stricks760@earthlink.net).)

  “They’ve pretty much nailed the 3.39 micron problem on red He-Ne tubes these days so magnets really aren’t needed on them. Even the new green tubes don’t have much of a problem – especially since the optic suppliers have perfected the mirror coatings. All of the good green mirrors are now done with Ion Beam Sputtering (IBS), as opposed to run-of-the-mill E-Beam stuff.However, you’ll probably see a benefit from magnets to suppress the 3.39µm line on the older He-Ne tubes.”

• While most inexpensive He-Ne tubes that produce linearly polarized light do so because of an internal Brewster plate and lasers with external mirrors have Brewster windows on the ends of the plasma tube, it is also possible to affect the polarization of the beam with strong magnets again using the Zeeman Effect.Where the capillary of the plasma tube is exposed as with many older lasers, and the magnets can be placed in close proximity to the bore, their strength can be much lower. A few commercial lasers (like the Spectra-Physics model 132) offered a polarization option which adds a magnet assembly alongside the tube. In this case, what is required is a uniform or mostly uniform field of the appropriate orientation rather than one that varies as for IR spectral line suppression though both of these could be probably be combined. However, the polarization purity with this approach never came anywhere close to that using a simple Brewster window or plate, found in all modern polarized He-Ne lasers.Also see the section: Unrandomizing the Polarization of a Randomly Polarized HeNe Tube.
• Two-frequency He-Ne lasers are used in precision interferometers for making measurements to nanometer accuracy. With these, the Zeeman effect is exploited to split the output of a single frequency He-Ne laser into a pair of closely spaced optical frequencies so that a difference or “split” frequency can be obtained using a fast photodiode. The most common are axial Zeeman lasers that use a powerful magnetic field oriented along the axis of the tube. For these, the “split” frequency is typically between 1.5 and 7.5 MHz (though it could be much lower but not much higher). Transverse Zeeman lasers use a moderate strength field oriented across the tube and have split frequencies in the 100s of kHz range. To stabilize these lasers, either a heater or piezo element is provided to precisely control cavity length.

In principle, varying fields from electromagnets could be used for intensity, polarization, and frequency modulation. I do not know whether any commercial He-Ne lasers have been implemented in this manner.

But if magnets were not originally present, the only situation where adding some may make sense is for older longer or “other colour” He-Ne tubes where a series of weak magnets may actually boost output power by 10 to 25 percent or more. On the other hand, most non-Zeeman stabilized He-Ne lasers do NOT like magnets at all. Even a relatively weak stray magnetic field from nearby equipment may result in a significant change in behaviour. However, unless ferrous metals are used in the laser’s construction, any change will likely not be permanent.

Typical Magnet Configurations

Here are examples of some of the common arrangements of magnets that you may come across. In addition to those shown, magnets may be present along only one side of the tube (probably underneath and partially hidden) or in some other peculiar locations. I suspect that for many commercial He-Ne lasers, the exact shape, strength, number, position, orientation, and distribution of the magnets was largely determined experimentally. In other words, some poor engineer was given a bare He-Ne tube, a pile of assorted magnets, a roll of duct tape, and a lump of modelling clay, and asked to optimize some aspect(s) of the laser’s performance. 🙂

• Transverse (varying field) – These will most likely be permanent magnets in pairs, probably several sets.Polarity may alternate with North and South poles facing each other across the tube forming a ‘wiggler’ so named since such a they will tend to deflect the ionized discharge back and forth though there may be no visible effects in the confines of the capillary:

                     N      S      N      S      N      S      N
               ||===================================================||
               ||======. .=================================. .======||
                     S |||  N      S      N      S      N  |_| S
                       '|'                                 '|'
  
  

  For some including the Spectra-Physics 120, 124, 125, and 127, the magnets are actually below and on one side. The objective is usually IR (3.39µm) suppression and the magnets are generally relatively weak (refrigerator note holding strength). Alternatively, North and South poles may face each other:

                     N      S      N      S      N      S      N
               ||===================================================||
               ||======. .=================================. .======||
                     N |||  S      N      S      N      S  |_| N
                       '|'                                 '|'
  
  

  With either of these configurations, after long hours of operation, there may be very pronounced brown deposits inside the bore that correlate with the pole positions.

• Transverse (uniform field). Here, the objective is to achieve a constant field throughout the entire discharge:

                     N      N      N      N      N      N      N
               ||===================================================||
               ||======. .=================================. .======||
                     S |||  S      S      S      S      S  |_| S
                       '|'                                 '|'
  
  

  This configuration is found in two very different situations. Strong magnets were used in laser like the Spectra-Physics 132P to polarize the beam. Weaker magnets are used in transverse Zeeman two-frequency He-Ne lasers.

• Axial – These will most likely be permanent magnet toroids (similar to magnetron magnets), though an electromagnetic coil (possibly with adjustable or selectable field strength) could also be used. Thus, the North and South poles will be directed along the tube axis:

                          +--+      +--+      +--+      +--+
                        N |  | S  N |  | S  N |  | S  N |  | S
                          +--+      +--+      +--+      +--+
              ||======================================================||
              ||====. .========================================. .====||
                    |||   +--+      +--+      +--+      +--+   |_|
                    '|' N |  | S  N |  | S  N |  | S  N |  | S '|'
                          +--+      +--+      +--+      +--+
  
  

  Other axial configurations with opposing poles or radially oriented poles may also be used or there may be a single long solenoid type of coil or cylindrical permanent magnet as for a two-frequency laser interferometer.

HeNe Laser Wavelengths

While what comes to mind when there is mention of a HeNe laser is a red beam, those with other wavelengths are manufactured.

• The most common HeNe lasers by far produce light at a wavelength of 632.8 nm in the red part of the visible spectrum. This is well into the region of the human eye’s high sensitivity (but not anywhre as good as green). Thus, a 1 mW red HeNe laser will appear brighter than a 4 mW diode laser operating at 670 nm. Although these are called red HeNe lasers, compared to the color of the 670 nm diode, their beam actually appears somewhat orange-red.
• Green (543.5 nm), yellow (594.1 nm), and orange (604.6 and 611.9 nm) HeNe lasers are also available but are not nearly as ‘efficient’ as the common red type since the spectral lines that need to be amplified are much weaker at these wavelengths. Thus, ‘other color’ HeNe lasers must be much larger for the same output power and use higher quality mirrors. Manufacturing yield is also lower and far fewer of these are produced. Taken together, the bottom line is that they are much more expensive either new or surplus.Note: Since the gain of these wavelengths is so low, they also have a shorter life and the chance of finding working surplus green or yellow HeNe lasers is much lower than for red. I would not recommend bidding on an eBay auction for one of these unless guaranteed to be working. The likelihood of the problem for an “unknown condition” green or yellow HeNe laser being just mirror alignment is small to none!
• IR (infra-red) HeNe laser tubes are manufactured as well (1,523.1 nm is most common probably because this wavelength is useful for testing of fiber optic data transmission systems). The other two common IR wavelengths are 1,152.3 and 3,391.3 nm. However, an invisible beam just doesn’t seem as exciting and these make truly lousy laser pointers!

Typical maximum output available from (relatively) small HeNe tubes (400 to 500 mm length) for various colors: red – 10 mW, orange – 3 mW, yellow – 2 mW, green – 1.5 mW, IR – 1 mW. Higher power red HeNe tubes (up to 35 mW or more and over 1 meter long) and ‘other-color’ HeNe tubes (much lower – under 10 mW) are also available. However, these will be very large and very expensive.

Tunable HeNe Lasers

If it were possible to select any available wavelength desired, then some people would be content beyond description. 🙂

A few tunable HeNe lasers have been produced commercially. These provide wavelength (color) selection with the turn of a knob. However, due to the low gain of most HeNe lasing lines, producing a useful tunable HeNe laser is not an easy task. Everything must be just about perfect to get the “other color” lines to lase at all, and even more so when a laser is to be designed to work at more than one wavelength with a TEM00 beam. The most widely known such laser (as these things go) is manufactured by Research Electro-Optics, Inc. (REO). It produces at least 5 of the visible wavelengths: normal red, two oranges, yellow, and green. A Littrow (or Brewster) prism with micrometer screw adjusters takes the place of the HR mirror in a normal HeNe laser. See the section: Research Electro-Optics Tunable HeNe Lasers.

There used to be a model ML-500 tunable HeNe laser from Spindler and Hoyer that did *14* lines between 611 nm and 1,523 nm. So no 604 nm orange, 594.1 nm yellow, 543.5 nm green, or 3.39 µm IR. The mirror set had to be changed to go between the visible and IR wavelengths. It used a Birefringent Filter (BRF) for wavelength selection instead of the Littrow prism in the REO tunable laser. A BRF has the advantage that there is no loss from a slightly incorrect Brewster angle for all but one wavelength, unavoidable with a Littrow prism. This is because the BRF is always set at exactly the Brewster angle. The birefringent crystal in the BRF filter produces a different optical delay for polarization components oriented in the direction of its slow and fast axes. Only when this difference is a multiple of a full cycle for any given wavelength, will the polarization be unchanged and thus result in minimal loss through the BRF. By rotating the BRF around its optical axis (still maintaining it at the Brewster angle to the laser’s optical axis), the wavelength where minimum loss occurs can be selected. In 1987, it was only $5,800 for laser with either wavelength range, an additional $750 for the other mirror set

I don’t know why Spindler and Hoyer would have admitted defeat in not including those other wavelengths as they were certainly known at the time. Perhaps, the losses through the two Brewster windows of their laser tube and the Brewster angled plate of the BRF compared to those of the Brewster window and Brewster prism of the PMS/REO tunable laser were just too high. Perhaps, their mirror coating technology was not as good as what PMS/REO had available.

Unfortunately, Spindler and Hoyer no longer makes this laser, only boring normal HeNe lasers and other optical equipment. However, a scan of the original ML-500 product brochure can be found at Vintage Lasers and Accessories Brochures and Manuals. With modern technology, a 17 line tunable HeNe laser should be possible. 🙂 A tube with internal mirrors and a BRF *inside* would reduce the number of Brewster angle reflective surfaces to only 2, compared to the 3 of the PMS/REO design. A magnetic coupling can be used to move the BRF from outside the tube. In addition, the mirrors can be recessed away from the ends of the tube so they don’t experience any high temperatures during the sealing process. The tube itself would be hard-sealed with frit or regular glass. Then optical contacting or leaky Epoxy seals can be avoided. Use a Brewster angle window to pass the laser beam out of the tube. One of the mirror mounts would be attached via a metal bellows to allow for alignment.

Exact Frequency/Wavelength of HeNe Lasers

There is, of course, no single precise HeNe wavelength since any given cavity will only oscillate at the permitted longitudinal modes and the gain curve is something like 1.5 GHz wide. Thus, for a common HeNe laser, there is no single wavelength and those that are present drift over time (mostly due to thermal expansion of the cavity).

For reference, here are the approximate red HeNe parameters close enough for Government work: 😉

• A vacuum wavelength of 633 nm corresponds to an optical frequency of 474 THz.
• A wavelength change of 1 nm at 633 nm corresponds to an optical frequency change of 749 GHz.
• An optical frequency change of 1 GHz at 633 nm corresponds to a wavelength change of 1.34 pm (picometers).
• A one part-per-billion (ppb) change in wavelength at 633 nm corresponds to an optical frequency change of 474 kHz.

Where more decimal points matter, a single mode frequency stabilized HeNe laser will have very nearly a constant single wavelength precise to 9 or more significant figures but it too will be affected by various physical parameters including the exact length of the laser’s cavity, gas pressure and He:Ne fill ratio, and temperature – there is no single correct answer!

For example, one typical stabilized HeNe laser from Hewlett-Packard, has a precise vacuum wavelength of 632.991372 nm. Another one from Melles Griot (as noted below) is 632.991058 nm in vacuum or 632.81644 nm in air (divide by the index of refraction of air, n=1.00027593).

(Portions from: Jens Decker (Jens.Decker@chemie.uni-regensburg.de).)

The Melles Griot catalog claims a nominal frequency of 473.61254 THz for their 05-STP series of frequency stabilized lasers. (Elsewhere in the same catalog they are more precise and lists 473.612535 THz for the 632.8 nm line.) Anyhow, with c = 2.997925E8 m/s this gives 632.991058 nm in vacuum or 632.81644 nm in air for n = 1.00027593 (formula from J Phys.E, vol. 18, 1985, pp. 845ff). To find reliable values for all the other HeNe lines is quite difficult. One has to compare a number of books to be sure whether the values are for air or vacuum.

(From: D. A. Van Baak (dvanbaak@calvin.edu).)

Well, here it is depending on the level of precision desired:

• If you want 4 significant digits, you have to know if it’s the wavelength in air, or in vacuum.
• If you want 6 significant digits, you need a single mode HeNe laser.
• If you want 7 significant digits, you have to stabilize the output to the center of the gain profile and you probably need to know the helium pressure and the neon isotope ratio.
• If you want 8 significant digits, you have to know the diameter of the beam since diffraction effects will change the wavelength.
• If you want 9 significant digits, you might need frequency, not wavelength, metrology.

The metrologists’ answer for a 632.8 nm HeNe laser stabilized to the a-13 component of the R(127) line of the 11-5 transition of the 127-Iodine dimer molecule is:

• Frequency = 473,612,214,705 kHz.
• Wavelength = 632,991,398.22 fm (femtometer = 10^-15 m).

under certain specified conditions, with uncertainty 2.5×10^-11. See: “Metrologia”, vol. 30., pp. 523-541, 1993-1994.

HeNe Laser Beam Characteristics

Compared to a diode laser, the beam from even an inexpensive mass produced HeNe tube is of very high optical quality:

• The width of the beam as it emerges from the tube is typically between .5 mm and 1.5 mm – the inside bore diameter of the capillary discharge tube.
• The beam from most HeNe lasers is already very well collimated even without external optics (unlike a laser diode which has a raw divergence measured in 10s of degrees). The divergence measured in milliradians (1 mR is less than 1/17th of a degree) is usually one of the tube specifications. A small HeNe tube may have a divergence of 1 to 2 mR.The minimum divergence obtainable is affected mostly by beam (exit or waist) diameter (wider is better). Other factors like the ratio of length to bore diameter (narrower is better) may also affect this slightly. The equation for a plane wave source is:

                                                       Wavelength * 4
      Divergence angle (half of total) in radians = --------------------
                                                     pi * Beam Diameter
  

  So, for an ideal HeNe laser with a .5 mm bore at 632.8 nm, the divergence angle will be about 1.6 mR. Note that although a wider bore should result in less divergence, this also permits more not quite parallel rays to participate in the lasing process. This assumes planar mirrors – which few HeNe lasers use. Where one or both mirrors are curved, the divergence changes. For example, it is common with HeNe tubes for the Output Coupler (OC) mirror to be ground slightly concave and for the High Reflector (HR) mirror to be planar. If the outer surface of the OC glass is not also curved to compensate for the negative lens that results, the beam will diverge at a much higher rate than would be expected for the bore diameter.HeNe laser tubes destined for barcode scanners often have a much higher divergence by design – up to 8 mR or more (where the optimal divergence may be as little as 1.7 mR or less). These tubes either have a negative curvature for the outer surface of the OC mirror glass (concave inward) or even an external negative lens attached with optical cement. See Uniphase HeNe Laser Tube with External Lens. The outer surface of OC in a normal HeNe tube will either be planar or slightly convex depending on whether the OC mirror is planar or slightly concave respectively. In the latter case, the convex surface precisely compensates for the extra divergence produced by the OC mirror curvature and results in a nearly optimally collimated beam. If the outer surface of your HeNe tube’s OC is concave, then it will have the high divergence characteristic. Note that the beam is still of very high quality but an additional positive lens approximately one focal length away from the OC will be required to produce a collimated beam.

  Also see the section: Improving the Collimation of a HeNe Laser with a Beam Expander.

• Common HeNe lasers are of two types: random polarized and linearly polarized, which refers to the polarization of the output beam. A random polarized laser generally doesn’t produce anything like rapidly fluctuation polarization. It simply means that nothing has been done to control the polarization. And for the red (632.8 nm) wavelength, most HeNe will actually produce two sets of linearly polarized modes that are orthogonal to each other and fixed to the physical structure of the tube. These will change in amplitude as the tube heats up and the cavity expands.For a short tube (e.g., 5 or 6 inches), this is easily observed by placing a polarizer in the beam. At certain orientations, the beam brightness will then appear to go through cycles – light, dark, light, etc. However, polarization can be affected by external means. See the section: Unrandomizing the Polarization of a Randomly Polarized HeNe Tube.
• HeNe tubes which generate a linearly polarized beam are also available. Rotating a polarizer in a linearly polarized beam will result in high transmission at one orientation and close to zero transmission 90 degrees to it. These tubes usually include a glass plate oriented at the Brewster angle in the beam path (inside the resonator). This results in the optical resonator favoring one polarization orientation and the beam then becomes almost 100 percent linearly polarized. Melles Griot puts this plate inside next to the HR mirror of a HeNe tube that is otherwise similar to a random polarized model. Other manufacturers like Hughes have used a tube with a Brewster window at the OC-end and fasten the OC mirror to it externally. And, some really old cylindrical Hughes laser heads use tubes with Brewster windows at both ends with the mirrors mounted in the metal end-caps of the case. See the section: What is a Brewster Window? for more information.
• Lasers with external mirrors and Brewster windows (plates at the Brewster angle attached to the ends of the tube) will be linearly polarized and really expensive. They will also be more finicky as there may be some maintenance – the optics will need to be kept immaculate and the mirror alignment may need to be touched up occasionally. However, the fine adjustments will permit optimum performance to be maintained and changes in beam characteristics due to thermal effects should be reduced since the resonator optics are isolated from the plasma tube. Some HeNe lasers have an internal High Reflector (HR) mirror at one end of the tube but a Brewster window and external Output Coupler (OC) mirror at the other end. These are also linearly polarized and only half as finicky. 🙂
• In the trivial triviality department, the largest commercial two-Brewster laser I know of is the Spectra-Physics model 125, rated at 50 mW (red, 632.8 nm) but often producing much more output power when new. The plasma tube in this beast is over 5 feet long. Jodon also manufactures a 50 mW HeNe laser. The smallest two-Brewster plasma tube I’ve ever seen was from a photo in a book on lasers from the 1960s. It was only about 4 inches in length.
• Inexpensive internal mirror HeNe tubes nearly always operate with multiple longitudinal modes and most have a TEM00 beam profile (though some, designed for maximum power output in a given size package, may have a wider bore and operate with multiple transverse modes – TEMxy where ‘x’ and ‘y’ are integers greater than 0). See the section: Instant HeNe Laser Theory for more information on laser mode structure.
• High precision or lab quality HeNe lasers may be of quite unconventional construction incorporating plasma tubes that differ substantially compared to these mass produced HeNe tubes – both electronically and optically. Not only may one or both mirrors be mounted external to the tube in many of these, even if both mirrors are internal, there may be interesting and strange electrical, optical, electro-optical, or magnetic devices added to implement external modulation, mode locking, stabilization, and additional high performance (and high cost) features. Consider such a HeNe laser to be quite a find! See the sections: Spectra-Physics 120, 124, and 125 HeNe Laser Specifications and Interesting and Strange HeNe Lasers and for some examples.

Ghost Beams From HeNe Laser Tubes

If you project the output from some HeNe laser tubes (as well as other lasers) onto a white screen a meter or so away, you may see a main beam and a weak beam off to the side a few cm away from it. Maybe even another still weaker one after that.

Most internal mirror HeNe tubes should not have any higher order transverse (non-TEM00) modes. And, for multimode tubes, such modes should show up as part of, or adjacent to the main beam anyhow.

One possible cause for this artifact is that the output-end mirror (Output Coupler or OC) has some ‘wedge’ (the two surfaces are not quite parallel) built in to move any reflections – unavoidable even from Anti-Reflection (AR) coated optics – off to the side and out of harm’s way. Where wedge is present, the small portion of the light that returns from the outer AR coated surface of the OC will bounce back to the mirror itself and out again at a slight angle away from the main beam. In a dark room there may even be additional spots visible but each one will be progressively much much dimmer than its neighbor. Note that if the laser had a proper output aperture (hole), it would probably block the ghost beams and thus you wouldn’t even know of their existence!

Without wedge, these ghost beams would be co-linear with the main beam (exit in the same direction) and thus could not easily be removed or blocked. This could result in unpredictable interference effects since the ghost beams have an undetermined (and possibly varying) phase relationship with respect to the main beam. Sort of an unwanted built-in interferometer! The wedge also prevents unwanted reflections from that same AR coated front surface back into the resonator – perfectly aligned with the tube axis – which could result in lasing instability including cyclic variations in output power.

Thus, the ghost beam off to one side is likely a feature, not a problem! The effects of wedge on both the output beam and a beam reflected from a mirror with wedge is illustrated in Effects of Wedge on Ghost Beams and Normal Reflections. Note that his diagrams shows the effect of a beam coming in from the right and reflecting off the mirror. Where the beam is from the tube itself, the main beam corresponds to the one marked “1st Back Surface”.

If it isn’t obvious from close examination of the output mirror itself that the surfaces are not parallel, shine a reasonably well collimated laser beam (e.g., another HeNe laser or laser pointer) off of it at a slight angle onto a white screen. There will be a pair of reflected beams – a bright one from the inner mirror and a dim one from the outer surface. As above, if the separation of the resulting spots increases as the screen is moved away, wedge is confirmed (there may be higher order reflections as well but they will be VERY weak – see below). Where the mirror is curved, the patterns will be different but the wedge will still result in a line of spots at an angle dependent on the orientation of the tube.

Wedge is often present on the other mirror (High Reflector or HR) as well (in fact, this appears to be more likely than the OC). Wedge at the HR-end won’t affect the output beam at all but performing the reflectance test using a collimated laser (as above) at a near-normal angle of incidence may result in the following:

• An intense spot in the center due to the reflection of the beam from the actual mirror.
• A weaker spot on the thinner side of the optic due to the reflection of the beam from its front surface.
• Several progressively weaker spots on the thicker side of the optic due to multiple internal reflections between its front surface and the mirror.

With the exaggerated amount (angle) of wedge in Effects of Wedge on Ghost Beams and Normal Reflections, another effect becomes evident: The weaker spots are spaced further apart. It is left as an exercise for the student to determine what happens when a laser beam is reflected at an angle from such a mirror! Note that his diagrams shows the effect of a beam coming in from the right and reflecting off the mirror. Where the beam is from the tube itself, the main beam corresponds to the one marked “1st Back Surface”.

The appearance resembles that of a diffraction grating on such a beam (but for entirely different reasons). The behavior will be similar for an OC with wedge but because the HR mirror isn’t AR coated, the higher order spots (from the HR) are much more intense.

It is conceivable that slight misalignment of the mirrors may result in similar ghost beams but this is a less likely cause than the built-in wedge ‘feature’. However, if you won’t sleep at night until you are sure, try applying the very slightest force (a few ounces) to the mirror mounts (the metal, not the mirrors as they are very fragile) in each while the tube is powered (WARNING: High Voltage – Use a well insulated stick!!!!).

• If the ghost beam or beams are caused by wedge, all the spots will get weaker but their relative intensity and separation won’t change significantly. The peak absolute intensity should be in the relaxed position.
• If the cause is poor mirror alignment, the shape, position, relative intensity, and even the number of visible ghost beams may change dramatically. The intensity of the main beam may increase when the mirror is deflected certain ways further confirming that a realignment is needed.

Depending on the type of laser you have, see the sections: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes, Quick Course in Large Frame HeNe Laser Mirror Alignment, and External Mirror Laser Cleaning and Alignment Techniques, for more information.

Another much simpler cause of an ugly beam from a HeNe (or other) laser is dirt on the outside of the output mirror since this will decrease the effectiveness of the AR coating. The dirt may also be on other external optics. Some HeNe laser heads have either a debris blocking glass plate glued at an angle to the end-cap or a neutral density filter to adjust output power. Even if AR coated, either of these may also introduce one or more ghost beams and if not perfectly clean, other scatter as well. I’m gotten supposedly bad HeNe lasers where the only problem was dirt on either the output mirror or external plate or filter.

(From: Steve Roberts.)

The mirror is wedged to cut down on the number of ghost beams, however even with a wedged mirror there is almost always one ghost. Nothing is wrong with your coatings on the mirror, it is simply a alignment matter. The mirrors need to be “walked” into the right position relative to the bore. There are many many paths down the bore that will lase, but only a few have the TEM00 beam and the most brightness, this generally corresponds to the one with minimum ghosts.

See the section: Quick Course in Large Frame HeNe Laser Mirror Alignment for more information.

Other Spectral Lines in HeNe Laser Output

While there is no such thing as a truly monochromatic source – laser or otherwise, the actual output beam of even an inexpensive HeNe laser is really quite good in this regard with a spectral line width of less than 1/500th of a nm. For a frequency stabilized HeNe laser, it can be 1,000 times narrower!

But if you look at the output of a HeNe laser with a spectrometer, there will be dozens of wavelengths present other than one around 632.8 nm (or whatever is appropriate for your laser if not a red one). Close to the output aperture, there will be a very obvious diffuse glow (blue-ish for the red laser) visible surrounding the actual beam. So why isn’t the HeNe laser monochromatic as expected?

With one exception, this is just due to the bore light – the spill from the discharge which makes it through the Output Coupler (OC) mirror. As your detector is moved farther from the output aperture, the glow spreads much faster than the actual laser beam and its intensity contribution relative to the actual beam goes down quickly. It is not coherent light but what would be present in any low pressure gas discharge tube filled with helium and neon. However, the presence of these lines can be confusing when they show up on a spectral printout.

The exception is that with a ‘hot’ (unusually high gain) tube or one with an OC that is not sufficiently narrow-band, one (though probably not more though not impossible) of the neighboring HeNe laser lines (e.g., for other color HeNe lasers) may be lasing though probably much more weakly than the primary line. For example, a red (632.8 nm) laser might also produce a small amount of output at 629.4 or 640.1 nm though this isn’t that common. For many applications, a bit of a “rogue” wavelength output is of little consequence and specifications for general purpose HeNe lasers usually don’t explicitly include any mention of them. However, rogue output will cause reduced accuracy in metrology applications and since they may not be TEM00, even where the beam is simply used for alignment.

I have a couple of 05-LHP-171 lasers that produce up to 10 percent of their output at 640.1 nm. The first is of unknown pedigree obtained in a lot laser junk from a well known laser surplus dealer. It may have been rejected for other reasons since the output at 632.8 nm is only about 4 mW when it should be well over 7 mW. The 632.8 nm is the normal TEM00 but the 640.1 nm beam may be TEM01 or TEM10 (2 modes) or even TEM11 (4 modes) depending on mirror alignment. With optimal mirror alignment for 632.8 nm, there may be no 640.1 nm at all. The other is a 25-LHP-171-249 system sold to a university lab. It has a manufacturing date of 2000, so this isn’t only a problem with old lasers as some people have claimed.

I have one ‘defective’ yellow (594.1 nm) HeNe tube that also produces a fair amount of orange (604.6 nm), and another that produces in addition some of the other orange line (611.9 nm).

While the probability of a commercial HeNe laser outputting at a rogue wavelength is low, where such a laser is used for measurements assuming pure 632.8 nm, errors could result. For more on this topic, see the paper:

• “Advice from the CCL on the use of unstabilized lasers as standards of wavelength: the helium-neon laser at 633 nm”, J. A. Stone, J. E. Decker, P. Gill, P Juncar, A Lewis, G. D. Rovera and M. Viliesid, 2009 Metrologia 46, 11-18.

In the course of research for this paper, the first author, Jack Stone, borrowed one of my interesting Melles Griot 633 nm lasers that produced 5 to 10 percent of its output at 640.1 nm! 🙂

And sometimes HeNe lasers are designed to produce more than one wavelength. PMS/REO used to produce such lasers, usually a visible and IR line, or a pair of IR lines. But one that I’ve acquired is a combination yellow (594.1 nm) and green (543.5 nm). Unlike many other PMS/REO lasers that produce multiple lines by accident :), this one was either designed that way or their Marketing department decided to convert a bug into a feature, since the laser head has a model number of LHGYR-0300M. Since these wavelengths are so far apart and are low gain, such an occurrence would normally not be likely, but I’ve also seen one where this was the case – a yellow laser that had a tiny bit of green.

I also have one that does orange (612 nm) and green (543.5 nm) but only under special conditions. It is a high mileage PMS/REO LHOR-0150M so it’s presumably supposed to be 612 nm only. But when run at lower than normal current, the green line pops up weakly. Just above the dropout current at 4.5 mA, it produces about 0.5 mW of orange and 0.05 nm of green. At 6.5 mA, it produces around 1 mW of pure 612 nm orange. I rather suspect this is a peculiarity of the tube running near end-of-life with a lower gas pressure. When new, it may not have been so interesting. 🙂

For more on multiline HeNes, see the sections starting with: The Dual Color Yellow/Orange HeNe Laser Tube. And to make your own (sort of), see the section: Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes.

(From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)

For gas lasers the plasma lines are typically 80 dB or more below the output (measured, of course, within the very small laser mode divergence). This is unlike most semiconductor lasers, which typically have broad ‘shoulders’ close in to the line, as well as ‘lines’ due to other modes and instabilities because the initial divergence of the diode is high, and spontaneous emission from the junction high, the broad background tends to be large.

For gas lasers it is usually in the form of narrow lines at remote wavelengths, very easily removed with an interference filter and/or spatial filtering in the *rare* cases where it matters. There is presumably a weak broad background from processes involving free electrons (bound/free and free/free), but I’ve never seen it even mentioned, let alone observed it. More likely to be significant in the high current density argon laser than the very low current density HeNe.

The only cases I have seen where the plasma lines caused problems were Raman measurements on scattering samples with photon counting detection, and weak fluorescence measurements which are similar.

In most cases scattered light in the monochromator is much more of an issue (hence double monochromators for Raman) and will obscure plasma lines in many cases.

Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes

As a practical matter, the only wavelength that is useful from an internal mirror HeNe laser is the one for which it was designed. (Or the pair in the case of a couple of Research Electro-Optics (REO) lasers.) However, it is often possible to at least obtain unstable lasing at other wavelengths by extending the cavity using an external mirror. The output power of the other lines can be anywhere from almost non-existent to greater than the power at the original wavelength. This probably works best obtaining a some red from a long “hot” yellow (594.1 nm) or orange (611 nm) tube since at least one mirror is likely coated broadband to include yellow through red. Due to the low gain of the non-red lines, going the other way – getting yellow from a red tube, for example – is not likely to succeed unless the tube is very long. But obtaining lasing at other red wavelengths – and even orange – may be possible with a moderate size red HeNe laser tube. Even a 1 mW tube may give you 1 or 2 other red lines. I doubt it will work at all with a green HeNe tube having mirrors that appear orange in transmission since both mirrors are probably too transparent at even the yellow wavelength (except possibly if two external mirrors are used). However, if a mirror is more red in transmission, there might be a chance. See the section: Instant HeNe Laser Theory for a table of HeNe lasing wavelengths and relative gains.

I’ve gotten most of the well known HeNe lasing lines in this manner including up to 4 mW of red from a 2 mW yellow HeNe laser, both orange lines, various other red lines, and one of the wavelengths that isn’t even mentioned in most texts dealing with HeNe lasers. More below. I’ve only heard of one instance of any yellow being produced from non-yellow tubes, that being a REO 612 nm laser. And I haven’t even attempted to obtain green from non-green tubes.

Here’s how to get other wavelengths from your HeNe laser. Either a bare tube or complete laser head can be used for these experiments.

• Position an adjustable mount with a red OC or HR mirror a few inches beyond the OC or HR of the tube. Depending on the actual reflectance curves of the tube’s mirrors, one end will be better than the other. An HR may have a better chance of obtaining the low gain wavelengths if it is broadband, but many found on commercial HeNe lasers have been designed with high reflectivity only at the desired lasing wavelength in order to suppress the others. For the best chance of obtaining the most wavelengths from a red HeNe laser tube, I’d suggest an external red HR mirror beyond the tube’s internal OC mirror. For an “other color” tube, which end is best may be a random function (see below). However, in general, the OC is preferred over the HR since the outer surface almost always has better optical quality and will be AR coated. The outer surface of an HR may have some wedge and its shape and coating are pretty much irrelevant to normal laser operation (some are not even polished), so could be quite poor inside a cavity.The Radius of Curvature (RoC) of the external mirror may need to be consistent with a stable resonator configuration for the overall cavity. I’m not entirely sure this matters that much (and the implication in the next section is that it may not), but I’d still go with a stable configuration given a choice. If you don’t want to perform the calculations, a mirror that should work would be one from a dead red HeNe laser at least as long as the tube you are using. Of those I tried that worked at all, minimizing the distance between the ecternal mirror and tube resulted in the best results but this may not always be true. A dielectric mirror is definitely preferred but a good quality aluminized front surface (planar) mirror should work, though it may not be as good.
• Place diffraction gratings in the beams from both ends of the laser so the spectra can be projected onto white cards. If you don’t happen to own a “real” diffraction grating, a junk CD or CD-R disc works quite well as a reflection grating. In conjunction with a weak positive lens inserted in the beam before the grating (approximately 1 focal length from the end of the tube), the lines can be narrowed to permit sub-nm resolution. There will be a bit of spread perpendicular to the spectrum due to the bit patterns encoded on the disc but this just makes the lines look more like lines. :)The quality of the beam from the end of the tube opposite where your external mirror is located will probably be better, especially if the mirror is beyond the HR of the tube (which may have some wedge and is not AR coated). However, the beam from the external mirror end is instructive at least in helping to adjust the alignment.
• Power up the laser and carefully adjust the external mirror so that the beam from the tube (leakage from HR or output beam from OC) reflects back on itself. As the adjustments pass through this exact point, there should be evidence of wavelength competition as a color change and/or intensity change of the beams at one or both ends of the laser. The size of the beam exiting the external mirror will also be a minimum at this point and there may be visible interference effects. As the tube heats and expands, the wavelengths will come and go as the modes compete for attention. Just touching the mirror mount will also result in similar effects. Nothing will likely be stable for more than a few seconds. When lasing at the non-design wavelengths is present, the intensity of the original color(s) will probably decrease, possibly substantially.

Using my Melles Griot 05-LYR-170 yellow HeNe tube which for my “broken” sample, actually lases a combination of yellow (594.1 nm) and orange (604.6 nm) from both ends (see the section: The Dual Color Yellow/Orange HeNe Laser Tube), it was quite easy to achieve red output, and all three colors were occasionally present at the same time – an impressive achievement for a HeNe laser. My setup is shown in 05-LYR-170 HeNe Laser Tube Mounted in Test Fixture for Multiline Experiments. The output from the tube’s OC was directed at an AOL CD used as a reflective diffraction grating with the first-order beam projected on a white card several feet away. An MSN CD would work just as well 🙂 but a CD-R or CD-RW may not. The lens from a pair of eyeglasses (mildly positive, about 4 diopters or 1/4 meter focal length) narrowed the spots to improve spectral resolution. This rig could easily resolve lines separated by less than 1 nm. The first external “red” mirrors I tried were from an SP-084 HeNe laser tube but due probably to their relatively short RoC, the 05-LYR-170 had to be pushed quite close to the mount to get any red output. Mirrors designed for a longer laser worked better but there wasn’t much difference between the behavior using an HR or OC (99 percent).

Then to add to the excitement, with a bit of twiddling, I was able to obtain the other orange line (611.9 nm) as well, and at times, all 4 lines were lasing simultaneously! As expected, this additional line was only present when using an external HR. Depending on the original makeup of the yellow and orange beam (for this tube, their absolute and relative intensities varied with time and were also a very sensitive function of mirror alignment), it was possible to get mostly red or to vary the intensities of the other colors, most easily suppressing yellow in favor of orange and red. The intensity of the red output was never more than 1 mW or so. Its transverse mode structure varied from TEM00 to a star pattern with nothing in the center. Strange. Due to both surfaces of the HeNe tube’s HR mirror reflecting some of the intracavity beam resulting in a multiple cavity interference effect, there was a distinct lack of stability. To help compensate for this, a micrometer screw to precisely adjust cavity length without affecting mirror alignment would have been nice.

I also tried this with the external mirror mounted beyond the tube’s OC mirror but although there was a definite effect on yellow and orange lasing, it wasn’t possible to obtain any red output. (For the 05-LYR-170, the OC already reflects red quite well and the HR doesn’t.) Finally, I replaced the red external mirror with a green HR (from a tube of about the same length) mounted beyond the 05-LYR-170’s OC (since its HR by appearance looked like it might be a good mirror for green). But, not surprisingly, while this could affect the lasing of the yellow and orange lines, I could detect no coherent green photons. However, I would expect that with a appropriately coated mirrors (or possibly two such mirrors, one beyond each end of the tube), obtaining lasing at the relatively high gain 640.1 nm red line would be easy – the usual “red” mirrors may deliberately kill this line to prevent it from lasing. Although I couldn’t detect any evidence of lasing at the other red lines of 629.4 nm and 635.2 nm, these should also be possible with appropriate mirrors as they have higher gain than the yellow and oranges. Another interesting one would be the “Border Infra-Red” line at 730.5 nm. Lasing at the IR lines might also be possible but they are so boring. 🙂

Next, determined to do something with a more normal HeNe laser tube, I tried a Siemens tube but that refused to do anything interesting. Then, I tried a Melles Griot 05-LHR-150 which typically outputs a 5+ mW red (632.8 nm) beam. Since the OC for this laser is probably around 99% reflective at most, peaking at 632.8 nm, I figured that it would be best to place the external mirror beyond the OC rather than the HR. And, with the same external HR as used above, it was possible to obtain 6 lasing lines, count’m 6: 629.4 nm, 632.8 nm, 635.2 nm, 640.1 nm, a line popping up around 650 nm (all variations on red), ****AND**** 611.9 nm orange! However, since the output is being taken from the HR, none of the colors was more than a fraction of a mW.

Lasing of the 650 nm line was hard to obtain – it only showed up for a few seconds off-and-on every few minutes and increasingly rarely after the tube warmed up. The exact wavelength is very close to 650 nm (649.98 nm) as determined later with an Agilant 86140B Optical Spectrum Analyzer (OSA) which is a lot more expensive than my AOL CD. 🙂 (The wavelength was referenced to the 632.8 line from the same laser resulting in a measurement error bound of +/- 0.02 nm assuming the 632.8 nm line is actually 632.8 nm. But since this could also be slightly shifted, the error may be higher.) Getting anything at 650 nm is really puzzling as there are no HeNe lasing lines between 640.1 nm and 730.5 nm. But I have no doubt it is a true lasing line since it was fluctuating independantly of the others (later confirmed, see below). And all those other lines were quite accurately located corresponding to their handbook wavelengths in the diffracted pattern (and later confirmed with the OSA). So there is little reason to suspect that the funny one isn’t as well. When present, it appeared as strong (or weak) as all the expected ones, (except of course, the original 632.8 nm line which was usually, but not always, the strongest). If 650 nm is not a HeNe lasing line – it’s certainly not in the sequence of energy level transitions that produce all the other visible HeNe lines – one possible explanation is that there is some trace element present inside the tube and that is what’s lasing, not neon. I figured this to be a distinct possibility since the particular tube I am using originally had gas contamination and I revived it by heating the getter. (See the section: Repairing the Northern Lights Tube.) Therefore, the 650 nm wavelength may not be present with another more normal tube. But as it turned out, contamination has nothing to do with it.

I don’t think the 730.1 nm line was present but given its low relative perceived brightness, it may not have been visible at all using my AOL Special CD diffraction grating but I couldn’t find it with the OSA either. It took awhile to detect the evidence of the 635.2 nm line which only appeared sporatically (but it is the lowest gain of all the known ones above).

A few days later, I tried the same experiment with a couple of my old Spectra-Physics 084-1 HeNe laser tubes which are of soft-seal design so have almost certainly leaked over time (but still work fine). With my “hottest” SP084-1 (about 2.9 mW), I could almost duplicate the results of the 05-LHR-150 including the funny line around 650 nm but minus anything at 635.2 nm. Using a more normal 2.4 mW SP084-1, it was possible to obtain (non 632.8 nm) lines at 629.4 nm and 640.1 nm. For these, an SP084-1 HR worked almost as well for the external mirror as the longer RoC HR I had been using with the 05-LHR-150. I then installed a SP098-1, a common hard-seal barcode scanner tube (this sample puts out about 1.4 mW). With that, the only additional line was at 640.1 nm. Which particular lines appear in each case seem consistent with the length of the tubes (and thus the single pass gain) and the relative gain of the lasing lines.

Some quick calculations predict that the real effect of the external HR mirrors is the obvious one – to increase the circulating power. A 1 percent OC (typical) followed by even a 90 percent external mirror would result in greater than a 99.9 percent effective mirror for a range of wavelengths/modes. An external 99.9 percent HR would result in an even better effective mirror. It looks like the reflectance peak is relatively broad with respect to wavelength (the transmission peak is rather narrow). Specific modes for each of the wavelengths will be enhanced or suppressed. This would also appear to be consistent with the apparent lack of need for the external mirror to result in a stable resonator. All it has to do is form a Fabry-Perot cavity.

These have to be classified right up there in the really fascinating experiments department. Seeing any HeNe laser operating with multiple spectral lines is really neat.

For more examples of these stunts using an already interesting “defective” HeNe laser, see the sections starting with: Melles Griot Yellow Laser Head With Variable Output and in particular, the section: External Mirror Therapy for Variable Power 05-LYR-171 Yellow Laser Head.

As always, depending on mirror reflectivity and other factors, your mileage may vary. But feel free to try variations on these themes. The results from using an HeNe HR beyond the OC of almost any red HeNe laser tube should be easily replicated (except perhaps for the funny 650 nm line). Almost any mirror will do something since even an aluminized mirror will be returning over 90 percent of the otherwise wasted photons to the cavity – enough to boost the gain of all but the weakest lines enough for lasing if everything lines up just right. Aside from getting zapped by the high voltage or dropping the tube on the floor, they are low risk, high reward experiments.

And, can you believe that people get stuff like this published in scholarly journals? I was recently sent an article entitled: “Yellow HeNe going red: A one-minute optics demonstration” by Christopher Hopper and Andrzej Sieradzan, American Journal of Physics, vol. 76, pp. 596-598, June 2008. Geez, they could have saved a lot of time and effort and come here instead. Or, perhaps they did. 🙂

(From: Bob.)

For neutral neon at low pressure, the lines 640.3 nm, 659.9 nm are listed. For neutral helium, there is one at 667.8 nm. None of the other noble gases have wavelengths listed this short. As far as ionized species go, singly ionized argon has a line at 648.30 nm. Singly ionized krypton has a hand full of lines from 647 nm to 657 nm. Finally, xenon has one at 652 nm.

For atmospheric gases, there is a singly ionized nitrogen line at 648.3 nm. There are no neutral lines of interest for atmospheric gases. The footnotes for the above line were listed as CW lasing in 0.02 torr of krypton. Whats the standard operating pressure of a HeNe laser? Not THAT far out of the ball park I would guess.

(From: Sam.)

The last one sounds promising and would make sense given the history of the particular 05-LHR-150 and the soft-seal design of the SP084-1. Though HeNe lasers operate in the 2 to 3 TORR range – about 100 times higher pressure, the partial pressure of any N2 contamination could very well be down around 0.02 Torr.

However, I now know exactly where the 650 nm line is coming from and it has nothing whatsoever to do with contamination. The exciting writeup from someone who beat me to this by about 15 years follows in the next section preceeded by a condensed version, below.

I’ve also found a commercial laser that appears to produce a very stable 650 nm line. See the section: The PMS/REO External Resonator Particle Counter HeNe Laser.

(From: Stephen Swartz (sds@world.std.com).)

Lasing of certain HeNe tubes at 650 nm is a known phenomenon and not just a hallucination. The 650 nm line which is never discussed in most standard texts is not due to a “normal” transition of neon. It comes instead from a Raman transition. The 650 nm line is not often observed but when it is it will always be seen simultaneously with operation on a multitude of other lines. A large number of other “unusual” colors have been seen over the years. Higher power tubes with mirrors that are excessively broadband are your best bet for observing them. Often these lines flicker on and off over a few seconds to minutes time scale. A diffraction grating is a good way to look for them.

(From: Someone at a major laser company.)

The 650.0 nm Raman line is a known problem in that it competes for power with the 632.8 nm line intermittently, particularly in long tubes with high circulating power. Polarized tubes are much less susceptible to this effect and using a lower reflectance for the OC mirror helps since it reduces circulating power without affecting output very much (over a reasonable range).

Bruce’s Notes on Getting Other Lines from Red (633 nm) HeNe Laser Tubes

This, to make a gross understatement, would appear to be the definitive word on coaxing other colors from surplus HeNe laser tubes. And I thought six lines (including the mysterious 650 nm line) was an achievement. 🙂

(From: Bruce Tiemann (BruceT@ctilidar.com).)

I have gotten many lines from many different HeNe lasers. In my experience almost every tube is capable of giving at least one other line than 633 nm. (Most wavelengths have been rounded to save bits. So, 632.8 nm becomes 633 nm.) I have never tried doing this with lasers that give other lines than 633 nm, but since that line has the highest gain, it should be no mean feat to at least get that line from lasers that are supposed to not give it. It is also not my experience that calculations to ensure resonator stability, etc., are necessary. Just try it! My best results, in terms of output power, were with a flat grating as the external feedback mirror, and my best results in terms of new lines was obtained with a flat dielectric mirror, formerly used as a facet in a polygonal scanning assembly. Flat mirrors are not stable at any separation for a diverging beam, and HeNe lasers are very rare that give converging beams for their output.

The home stuff had the mirrors on blocks, with the steering accomplished by adjusting the HeNe tube by lifting one or the other end of the tube with sheets of paper, and the azimuth by moving the laser tube back and forth. The lab experiments were done with “real” mirror mounts, supplemented by a single PZT that tilted the feedback mirror a few microns.

(I like PZTs a great deal, and would like to observe that you can get PZT elements from little piezo alarms, from which the useful element can be extracted with some hand-tools and the mind-set of a 9-year-old kid dissecting a bug. 🙂 These are only about $1 each, as opposed to tens to hundreds of bucks for “real” PZTs that you buy from Thor, etc. One of them and a 0 to 50 VDC power supply can precision-wiggle a mirror on the micron scale, which is all that is needed for these experiments.)

(From: Sam.)

I have indeed done something similar using the piezo beeper from a dead digital watch to move a mirror in a HeNe laser based Michelson interferometer. With 0 to 25 V, it went through 4+ fringes which means over 2 full wavelengths at 633 nm. The configuration in these is called a “drum head” piezo element because the movement resembles that of a musical (depending on your point of view!) drum head with the most shift in the center. The piezo material itself doesn’t change by very much in thickness but is constructed so it distorts to produce the shape change. With care, the piezo material can be cut to size or drilled to pass light through its center. Much more voltage could have been safely applied if needed.

(From: Bruce.)

Something I also did is cast the spots from a smaller (approximately 3/4 m) spectrometer directly onto the CCD element of a small camera with no lens. I also fabricated a beam block by taping little wires to the side of a block, that would protrude up just in the locations of the very bright lines, like 633, 650, and 612 nm, to block them, but letting light of other colors pass in the ample space between the wires. You could still see when the bright lines were on from light leaking around the wires, but it wouldn’t wash out the image when they were.

In this case, when the feedback mirror was tilted, speckle, which was cast everywhere, would kind of shift around all over the place, but the new lines looked like ghostly bullseyes, which would breathe in and out as the mirror was tilted, but remain in the same location unlike the speckle. This was an easy way to see the weakest lines like 624 nm, and it was also how I discovered 668 nm, the CCD being more sensitive than the eye in the deep red. (I searched for but did not find the normal laser line 730 nm even with this very sensitive method.)

• “Normal” laser lines: The multiplet that gives 633 nm includes a total of nine lines, ranging from 543 nm at the green end to 730 nm at the far-red end. In between are yellow (594 nm), orange (605 and 612 nm), and several reds (629, 633, 635, and 640 nm). I have never obtained the yellow, green, or far-red line from any 633 laser but I have gotten all the others.
• Grating feedback: My best results are with a 5 mW Melles Griot 05-LHR-551 (or similar) laser. For maximum power output on these lines, a flat aluminum-coated grating, unblazed and with low (less than 10%) diffraction efficiency, gives near 1 mW power on 612 and 640 nm (as well as 650 nm, see below), when it is used to exactly retro-reflect the 633 nm beam back into the bore. The grating handily disperses the different colors off to the side. Sub-mW output power is also available on 629 and 635 nm. The beams sometimes wink on and off, but contrary to one’s impression they are on more often than they are off, and represent fairly stable and reliable outputs. I have also used a blazed 600 l/mm Edmund grating in Littrow, meaning, slanted so the first order is returned to the output coupler, and have thereby obtained operation at 612 and 640 nm, one at a time, at reduced output power, though of course with 633 nm on all the time.
• 640.1 nm: This line experiences anomalous dispersion from a nearby line, and therefore experiences gas lensing in the bore. Hence, it will oscillate in some marginally unstable (2-mirror) resonators even when 633 nm won’t. In a 3-mirror system, 2 from the laser tube and a third added by the experimenter, 640 nm is often the line that will oscillate with the least feedback. One of my 5 mW lasers will lase this line with an uncoated glass microscope slide, or even a plastic ruler or plastic box-lid as the third (feedback) mirror, up to a distance of several inches from the output coupler. With an uncoated Edmund 1/10 wave optical flat, 640 nm would oscillate with the surface located up to some 1.2 m away from the output coupler of the laser. That the resulting resonator is unstable can be clearly seen by the fact that the retro-reflected beam from the flat grossly overfills the size of the beam exiting the output coupler, nevertheless, 640 nm operation can be verified by looking at the diffracted beam from a grating, located behind the optical flat.That 640 nm line would lase even with a plastic ruler or similar non-mirror mirror and could be established by hand-holding the piece of plastic in the beam, braced against the laser tube.
• Dielectric feedback mirrors: Most small, 1 mW metal-ceramic tubes won’t give any other lines with a metal grating as the feedback element, so it is natural to try to coax them out with higher R. With use of a max-R dielectric mirror, almost every laser I have tried has given at least one other line. 640 nm is probably the most common next line to get, though there are some tubes that give many others but not that one, go figure. One small metal-ceramic tube would only give 629 and 635 nm, no others, even though these are weak lines. Probably has to do with the narrow-band coating on the back mirror. Interestingly, this laser would only give them periodically, with the 633 nm mode, normally TEM00, splitting up and becoming complicated, at which point the other lines would come on. When 633’s mode started to simplify, the others would vanish. One is drawn to think that the max-R mirror plus the laser output coupler forms a Fabry-Perot cavity, that when it becomes resonant at 633 nm, loses reflectivity there and thereby gives gain to the other lines, as well as inclines 633 to find a higher-order mode where the cavity is still reflective.
• 604.6 nm: My “best” tube gives 605 nm, only in one exact position of feedback mirror only a few mm from the output coupler,. and then, curiously, only when the feedback mirror is misaligned such that the appearance of the output, seen in transmission through the dielectric mirror, is a contiguous line of spots, instead of just the dot in the middle. This line competed strongly with 612, trading intensity and almost never being on at the same time. When 605 was on, it was just as bright as 612, which is curious given how reluctant it was to lase.
• 650.0 nm: Many 5 mw lasers give a line at 650.0 nm as well. This line isn’t a neon (Ne) transition, and isn’t due to an impurity either. Nor is it widely known. I demonstrated this line, and some of the usual other ones, to an astonished audience at Japan’s NIST, the National Research Laboratory for Metrology in Tsukuba, using one of their own “single line” HeNe lasers and one of their dielectric mirrors. It’s an electronic Raman line, pumped by 633 nm. The 2s states of Ne, which are what the 2p lower laser levels dump into, include two metastable states, from which it is forbidden to drop into the ground state. Hence, they build up population in the plasma. If a Ne metastable interacts with a 633 photon, it can take some of the energy to promote the atom to a higher state, and scatter the photon with the correspondingly lesser amount of energy, in this case at 650 nm. The energy difference between the 1S5 and 1S4 states, about 417 cm^-1, is the same as the energy difference of the photons, 633 nm compared to 650 nm. This line was only discovered around 1985!The gain-bandwidth of the Raman transition is only 60 MHz wide, so the cavity modes for 633 nm must line up with the cavity modes at 650 nm, to within this uncertainty, in order for 650 nm to oscillate. Considering that the Doppler bandwidth is more like 1,500 MHz (1.5 GHz), and the FSR of the laser is ~hundreds of MHz, that is only rarely the case. Hence, 650 nm comes and goes, most of the time being gone. And when it’s gone, it’s gone. When the laser warms up, however, the cavity expands, and the 633 and 650 nm modes sort of vernier past each other, sometimes bringing them into alignment in difference-frequency space. When they align, 650 nm oscillates. The observed behavior is that 650 nm more rapidly blinks on when the laser is warming up, but only for short periods, and then as the tube comes closer to a steady-state temperature, the periods become less frequent, but 650 nm lasts for a longer duration each time. Eventually, at the steady-state condition, 650 nm will be gone, or more rarely, may persist. However, temperature control of the laser can cause 650 nm to become steady, in the low-tech way of putting a blanket made of paper sheets or something over the laser tube, to stabilize the laser tube temperature to the next-higher value that supports 650 nm oscillation, or in the higher-tech case with a heater tape and thermistor and temperature control unit. When 650 nm goes, it is strong, and one 5 mW tube gives nearly 1.5 mW of output power at 650 nm, when the feedback element was a metal grating, and the output was taken from the first order. It is also perceptibly a deeper red color than 633 or even 640 nm, to me.I (Sam) have tested two external PMS/REO particle counter assemblies lasing on up to 6 normal HeNe lines (605, 612, 629, 633, 635, 640) and the 650 nm Raman line where the 650 nm line was present 100 percent of the time with little variation in intensity. No stabilization of any kind is involved and the behavior is little changed from power-on to thermal equilibrium. This seems to directly contradict the need to simultaneous resonance mentioned above and in the papers. On one sample, I believe the 650 nm line was actually the strongest one, stronger even than the 633 line. On that laser, there was even the occasional hint of another strange line at around 653 nm. Also see the section: The PMS/REO External Resonator Particle Counter HeNe Laser.
• 4-wave mixing lines: Even less well known than 650 nm is that some HeNe lasers can give 4-wave mixing lines when 650 nm also oscillates. I have gotten 10 such lines from my “good” laser, many at one time. The most easily seen one is at 597 nm, and results from the addition of 417 cm^-1 to a 612 nm photon. It can be thought of as a sideband of 612 nm, modulated by the 11.7 THz modulation set up by the difference frequency between 633 nm and 650 nm. Thus, to see 597 nm, one needs both 650 nm and also 612 nm to be oscillating. It also needs the feedback element to be a max-R dielectric mirror – the metal grating doesn’t work, even though it easily gives 612 nm and 650 nm at the same time. The 4-wave outputs, taken in transmission through the max-R feedback mirror, or through the high-R mirror on the back of the laser tube, are VERY weak, measured in 10s of nW at most, and dwindling down into the pW for the weakest ones. (This power level may be compared with the 40 nW that results from a 1 mW HeNe beam reflecting off a 4% reflector, an uncoated glass surface.) The stronger ones, such as 597 and 613 nm, can be easily as dots on white paper in a slightly darkened room, but the weakest ones, such as 589 and 624 nm, are best seen by looking directly into the grating, with dark-adapted eyes in a darkened room, in which case they look like star-disks that come and go as the feedback mirror is wiggled, differently from the 633 speckle. 597 nm is also perceptibly different color than even 612 nm. It looks “yellow” in comparison to the other orange and red spots from the HeNe, though the true color is more orange than that.These lines were best observed with the high-R feedback mirror located within about 6 inches of the output face of the laser, closer tending to be a bit better. Except for 589 nm, which required 605 nm to be oscillating, and this only occurred for one exact spacing of feedback mirror about 1.5 cm away from the output coupler, or about 1 mm away from the output flange of the tube, which I didn’t remove. (I did, however, find out that you can take a laser tube to the university infirmary, and ask to have it X-rayed, to determine the extent of the internal glass envelope within the aluminum outer casing, and they would only charge you $10 for the cost of the X-ray film and processing, which is not bad for a doctor visit including X-rays.)To my knowledge, these lines are my discovery.

  A brief table shows the relationship between “pump” lines and 4-wave mixing lines, observed on one tube. Upper Sideband is toward shorter wavelengths from the pump; Lower Sideband is toward longer wavelengths of the pump (all values in nm):

     Upper       Pump       Lower
    Sideband  Wavelength  Sideband
   --------------------------------
     589.7       604.6      -----
     596.6       611.9      627.8
     613.3       629.4      646.4
     616.5       632.8     (650.0)
     618.8       635.2      652.5
     623.4       640.1      -----
    (633.8)      650.0      668.1    
  

  (632.8 and 650.0 nm are parenthesized since they are associated with the genesis of the 4-wave mixing lines.)

All in all this laser produced 17 different lines, many at one time, from a “single line” 633 swap-meet laser. 🙂

References:

The 650 nm discovery paper is:

• Assendrup, J.; Grover, B.; Hall, L.; Jabr, S., “CW Helium-Neon Raman Laser”, Applied Physics Letters; 1/13/86, vol. 48, is. 2, p86, January 1986.Abstract: Continuous lasing has been observed at 650 nm with a helium-neon electrical discharge placed in an ultrahigh finesse optical cavity. This new lasing line is attributed to a Stokes-Raman process between the 1s5 and 1s4 electronic states of neon atoms pumped by the 632.8-nm neon lasing line. A gain calculation based on a near-resonant stimulated electronic Raman process predicts a lasing threshold for the 650-nm line near that measured. Lasing output power was measured as a function of discharge current and helium-neon gas pressure for the pump line and for the Stokes line.
• Huang, Zhiwen; Zhao, Suitang; Jin, Haoran, “650 nm CW He-Ne Raman Laser”, (Chinese Journal of Lasers, vol. 15, Nov. 1988, p. 648-651), Chinese Physics – Lasers (ISSN 0887-3518), vol. 15, Nov. 1988, p. 803-806. Translation.Abstract: The 650 nm laser line of a simultaneously operating six-wavelength He-Ne laser was studied experimentally. It is shown that the precise lasing wavelength at 650 nm should be 650.00 + or – 0.05 nm and that this laser line is the result of the stimulated Raman scattering of the 632.8 nm transition between the 1S5 and 1S4 states. The characteristics of the Raman emission are studied and the gain is obtained.
• Peter Franke, Alfred Feitisch, Fritz Riehle, Kegung Zhao and Jurgen Helmcke, “Simultaneous cw laser emission including a Raman line of a He-Ne laser at six wavelengths in the visible range”, Applied Optics, vol. 28, no. 17, 1 September 1989, pp. 3702-3707.Abstract: Simultaneous CW laser emission has been observed in a He-Ne discharge at 611.8-, 629.3-, 632.8-, 635.1-, 640.1-, and 650.0-nm wavelengths. The output power and the mode spectra have been investigated for various operational conditions. Spontaneous mode locking of the different lines has been observed. The Raman transition (650.0 nm) pumped by the strong intracavity radiation at 632.8 nm has been investigated in detail and its relevance for a secondary multiwavelength standard is discussed.

Miscellaneous Comments on Getting Other Lines from HeNe Laser Tubes

(From: Flavio Spedalieri.)

I have a small Yellow Tube – 05-AYR-006 (as a combo with power supply 05-LPM-496-037). This tube is physically the same size at the 1mW reds, but has a larger bore resulting in multimode output.

I have managed to get the red (632.8nm) line to lase and perhaps orange lines by placing a HR from a 632.8nm HeNe at the HR end of the yellow tube.

Further, I obtained a broadband mirror from an Argon Laser tube, the OC worked best at the OC end of the Yellow tube, have got the laser to output green…

The mirrors hand-held – next to build a small external resonator assembly.

About the Waste Beam from a HeNe Laser

The so-called High Reflector (HR) or totally reflecting mirror in a HeNe laser isn’t really perfect, though the actual reflectivity is generally 99.95 percent or better. For a 1 mW laser tube with a 99 percent Output Coupler (OC) mirror, there is about 100 mW of intracavity power. Of this, about 50 uW will exit the rear through a 99.95 percent HR mirror. Unless the back of the HR mirror is painted or covered, there is always some small beam exiting the rear of the laser.

Normally, what comes out in that direction is, well, waste, and is of no consequence. But, there are times where it’s convenient to use this low power beam as a reference, expecting its power to track that of the main output beam. Unfortunately, it is sometimes not well behaved in this regard.

In constructing some amplitude stabilized HeNe lasers which depend on the waste beam feeding a photodiode for their feedback loop, an annoying characteristic of the waste beam has become evident with some otherwise perfectly normal and healthy HeNe laser tubes. Namely, that the relative power in the waste beam and the main beam does not remain constant as the tube warms up. In fact, one tube I was using had a variation of almost 2:1 in relative waste beam and output beam power depending on the tube’s temperature. This is probably due to one or both of the following:

1. Variation in mirror reflectivity. Designing and manufacturing high reflectivity mirror coatings is somewhat of an art and they don’t always come out right. There may be ripples, a slope, or other variations in the reflectivity-versus-wavelength function. For an HR mirror on a 1 mW tube of, say, 99.97 percent resulting in 30 uW, a change of only 0.01 percent would add 10 uW to the waste beam.
2. Lack of wedge or insufficient wedge between the inner and outer surfaces of the HR mirror. This will result in an etalon effect, effectively modulating the reflectance as a periodic function of temperature by perhaps 10 or 20 percent, which would appear as a similar change in the waste beam power. From room temperature to the operating temperature of a typical enclosed HeNe laser head, the power variation would go through several cycles.

The coating problem is more likely to result in a strictly increasing, or at least slow change in waste beam power with higher temperature while the etalon would be periodic with temperature going through several cycles, it might be possible to determine which of the two effects is present.

Normally, the waste beam is not used for anything and no one cares. Though there will also be a change in the power of the output beam (inversely relative to the waste beam) from these issues, it will be too small to be detectable without careful measurements, being swamped by the normal mode sweep power variations. But when the waste beam is used as the amplitude reference in a stabilized laser, the supposedly stabilized output will vary based on the relative waste beam power. That 10 uW change would result in the output power changing by 33 percent.

For some plots the mode sweep of normal and naughty tubes, see the section: Plots of HeNe Laser Power Output and Polarized Modes During Warmup. In particular, compare the plots of the Spectra-Physics 088 with those of the tubes that immediately follow it.

Instant He-Ne Laser Theory

The term laser stands for “Light Amplification by Stimulated Emission of Radiation”. However, lasers as most of us know them, are actually sources of light – oscillators rather than amplifiers. (Although laser amplifiers do exist in applications as diverse as fibre optic communications repeaters and multi-gigawatt laser arrays for inertial fusion research.) Of course, all oscillators – electronic, mechanical, or optical – are constructed by adding the proper kind of positive feedback to an amplifier.

All materials exhibit what is known as a bright line spectra when excited in some way. In the case of gases, this can be an electric current or (RF) radio frequency field. In the case of solids like ruby, a bright pulse of light from a xenon flash lamp can be used. The spectral lines are the result of spontaneous transitions of electrons in the material’s atoms from higher to lower energy levels. A similar set of dark lines result in broad band light that is passed through the material due to the absorption of energy at specific wavelengths. Only a discrete set of energy levels and thus a discrete set of transitions are permitted based on quantum mechanical principles (well beyond the scope of this document, thankfully!). The entire science of spectroscopy is based on fact that every material has a unique spectral signature.

The HeNe laser depends on energy level transitions in the neon gas. In the case of neon, there are dozens if not hundreds of possible wavelength lines of light in this spectrum. Some of the stronger ones are near the 632.8 nm line of the common red HeNe laser – but this is not the strongest:

The strongest red line is 640.2 nm. There is one almost as strong at 633.4 nm. That’s right, 633.4 nm and not 632.8 nm. The 632.8 nm one is quite weak in an ordinary neon spectrum, due to the high energy levels in the neon atom used to produce this line.

(The relative brightnesses of these don’t appear to be accurate though at present.) More detailed spectra can be found at the: Laser Stars – Spectra of Gas Discharges Page. And there is a photo of an actual HeNe laser discharge spectra with very detailed annotation of most of the visible lines in: Skywise’s Lasers and Optics Reference Section. The comment about the output wavelength not being one of the stronger lines is valid for most lasers as if it were, that energy level would be depleted by spontaneous emission, which isn’t what is wanted!

There are also many infra-red lines and some in the orange, yellow, and green regions of the spectrum as well.

The helium does not participate in the lasing (light emitting) process but is used to couple energy from the discharge to the neon through collisions with the neon atoms. This pumps up the neon to a higher energy state resulting in a population inversion meaning that more atoms in the higher energy state than the ground or equilibrium state.

 

It turns out that the upper level of the transition that produces he 632.8 nm line (as well as the other visible He-Ne lasing lines) has an energy level that almost exactly matches the energy level of helium’s lowest excited state. The vibrational coupling between these two states s highly efficient.

1. A DC electrical discharge or RF field excites He atoms to the 2s energy state.
2. Collisions efficiently transfer energy raising Ne atoms to the 3s2 energy state. Note the relatively high energy levels involved – over 20 eV for the upper energy states.
3. Stimulated emission (lasing) causes a drop to one of several Ne 2p states.
4. Radiative decay (spontaneous emission) drops Ne from the terminal lasing state to the 1s state.
5. Collisions with the tube wall drops Ne from the 1s state to the Ground state.

For 632.8 nm, one mirror will be highly reflective at 632.8 nm (typically 99.9 percent or better). This is the “High Reflector” or HR. The other mirror will have a typical reflectivity of 99 percent at 632.8 nm. This is the “Output Coupler” or OC from which the useful beam emerges. In order to suppress lasing at other wavelengths, the mirrors will generally be designed to have lower reflectivity there. (Though given the low gain of all the He-Ne lasing lines, especially the “other colour” lines, this isn’t much of a problem at 632.8 nm.)

The rate at which (4) and (5) can take place ultimately limits the power of a He-Ne laser and explains why increasing the excitation (1) actually reduces power above some optimum level.

The gas mixture must be mostly helium (typically 5:1 to 10:1, He:Ne), so that helium atoms can be excited. The excited helium atoms collide with neon atoms, exciting some of them to the state from which they can radiate at 632.8 nm. Without helium, the neon atoms would be excited mostly to lower excited states responsible for non-laser lines. And the gas mixture has to be super pure as any contamination results in excitation of rogue atoms (like H, O, and N) to lower energy states where all that will happen is that they will glow like a poorly made neon sign.

A neon laser with no helium can be constructed but it is much more difficult and the output power will be much lower without this means of energy coupling. Therefore, a He-Ne laser that has lost enough of its helium (e.g., due to diffusion through the seals or glass) will most likely not lase at all since the pumping efficiency will be too low.

However, pure neon will lase superradiantly in a narrow tube (e.g., 40 cm long x 1 mm ID) in the orange (611.9 nm) and yellow (594.1 nm) with orange being the strongest. Superradiant means that no mirrors are used although the addition of a Fabry-Perot cavity (e.g., mirrors!) does improve the lateral coherence and output power. This from a paper entitled: “Super-Radiant Yellow and Orange Laser Transitions in Pure Neon” by H. G. Heard and J. Peterson, Proceedings of the IEEE, Oct. 1964, vol. #52, page #1258. The authors used a pulsed high voltage power supply for excitation (they didn’t attempt to operate the system in CW mode but speculate that it should be possible).

(From: Steve Roberts.)

“Various IR lines will lase in pure neon, and even the 632.8 nm line will lase, but it takes a different pressure and a much longer tube. 632.8 nm also shows up with neon-argon, neon-oxygen, and other mixtures. Just about everything on the periodic table will lase, given the right excitation. See “The CRC Handbook of Lasers” or one of the many compendiums of lasing lines available in larger libraries. These are usually 4 volume sets of books the size of a big phone book just full of every published journal article on lasing action observed. It’s a shame that out of these many thousands and thousands of lasing lines, only 7 different types of lasers are under mainstream use.

There are many possible transitions in neon from the excited state to a lower energy state that can result in laser action. (Only the three found most commonly in commercial He-Ne lasers are shown in the diagram, above.) The most important (from our perspective) are listed below:

      (1)         (2)           (3)           (4)          (5)         (6)
     Output       HeNe       Perceived       Lasing      Typical     Maximum
   Wavelength  Laser Name    Beam Colour   Transition   Gain (%/m)  Power (mW)
 ------------------------------------------------------------------------------
     543.5 nm    Green         Green        3s2->2p10   0.52   0.59    2 (5)  
     594.1 nm    Yellow    Orange-Yellow    3s2->2p8    0.5    0.67    7 (10)
     604.6 nm                  Orange       3s2->2p7    0.6    1.0     3
     611.9 nm    Orange      Red-Orange     3s2->2p6    1.7    2.0     7
     629.4 nm                Orange-Red     3s2->2p5    1.9    2.0
     632.8 nm     Red          "    "       3s2->2p4   10.0   10.0    75 (200)
     635.2 nm                  "    "       3s2->2p3    1.0    1.25
     640.1 nm                   Red         3s2->2p2    4.3    2.0     2
     730.5 nm             Border Infra-Red  3s2->2p1    1.2    1.25    0.3
     886.5 nm                  "    "       2s2->2p10   1.2    1.25    0.3
   1,029.8 nm   Near-IR      Invisible      2s2->2p8    ???
   1,062.3 nm    "   "         "   "        2s2->2p7    ???
   1,079.8 nm    "   "         "   "        2s3->2p7    ???
   1,084.4 nm    "   "         "   "        2s2->2p6    ???
   1,140.9 nm    "   "         "   "        2s2->2p5    ???
   1,152.3 nm    "   "         "   "        2s2->2p4    ???            1.5
   1,161.4 nm    "   "         "   "        2s3->2p5    ???
   1,176.7 nm    "   "         "   "        2s2->2p2    ???
   1,198.5 nm    "   "         "   "        2s3->2p2    ???
   1,395.0 nm    "   "         "   "        2s2->2p?    ???            0.5
   1,523.1 nm    "   "         "   "        2s2->2p1    ???            1.0
   3,391.3 nm    Mid-IR        "   "        3s2->3p4    ???  440.0    24

Notes:

 

• Output Wavelength is approximate. In addition to slight variations due to actual lasing conditions (single mode, multimode, doppler broadening, etc.), some references don’t even agree on some of these values to the 4 or 5 significant digits shown.
• He-Ne Laser Name is what would be likely to be found in a catalogue or spec. sheet. All those that have an entry in this column are readily available commercially.
• Perceived Beam Colour is how it would appear when spread out and projected onto a white screen. Of course, depending on the revision level of your eyeballs, this may vary someone from individual to individual. 🙂
• Lasing Transition uses the so-called “Paschen Notation” and indicates the electron shells of the neon atom energy states between which the stimulated emission takes place.
• Typical Gain (%/m) shows the percent increase in light intensity due to stimulated emission at this wavelength inside the laser tube’s bore. This is the single pass gain and will be affected by tube construction, gas fill ratio and pressure, discharge current, and other factors. The first column is from various sources. The second column is from Hecht, “The Laser Guide Book”. However, a newer text: Mark Csele, “Fundamentals of light sources and lasers” (ISBN 0-471-47660-9, Wiley-Interscience, 2004) lists the typical gain as 1.2 to 1.5 at 633 nm. And measurements by myself and others seem to show that this slightly higher value may be more accurate, at least under some conditions.
• Gain at 1,523 nm may be similar to that of 543.5 nm – about 0.5%/m. Gain at 3,391 nm is by far the highest of any – possibly more than 100%/m. I know of one particular He-Ne laser operating at this wavelength that used an OC with a reflectivity of only 60% with a bore less than 0.4m long. Yet, the output power of the largest 3,391 nm commercial He-Ne laser is still only a fraction of that at 632.8 nm.

Maximum Power shows the highest output power lasers commercially available in a TEM00 beam for each wavelength. The first number is rated power while the number in () is achieved output power for a particularly lively tube. Lasers operating with multiple (spatial) modes (non-TEM00) may have somewhat higher output power.

The most common and least expensive He-Ne laser by far is the one called ‘red’ at 632.8 nm. However, all the others with named ‘colours’ are readily available with green probably being second in popularity due to its increased visibility near the peak of the of the human eye’s response curve (555 nm). And, with some He-Ne lasers with insufficiently narrow-band mirrors, you may see 640 nm red as a weak output along with the normal 632.8 nm red because of its relatively high gain. There are even tunable He-Ne lasers capable of outputting any one of up to 5 or more wavelengths by turning a knob. While we normally don’t think of a He-Ne laser as producing an infra-red (and invisible) beam, the IR spectral lines are quite strong – in some cases more so than the visible lines – and He-Ne lasers at all of these wavelengths (and others) are commercially available.

The first gas laser developed in the early 1960s was an HeNe laser operated at 1,152.3 nm. In fact, the IR line at 3,391.3 is so strong that a HeNe laser operating in ‘superradiant’ mode – without mirrors – can be built for this wavelength and commercial 3,391.3 nm HeNe lasers may use an output mirror with a reflectivity of less than 50 percent. Contrast this to the most common 632.8 nm (red) He-Ne laser which requires very high reflectivity mirrors (often over 99 percent) and extreme care to minimize losses or it won’t function at all.

When the He-Ne gas mixture is excited, all possible transitions occur at a steady rate due to spontaneous emission. However, most of the photons are emitted with a random direction and phase, and only light at one of these wavelengths is usually desired in the laser beam. At this point, we have basically the glow of a neon sign with some helium mixed in!

To turn spontaneous emission into the stimulated emission of a laser, a way of selectively amplifying one of these wavelengths is needed and providing feedback so that a sustained oscillation can be maintained. This may be accomplished by locating the discharge between a pair of mirrors forming what is known as a Fabry-Perot resonator or cavity. One mirror is totally reflective and the other is partially reflective to allow the beam to escape.

One mirror may be perfectly flat (planar) or both may be spherical with a typical Radius of Curvature (RoC = 2 * focal length) slightly longer that the length of the cavity (L) or even longer. Where both mirrors have an RoC equal to L, the configuration is called ‘confocal’ (the focii of the two mirrors are coincident), but it is marginall stable, so the RoCs will be at least slightly longer than L. A cavity with two planar mirrors is borderline stable and essentially impossible to align or maintain in alignment over time, so it is never used in He-Ne lasers (but is in some pulsed solid state and other lasers). Curved mirrors result in an easier to align more stable configuration but are more expensive than planar mirrors to manufacture and are not as efficient since less of the lasing medium volume is used (think of the shape of the beam inside the bore). The confocal arrangement represents a good compromise between a true spherical cavity (r = 1/2 * L) which is easiest to align but least efficient and one with plane parallel mirrors (f = infinity) which is most difficult to align but uses the maximum volume of the lasing medium. (But as noted above, for a practical confocal cavity, RoCs slightly longer than L are used to assure stability.)

These mirrors are normally made so that the two mirrors together has peak reflectivity at the desired laser wavelength. (For technical reasons, it’s sometimes easier to make mirrors like cliffs – high reflectivity that drops to low reflectivity at a given wavelength, in either direction – than to guarantee a particular peak reflectivity.) When a spontaneously emitted photon resulting from the transition corresponding to this peak happens to be emitted in a direction nearly parallel to the long axis of the tube, it stimulates additional transitions in excited atoms. These atoms then emit photons at the same wavelength and with the same direction and phase. The photons bounce back and forth in the resonant cavity stimulating additional photon emission. Each pass through the discharge results in amplification – gain – of the light. If the gain due to stimulated emission exceeds the losses due to imperfect mirrors and other factors, the intensity builds up and a coherent beam of laser light emerges via the partially reflecting mirror at one end. With the proper discharge power, the excitation and emission exactly balance and a maximum strength continuous stable output beam is produced.

Spontaneously emitted photons that are not parallel to the axis of the tube will miss the mirrors entirely or will result in stimulated photons that are reflected only a couple of times before they are lost out the sides of the tube. Those that occur at the wrong wavelength will be reflected poorly if at all by the mirrors and any light at these wavelengths will die out as well.

Summary of the He-Ne Lasing Process

The He-Ne laser is a 4 level laser (see the table above for the specific energy level transitions for the common wavelengths):

• Collisions with excited helium atoms raise the neon atoms from level 1 (ground state) to level 4 (which is the 3s state for visible wavelengths).
• The visible lasing transitions are from the 3s to various 2p states (depending on wavelength) or level 3.
• The neon atoms then decay rapidly to the 1s state or level 2.
• Return to the ground state or level 1 is aided by collisions with the He-Ne laser tube’s bore/capillary walls.

For most common IR wavelengths, level 4 is the 2s state and level 3 are various 2p states. However, the very strong 3.93 um line originates from the 3s state just like the visible wavelengths – and is the reason it competes with them in long He-Ne tubes and must be suppressed to optimize visible output.

The ‘s’ states of neon have about 10 times the lifetime of the ‘p’ states and thus support the population inversion since a neon atom can hang around in the 2s state long enough for stimulated emission to take place. However, the limiting effect is the decay back to level 1, the ground state, since the 1s state also has a long lifetime. Thus, one wants a narrow bore to facilitate collisions with its walls. But this results in increased losses. Modern He-Ne lasers operate at a compromise among several contradictory requirements which is one reason that their maximum output power is relatively low.

Approximate Reference Values for the Red (632.8 nm) He-Ne Laser

Here are some common values and relationships that may come in handy when doing calculations. These are not the most exact since they may depend on other factors like the precise gas-fill and environmental conditions but are generally good enough for government work. 🙂

• Wavelength: 632.8 nm.
• Optical Frequency: 474 THz.
• Gain Bandwidth of Neon: 1.6 GHz or 2.136 nm.
• 1 nm at 632.8 nm: 749 GHz.
• 1 GHz at 632.8 nm: 1.335 nm.

Longitudinal Modes of Operation

The physical dimensions of the Fabry-Perot resonator impose some additional constraints on the resulting beam characteristics.

While it is commonly believed that the 632.8 nm (for example) transition is a sharp peak, it is actually a Gaussian – bell shaped – curve. (Strictly speaking, it is something called a “Voigt distribution” which is a combination of Gaussian and Lorentzian – but that’s for the advanced course. Gaussian is close enough for this discussion since the discrepancy only shows up way out in the tails of the curve.) In order for a linear or (Fabry-Perot) cavity to resonate strongly, a standing wave pattern must exist. This will only occur when an integral number of half wavelengths fit between the two mirrors. This restricts possible axial or longitudinal modes of oscillation to:

                   L * 2                 c * n 
             W = ---------    or   F = --------- 
                     n                   L * 2

Where:

• L is the distance between the mirrors (m).
• W denotes the possible wavelengths of oscillation (m).
• n is a large integer (order of 948,000 for W around 632.8 nm, L = .3 m).
• F denotes the possible frequencies of oscillation (Hz).
• c is the speed of light (approximately 300 million m/s).

The laser will not operate with just any wavelength – it must satisfy this equation. Therefore, the output will not usually be a single peak at 632.8 nm but a series of peaks around 632.8 nm spaced c/(2*L) Hz apart. Longer cavities result in closer mode spacing and a larger number of modes since the gain won’t fall off as rapidly as the modes move away from the peak. For example, a cavity length of 150 mm results in a longitudinal mode spacing of about 1 GHz; L = 300 mm results in about 500 MHz. The strongest spectral lines in the output will be nearest the combined peak of the lasing medium and mirror reflectivity but many others will still be present. This is called multimode operation.

Think of the vibrating string of a violin or piano. Being fixed at both ends, it can only sustain oscillations where an integer number of cycles fits on the string. In the case of a string, n can equal 1 (fundamental) and 2, 3, 4, 5 (harmonics or overtones). Due to the tension and stiffness of the string, only small integer values for n are present with a significant amplitude. For a He-Ne laser, the distribution of the selected neon spectral line and shape of the reflectivity function of the mirrors with respect to wavelength determine which values of n are present and the effective gain of each one. And n will be much greater than 1!

For a typical HeNe laser tube, possible values of n will form a series of very large numbers like 948,161, 948,162, 948,163, 948,164,…. rather than 1, 2, 3, 4. 🙂 A typical gain function showing the emission curve of the excited neon multiplied by the mode structure of the Fabry-Perot resonator and the reflectivity curve of the mirrors may look something like the following:

                |                  632.8 nm
               I|                     .
                |                  |  |  |
                |               |  |  |  |  | 
                |            |  |  |  |  |  |  |  
         _______|______.__|__|__|__|__|__|__|__|__|__._______
           n=948,166  -5 -4 -3 -2 -1 +0 +1 +2 +3 +4 +5

Or, see the following for some slightly more aesthetically pleasing diagrams of the longitudinal modes of random polarized He-Ne lasers. 🙂

Mode Sweep

Since the mode locations are determined by the physical spacing of the mirrors, as the tube warms up and expands, these spectral line frequencies are going to drift downward (toward longer wavelengths). However, since the reflectivity of the mirrors as a function of wavelength is quite broad (for all practical purposes, a constant), new lines will fill in from above and the overall shape of the function doesn’t change.

In the diagrams above, a single arbitrary mode position is shown, but for well behaved lasers, the lasing lines will move smoothly through the gain curve as the laser warms up. This is called by various names including “mode sweep” and “mode cycling”. While present with most lasers, the effects are quite striking with low to medium power He-Ne lasers due to their relatively narrow neon gain bandwidth (which is only a small multiple of the longitudinal mode spacing in low to medium power He-Ne lasers), the rather fortuitous phenomenon that for red (633 nm) He-Ne lasers at least, adjacent longitudinal modes tend to be orthogonally polarized, and nearly ideal behaviour in other respects with the Physics mostly cooperating. (Murphy has seen the LASER DANGER signs and stays away!) Much more on all this below (except perhaps for Murphy).

In the nice diagram above 🙂 of the 8 mW laser, there are 5 longitudinal cavity modes that see gain above the lasing threshold (the right-most just barely). These become lasing modes (red and blue) producing a total output power of somewhat over 8 mW in this specific example. For the 30 mW laser, there are twice as many lasing modes one half the distance apart, and each mode has more power. Interestingly, adjacent modes in a so-called “random polarized” red (632.8 nm) He-Ne laser are almost always orthogonally polarized, with the polarization axes fixed relative to the tube. (Here, one of them is arbitrarily referenced as 0 degrees, more on this later). As the distance between the mirrors is increased, the number of oscillating modes increases as well, though the actual power in each mode increases only slightly.

One complete cycle (red or blue) represents a change in cavity length of one wavelength (at 633 nm) and a change in optical frequency of 2 times the mode spacing of c/2L. The additional factor of 2 arises because the adjacent modes of the red (633 nm) HeNe are orthogonally polarized. This is not true with most other lasers and even HeNe lasers at other wavelengths. Note that while the profile of the mode sweep is affected by the neon gain curve, the period is NOT directly related to it, only c/2L.

However, note that as HeNe lasers get longer, mode competition results in greater and greater instability, so don’t expect to see a nice orderly march with a Spectra-Physics 127 (39 inch cavity). In fact while the envelope of the modes will generally follow the gain curve, each mode will be jumping up and down in a quasi-chaotic dance! Instability may appear in the display of a Scanning Fabry-Perot Interferometer (SFPI) when viewing the longitudinal modes of a 633 nm HeNe with tubes rated at 7 to 10 mW. 5 mW lasers are usually quite clean while 35 mW lasers can be a real mess.

For very short HeNe tubes, the width of the gain curve may be similar to or even narrower than the spacing between modes. With those, the output power will become very low or go to zero during portions of the mode sweep. Very few HeNe lasers were produced with cavity lengths where this would be an issue since maximum output power would be very low. The only one I know of was the Spectra-Physics 119 stabilized laser with a 100 mm cavity length (mode spacing of 1.5 GHz). The very short cavity was required to provide special characteristics for this system.

In fact, it’s often possible to go so far as to identify a specific manufacturer and even model of a HeNe laser tube based solely on the plots of its polarized mode sweep, providing a sort of “fingerprint” for lasers. 🙂 For example, the type of tube installed in a Zygo or Teletrac/Axsys stabilized laser can be determined without opening the case!

These tubes are all physically similar yet have dramatically different mode sweep plots. And, it’s often possible to determine key information about the health of a laser tube by comparing its mode sweep with that of a new one. Over most of its life, the general shape will remain the same, but as the power declines, in addition to the total height of the plot decreasing, the amplitude of the variation (i.e., the AC component) relative to the total will increase. However, near end-of-life when power is way down and fewer modes are oscillating, the distinctions will tend to disappear.

For very long tubes like the 30 mW one in the example above where there are many longitudinal modes, the actual appearance of mode sweep may be rather chaotic as power shifts among the modes in a random dance. When I first observed this behaviour with a Melles Griot 05-LHP-928 (35 mW) He-Ne producing over 40 mW, I thought it might have been defective in some way despite the high power. But two other healthy samples behaved in a similar manner. So, don’t expect to see nice well behaved marching modes for these high power lasers. There is often a hint of instability even in shorter tubes though it may be subtle – a few percent variation in the peak amplitudes not attributable to other causes like normal movement under the gain curve or power supply ripple.

The effects of mode sweep are more dramatic with short low pressure carbon dioxide (CO2) lasers because for a given resonator length, the ratio of wavelengths (10,600 nm for CO2 compared to 632.8 nm for He-Ne means that the longitudinal mode spacing is 16.7 times larger). In these cases, the laser output will turn on and off as it heats up and the distance between the mirrors increases due to thermal expansion. For this to happen in a 632.8 nm He-Ne would require the tube to be less than about 75 mm (3 inches) in length.

A linearly polarized He-Ne laser would have the same longitudinal mode spacing, but all the lasing modes would have the same polarization orientation (red or blue) as shown in the diagrams and animations, above.

So, someone with red/blue color-blindness (if there is such a thing) would see the diagrams for all them as being linearly polarized!

A label on the polarized laser will indicate the plane or orientation of polarization of the output beam. For a random polarized He-Ne laser, a polarizer oriented at 45 degrees with respect to the plane of polarization would produce an output with respect to mode sweep that is similar to that of a linearly polarized laser, except that even with an ideal polarizer, the output power would be cut in half.

Now for some actual numbers: The Doppler-broadened gain curve for neon in a red (632.8 nm) He-Ne laser has a Full Width Half Maximum (FWHM, where the gain is at least half the peak value) on the order of 1.5 or 1.6 GHz. So, for a 500 mm long (high gain) tube with its mode spacing of about 300 MHz (similar to what is depicted above), 5 or 6 lines may be active simultaneously and oscillation will always be sustained (though there would be some variation in output power as various modes sweep by and compete for attention). However, for a little 10 cm tube, the mode spacing is about 1,500 MHz. If this laser were to be really unlucky (i.e., the distance between mirrors was exactly wrong) the cavity resonance might not fall in a portion of the gain curve with enough gain to even lase at all! Or, as the tube heats up and expands, the laser would go on and off. There are very few commercial He-Ne laser tubes that short. It is possible to widen the gain curve somewhat by using a mixture of neon isotopes (Ne²⁰ and Ne²²) rather than a single one since the location of their peak gain differ slightly. This would allow a smaller cavity to lase reliably and/or reduce amplitude variations from mode sweeping in all size He-Ne lasers. The actual lasing threshold will also determine the effective width of the neon gain curve over which lasing occurs, so it may be wider than the FWHM.

A high speed silicon photodiode and oscilloscope or RF spectrum analyzer can be used to view the frequencies associated with the longitudinal modes of a He-Ne laser. The clearest demonstration would be using a short tube where at most two longitudinal modes are active. This will result in a single difference frequency when both modes are lasing. A polarized tube is best as it forces both modes to have the same polarization as a photodiode will not detect the difference frequencies for orthogonally polarized modes. Adjacent longitudinal modes of random polarized tubes are almost always orthogonally polarized (for a 633 nm He-Ne at least). But, adding a polarizer at 45 degrees to the polarization axes can compensate for this with a slight loss in signal strength. Without a polarizer, the beat frequencies of a random polarized laser will tend to be at multiples of twice the mode spacing since only those modes with the same polarization orientation beat with each-other in the photodiode. (If measured very accurately, it will be seen that these frequencies will not generally be exactly at multiples of the mode spacing based on c/2L and will vary slightly during mode sweep. The is due to mode pulling or pushing effects, reserved for the advanced course!)

Passive stabilization (using a structure made of a combination of materials with a very low or net zero coefficient of thermal expansion or a temperature regulator) or active stabilization (using optical feedback and piezo or magnetic actuators to move the mirrors, or a heating element to control the length of the entire structure) can compensate for these effects. However, the added expense is only justified for high performance lab quality lasers or industrial applications like interferometric based precision measurement systems – you won’t find these enhancements on the common cheap He-Ne tubes found in barcode scanners.

Thus, a typical HeNe laser is not monochromatic though the effective spectral line width is very narrow compared to common light sources. Additional effort is needed to produce a truly monochromatic source operating in a single longitudinal mode. One way to do this is to introduce another adjustable resonator called an etalon into the beam path inside the cavity. A typical etalon consists of a clear optical plate with parallel surfaces. Partial reflections from its two surfaces make it act as a weak Fabry-Perot resonator with a set of modes of its own. Then, only modes which have the same optical frequency in both resonators will produce enough gain to sustain laser output.

The longitudinal mode structure of an optional intra-cavity etalon might look like the following (not to scale):

                |                  632.8 nm
               I|      .              .              .
                |      |              |              |
                |      |              |              |
                |      |              |              |
         _______|______|______________|______________|_______
           m=13,542   -1             +0             +1

Notice that since the distance between the two surfaces of the etalon is much less than the distance between the main mirrors, the peaks are much further apart (even more so than shown). (The etalon’s index of refraction also gets involved here but that is just a detail.) By adjusting the angle of the etalon, its peaks will shift left or right (since the effective distance between its two surfaces changes) so that one spectral line can be selected to be coincident with a peak in the main gain function. This will result in single mode operation. The side peaks of the etalon (-1, +1 and beyond) will may coincide with weak peaks in the main gain function shown above but their combined amplitude (product) is insufficient to contribute to laser output.

This example is based on the same 30 mW laser as in the diagram in the section: Longitudinal Modes of Operation. Adding an etalon inside the cavity introduces an additional loss function with peaks every GHz or so. (Note that such an etalon would be about 15 cm long, so the plasma tube for this laser needs to be short enough to allow for that much space between it and one of the mirrors, but that’s just a detail!) Only where the product of the original net (round trip) gain and the etalon transmission is above one will the laser lase. For this example, there is only place where a cavity mode and etalon mode coincide – just to the left of centre of the neon gain curve peak. And, now that there is only a single mode oscillating, it will have an output power of over 15 mW, rather than the ~3 mW or less in each of several multiple modes. There is always some loss in adding an etalon, so the full 30+ mW originally present isn’t usually possible, though the ~50 percent reduction in output power shown here may be excessive.

(From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)

The standard, small He-Ne laser normally lases on only one transition, the well known red line at about 632.8 nm.

The He-Ne gain curve is inhomogeneously Doppler-broadened with a gain bandwidth of around 1.5 GHz (at 632.8 nm). (The width of the Doppler-broadened gain curve depends on the lasing wavelength. At 3,391 nm, it is only about 310 MHz.) For a typical laser, say 30 cm long, the axial modes are separated by about 500 MHz. Typically, two or three axial modes are above threshold, in fact as the laser length drifts you typically get two modes (placed symmetrically about line centre) or three modes (one near centre, one either side) cyclically, and a slow periodic power drift results. Shorter lasers, less modes, more power variation unless stabilized. But it needs a huge He-Ne laser to get ten modes, and since they are closer of course they still only spread over the 1.5 GHz line width.

Most He-Ne lasers which do not contain a Brewster window or internal Brewster plate are randomly polarized; adjacent modes tend to be of alternating orthogonal polarizations. (Note that this is not necessarily true for He-Ne lasers operating at wavelengths other than 632.8 nm and/or can be overridden with a transverse magnetic field, see below.

Some frequency stabilized HeNe lasers are NOT single mode, but have two, and the stabilization acts to keep them symmetrical about line centre – i.e., both are half a mode spacing off line centre. A polariser will then split off one of them or a polarizing beamsplitter will separate the two.

(From: Sam.)

The party line is that adjacent modes in a He-Ne laser will be of orthogonal polarization. However, I’ve seen samples of small (e.g., 5 or 6 inch) random polarized tubes only supporting 2 active modes where this is not the case – they output a polarized beam that remains stable with warmup and in any case, applying a strong transverse magnetic field will override the natural polarization. So, it’s not a strong effect. Only if everything inside the tube is reasonably symmetric, will the modes alternate. Modes may also remain one polarization as they move through part of the gain curve and then abruptly – and repeatably – flip polarization. But the majority of tubes are well behaved in this regard.

Resonator Length and Mode Hopping

Here are some additional comments that address the common fear of the novice laser enthusiast that the resonator length has to be stabilized to the nm or else the laser will blink off.

(Portions from: Steve Roberts.)

Flames expected, as I’m ignoring some of the physics and am trying to explain some of this based on what I observe, aligning and adjusting cavities on He-Ne and argon ion lasers as part of repairing them. Anyone who only goes by the textbooks has missed out on the fun, obviously having never had to work on an external mirror resonator. It can be quite a education!

Due to the complex number of possible paths down the typical gain medium, you will see lasing as long as the mirrors are reasonably aligned. The cavity spacing is not always that critical and will change anyway as the mirror mounts are adjusted (there will always be some unavoidable translation even if only the angle is supposed to be changed). No, lasers don’t really flash on and off in interferometric nulls as you translate the mirrors – they instead change lasing modes. They will find another workable path. You will in some cases see this as a change in intensity but it is more properly observed on a optical spectrum analyzer as a change in mode beating. Eventually you can translate them far apart enough that lasing ceases, but this is a function of your optics not the resonator expansion.

I have seen what you fear in some cases by adding a third mirror to a two mirror cavity with a low gain medium such as He-Ne where the third mirror can be positioned in such a way to kill many possible modes. This usually occurs when I use a He-Ne laser to align an argon laser’s mirrors and the HeNe laser will flicker from back reflections. See the section: External Mirror Laser Cleaning and Alignment Techniques. But unless you have a extremely unstable resonator design, translation will just cause mode hopping, this becomes important on a frequency stabilized or mode locked laser if you have a precision lab application. Otherwise, most commercial lasers are not length stabilized in the least. There are equations and techniques for determining if you have a stable optical design – stable in this case meaning it will support lasing over a broad range of transverse and longitudinal modes. For examples see any text by A. E. Siegman or Koechner. If your library doesn’t have any similar texts, find a book on microwave waveguides. It might aid you in visualizing what is going on.

Either an intracavity etalon or active stabilization systems are usually used on single frequency systems anyway, by either translating the mirror on piezos or by pulling on mirror supports with small electromagnets, or in the case of smaller units, heaters to change the cavity length on internal mirror tubes. An etalon is basically a precision flat glass plate in the lasing path between the mirrors, its length is changed by a oven and it acts as a mode filter.

Length stabilization to the 50 or 100 nm you might have expected to be needed would be gross overkill anyhow, and would be impossible to achieve in practice by stabilizing the resonator alone. Depending on the end use of the product, most lasers are simply built with a low expansion resonator of graphite composite or Invar, although in many products a simple aluminium block or L shape is used, a few rare cases use rods made of two different materials designed to compensate by one short high expansion rod moving the mirror mount in opposition to the main expansion. A small fraction of a millimeter is a more reasonable specification.

(From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)

The basic idea, that the laser can only work at the frequencies where an integral number of half waves fit in the cavity, is perfectly correct. The separation between adjacent modes is just 1/(2*L) where L is the cavity length in cm. From this we get the separation in ‘wavenumbers’. One wavenumber is 30 GHz, so in more usual units it is just 30 GHz/(2*L). Or, to make it easy, in a 50 cm long laser the modes are 300 MHz apart. That is not very far optically.

The laser operates by some molecule, gas, ion in a crystal, etc. making a transition between two levels. But those levels are not perfectly ‘sharp’; we say they are ‘broadened’. The reason can be many things:

• In a gas – Doppler (or temperature) broadening. The molecules move about randomly, and the light is Doppler shifted a random amount.
• Collision (pressure) broadening. Collisions either relax or dephase the state – i.e., ‘mess it up’ and broaden it!
• In a solid various things can happen, but for example in a glass different laser ions are in slightly different positions, and this causes them to have slightly different energies.

In any case no transition is *perfectly* sharp, the fact that it has a finite lifetime gives it a certain width, but this is not often the real limit, something else is usually more important.

These broadening mechanisms ‘blur out’ the line – we see optical gain over that *range* of frequencies, the gain bandwidth.

An example is carbon dioxide. The ‘natural width’ is very small, of order Hz. The Doppler width at 300 °K is about 70 MHz. The collision-broadened width increases about 7 MHz/Torr; so well below 10 Torr the width is Doppler-limited, ~70 MHz; above 10 Torr pressure broadened (e.g. ~700 MHz at 100 Torr).

If I take a typical He-Ne laser it might ‘blur’ out over a GHz or so – **more** than that 300 MHz mode spacing – so there are *always* two or thee modes within the ‘gain bandwidth’ and it will always lase. For a glass laser there might be *thousands* of modes, because the glass gain is very wide indeed.

But there *are* cases that go the other way. For carbon dioxide, at low pressure, the line is Doppler-broadened and about 70 MHz wide, much **LESS** than that 300 MHz mode spacing. So short carbon dioxide lasers really do turn on and off as the cavity length changes, and you have to ‘tune’ the cavity length to get a mode inside the gain width. This mainly happens with short, gas lasers in the infrared.

For a *high pressure* CO2 laser at 760 Torr (1atm), the line width is several GHz, much more than the mode spacing, so the effect disappears.

Observing Longitudinal Modes of a He-Ne Laser

Monitoring the output power of any He-Ne laser while it’s warming up will show a variation in output power due to longitudinal mode cycling. There is even a specification called the “Mode Sweep Percentage” which indicates how large the variation is in relation to the output power. For short tubes, the power fluctuations can approach 20 percent; for long tubes, they may be less than 2 percent.

There are many ways to actually “see” the modes of a laser including the use of an instrument called a Scanning Fabry-Perot Interferometer. However, for a short tube with only 1 or 2 modes, it’s quite straightforward to interpret what’s going on from the output power and polarization alone. All that’s needed is a photodiode and multimeter (or continuous reading laser power meter), and polarizing filter. (A lens from a pair of polarized Sun glasses or a photographic polarizing filter will do.) The power monitor can be set up in the output beam and the polarizing filter in the waste beam from the HR mirror. Alternatively, a non-polarizing beamsplitter can be used to provide the two beams. Adding a polarizing beamsplitter oriented so that it separates the two polarization orientations in one of the beams can simplify the interpretation of the polarization changes.

Changing the orientation of the polarizer will affect the amplitude of the intensity variations. For most red He-Ne lasers, the longitudinal modes will generally remain at two fixed orthogonal orientations, with adjacent modes usually being orthogonal to each other. As the tube heats and the cavity length increases, the modes march along under the gain curve with those at one end disappearing and new ones appearing at the other end as described above. But for well behaved tubes, they don’t flip polarization. When the polarizer is oriented at 45 degrees to the polarization axes of the tube, the reading will remain constant. When aligned with the polarization axes of the tube, the reading will fluctuate the most.

As a specific example, consider an He-Ne laser tube with a mirror spacing of 120 mm (about 4.75 inches, one of the shortest commercially available laser tubes). This corresponds to a mode spacing of about 1.25 GHz – rather close to the FWHM of 1.5 to 1.6 GHz for the neon gain bandwidth. With this tube, at most 2 modes will be oscillating at any given time. When the output power and polarization is monitored while the tube is warming up, a very distinctive behaviour will be observed. One might think that it should be a periodic variation in output power with a simple sinusoidal or similar characteristic. However, there will actually be two peaks for each cycle: A large one corresponding to when there is a single lasing mode at the centre of the gain curve, and a smaller one when there are two modes symmetric around the centre of the gain curve. For most tubes, the polarization of adjacent modes is orthogonal and will remain fixed with the mode. So, as the modes cycle under the gain curve successive large peaks will have opposite polarization. The small peaks will have equal components of both polarizations. Even though two modes are oscillating, the gain for each one is so much closer to the lasing threshold that their combined power is still lower than for the single mode at the peak of the gain curve. There may also be rather sudden changes in output power as modes on the tails of the gain curve come and go. However, for some tubes which are affectionately called “flippers”, the polarization of the modes will tend to suddenly change orientation as they move through the gain curve. This should also be apparent when viewing the beam through a polarizing filter.

Waveforms and RF Spectrum of Longitudinal Modes

While the beam from a healthy He-Ne laser appears by eye to be constant (except possibly for the normal variation in output power during mode sweep), only a single frequency laser has an output which is truly DC. With a high speed photodiode and basic test equipment, a great deal of information can be determined as a result of the interaction among the multiple longitudinal modes (also called axial modes) that are present in all but the shortest He-Ne lasers (or stabilized single frequency lasers). OK, well perhaps this requires some not quite so basic test equipment like a high speed oscilloscope and/or RF spectrum analyzer. 🙂 While these instruments may not be something you have handy, if you’re friendly with someone in a research lab at a local college or university, they may have may be able to help and then everyone could learn a lot from some simple experiments! 🙂

The photodiode (PD) must have a frequency response that extends beyond at least the longitudinal mode spacing of the laser. A fancy costly one may not be essential, only that the PD is quite small. One with a 1 GHz response is typically around 1 mm square, with the frequency response being roughly inversely proportional to area. Candidate PDs may turn up in all sorts of equipment, even old optical mice. The PD should be back-biased with a few volts to improve frequency response and set up to drive into a 50 ohm load terminating at the scope input. Basing the circuit on something like the Thorlabs DET10A would be perfect. (Search for this on the Thorlabs Web site. The spec sheet will have the circuit diagram.)

The first approach is to view the resulting mode beating on a fast oscilloscope. For a random polarized laser, a linear polarizer will be required in front of the PD oriented at 45 degrees to the principle polarization axes of the laser to force adjacent modes that are usually orthogonal to have the same polarization at the PD. The adjacent longitudinal modes will then produce a beat equal to their difference frequency. There will also be weaker beats from all other combinations of modes. Common HeNe lasers have a fundamental mode spacing of between 1.5 GHz (for a tiny 0.5 mW barcode scanner tube, around 10 cm between mirrors) and 161 MHz (for a 35 mW SP-127, around 95 cm between mirrors).

This laser is rated 5 mW with a mode spacing of 438 MHz (around 58 cm between mirrors). The waveforms were taken using a Thorlabs DET210 photodetector and my special edition laser-zapped Tektronix 2467 oscilloscope – formerly resident in the test department of a major laser manufacturer – evident from the 5 unsightly black blobs on the lower part of the screen where the CRT phosphor has been blown away by a high power pulsed laser! 🙂 While the fundamental can usually be seen, information about any higher difference frequencies is hard to interpret. And even this relatively fast scope doesn’t have much sensitivity beyond the 438 MHz fundamental. The screen shots are in no particular order in the montage other than to make the sequence somewhat pleasing. 🙂 This is further complicated by higher order effects like mode pulling, which slightly shift the positions of the modes based on their location relative to the centre of the neon gain curve. Thus, beyond confirming that the mode spacing is as expected, not much more can be easily determined and switching to the frequency domain will be more fruitful.

The output from the PD may also be applied to an RF spectrum analyzer, there will be significant power detected at the longitudinal mode spacing and its harmonics (hundreds of MHz or more) due to beating between longitudinal modes, as well as under 1 MHz (due to second order beats and mode pulling).

The above image shows the primary beat signal for the same laser head using a Thorlabs DET210 1 GHz silicon photodetector and an HP 8590L RF spectrum analyzer. (As with the scope, for a random polarized laser, a polarizer would need to be placed in front of the PD oriented at 45 degrees to the polarization axes to detect adjacent beats.) The centre frequency is around 437 MHz and the span is 1 MHz. (Each box is 100 kHz.) (The spec’d value for the mode spacing of this laser is 438 MHz but it’s possible the spectrum analyzer is in need of calibration! Otherwise, complain to Melles Griot!) The sequence of screen shots show about half the full mode sweep cycle.

If there were no mode pulling, the display would always look like the one in the upper left corner (or even narrower) – a single frequency. However, the individual modes move slightly compared to the cavity resonances, so the spectrum spreads out as a function of the position of the modes on the neon gain curve.

Interestingly, the display remains where there is a single narrow peak for longer than could be accounted for based on the normal speed with which the frequencies are changing. In fact, it’s impossible to capture a situation where the peak is just slightly wider – it snaps from a FWHM of about 1/5 box (top left in composite photo) to approximately 1 box (top centre) and vice-versa. Nothing in between ever appears. This suggests that there is a self-locking process taking place, as mentioned in the previous section.

When set for a frequency range covering 0 to 200 kHz, peaks are present similar to what appears on the right side of those shown above. But a linear He-Ne laser power supply had to be used to avoid seeing the ripple frequency and harmonics of the switchmode brick overlaid on the beats! There are multiple strong beats at around 874 MHz as well, 2 times the mode spacing. They vary a way similar way as the others. This makes sense since there are are 3 longitudinal modes oscillating most of the time, with 4 modes for a brief period during mode sweep. The spectrum analyzer also claims there are weak peaks at around 1,311 MHz and 1,748 MHz during most of the mode sweep cycle, not simply that period where the self mode-locking takes place. However, it’s not clear where these originate, or if they are even real. To be direct, the one at 1,748 MHz would require 5 modes to produce a beat at 4 times the mode spacing of 437 MHz. But there are never 5 modes present, let alone for most of the cycle. Perhaps they are the sum of second-order beats. Or, they could simply be an artefact of the analyzer, perhaps leakage from an internal mixer.

The above image shows scope display for a JDS Uniphase 1145P, with a mode spacing of 438 MHz (around 34 cm between mirrors). The additional complexity is due both to the lower beat frequencies (and thus better response of the scope) as well as the greater number of modes oscillating. An RF spectrum of this laser would have many more peaks closer together, but would look generally similar to that of the 5 mW laser.

Mode spacing of 687 MHz (around 22 cm between mirrors). With only 3 longitudinal modes oscillating (and stressing the bandwidth capabilities of the poor scope), the display is a fairly clean sine wave. The only obvious difference during mode sweep is that the amplitude changes slightly. However, most of the time, it is a relatively clean sine wave and for a higher power healthier tube, the reduction in amplitude is not that great as in this example.

Transverse Modes of Operation

Lasers can also operate in various transverse modes. Laser specifications will usually refer to the TEM00 mode. This means “Transverse Electromagnetic Mode 0,0” and results in a single beam. The long narrow bore of a typical HeNe laser forces this mode of oscillation. With a wide bore multiple sub-beams can emerge from the same cavity in two dimensions. The TEM mode numbers (TEMxy) denote the number (minus one) or configuration of the sub-beams.

Here is a rough idea of what transverse modes might look like for a rectangular cavity:

                        O        OO        OOO      Each 'O' represents
     O        OO        O        OO        OOO       a single sub-beam.

   TEM00     TEM10    TEM01    TEM11      TEM21

I have only shown the rectangular case because that’s the only one I could draw in ASCII!

Other (non-cartesian) patterns of modes will be produced depending on bore configuration, dimensions, and operating conditions. These may have TEMxy coordinates in cylindrical space (radial/angular), or a mixture of rectangular and cylindrical modes, or something else!

To achieve high power from a He-Ne laser, the tube may be designed with a wider but shorter bore which results in transverse multimode output. Since these tubes can be smaller for a given output power, they may also be somewhat less expensive than a similar power TEM00 type. As a source of bright light – for laser shows, for example – such a laser may be acceptable. However, the lower beam quality makes them unsuitable for holography or most serious optical experimentation or research. An example of a high power multimode He-Ne laser head is the Melles Griot 05-LHR-831 which has a rated output power of 25 mW. Compared to their 05-LHR-827 which is a 25 mW TEM00 laser head, the multimode laser is about 2/3rds of the length and runs on about 3/5ths of the operating voltage at lower current.

(Note that it is easy in principle to convert the output of a TEM00 laser into multimode by using a length of fiber-optic cable with lenses at each end to focus the beam into it and collimate the beam coming out. If the core diameter of the fiber is greater than that needed for the fiber itself to be single mode, then the result will be that multiple modes will propagate inside and the output will be multimode. To assure single mode propagation at 632.8 nm with the index of refraction of a typical glass fiber, a 4 um or smaller core is needed. The actual core diameter of the fiber will determine how many modes are actually generated. A core diameter of 10 um will result in a few modes while one of 125 um will produce dozens of modes. Why this would be desired is another matter.) However, all these modes will be exactly the same wavelength since they originate from a single TEM00 beam.

Sometimes, laser companies don’t quite get it right either and a laser tube that is supposed to be TEM00 may actually be multi-transverse mode all the time or whenever it feels like it (e.g., after warmup). I have a 13.5 mW Aerotech tube that is supposed to be TEM00 but produces a beam that has an outer torus (doughnut shape) with a bright spot in the middle. I’ve also seen an apparently factory-new Uniphase green He-Ne laser that produces a similar doughnut beam. Both of these are probably the result of one or both mirrors having a radius of curvature that is too short for the bore diameter. They may have been manufacturing goofups. Everyone can have a bad day, even if it results in a bunch of dud lasers. 🙂 Good for us though. Everyone (well everyone who cares!) has seen a nice TEM00 He-Ne laser. How many have one that does three wavelengths with different mode structures! 🙂

Note that the mode structure implies nothing about the polarization of the beam. Single mode (TEM00) and multimode lasers can be either linearly polarized or randomly polarized depending on the design and for the multimode case, each sub-mode can have its own polarization characteristics. HeNe (and other) lasers will be linearly polarized if there is a Brewster window or Brewster plate inside the cavity. The majority of HeNe laser tubes produce a TEM00 beam which has random polarization. For internal mirror tubes, linear polarization may be an extra cost option. External mirror HeNe lasers also generally produce a TEM00 beam but are linearly polarized since the ends of the tube are terminated with Brewster windows.

A fast photodiode (PD) and oscilloscope or RF spectrum analyzer can be used to view the frequencies associated with transverse modes. The transverse difference frequencies are very low compared to the longitudinal mode spacing so a really high speed PD isn’t needed. A response of a few MHz should be sufficient. Typically less than 2 mm square silicon PD will have an adequate frequency response if back biased. But the modes do have to overlap on the detector so it may be necessary to spread the beam of a multimode He-Ne laser using a lens. A polarized tube is best as it forces the modes to have the same polarization (a PD will not detect the difference frequencies for orthogonally polarized modes). But, adding a polarizer can partially compensate for this, though the polarization may drift with a randomly polarized laser.

Multi-Transverse Mode He-Ne Lasers

As noted, most He-Ne lasers are designed to operate with a single transverse (spatial) mode or TEM00. However, to obtain the highest power for a given tube size or by a goof-up in design, a higher order mode structure may be produced. A non-TEM00 mode may be present if:

• The bore is too short.
• The bore is too large in diameter.
• The mirror RoC is too short.
• The mirror is too far from the bore-end (same effect as bore being too short).

All of these are really somewhat equivalent and simply mean that more than one mode fits inside the available active mode volume.

• For a laser designed to be multimode, a low order mode pattern is typical, though it may not look like the examples in textbooks. Mode patterns that resemble hexagonal close-packed honey combs are often the rule rather than the exception since the circular bore doesn’t really favour Cartesian modes like TEM11 or TEM22. And tubes designed to be multimode will probably have higher order modes than TEM01 or TEM10. Multimode He-Ne lasers typically have 50 to 100 percent more output power for their length than equivalent lasers operating TEM00.
• For a laser with a very wide bore like one using the Melles Griot 05-LHB-570 one-Brewster laser tube and a short RoC mirror, a high order mode structure will be produced with a dozen or more individual spots in a more or less random (possibly sort of hexagonal) pattern.Where there is access to the inside of the cavity (as with a one-Brewster tube), a laser that operates multimode can be forced to operate TEM00 with a stop (aperture) between the external mirror and tube-end. However, there will be a (possibly substantial) reduction is output power. Where both mirrors are external, it may be possible to substitute longer RoC mirrors to force TEM00 mode (again at the expense of some output power).
• For a laser that is supposed to be TEM00 but due to a design error isn’t, a TEM01 or TEM10 pattern is typical or it may produce a beam shaped like a doughnut (torus) with or without a spot in the middle. I have several long Aerotech He-Ne lasers heads rated 12 mW (actual output power around 13.5 mW) that produce a beam like this. This was either by design, or an “oops” and the heads were relabelled with an “M” indicating multimode.
• Sometimes, slight misalignment of the mirrors will produce a multimode (probably TEM01 or TEM10) beam in a laser that otherwise operates reliably TEM00. A warped or misaligned bore may also do this.

Note that a speck of dirt or dust on the inside of a mirror or window (if present), or damage to an optical surface, can result in a multi-transverse mode beam even if the bore and mirror parameters are correct for TEM00 operation. Unfortunately, convincing a bit of dust to move out of the way isn’t always easy on the inside of an internal mirror He-Ne laser tube! Yes, though not common, it can happen. This is one reason not to store tubes vertically. I’ve heard of people successfully using a Tesla (Oudin) coil to charge up the errant dust particle, causing it to just out of the way via electrostatic repulsion. Your mileage may vary. 🙂

Coherence Length of HeNe Lasers

Common He-Ne lasers have a coherence length of around 10 to 30 cm. By adding an etalon inside the cavity to suppress all but one longitudinal mode, coherences lengths of 100s of meters are possible. Naturally, such He-Ne lasers are much more expensive and are more likely to be found in optics research labs – not mass produced applications. However, slightly less exotic and expensive stabilized He-Ne lasers are readily available which oscillate on two orthogonal longitudinal modes and locked so they are in a fixed position. When one mode is blocked with a polarizer, the resulting beam is then (nearly) single frequency with a coherence length of hundreds of meters – much longer than can even be measured without a great deal of effort and expense.

The following actually applies to all lasers using Fabry-Perot cavities operating with multiple longitudinal modes. It was in response to the question: “Why does the coherence length of a He-Ne laser tend to be about the same as the tube length?” The answer is that the coherence period is equal to the tube length but the useful coherence length is generally less (except for the special case above of a single mode).

(From: Mattias Pierrou.)

In a He-Ne laser you typically have only a few (but more than one) longitudinal modes. These cavity modes must fulfil the standing-wave criterion which states that must be an integer number of half wavelengths between the mirrors. In the frequency domain this means that the ‘distance’ between two modes is delta nu = c/(2L), where L is the length of the laser.

The beat frequency between the modes gives rise to a periodic variation in the temporal coherence with period 2L/c, i.e. full coherence is obtained between two beams with a path-difference of an n*2L (n integer).

If you have only one frequency, the coherence length is infinite (that is, if you neglect the spectral width of this mode which otherwise limit the coherence length). If you have two modes, the coherence varies harmonically (like a sinus curve).

The more modes you have in the laser, the shorter is the regions (path-length differences) of good coherence, but the period is still the same.

You can try this by setting up a Michelson interferometer and start with equal arm-lengths which of course gives good coherence. Then increase the length of one arm until the visibility of the fringes disappear. This should occur for a path-difference slightly less than 2L (remember that the path-difference is twice the arm-length difference!). If there are only two modes is the laser the zero visibility of fringes should occur at exactly 2L. Now continue to increase the path-difference until you reach 4L (arm-length difference of 2L). You should again see the fringes clearly due to the restored coherence between the beams.

Ripple, Noise, and Other Artifacts in HeNe Lasers

While one normally thinks of a He-Ne laser as a constant or “Continuous Wave” (CW) source, this is far from the case over a wide range of time scales from nanoseconds to hours. Only to our long obsolete Mark I eyeballs does a typical He-Ne laser really look like the DC equivalent of light. 🙂

Some of these artefacts result from implementation but others are fundamental to the lasing process. The following apply to normal He-Ne lasers (not those involving Zeeman splitting or something more exotic):

• Mode sweep (seconds to hours):
• Longitudinal mode beating (less than 100 MHz to 1.5 GHz):
• Transverse mode beating (1 MHz to 100s of MHz):
• Plasma oscillations (100 kHz to several MHz):
• Plasma instability:
• Higher order mode pulling (1 kHz to 1 MHz):
• Mirror birefringence (100 kHz to 1 MHz):
• HV power supply line frequency ripple (50/60 Hz and harmonics):
• HV power supply switching frequency ripple (20 to 100 kHz and harmonics):
• Mode flipping (0.1 s to minutes):

The closest to a constant output would probably be an intensity stabilized He-Ne operating on a single longitudinal mode, often called “single frequency” (which is close but even that is not totally accurate), with a highly filtered He-Ne laser HV power supply.

Longitudinal Mode Pulling

While introductory textbooks may state that lasers oscillate on multiples of the cavity resonance frequency, c/2L, it turns out that this is actually not true in most cases. (The exceptions would be where the gain curve is essentially flat but that’s another story.) Longitudinal modes that aren’t exactly centered on the gain curve will be at frequencies very slightly offset from these, pulled toward the center of the gain curve with those that are farthest away seeing the most shift. This is a well known effect called “mode pulling” with highly developed theory to back it up. (Mode pulling isn’t unique to lasers. For example, a quartz crystal oscillator can be tuned over a small range using an external capacitor even though its mechanical resonance frequency differs from the output frequency.)

Although the math can get to be rather hairy, one way of thinking of mode pulling is that the cavity bandwidth has a finite extent which depends primarily on the reflectivity of the mirrors and cavity length. So, if the net gain is greater slightly off to one side due to the position of the gain curve relative to the cavity resonance, the lasing line will be shifted in that direction.

When the laser beam hits a high speed photodetector like a photodiode, which is a non-linear (square law) device, in addition to the DC power term, there are the primary difference frequencies which are close to multiples of c/2L (but not exactly due to mode pulling), but also the differences of the difference frequencies – the second order intermodulation products – which will be at (relatively) low frequencies compared to c/2L. As the cavity length changes and the lasing modes drift across the gain curve, the mode pulling effect on each one varies slightly. But, small differences between large numbers can result in dramatic changes in these second order terms, rapidly rising and falling in frequency, and coming and going as modes drop off one end of the gain curve and appear at the other. The amplitude of the second order beat will be much lower than that of the primary beat but is still detectable with a spectrum analyzer, or in some cases with an audio amplifier.

For a He-Ne laser, the range of second order frequencies is typically in the 1 kHz to 100s of kHz range while for a solid state laser it will be in the MHz to 10s or 100s of MHz range. Note that there will generally not be any beat in the range from 0 Hz to some minimum frequency (e.g., 1 kHz or so in the case of the He-Ne laser) as would be expected when the modes are almost symmetric on either side of the gain curve where there would be very low second order frequencies. Apparently, a self mode-locking effect occurs to force these to be exactly zero frequency over a small range of mode positions. This behaviour can easily be observed in the mode beat RF spectrum of a medium power (e.g., 5 mW) He-Ne laser. See the next section.

For these second order beat frequencies to be present, the laser has to be able to oscillate on at least 3 longitudinal modes simultaneously. (With only 2 modes, there will be only a single difference frequency.) The Doppler-broadened gain curve of neon for the He-Ne laser is about 1.5 GHz Full Width Half Maximum (FWHM) at 632.8 nm. To get 3 modes requires the modes to be less than about 500 MHz apart implying a c/2L tube length of about 30 cm or more – typical of a 5 mW or more (rated) He-Ne laser. It should be polarized to force all modes to be of the same polarization – orthogonal polarizations do not mix in a photodetector. For a randomly polarized laser which typically produces alternating polarizations for adjacent modes, a longer tube length would be required to guarantee enough same-polarized modes and/or a polarizer at 45 degrees to the beam polarizations could be added (but this would cut the power to the photodiode by 50 percent or more).

This effect can be demonstrated using a medium length He-Ne laser, high speed photodiode, and audio amplifier. Initially when the laser is turned on and is heating up and expanding the fastest, they may sound like clicks or pops or just non-random noise. As the expansion slows down, more distinct chirps and other interesting sounds will appear. The complexity of the symphony will also depend on the tube length and thus how many modes are oscillating.

A more precise way to look at mode pulling would be to monitor the beat frequencies produced by a high speed photodiode using an RF spectrum analyzer. By expanding the region around c/2L, the changes during mode sweep will be clearly evident. There will be smooth movement as well as sudden shifts corresponding to mode hops. I even did this by beating not a single laser, but two identical stabilized He-Ne lasers against each-other. With two modes from each laser, there are then as many as 6 beat frequencies if 45 degree polarizers are placed in front of each laser and they are then combined in a non-polarizing beam-splitter. I’ll leave analysis of this behaviour as an exercise for the student. It is at first a bit confusing, but with some thought, makes perfect sense. Simply concentrating on the mode pulling of each laser’s longitudinal mode where one laser was locked and the other was allowed to mode sweep yielded a shift of about 500 kHz.

(From: Roithner Lasertechnik (office@roithner-laser.com).)

You can “listen” to a single mode He-Ne tube: Take an X-rated photodiode and an AC power amplifier – guide a small part of the He-Ne laser beam to the photodiode (don’t let it saturate!) – and listen to the “chirping oscillations” during warming up with a speaker. Hint: There are no birds inside the tube. 😉 But it sounds similar! Looks like sin(x)/x.

Low Level Oscillation due to Mirror Birefringence

This is a phenomenon whereby a low level oscillation in output power at hundreds of kHz is present during part of the mode sweep. It’s not something you would see by eye or likely run into by accident, as the variation is typically only around 1 percent of the total power of the tube. But it is in the category of “interesting”. 🙂 In fact, the oscillation was detected only because the spectrum generated by an optical instrument using a laser with this affliction as a wavelength reference was being corrupted by spikes at frequencies that should not have been there. It would go unnoticed by at least 99.999% of the users of He-Ne lasers. 😉

For the following, refer to [download id=”5602″] and click on the last slide to enter the animation. Though slightly shorter, the tube simulated in the animation will have mode sweep similar to that of the tubes in question, with no more than 3 modes present at any time. The red and blue modes denote the two orthogonal polarization axes of the tube called “p” and “s”.

• The tubes in which these low level oscillations have been observed are all between around 9.0 and 10.5 inches (approximately 225 to 260 mm) in length with random polarization. These have at most 3 modes. It may be present in longer tubes with more modes but this has not been confirmed. Tubes with fewer than 3 modes are immune.
• So far, only the Zygo 7701/2 and another unidentified Zygo tube, and the Siemens/LASOS LGR-7621s have unequivocally exhibited these oscillations. Tests with the Melles Griot 05-LHR-038 and 05-LHR-117, Spectra-Physics 088-2, and Siemens LGR-7631A resulted in no detectable oscillation even though these tubes are physically similar. When a tube exhibits oscillations, all samples of the same model will do so as well. At least until contradicted. 🙂
• The oscillation will be present ONLY where 3 longitudinal modes are present – there are 2 blue modes on either side of a red mode, or 2 red modes on either side of a blue mode in the animation. Most of the time, it will appear instantly as soon as a third mode pokes its head above the noise. But in some instances, there will be a delay, and then appear instantly. (Not ramp up to any degree.)
• The phase of the oscillation is opposite for p and s polarization and their amplitude is similar. Thus, unless only p or only s is selected with a linear polarizer, little or no oscillation may be detected. This means that the p modes are changing in amplitude 180 degrees out of phase with respect to the s modes. And this has been confirmed by observing the p and s oscillations simultaneously using a Polarizing Beam-Splitter (PBS) and two biased photodiodes, while also viewing the longitudinal modes on a Scanning Fabry-Perot Interferometer (SFPI). This would then seem to be some sort of mode competition between the p modes and s modes. (I’ve heard that this opposite phase may not be true with every sample of the same model where the oscillations occur but until I see it with my own eyes, I’ll continue to make the claim!)
• The p-p amplitude of the oscillation is only on the order of 1 percent of the total output power of the tube. (See why I said you probably wouldn’t see this!) For a tube outputting 3.5 mW total with 2 to 2.5 mW peak in each polarization, this ends up being about 10 mV p-p from the photodiodes. Its amplitude varies somewhat, peaking near the center of the period in which the oscillation is present. The variation may be small or as much as 2:1 depending on the specific laser tube, but does NOT track the amplitude of either of the side modes. The signal appears almost instantly at a minimum of 1/2 its maximum.
• The frequency of the oscillation is typically between 200 and 600 kHz and differs for each sample of the laser tube (even of the identical model) and possibly depending on whether a p or s mode is in the center (meaning the orientation is a factor). Thus, it is dependent on a physical characteristic of the tube. It also depends on the temperature of the tube to some degree. (No pun….) Here are some *very* rough estimates of the frequencies ranges for several tubes:

    ID    Make/Model       Length   Oscillation #1  Oscillation #2
   ----------------------------------------------------------------
     1    LASOS LGR-7621   260 mm    270-280 kHz     490-510 kHz
     2      "    "   "      "  "     293-307 kHz     436-443 kHz
     3    Zygo 7701/2      235 mm    496-509 kHz     657-665 kHz
     4      "     "         "  "     398-420 kHz     594-624 kHz
     5    Zygo Unknown     225 mm    200-360 kHz     200-360 kHz
     6      "     "         "  "     280-460 kHz     295-460 kHz
     7      "     "         "  "     544-595 kHz     570-612 kHz
  

  Interestingly, both oscillation frequencies for the Zygo Unknown model tubes were similar, and in one case identical within the uncertainty of my measurements. This was also the only sample of those tubes that I had added “wedge” to the HR mirror since they lacked it for some reason. So possibly the slight back-reflections from the outer surface of the mirror substrate do have some minor effect.

  Additional frequencies at around 1.95 MHz were seen on some of these tubes but they were at much lower levels, and possibly NOT correlated with the presence of 3 longitudinal modes. Another mystery.

• Except for where the oscillation nearly abruptly appears or disappears (which can have a huge change), the frequency increases or decreases more or less monotonically by up to 10s of percent during the period in which it is present. Whether it increases or decreases depends on if the p or s polarization is in the center, another indication of a physical dependence on tube construction. The monotonicity also indicates an asymmetry in the behaviour with respect to the gain curve or something related to it. Note that the LASOS 7621s and Zygo 7701/2s only varied by 15 to 30 kHz while some of the unknown Zygo tubes varied by almost 200 kHz.

Since the precise behaviour depends on whether the p and s polarization is in the center, and polarization axes are locked to tube orientation, the current hypothesis is that these result from (or are at least affected by) mirror birefringence. Mirror birefringence results from mirror coating processes that are not perfectly symmetric due a crystalline structure or whatever. The result in an effective depth of reflection – and thus cavity length – that depends on orientation, with a typical variation of a fraction of 1 nm.

1 nm of cavity length change at 633 nm for a tube with a cavity length of 9 inches (around 225 mm) represents a frequency change of approximately 2 MHz. So, the observed shifts being in the 100s of kHz would be consistent with p and s modes being shifted off of normal c/2L cavity resonances by a fraction of 1 nm, resulting in some sort or low level mode competition. Perhaps the LASOS and Zygo tubes differ in whether 1 or both mirrors have significant birefringence. But exactly how this all works beyond the above statments would be total hand waving, as if it isn’t already. 😉

Not all coating processes exhibit birefringence but it’s quite possible those used in most common He-Ne laser tubes do since they tend to have fixed polarization axes. And there can also be birefringence resulting from other asymmetries in the tube construction, though they may be smaller. Known exceptions with very low birefringence are REO (based on their own claims and that where fixed polarization axes are required as with most stabilized He-Ne lasers, REO tubes must be more complex to get around this “feature”) and HP/Agilent Zeeman He-Ne tubes (which have very peculiar mode sweep without their Zeeman magnets). Because REO and HP/Agilent tubes are so strange to begin with, it would not be possible to test them for this type of oscillation. Nor is there any easy way of determining if those tubes that don’t exhibit the oscillations have lower or no birefringence. But there must be some asymmetries because they all have fixed polarization orientation.

In coming to the conclusion of mirror birefringence or something related, the following have been ruled out:

• Higher order longitudinal mode beating: While the primary longitudinal modes differ in frequency by close to cavity mode spacing of c/2L, they aren’t precisely c/2L due to mode pulling effects, which can result in them being off by frequencies in the 100s of kHz range. Then the second order beats – the beats of the beats – could have a difference frequency in this range. However, this would not explain the opposite phase or how a single mode could have the oscillation. And, it would not explain why the low level oscillation is absent from other similar tubes.
• Transverse mode beating: Not only would it be extremely unlikely for all of these TEM00 tubes to have significant higher order transverse modes, but an explanation involving them would also not be able to account for the opposite phase behavior or the oscillation from a single longitudinal mode. LASOS tube ID #2 did have a non-TEM00 mode of about 2 percent of total power that appeared for a portion of the mode sweep cycle, but it’s occurrence did not coincide with the low level oscillation. LASOS tube ID #1 had a barely detectable non-TEM00 mode of about 0.05 percent, too small to be a factor. Any non-TEM00 modes in the other tubes were even smaller or non-existent.
• Zeeman splitting: There are no magnets or magnetized material in the vicinity. Also, the amplitude of the oscillation is a maximum when a polarizer passes the entire center mode leaving no opportunity to combine two sub-modes in the PD. The mode itself could be split somehow into two sub-modes that cannot be resolved on the SFPI, but (1) there is no basic principle to support this, (2) it doesn’t explain how the side modes have the same frequency, and (3) that they are out of phase.
• Plasma oscillations: While these may occur in the 100s of kHz range, they do not depend on mode position. And in addition, varying the power supply current has essentially no effect on the observed behaviour with respect to the low level oscillations.
• Harmonics of the power supply ripple: Multiple power supplies have been tested with these tubes and there was absolutely no difference, as well as having no correlation to mode position.
• Back-reflections: This is sort of grasping at optical straws, but to rule it out, I added wedge to the HR mirror on one Zygo tube that didn’t have it. This minimizes reflections from the uncoated outer surface of the mirror substrate from getting back into the cavity. As expected, there was no change.

While I believe the same phenomenon is responsible for these oscillations in LASOS and Zygo tubes, there are differences. Notably with the Zygo tubes, the frequency changes by almost 2:1 compared to only 20 kHz or so for the LASOS tubes. On one sample, ranging from around 200 kHz to 360 kHz. And compared to the LASOS tube where the two frequencies differed by almost 2:1 (around 280 and 500 kHz) they are similar enough for the Zygo tube that they may in fact be the same, but alternately increasing and decreasing depending on whether a p or s mode is in the center.

I’m sticking with mirror birefringence for now. It may be a feature, not a bug. 🙂 Specifically, that a higher birefringence coating on one or both mirrors is used to lock the polarization orientation, required to be able to use these tubes in stabilized He-Ne lasers, though I’m not sure LASOS and Zygo are that sophisticated!

The test to reproduce this phenomenon is relatively simple and doesn’t require fancy expensive instrumentation. The main components are a polarizer or polarizing beam-splitter, a biased silicon photodiode (almost any type will do using a circuit similar to what’s in a Thorlabs DET210 or DET10A (Google will find it), and an oscilloscope (almost any type since the frequencies involved are under 1 MHz). Terminate the output in 2k to 5k ohms and set the scope for a vertical sensitivity of a few mV/division, AC-coupled, and a sweep speed of a few µs/division. The tubes should be red (633 nm), random polarized, and healthy. (I have no idea what happens with other colour tubes!) They should be between 8 and 10 inches in overall length. Shorter tubes will have only 2 modes at most. Longer tubes with more modes may work, but that is left as an exercise for the student. 🙂 Since the signal is so low level – around 10mV even from a tube producing 3 or 4 mW, eliminating other sources of ripple and noise can be a challenge. Specifically, the current ripple from the He-Ne laser power supply may overwhelm and bury the low level oscillation signal. A highly filtered linear supply is best though some modern switchmode “bricks” have decently low ripple. And an external ripple reducer can be added to these. The photodiode will also need to be blocked from room light as the variation in intensity from lamp ballasts will be picked up. Power everything up and look for a clear oscillation in the 100s of kHz range that comes and goes, at a decreasing rate as the tube warms up. Rotate the tube over 45 degrees to align for maximum signal. If you have an SFPI, use a non-polarizing beam-splitter to sample a portion of the tube’s output for it to show how the oscillation correlates with the number of modes and their position. As noted above, only selected tubes exhibit this phenomenon and unfortunately, they are not the most commonly available ones. So your mileage – and success – may vary. 🙂

What is Mode Locking?

The normal output of a He-Ne or other CW laser is a more or less constant intensity beam. Although there may be long term variations in output power as well as short term optical noise and ripple from the power supply, these are small compared to the average intensity. Mode locking is a technique which converts this CW beam to a periodic series of very short pulses with a length anywhere from picoseconds to a fraction of a nanosecond. The separation of the pulses is equal to the time required for light to make one round trip around the laser cavity and the pulse repetition rate (PRF) will then be: c/(2*l). For example, a laser resonator with a distance of 30 cm (1 foot) between mirrors, would have a mode locked PRF of about 500 MHz.

Mode locking is implemented by mounting one of the mirrors of the laser cavity on a piezo-electric or magnetic driver controlled by a feedback loop which phase locks it with respect to the optically sensed output beam.

Without mode locking, all the modes oscillate independently of one another with random phases. However, with the mode locked laser, all the cavity modes are forced to be in phase at one point within the cavity. The constructive interference at this point produces a short duration, high power pulse. Destructive interference produces a power of almost zero at all other points within the cavity. The mode locked pulse then bounces between the two laser mirrors, and a portion passes through the output coupler on each pass.

As a practical matter, you probably won’t run into a mode locked He-Ne laser at a garage sale!

Cavity Dumped Pulsed He-Ne Laser

Here’s another one that won’t turn up at a swap meet. Inside the cavity of a typical He-Ne laser, the circulating power is 50 to 1,000 times the output power. If only all those photons could be accessed! Well, it turns out there is a way to do this sort of, at least in principle and for a short time. It’s called “cavity dumping”. The idea is use a high speed optical switch to briefly divert the Intra-Cavity (IC) beam outside the cavity. This sounds simple, right? 🙂 There are just a couple of problems. With the low gain of the typical He-Ne laser, any optics inside the cavity has to either be at the Brewster angle or have very good AR-coated surfaces so as not to significantly impact circulating power, or kill lasing entirely. And since any practical He-Ne laser is limited in size perhaps 2 meters at most, the switching has to take place in nanoseconds to get any significant fraction of the IC power to exit the laser.

The optical switch is the key to making this work. Either it has to actually deflect the beam or change its polarization so some other optical element will then reflect it out of the cavity. There are devices like Kerr cells and Pockels cells that could potentially be fast enough but they require high voltage to operate and may have excessive losses. Other approaches use an Acousto-Optic Modulator (AOM) as a deflector to divert the IC beam just enough to be reflected out of the cavity. However, AOMs don’t operate instantaneously since they depend on a high frequency acoustic wave to propagate in their crystal. Even a high performance AOM would require 10s or even 100s of nanoseconds to switch states.

I’m sure a literature search would turn up some mouldy papers describing the cavity dumping technique with a HeNe laser, but a “proof of concept” experiment was performed recently by Kevin Zheng, a very talented high school student, while at the Stony Brook Laser Teaching Center. See Kevin’s Research Journal and Poster. While the performance wasn’t that fabulous (forget any ideas of a hole burning HeNe laser!), just being able to get this work at all with relatively limited resources is impressive.

He-Ne Laser Output Power Fluctuation During Warmup

While not generally visible by eye alone except possibly for very short or tired (low gain) He-Ne lasers, there is a quasi-periodic variation of output power with time. For the typical He-Ne laser tube shortly after turn-on, the frequency is quite rapid (a cycle every few seconds) and gradually slows down as the tube temperature reaches a steady state value (after a half hour or more).

Note that while the frequency of the power variations in output power of a He-Ne laser goes to beyond the GHz range, the following deals with what can be seen by human eyeballs with the aid of only a photodiode and multimeter or chart recorder (or a PC with a data aquisition module).

Here is a plot of the measured output power of mid-size (probably around 5 mW) He-Ne laser tube from power-on to 20 minutes:

(Plot courtesy of Ryan Haanappel). (Though typically, the output power starts out at a much higher value, often above 75 percent of its value after full warmup. But there are exceptions.) Many more plots can be found later in this section.

Examining the actual plot of output power versus time such as shown below:

(or careful observation of laser power meter readings) of a He-Ne laser reveals that the curve is not simple but may include several types of behaviour:

• Long term trend in output power: With a laser that is in good condition, this is a generally increasing function until it levels off after warmup. The dominant effect is that as the laser tube heats, various parts expand and the laser approaches optimal alignment (which should have been the way it was originally adjusted during manufacture). If the mirror alignment is not quite correct, power may go up and then down, or just down, and/or may never reach rated power. If the cause is alignment, gently pressing side-ways on one of the mirror mounts at the correct angle with the correct force (with an insulated tool!) should result in near maximum power even when just powered on.There is also usually an increase of power due to the heating of the laser tube (independent of thermal expansion effects) as well. But this may be only a fraction of the effects of alignment and is related to the increase in internal temperature and pressure.In addition, especially with soft-seal tubes, there may be a power increase as the cathode, acting as a weak getter, removes contaminants from circulation that may have accumulated from a period of non-use. Or depending on how far gone they are, the power may go down as various parts outgas from the heat!Depending on the particular laser, the initial output power can be very low even where the final output power exceeds rated power. Striking examples can be found in a non-negligible percentage of long JDS Uniphase He-Ne lasers like the 1145P. With these, a cold power of 1 or 2 mW for a laser that reaches 24+ mW after 20 minutes isn’t unheard of. The dominant cause is a change in mirror alignment.
• Short term variations in output power: As the laser tube heats and expands, the longitudinal modes of the laser drift across the 1.5 GHz neon gain curve. The output power varies depending on where they are and may change suddenly as a mode drops off one end or appears at the other. For a 6 inch tube (c/2L=0.5 GHz), there are 1 or 2 active modes; for a 24 inch tube (c/2L=125 MHz), there may be 8 or 10 active modes. The short tube will usually have much more dramatic variations in power due to this mode cycling. Specifications range from 20 percent (for 5 inch tubes) to less than 2 percent for long ones. For a short random polarized tube, the polarized modes may vary by 100 percent. Note that the variations may not be of a single frequency but often exhibit the double-dip behaviour shown in the plot, above. This is probably due to the number of modes oscillating and how they are centered on the gain curve. There may also be a slight difference on alternate cycles due to the polarizations of adjacent modes seeing slightly different gain.These effects are collectively called “mode sweep” or “mode cycling”. Plotting and analyzing this behaviour for various tubes under a variety of conditions can be quite fascinating. More in the next section.
• Medium term variations in output power: (These are generally less common with lasers that are properly designed and manufactured.) There are two common types of medium term effects:
  1. IR Mode competition: This is generally only a problem with higher power lasers. It is usually at the high gain 3.39 um mid-IR He-Ne lasing wavelength and may result in an additional variation in output power. While longer tubes generally are designed with mirror coatings highly transparent at the 3.39 um wavelength and/or with IR suppression magnets, their effect isn’t always perfect. These power variations will be at a frequency 0.633/3.39 (for 633 nm, red He-Ne lasers) of the fundamental frequency of the primary mode cycling, above. The effect will be negligible for red He-Ne lasers of less than 15 or 20 mW but may appear in relatively short “other colour” (e.g., yellow) He-Ne lasers due to the low gain of the lasing wavelength. A simple test is to carefully place a few reasonably powerful magnets at several locations near the tube. If the problem is mode competition, the amplitude of the variations should be reduced and output power at the design wavelength may increase by 10 or 20 percent or more. Modern He-Ne lasers rarely suffer from IR mode competition.
  2. Non-wedged mirror substrates: Another source for power variations on a similar time scale is a lack of wedge in the HR and/or OC mirror glass substrates. Wedge means that the two surfaces are ground at a slight angle. Without wedge, the mirror coating and the uncoated outer surface (HR) or AR-coated outer surface (OC) form an etalon whose transmission varies as the glass expands during warmup due to constructive and destructive interference within the mirror glass. The result is that the power of the waste beam from the HR mirror may vary periodically by an amount much larger than that of the mode sweep specification, and it doesn’t track the power of the main beam as would be expected. Lack of wedge for the HR mirror is particularly nasty since the approximately 4 percent Fresnel reflection from the outer suface is enough to vary the transmission through the HR – and thus the waste beam power – by a ratio of up to 2.25:1. Waste beam power variation due to lack of wedge for the OC mirror is less severe since the AR coating results in less reflection but still may be 10 percent or more. While most applications don’t care what the waste beam does, there may also be ripples in the main beam of 1 or 2 percent. These will be superimposed on all the other power variations described above. For an enclosed cylindrical laser head, there may be up to 10 or more of these power variation cycles while a bare tube will go through fewer cycles because its temperature increase is not as great. These small power variations may be considered normal behaviour for short tubes where it is much less than those due to mode sweep, and no one (at least almost no one!) cares what the waste beam does. Mass produced barcode scanner tubes – built for low cost, not highest performance – are most likely to have this problem but many of them not too bad. Higher quality tubes will have both made with wedged substrates. Plots of power output during warmup for a few typical cases are also shown below.

Goofups in design and manufacturing can result in various combinations of these and other effects, though for the most part, He-Ne laser companies generally know what they are doing! 🙂 But see the plots below for both normal and abnormal behaviour, and a link near the end of the section for a case study of one dramatic example of an “oops”. 🙂

Plots of HeNe Laser Power Output and Polarized Modes During Warmup

Here are some plots of power output versus time for a variety of typical He-Ne laser tubes and heads from nearly the shortest available through mid-size. (For longer tubes, the appearance will be very similar, but with even a smaller short term fluctuation in power.) The shape of the plots is mostly the result of what’s called “mode sweep” or “mode cycling” as the longitudinal modes of the laser move with respect to the neon gain curve due to thermal expansion of the laser cavity. However, where the plot covers a long time (e.g., most of the warmup period), there will generally be an increasing trend in output power due to other factors as noted above.

Most of these are from Melles Griot but the behaviour of lasers from other manufacturers will be relatively similar, though the detailed shape of the individual polarized modes (more below) can differ significantly. The majority are healthy samples but a few show some rather dramatic peculiarities. There are also plots of a Coherent model 200 and Hewlett Packard model 5517A frequency stabilized He-Ne lasers from power-on to locking.

Plots such as these are almost like fingerprints for He-Ne lasers. Many of the physical characteristics of the laser can be determined by their appearance, and some features are unique to a particular model or manufacturer.

For most of the plots, my “instrumentation” consisted of a pair of $2 photodiode feeding two of the analog inputs of a DATAQ RS232 Chart Recorder Starter Kit attached to my ancient 486DX-75 Kiwi laptop running Win95. The photodiodes are reverse biased by 30VDC from a +/-15VDC power supply with a variable load resistor to set the calibration. The output is taken between the junction of the resistor and the photodiode, and power supply common (0 VDC). One channel is shown below:

               R1     PD1
 +15 VDC o----/\/\----|<|----+
              100            |
                             /
                             \<----------+----+---o A/D Input (+/-10 V range)
                             / R2        |    |
                             \ 25K       |    /
                R3           |       C1 _|_   \ 200K ohms (Zin of A/D module)
 -15 VDC o-----/\/\----------+      1uF ---   / 
               68K                       |    \
                                         |    |
   0 VDC o-------------------------------+----+----o A/D Ground

The values shown were selected for lasers with a maximum power output of around 1 mW. For higher power lasers, R2+R3 can be decreased or an attenuation filter can be placed in the beam. The later is preferred to avoid shifting the 0 mW reference level, and is what I did for most of the plots.

The capacitor across the input is intended to minimize noise pickup. The resulting filter rolls off at around 0.1 Hz. For reasonably well behaved He-Ne lasers, even during the initial warmup period, this bandwidth is more than adequate. The sampling rate for all the plots is at least 10 Hz to allow for averaging since the A/D seems to have an uncertainty of about 2 LSBs.

In most cases, the two photodiodes are positioned at the outputs of a Polarizing Beam-Splitter (PBS) cube and the laser tube is oriented so that they are aligned with its natural polarization axes.

For monitoring power from the waste beam (which is much lower), a dedicated beam sampler assembly was constructed, which along with a photodiode preamp, enabled power levels as low as a few uW to fully utilize the 20 V p-p range of the A/D.

Some of the plots have been acquired with the same photodiodes, but feeding a dual channel preamp and also a summer tp compute the total power without requiring a separate photodiode channel. (Of course, for this to be meaningful, the photodiodes and premaps have to be set up so the two channels have equal gain.) The premap results in lower noise in the plots especially for low power lasers.

And as of 2011, I’ve “upgraded” to USB versions of the DATAQ device (DI-158U and DI-145) and laptops that are only 10 years old. 🙂 The original DATAQ RS232 module died and the Kiwi laptop doesn’t have USB and is falling apart (though is still usable). The PDs are now each attached to a general purpose trans-impedance amp rather than the simple bias network. [See the section: Sam’s Photodiode Preamp 1 (SG-PP1).] And in addition to channels 1 and 2, the outputs also feed a pair of resistors and a pot to adjust balance to channel 3 so their sum (which would usually be total power) is always present.

Although some of these plots aren’t as nicely annotated as the one above, zero power is near the bottom of the plot so relative power variations can still be easily seen (who cares about absolute power anyhow!) and the time/division is indicated. The plots are arranged by increasing laser tube length.

For the following, “Total” means all the power in the beam; “Polarized” means a polarizing beam-splitter is used to separate the two orthogonally polarized modes with either one or both plotted. (This is Only done for random polarized lasers.) The scale factor for the “polarized” plot has been adjusted so that the peak amplitude is approximately the same as for the “total” plot for ease of viewing. However, it should be understood, that the sum of the power in the two orthogonal polarizations must add up to the total power. All are red (632.8 nm) HeNe lasers unless otherwise noted.

Note: I have “edited” (doctored?) some of the plots to clean up unsightly randomness and other blemishes, mostly due excessive electrical noise at low optical power levels. However, the important features are unchanged.

• 

  This is the shortest commercial HeNe laser I know of with a total length of about 4-5/8 inches. The rated output power is 0.5 mW but this sample produces about 0.8 mW. Only 2 longitudinal modes will be oscillating with a mode sweep of about 10 percent. The large peaks correspond to a single mode being at the center of the gain curve. If the distance between the mirrors were much shorter, it wouldn’t be possible for even 2 modes to coexist and the output would turn off for a portion of the mode cycle. Note that the actual tube tested was a Siemens 007 but it has identical specs to the Melles Griot 05-LHR-007 so I figured it’s better to be consistent. 🙂 It was necessary to construct a tent over this (and other) bare tubes to even get this far without random air currents affecting the temperature too much. (The peaks beyond 20 minutes where this and the next run terminated were unrecognizable!) An enclosed laser head would take much longer to stabilize but be more tolerant of ambient conditions.

• 

  This is the same tube but with a non-polarizing beamsplitter followed by orthogonal polarizing filters inserted in the beam. The orientation of the polarizing filters is adjusted for minimum transmission when its mode is not present since as can be seen, the power actually goes to 0 mW for about half the period of each polarized mode. Alternate similar height peaks on the total power plot correspond to the same mode polarization. A careful examination will confirm that they actually alternate very slightly in amplitude due to minor variations in gain as a function of polarization. (I have adjusted the scale factors to make the plot looks similar.) The reason why the peak spacing on the two plots differs is that the tube was likely not quite at the same temperature when each run was started.

• 

  The 05-LHR-640 is very nearly second shortest commercial HeNe laser tube as shown below

  

  It is only about 5 inches in total length and as with the 05-LHR-007, only 2 longitudinal modes are oscillating. The rated output power is 0.5 mW but this sample produces about 1.2 mW. As can be seen in this plot, the “mode sweep percentage” is still rather modest – about 5 to 7 percent. The specification for this tube allows for up to 20 percent. This bare He-Ne laser tube has nearly fully stabilized in under 20 minutes.

• 

  . This is the same tube with the polarized modes separately plotted. Similar comments apply for this tube as for the 05-LHR-007, above.

• 

  Above shows the two polarized modes and total power in a laser with a cavity length of about 7 inches. This is just about at the point where 3 modes can barely lase simultaneously when one is centered on the neon gain curve.

• 

  This uses a slightly longer tube – almost 8 inches – which probably supports three simultaneous lasing modes. The rated output power is 1.5 mW but this sample produces about 2.0 mW. Since the 05-LHP-131 is a polarized laser, the polarization of all modes in the output beam is the same. As the length of the laser tube increases, the amplitude of the short term power variations will tend to decrease – for this laser head, it is under 2 percent. This is actually quite good – well below the 10 percent in the specifications. Being an enclosed laser head, even after 30 minutes, the power hasn’t fully stabilized.

• 

  This is a slightly longer random polarized tube, about 8.5 inches between mirrors. In addition to having been used by the hundreds of thousands in barcode scanners peaking during the 1980s, tube like this was used in the Spectra-Physics 117 and SP-117A (and similar Melles Griot 05-STP-901) stabilized HeNe laser. Note the well behaved mode sweep with smoothly varying polarization components and no flipping. A closeup is shown below

  

• 

  While nearly identical in length to the SP-088, note the dramatic difference in the shape of the mode waveforms. Each is nearly a perfect triangle wave like the Melles Griot 05-LHR-911, above. This is especially evident near the end of the warmup period but it is very similar throughout. This shape seems to be characteristic of all these Siemens tubes as I’ve tested several with essentially the same mode waveforms. The mode shape of the Uniphase 098, another barcode scanner tube, is identical. The resonator length of the 088 and 098 is virtually identical, while that of the LGR-7641 is about 1/8″ longer. The cause is unknown but based on this, it doesn’t seem to be related to the size. A wild guess would be that it has something to do with the isotopic purity (or lack thereof) or ratio of the gas-fill resulting in a wider neon gain curve curve so that as with the 05-LHR-911, 3 longitudinal modes can just barely lase simultaneously when one is centered.However, the dramatic variation in mode amplitude over the course of warmup is an artefact of the way that data is being collected for this run and a peculiarity of the tube that doesn’t noticeably affect its useful output. Rather than using the output beam, the P and S Modes are taken from the waste beam leaking through the HR mirror at the back of the laser. The Total Power (Waste) is then simply the sum (in an op-amp) of the modes. Compare this to the Total Power (Output) curve, which was measured from the main beam. The cause of the rear beam power variation is interference from multiple internal reflections in the HR mirror glass – between the HR coated inner surface and the uncoated outer surface. The result is a weak Fabry-Perot etalon which varies the effective reflectance of the HR mirror. It doesn’t take much: A change from 99.975% to 99.95% would double the waste beam power – from about 15 uW to 30 uW. The 15 uW lost from the main beam power of about 1 mW is almost undetectable on the plot. The HR mirror glass is apparently not wedged on these tubes so the surfaces are very parallel. And indeed there was no ghost beam next to the waste beam as would be the case if wedge was present. The cause was confirmed by putting a dab of 5 minute Epoxy on the outer surface of the mirror. The Epoxy is smooth and clear enough to pass sufficient power for the photodiodes (though it is reduced), but the Epoxy surface is lumpy enough to greatly reduce the power variation. Why? The glass and Epoxy are fairly closely index matched so that the dominant reflection is no longer from the planar glass surface but from the lumpy surface of the Epoxy. There is minimal reflection directly back along the optical axis and thus minimal etalon effect resulting in a reduction of power variation from nearly 100 percent to under 10 percent. Using Norland 65 UV cure optical cement to glue an angled plate to the HR mirror reduced the ripples even more as shown below.

  

  .

• 

  is another similar (basically interchangeable) barcode scanner tube with the same malady as the LGK-7641: No HR wedge and large variation in waste beam power due to etalon effects. Insulating the same tube by installing it in a head cylinder allows for several cycles of the waste beam variation as shown below:

  

  A close examination of the Total Power (Measured) shows small dips representing the power being stolen by the waste beam from main beam! The measured output power is about 1 mW. The amplitude of the waste beam power variation for this tube is from around 5 uW to 10 uW.

• 

  and

  

  are examples of common short (6 inch) Melles Griot barcode scanner tubes with varying degrees of the same problem. #1 has nearly the theoretical maximum waste beam power variation ratio of 2.25:1.

• 

  shows the behaviour of another virtually identical short (6 inch) tube with a small amount of wedge. But it’s enough to virtually eliminate power fluctuations in the waste beam. The residual ripples may actually be due to lack of wedge in the OC mirror and adding an angled plate to the OC mirror with optical cement actually made the ripples larger as shown below

  

  . This is somewhat as expected since it’s not possible to index match to an AR-coated surface without removing the AR coating. So, the reflections there would increase.

• 

  has even less. But another Uniphase tube had among the worst case as shown in

  

  These were identical model numbers used in the identical barcode scanners.

• 

  This is a very common medium size HeNe laser with a tube about 13 inches in length so 4 or 5 modes are oscillating. The rated output power is 5 mW but this sample produces about 7.5 mW. The power fluctuations are virtually undetectable on these plots – well under 1 percent.

• 

  This is the same laser head but with the two orthogonal polarizitions separated (as described for the shorter tubes, above) and oriented for maximum variation (“ripple”). They are plotted separately to reduce clutter. Since there are always modes of both polarization present regardless of polarizer orientation, the output power in doesn’t go to zero as with the shorter laser but their ripple is almost perfectly complementary. As expected, the size of the fluctuations in each polarization – 5 to 10 percent – is more in line with the total power behavior of a laser with only 2 or 3 modes. Even this amplitude seems remarkable given the almost perfectly smooth behavior of the total (randomly polarized) power. If the plots are examined very carefully, it will be noted that their envelopes are not identical – there is a very subtle slow variation over the course of the warmup period. This may be attributed to a small rotation of the polarization axes as the tube expands. With some samples of these lasers, it can be much more dramatic including polarization flips whenever it feels like it. But such behavior is still considered normal since for a random polarized laser, only the total power really matters, not any peculiar gyrations the modes may go through.

• 

  This is a typical yellow (594.1 nm) HeNe medium-long HeNe laser. The rated output power is 2.0 mW but this sample produces about 4.1 mW. Due to the low gain of the yellow lasing line, the amplitude of the power fluctuations is somewhat greater than for even the slightly shorter 05-LHR-151 red laser, above.

  

  The same laser with a polarizing filter in the beam. The fluctuations are larger as expected both because of the fewer modes in the polarized beam, and the lower gain of the 594.1 nm lasing line.

• Not all lasers are manufactured properly. For example, one sample of a laser similar to the 05-LYR-171 exhibits a slow large amplitude oscillation in power. Compare:

  

  and

  

  with the plots above. The plots in blue are for the normal output beam from the front (OC) of the laser. The plots in red are for the excessively large waste beam from the rear (HR) of the laser – a clue to the part of the cause of the power oscillations. That has to be one spectacular screwup. I wonder how many laser heads were built that way. 🙂

• 

  . Finally, here is how a laser with active mode stabilization behaves. This laser is designed to provide a single longitudinal mode output with a frequency stability on the order of 1 MHz. Since the laser head has optics to separate the modes with orthogonal polarization, the raw beam already varies by more than 2:1 in output power without any additional polarizer. Yes, that is the actual spread – the vertical scale hasn’t been stretched! The actual HeNe laser tube inside is a specially selected Melles Griot 05-LHR-120, which by itself would have a normal mode sweep with a small ripple. From a cold start to lock takes about 20 minutes.

  

  This plot zooms in on the last two cycles. Notice that there is a slight distortion on the rising part of the second cycle in the plot. That is probably when the active feedback is switched on. Before then, the heater is simply running at a constant current to bring the tube up to operating temperature. It only takes less than one full additional cycle to achieve lock. The amplitude is then quite stable (uncertainty of less than 0.5 percent on the plot), but the frequency stability which is d(power)*slope(frequency/power), will be under 0.125 percent of the mode spacing of around 750 MHz, so less than 1 MHz.

• 

  Above shows how another even more highly stabilized (or at least more expensive!) laser behaves. Note that the entire warm up period from laser on to locked is only around 3.5 minutes because the heater for the active mode control is inside the laser tube, wrapped directly around the bore. The control algorithm during warm up is also a bit more sophisticated, pausing periodically to determine the “mirror spacing rod” temperature by measuring the heater resistance, and entering a “fine adjust” mode about half way through. In fact, from the relative shapes of the red and blue mode cycles, it can be seen that for this particular laser, during most of the time from power on (a cold start) to lock, the tube is heating (about 75 cycles), but it switches to steady cooling (about 6 cycles) just before locking.

• 

  Above shows the 5 mode cycles just before locking and the final transition to the locked state. The peculiar shape of these Zeeman-split modes is clearly evident in this expanded view.The actual beat frequency is shown for the last few cycles and after locking in both these plots. This is the actual measured frequency captured along with the F1 and F2 modes, and total output power. (Showing the frequency plot earlier would be a mess.) The beat only appears for a small percentage of the mode cycles with some variation during the time it is present. The warm up and locking algorithm is partially responsible for the distorted nature of these plots compared to those for “normal” unstabilized He-Ne lasers or even other common stabilized He-Ne lasers due to the periodic pauses and switching from heating to cooling that may occur. However, the peculiar shape of the mode cycles themselves is due to the fact that these are not linearly polarized modes as with all the previous lasers. Rather, they are Zeeman-split modes distorted by a magnetic field and include (mostly) components differing in frequency by at most a few MHz, rather than the normal longitudinal mode spacing, which is 1.2 GHz for this laser.

• 

  This is a very naughty laser. 🙁 🙂 As can be seen, for most of the mode cycles during the warm up period, rather than the two orthogonal polarizations (called P and S) changing smoothly as they cycle, each one moves to a distinct point and then instantaneously swaps places with the other one. Tubes with flipper behaviour will generally not be useful for a stabilized He-Ne laser but may be perfectly satisfactory when simply used as a light source without polarizing optics.However, near the very end of the warm up period (measured in terms of mode cycles, not time) something very interesting occurs: The tube seems to have reverted to being well behaved! This only happens when the tube is approaching thermal equilibrium where each complete mode cycle is taking over 90 seconds. There are perhaps 3 or 4 beyond what is on the plot but the tube temperature is so close to its final value that any disturbance like moving near the laser head will disrupt the sequence. This behaviour is consistent from run to run. The cause is unknown, nor is it known whether the tube would continue to behave if stabilization was attempted. But it might since the operating temperature will be somewhat above the natural point of thermal equilibrium.

  

  Above is a close up of the mode variations when flipping. The shapes are nearly identical from the start of warm up until the transition to normal behaviour. Also note that the frequency of the mode cycles for a flipper is double that of a normal tube – each mode would normally be what resulted from tracing the continuous curve and not taking the discontinuities as is evident below:

  

  So following red-blue-red, etc., ignoring the green lines.

  

  Above is a closeup of the point where flipping ceases. Note that the “envelope” of the mode plot is virtually unchanged at this point but the green transitions have disappeared. At the transition point, the period of a full mode sweep cycle is about 80 seconds. There are then an additional 10 full cycles (only 4 or 5 are shown) requiring about an hour until thermal equilibrium.

• 

  Above is an example of consistent flipper behavior. The character of the plot does not change from power-on to full warmup. There is a consistent flip at the same point in the mode sweep cycle. Interestingly, there is a slight asymmetry in the mode sweep envelope, unusual for most HeNe lasers. Whether this has any profound cosmic significance is unknown. 😉 These tubes were found in some barcode scanners due to their physical robustness – They could fall on the floor – or probably be used as hammers – without sustaining any damage! See the section: Metrologic Steel-Ceramic Hard-Seal He-Ne Laser Tubes.

These have all been high quality HeNe lasers and except as noted, have relatively predictable mode performance. For information and plots for a really ill-mannered beast, see the section: Far East HeNe Laser Tubes.

Mode Competition in Short He-Ne Lasers

If you haven’t been wondering why some of the output power plots are so strange, you should be. 🙂

The primary reason that the output power in any give longitudinal mode doesn’t vary in a nice smooth (Gaussian) manner is due to mode competition. If not for mode competition, the gain would not saturate and be the same for all modes. Everyone would thus trace out the envelope of the neon gain curve. However, since the lasing modes are actually competing for a limited resource – the atoms in the upper lasing state – whenever there are more than one mode present, they have to be nice and share. This is most dramatic when only 2 or 3 modes are present since each one has a large fraction of the total output power. With those, the shapes of the envelopes of the polarized output power curves can be decidedly non-Gaussian. And for Zeeman-split lasers, downright weird. But once the various regions are understood – where there are 1, 2, 3, or more modes competing – then the resulting shapes make more sense:

1 mode: The output power will change smoothly during mode sweep roughly following the profile of the Gaussian neon gain curve (minus the lasing threshold). The only way a real laser could be single mode throughout mode sweep would be either for the cavity to be around 10 cm or less (in which case lasing may cease entirely for a part of mode sweep) or for there to be an additional means of forcing SLM operation (such as an etalon inside the cavity). But slightly longer tubes will operate with a Single mode over a portion of mode sweep with 2 modes for the remainder.

Plot of Mode Sweep of Typical 1 mW Random Polarized He-Ne Laser Tube shows the appearance for a Melles Griot 05-LHR-007, the shortest modern laser tube I’m aware of. Over approximately 50 percent of the mode sweep cycle, it is pure single mode with power sharing during the remainder.

2 modes: When a second mode appears, it will start eating into the power of the first mode. Where the modes are balanced on either side of the neon gain curve, their power will be equal. Between these 2 points, they will share power. The total output power may remain relatively constant or increase slightly when equal (usually up to around 20 percent). Tubes with a cavity length of 12 to 16 cm will operate with 1 or 2 modes.

3 modes: When a third mode appears, it will start eating into the power of the other two. The relative and total power will depend on their location on the neon gain curve and is at the very least, not intuitively predictable. 🙂 Tubes with a cavity length of 20 to 25 cm will operate with 2 or 3 modes during mode sweep.

Plot of Mode Sweep of Typical 3 mW Random Polarized He-Ne Laser Tube shows the appearance for a Spectra-Physics 088 (same as the Melles Griot 05-LHR-088) used in the SP-117/A/B/C and Melles Griot 05-STP-901 stabilized lasers. It is similar a common barcode scanner tube. At the peaks of the polarized modes (minimum for total power), there are 2 modes. Where the polarized modes cross, there are 3 modes. The overall shape of the mode sweep depends on many factors including the exact length of the cavity which determines where it switches from 2 to 3 modes.

4 or more modes: The same general rules apply, but since the contribution of each mode is smaller, the effects of mode competition are also smaller and more difficult to see and interpret.

Inhomogeneous Broadening in Neon and Mode Sweep

The shape of the neon gain curve is by now familiar, but what does it really mean? The popular notion of it being the result of some magical process is fine as a first step, but doesn’t help in attempting to understand how it is affected by wavelength, or for explaining phenomena like the Lamb Dip.

What is really being depicted in the gain curve is a combination of a curve derived from what’s called the “natural line width of neon” which is homogeneously broadened, and the distribution of atomic velocities of excited neon atoms as they translate into a distribution of Doppler shifts in optical frequency.

Ignoring Special Relativity (which is acceptable for the velocities involved), the Doppler shift in optical frequency is equal to the relative velocity of the excited atom divided by the speed of light multiplied by the optical frequency or:

              va
  Δf = -f0 * ----
              c

Where:

• Δf is the optical frequency shift.
• f₀ is the original optical frequency.
• v_a is the velocity of an atom in the upper lasing energy state relative to a photon traveling along the axis of the laser tube.
• c is the speed of light.

At any temperature above absolute zero, all atoms are in motion and have a probabilistic distribution of velocities (speed and direction), which all contribute to the Doppler broadening. For a Fabry-Perot (linear) cavity, the photons traveling in either direction “experience” the relative speed of the excited atoms. Stimulated emission will only occur when the Doppler-shifted energy of the photon matches a possible lasing transition of an excited atom. The width of the Doppler broadening is directly proportional to optical frequency, but it is also affected by other factors including temperature and pressure, since these impact the distribution of atomic velocities. The shape of the Doppler broadening curve is then the result of the aggregate of the motion of all the atoms available for stimulated emission. And the width of the inhomogeneously broadened neon gain curve is the width due to homogeneous line-width of neon plus the inhomogeneous Doppler broadening. Since they are added like independent noise souces using the square-root of the sum of the squares, the increase in neon gain bandwidth due to the homogeneous line-width is quite small (just over 5 percent even at 3,391 nm). Thus, the change is close to 1/5th even if the homogeneous part is ignored.

Assuming the FWHM value of 1.6 GHz for the entire inhomogeneously Doppler-broadened gain bandwidth of the common red wavelength of 632.8 nm, at the mid-IR wavelength of 3,391 nm it is only 315 MHz. And at the green wavelength of 543.5 nm it is about 1.86 GHz. The optical frequency difference between cavity modes (c/2L) is only dependent on cavity length and the speed of light. Thus, the number of lasing modes possible for a given cavity length decreases as the gain bandwidth becomes narrower at longer wavelengths.

Note that the lasing modes themselves will have a very narrow bandwidth – possibly as small as 5 kHz or even lower for a laser operating with a single mode. At that point, physical vibrations, laser power supply noise, and other external effects are the limiting factors, not the theoretical minimum bandwidth for the HeNe laser which is under 1 Hz! (Schawlow-Townes linewidth). I originally thought that finding values for the bandwidth of commercial HeNe lasers would be straightforward, but it seems to be near impossible. The only specifications I am aware of from a laser manufacturer are in Laboratory for Science brochures. The best is for their model 220 Ultra Stable HeNe laser, which lists 5 kHz over a period of 1 second. But the value for the type of HeNe laser tube that used to be found in barcode scanners may not be all that much greater if it is mounted to minimize vibrations and driven with a well filtered HeNe laser power supply.

One would expect that with the much smaller gain bandwidth at 3,391 nm of 315 MHz, there would be fewer longitudinal modes oscillating compared to 632.8 nm. Or equivalently, a laser tube would need to be much longer for the same number of modes to fit within the FWHM of 315 MHz. But because of the very high gain at 3,391 nm, the lasing threshold will be lower and thus the effective gain bandwidth of the neon gain curve is going to be wider. I do not know by how much, but with a potential gain over 40 times that of the 632.8 nm transition, it could be very significant. There might even be more modes than at 632.8nm.

Due to the longer wavelength, mode sweep for a laser tube at 3,391nm will have a complete cycle that is over 5 times as long as one at 632.8nm. These same numbers would apply to mode competition at 3,391 nm interfering and stealing power from a 632.8nm laser.

Number of Longitudinal Modes at Other He-Ne Wavelengths

As described above, the gain bandwidth of neon is roughly inversely proportional to the wavelength (or proportion to the frequency) of the lasing transition. However, this assumes that the lasing threshold is at the same location relative to the peak of the neon gain curve, often specified as the Full Width Half Maximum or FWHM. At 632.8nm, this turns out (not coincidentally!) to be reasonable and results in the expected number of lasing modes and mode sweep plots to go along with them.

For very low gain wavelengths like green (543.5nm) and yellow (594.1nm) – which may have 1/10th the gain or less compared to the common red (632.8nm) wavelength, the lasing threshold will be far higher on the roughly Gaussian shaped gain curve, where it is narrower. So, while the FWHM of the neon gain curve may be slightly wider at these wavelengths, fewer modes will be oscillating because of the narrowing due to the higher lasing threshold. However, until the lasing threshold approaches the peak of the gain curve, the reduction in number of modes won’t be that dramatic. And every effort is made to eliminate losses inside the cavity for these low gain lasers, so in fact, the lasing threshold may not even get that high relative to the peak during the expected life of the laser.

For very high gain wavelengths, the reverse will happen. There’s really only one – the mid-IR transition at 3,391nm which behaves more like a solid state laser with a gain over 40 times that of 632.8nm. The lasing threshold will be much lower on the gain curve extending the useful region well out into the tails of the distribution. In this situation, many more modes could end up oscillating than would be accounted for by the much narrower FWHM of the neon gain curve of 315MHz – roughly 1/5th the width compared to 632.8nm. If calculations based solely on this small gain bandwidth were valid, a 75cm 3,391nm laser would have a similar number of longitudinal modes to a 14 cm 632.8nm He-Ne where there are only 1 or 2 active modes at any given time. Since 3,391nm lasers much shorter than 75 cm are commercially available and don’t have dramatic variations in output power with mode sweep, this must not be the case. For example, REO has one with a cavity length of less than 50cm and maximum power variation of 5 percent, which implies that there are several longitudinal modes always present.

Here are results so far:

• 543.5 nm (green): TBD.
• 594.1 nm (yellow): TBD.
• 604.6 nm (orange): TBD.
• 611.9 nm (orange): TBD.
• 632.8 nm (red): See other info below and in the sections starting with Inexpensive Home-Built Frequency or Intensity Stabilized HeNe Laser.
• 1,152 nm (near-IR): TBD. Other than my rebuilt SP-119 laser head, a sample of one of these may be difficult to find. The only thing that can be said about the IR SP-119 is that it is short enough that lasing ceases entirely for a portion of the mode sweep.
• 1,523 nm (near-IR): Initial testing of a Melles Griot 05-LIR-150 with a cavity length of 34.2 cm and a strategically placed magnet seem to show that its behavior is similar to that of a 632.8 nm laser with a cavity length of 20 or 25 cm. But, the amplitude of the two polarizations are not equal implying that it is probably operating at least in part as a transverse Zeeman laser, which isn’t that surprising given the magnet. However, with 3 strategically placed magnets, the behavior reverts back to what would be expected of a 633 nm tube of 20 or 25 cm with two pure orthogonally polarized modes separated by the longitudinal mode spacing of the tube are present for most of mode sweep with just a hint of a third mode when one is near the center of the neon gain curve. So, it would look like:

  

  which is the same diagram as

  

  with different numbers.) With my $2 SFPI modified for 1,5XX nm operation (replaced PD with cut-open germanium transistor photodiode), I have confirmed that the modes of the 05-LIR-151 are also similar in number and appearance to those of a 20 to 25 cm 632.8 nm laser. There are 2 modes most of the time with 3 appearing briefly when a mode is close to the center of the gain curve.

• 3,391 nm (mid-IR): TBD, maybe.

Intensity Stability of He-Ne Lasers

There are at least three kinds of intensity variations present with He-Ne (or other gas) lasers: long term as various longitudinal modes compete for attention, short term due power supply ripple or discharge instability, and beat frequencies between modes that are active.

Common internal mirror He-Ne laser tubes include a specification called “Mode Cycling Percent” or something similar. This relates to the amount of intensity variation resulting from changes in longitudinal modes due to thermal expansion. Typical values range from 20 percent for a small (e.g., 6 inch, 1mW) tube to 2 percent or less for a long (e.g., 15 inch, 10mW) tube. These take place over the course of a few seconds or minutes and are very obvious using any sort of laser power meter or optical sensor. Even the unaided eyeball may detect a 20 percent change. The more modes that can be active simulataneously, the closer those that are active can be to the same output power on the gain curve. Very short tubes or those with low gain (other wavelengths than 632.8nm or due to age/use or poor design) may vary widely in output intensity or even cycle on and off due to mode cycling. (Note that since the polarization for each mode may be different, reflecting the beam of one of these He-Ne lasers from a non-metallic reflective surface (which acts somewhat as a polarizaer) can result in a large variation in brightness as the dominant polarization changes orientation over time.) Trading off between tube size and mode cycling intensity variations is one reason that He-Ne tubes with otherwise similar power output and beam characteristics come in various lengths.

There are also stabilized He-Ne lasers which use optical feedback to maintain the output intensity with a less than 1 percent variation. (They usually also have a frequency stabilized mode but can’t do both at the same time.) An alternative to doing it in the laser is to have an external AO modulator or other type of variable attenuator in a feedback loop monitoring optical output power. See the next section for more info.

Short term changes in intensity may result from power supply ripple and would thus be at the frequency related to the power line or inverter. These can be minimized with careful power supply design.

Intensity variations at 100s of MHz or GHz rates result from beats between the various longitudinal modes that may be simultaneously active in the cavity. For most common applications, these can be ignored since they will be removed by typical sensor systems unless designed specifically to respond to these high beat frequencies.

Also see the section: Amplitude Noise.

Stabilized Single Frequency He-Ne Lasers

The common red (633 nm) He-Ne laser, while highly monochromatic, generally does not produce just a single frequency (or equivalently, wavelength) of light. As noted in the section: Longitudinal Modes of Operation, several closely spaced frequencies will generally be active at the same time and their precise values and intensities will change over time. For many applications, this doesn’t matter. However, for others, it makes such a laser useless.

If you have, say, $5,000 to spend, you can buy a red (633nm) He-Ne laser that actually produces a single frequency with specifications guaranteed stable for days and that don’t change over a wide temperature range. While the operation of such a He-Ne laser is basically the same as the one in a barcode scanner (and in fact may use the identical model He-Ne laser tube!), several additional enhancements are needed to eliminate mode sweep and select a single output frequency. Simply constructing the laser cavity of low thermal expansion materials isn’t enough when dealing with distances on the order of a fraction of a wavelength of light! Active feedback is needed. The most common implementation of these lasers starts with a short red (632.8 nm) tube that can only oscillate on at most 3 longitudinal modes. (For technical reasons, stabilized lasers at the other common visible and IR He-Ne wavelengths are more difficult to implement and are much less common. More on this below.) It then adds optical feedback to keep them in a fixed location on the He-Ne gain curve by precisely adjusting the distance between the mirrors over a range of about 1/2 the lasing wavelength. This is most often done with a heating coil (inside or outside the tube), but a PieZo Transducer (PZT, an expensive version of the beeper element in a digital watch) may also be used. The PZT reduces the time for the system to stabilize to a few seconds, compared to up to 30 minutes for the heater. But, for a laser that will be left on continuously, this probably doesn’t matter. Some lasers use a means of cooling in addition to the heater like a piezo fan, probably to allow the laser to run stably over a wider temperature range. And a few including the Melles Griot 05-STP-909/910/911/912 (originally based on the Aerotech Syncrolase 100) use a miniature RF induction heater surrounding the HR mirror mount to control only its length, not that of the entire tube. With direct heating of such a small mass, the response is quite fast. This also makes for a more compact package than a full tube heater.

Many schemes work well and it’s amazing how dirt simple these really are considering their hefty price tags! It’s easy to build perfectly usable systems from a common surplus HeNe laser tube and a few common junk parts.

The common ones are listed below:

  Type of Stabilization Technique                  Variation  Precision
 -----------------------------------------------------------------------
  Normal (multimode) HeNe laser                       ---        ---
  Single mode without stabilization                 1.5 GHz     3x10-6
  Single mode amplitude stabilization                10 MHz     2x10-8
  Lamb dip stabilization                              5 MHz     1x10-8
  Gain peak stabilization                             5 MHz     1x10-8
  Dual mode polarization stabilization                1 MHz     2x10-9
  Second order beat stabilization                   200 kHz     4x10-10
  Zeeman beat frequency stabilization               100 kHz     2x10-10
  External reference (iodine) cell stabilization     <5 kHz     1x10-11
  External reference (F-P resonator) stabilization   <1 Hz      1x10-14

Note that an etalon inside the laser cavity could also be used to select out a single longitudinal mode. For high power lasers which would require long tubes supporting many modes, this would be needed with both the overall mirror spacing and etalon being feedback controlled. But for low power lasers (e.g. 1 to 3 mW), the use of a short tube to limit the number of modes in conjunction with basic feedback control is a much less complex lower cost approach.

Stabilized lasers (or anything that needs to be regulated to some precision) can be classified as two types. The technique is “intrinsic” – basically derived from an internal reference – if what is used to regulate the device is a fundamental property of its construction – the laser physics in this case. It is “extrinsic” if some external reference is used. Most commercial stabilized HeNe lasers are of the first type since they exploit the known and essentially fixed frequency/wavelength and shape of the neon gain curve in the E/M spectrum. Additional techniques may be used to further reduce the uncertainty.

Most common commercial stabilized HeNe lasers are red at 633 nm, partially because of all the available HeNe wavelengths with a single frequency output power of less than 2 mW. Systems like this are both relatively easy to implement and generally useful for a wide range of applications. The approaches usually fall into one of two subclasses:

1. One or Two Mode stabilized systems: These use random polarized HeNe laser tubes that are short enough that only a few modes will oscillate at the same time. Adjacent modes of a random polarized HeNe laser tube are almost always orthogonally polarized. So, where two modes are oscillating, separate signals corresponding to the amplitude of each mode can be easily obtained by feeding a pair of photodiodes from a polarizing beamsplitter. (If a tube has modes that aren’t orthogonally polarized or that behave strangely, it gets recycled into another application or the dumpster.) The signals may be obtained from the waste beam out of the HR mirror of the laser or by sampling a portion of the output beam. Either one or both of the photodiode signals can then be used for the feedback loop depending on whether intensity or frequency stability is most important. Note that under some conditions, up to 3 or even 4 modes may be permissible in a tube that is to be used for these purposes. More below.
   • Where the best frequency stability is desired, the ratio of the mode signals (usually made 1:1) is used in the feedback loop. This results in better absolute frequency stability since this ratio is independent of the actual output power, which may change as the tube warms up and ages due to use. With a ratio of 1:1, the two modes are parked equally spaced on either side of the gain curve. Even if the tube oscillates on 3 modes if one is near the center of the gain curve (1 strong one and 2 weak ones), there will only be 2 modes when stabilized. The overall approach is shown below:

     

     Commercial examples include the Coherent 200, Spectra-Physics 117/A/B/C (and identical Melles Griot 05-STP-901), REO SHL. Axsys/Teletrac 150, and many others.Some inexpensive (this is relative!) stabilized HeNe lasers only use a single mode for frequency locking. When on the slope, this will be reasonably stable after warmup once the output power has reached equilibrium.

   • When the best intensity stability with a polarized output is desired, the signal from a single mode (one photodiode channel) is compared to a reference voltage and this becomes the error signal in a feedback loop to put its mode near the center of the gain curve. Even if the tube oscillates on up to 4 modes if there are two on either side of the gain curve, with one near the center of the gain curve when stabilized, there will be at most 2 weaker modes on the tails of the gain curve. Since these will be orthogonally polarized to the dominant center mode, they can be blocked by the output polarizer. The overall approach is shown below:

     

     Commercial examples include the Spectra-Physics 117A (and identical Melles Griot 05-STP-901), and REO SHL.

   • When the best intensity stability of the total output (without regard to polarization) is desired, a non-polarizing beam sampler is used or the signals from the two photodiode channels are summed and compared to the reference. I am not aware of any commercial lasers using this approach.
2. Zeeman split systems: A magnetic field is used to create a pair of lasing modes that differ from each other by a relatively small frequency. The stable optical frequency along with the Zeeman difference frequency are used for a variety of metrology applications. These may be classified as either axial or transverse based on the orientation of the magnetic field:
   • Axial: Like the single mode systems described above, the tube length is such that only a single longitudinal mode will oscillate. However, a powerful axial magnetic field splits this single mode into two sub-modes with counterrotating circular polarization states. When passed through a polarizer at the output, this results in a beat frequency in the 100s of kHz to several MHz range (depending on the magnetic field strength and other factors) which may be used to derive the stabilization feedback signal and is also key to the measurement technique for which these are designed. The overall approach is shown below:

     

     Commercial examples include the HP/Agilent 5501B, 5517, 5518A, and 5519A/B (though the heater is actually *inside* the tube for these); Excel 1001; Zygo 7705; and others.

   • Transverse: Like the two mode systems described above, the tube length is such that a pair of modes can oscillate when straddling the gain curve but only a single mode when at the peak. A moderate transverse magnetic field in conjunction with the natural birefringence of the mirror system results in a beam frequency in the 10s to 100s of kHz range. Since the beat frequency varies slightly with the mode position, it may be used in a PLL feedback loop for frequency stabilization. One example is the Laboratory for Science model 220.

Most commercial stabilized He-Ne lasers for general laboratory applications are of type (1) and operate with 2 orthogonal modes for frequency stabilization, though some use 1 mode for intensity stabilization (or can select between them with a switch). (Regardless, only a single longitudinal mode – thus a single optical frequency – may be allowed to exit the laser, the other being blocked with a polarizer.) These include the Coherent 200, Spectra-Physics 117 and 117A (and the identical Melles Griot 05-STP-901), many from Zygo, and various models from REO, Thorlabs, and others. For example, in the Melles Griot 05-STP-901 frequency and intensity stabilized He-Ne lasers (no longer in production), the laser cavity permits a pair of orthogonal polarized longitudinal modes to be active and can provide very precise control by straddling these on either side of the gain curve (frequency stabilized mode) or a single longitudinal mode that is also used for the output on one side of the gain curve (intensity stabilized mode). Those from other companies are generally similar.

All the interferometry lasers manufactured by Agilent (formerly Hewlett Packard), Excel, and one model from Zygo (the 7705) are of type (2). While lasers from Teletrac/Axsys, Optodyne, Renishaw, and others are type (1).

And there are hybrid approaches. For example, the Zygo 7701/2/12/14 lasers generate and lock a single frequency via dual mode stabilization, But it is split into two using an Acoutso-Optic Modulator (AOM) rather than the Zeeman effect.

For some photos of the (quite simple) Zeeman split stabilized He-Ne tube used in the Hewlett-Packard 5517 laser head, see the Laser Equipment Gallery (Version 1.86 or higher) under “Assorted Helium-Neon Lasers”. And for more information on these lasers, see the sections starting with: Hewlett-Packard/Agilent Stabilized HeNe Lasers.

It isn’t really possible to convert an inexpensive HeNe tube that operates on many longitudinal modes into a single frequency laser. Adding temperature control could reduce the tendency for mode hopping or polarization changes, and the addition of powerful magnets can force a polarized beam. But, selecting out a single longitudinal mode would be difficult without access to the inside of the tube. However, if the HeNe tube is short enough that the mode spacing exceeds about 1/2 the Doppler-broadened gain bandwidth for neon (about 1.5 GHz), it will oscillate on at most 2 longitudinal modes at any given time and these will each be linearly polarized and usually orthogonal to each-other. Then, stabilization is possible using very simple hardware. In fact, even if the mode spacing is a bit smaller – down to 500 or 600 MHz – then only 2 modes will be present most of the time but 3 may pop up if one is close to the center of the gain curve. This, too, is an acceptable situation since the tube can be stabilized with the modes straddling the gain curve and then only 2 modes will oscillate. For intensity stabilization, 4 modes may even be permitted. Note that while the modes of a random polarized and linearly polarized tube are similar (except for polarization), a random polarized tube is desirable to be able to use a tube that supports 2 modes with the benefits they provide, while being able to eliminate the second mode from the output.

It may be possible with a combination of what can be done externally, as well as control of discharge current, to force a situation where gain is adequate for only 1 or 2 modes even for a longer tube. Whether this could ever be a reliable long term approach for a HeNe tube that normally oscillates in many longitudinal modes is questionable. What I don’t think will have much success are optical approaches such as feeding light back in through the output mirror. Doing this would likely have the exact opposite of the desired effect but may work in special cases (it’s called injection locking and is used with great success for other applications).

Coherent, Melles Griot, Spectra-Physics, and others will sell you a small stand-alone stabilized HeNe laser for $5,000 or so and Agilant (HP) and others have interferometers and other similar equipment which includes this type of laser (and are even more expensive!). Other manufacturers includ Zygo, Teletrac, Nikon, Micro-g Solutions, SIOS, NEOARK, and REO. The lab lasers that I’ve seen all use short HeNe tubes with thermal feedback control of the resonator length and all operate at the red HeNe wavelength (632.8xxxxxx nm to 8 or more significant figures). The Spectra-Physics model 117A/118A laser actually uses a lowly SP088-2 tube similar to those in older grocery store barcode checkout scanners as its heart. A tube like this is visible below:

However, some do employ a custom tube with the heater inside to greatly speed up response and reduce heat dissipation to the outside. A stabilized HeNe laser for green or other color visible HeNe wavelength or one of the IR wavelengths is also possible using the same techniques.

As noted above, the actual stabilization mechanism for the general purpose stabilized lasers may be the ratio of amplitudes of two longitudinal modes (which is better for frequency stabilization) or the amplitude of one mode (which is better for intensity stabilization). These are usually stable to within a few parts in 10⁹. However, the frequency drift when intensity stabilized is still not much – probably less than 1 part in 10⁸. Output power variation may be 0.2 percent if intensity stabilized and 1 percent if frequency stabilized. Some allow either method to be selected via a switch, as well as providing for an external tuning input to vary the frequency over several hundred MHz. (However, due to the thermal control most often used, the response time is not exactly fast.)

The Zeeman split interferometer lasers may lock the difference frequency to a crystal clock, though most seem to use the basic polarized modes for stabilization, with the Zeeman beat used only as the reference for the interferometer. A few do lock the Zeeman frequency to a PLL. One of these was the Laboratory for Science Model 220. (Laboratory for Science is now out of business).

More sophisticated schemes with even better precision and lower long term drift may lock to the “Lamb Dip” at the center of the neon gain curve or to one of the hyperfine absorption lines of an iodine vapor other type of gas cell, achieving stabilities on the order of 1 part in 10¹⁰ or even better.

Due to the performance, simplicity, reliability, and relatively low cost of stabilized HeNe lasers, they are still often the preferred frequency reference for many applications. As noted, a typical system might go for $5,000. While this may seem high, it is small compared to many other technologies. The cost is not the result of expensive components or complex manufacturing, but more to the relatively limited number of units produced. If stabilized HeNe lasers were as popular as laser pointers, they would probably cost under $100

Digital Control of Stabilized He-Ne Lasers?

These types of lasers have been designed using simple analog techniques for over 35 years. So why change? A few op-amps, a monostable or two, and a handful of discrete parts is sufficient for any conceivable level of performance in a mode-stabilized HeNe laser. There are at most two signals that need to be monitored (the polarization modes) with the objective of maintaining them equal or in a fixed ratio. Yet, I’ve seen at least 3 examples of dual polarization mode stabilized He-Ne lasers that have gone from a simple analog approach to a much more complex digital approach, apparently with no obvious technical justification:

• HP/Agilent 5517: Xylinx or similar FPGA.
• Zygo 7702: Motorola 68HC11 microprocessor.
• Teletrac/Axsys: Microchip PIC16C73A-20/SP PIC.

All are basic mode stabilized He-Ne lasers. The 5517 is a Zeeman-split laser but the stabilization is mode-based.

The redesign in each case must have cost a fortune. Since none of these lasers had many adjustments in their analog designs, ease of manufacturing is probably not the justification. And there is no need for preventive maintenance as components age – lasers like this will run for years on-end without any adjustments. Cost of components is also not a viable excuse as jelly bean op-amps and other common parts are adequate for any of these lasers. Nor do any require an external computer interface like more complex lasers.

However, one obvious benefit from the company’s point of view is serviceability, or lack thereof for anyone not supported by the manufacturer. The new designs are virtually impossible to troubleshoot and repair without detailed service information, and possibly support software. Unless the problem is obvious like a broken wire or blown fuse, attempting to find an electronic fault in these high density surface mount PCBs controlled by firmware programs is just about impossible. And Marketing can promote the “benefits” of digital technology, as bogus as that may be here. If anything, the additional electrical noise from digital signals is a detriment. Digital has to be better, right? 🙂

Iodine Stabilized He-Ne Lasers

Unlike the more common He-Ne stabilized lasers like those that lock to some intrinsic feature of the lasing process like the neon gain curve, an Iodine Stabilized HeNe Laser (ISHL) uses a external gas cell containing iodine vapor, so that a line in the iodine absorption spectrum is used as the reference wavelength. In principle, this provides an improvement in long term wavelength accuracy of 1 to 2 orders of magnitude – down to 0.1 parts per billion, corresponding to a few 10s of kHz – or better.

An ISHL operating on the common red (633 nm) wavelength consists of a He-Ne laser tube with one or two Brewster windows, a gas cell containing iodine at low pressure, and at least one external mirror on a PieZo Transducer (PZT) for fine cavity length control. The iodine cell needs to be installed inside the laser cavity to benefit from the high intra-cavity circulating power as the sensitivity in the vicinity of 633 nm is very low. However, when operating on the green (543.5 nm) wavelength, the cell can be external despite the lower power generally achievable with green, because the sensitivity is higher.

The basic principles of operation for an ISHL are rather straightforward: The iodine (or actually I₂) has a very complex absorption spectra with hundreds of absorption lines. A very small portion of it is shown below:

By dithering the laser cavity length via a PZT, a lock-in amplifier (also known as a phase sensitive detector or synchronous demodulator) can maintain the wavelength at the very center of any selected absorption peak (or dip, depending on your point of view!). The challenging part is to be able to reliably select a specific absorption line to lock to. So, although locking to a given line is fairly simple, the overall electronics can get to be quite complex if automatic line selection is desired, though nowadays, an embedded microcomputer does the line selection.

Here are some photos of an iodine stabilized laser based on the classic NIST (National Institute of Standards and Technology, formerly the National Bureau of Standards) design originally described in the paper: Howard P. Layer, “A Portable Iodine Stabilized Helium-Neon Laser, “IEEE Trans. on Inst. and Meas, IM-29, pp358-361, 1980. The photos are actually of two different samples of the NIST design. The first one is of a complete laser head while the others are of a physically similar resonator only where it’s easier to see the individual components.

• 

  The overall appearance is unremarkable with a shutter at the front (the round black thing) and several cables coming out the back (hidden). Leveling “feet” would often be installed be installed in the cast tabs for precise alignment. It is not known if this was a commercial product or built by NIST or perhaps even Hewlett Packard based on the NIST design. But there was an Agilent inventory sticker on the cover, so perhaps this very laser was used to certify HP/Agilent metrology lasers like the 5517A! 🙂 In fact, the base of this laser bears a striking resemblence to the 5517A (though the dimensions don’t match). It’s a combination of a cast and machined assembly, clearly not made for a one time research project. It may in fact be a Frazier Model 100 FISL (or the NIST version they copied) as the head looks identical to the Frazier laser down to the pattern of holes in its cover. 🙂

• 

  The glow of the Melles Griot 05-LHB-290 two-Brewster HeNe laser tube can be seen within the resonator structure.

• 

  The resonator is a rather massive metal structure about 18 inches long.

• 

  Most of the Melles Griot 05-LHB-290 two-Brewster tube is hidden, but it is mounted via compression O-ring fittings with the high voltage supplied via the BNC connector. The gray blobby thing houses the ballast resistors.

• 

  The iodine cell is mounted via compression O-ring fittings between the two-Brewster HeNe laser tube and one of the mirrors. The gold connectors (1 of 2 are visible) are for temperature control of the iodine cell, and possibly a photodiode for monitoring the fluorescence.

• 

  This shows the same iodine cell having been removed from the ISHL resonator, being excited by a separate green HeNe laser. The yellow-green (with some red) fluorescence inside the iodine cell means some green light is being absorbed and would show up as a reduction in transmitted beam power. (Fluorescence from a 633 nm beam would be in the IR and boring.)

• 

  This has an angled plate to provide a small portion of the output beam to a silicon photodiode. Both mirrors are mounted on PZTs for cavity length control and dither (though it’s not clear why a single PZT wouldn’t suffice for both functions).

Although the laser head does not presently lase, I am hopeful that it will someday. The discharge color of the HeNe laser tube is normal and there is no visible brown crud in the bore indicating that it should be healthy. The iodine cell still has iodine in it based on its response to a green (532 nm) DPSS laser pointer beam. This thing has probably been sitting on a warehouse for years, if not decades (next to the lost Ark), so the non-lasing condition isn’t exactly a surprise. However, there seemed to be some type of contamination inside one of the B-windows. So, it may require a replacement 05-LHB-290. I do have one that lases, though it’s a bit weak. However, the NIST paper states that the reflectivity of the OC mirror is only 93 percent, presumably to force single longitudinal mode operation by reducing gain, but this also dramatically reduces output power. And the tube would need to be quite healthy to lase at all. Replacing that mirror with a 99 percent OC might be an option. Then mirror alignment or some other means could be used to force SLM. It would seem like a more logical solution to force SLM would be to add a PZT-controlled etalon that tracks cavity length tuning. Then, the output power would be close to the maximum available from the tube – 5 to 10 times higher than this design produces. But I’ve not seen that anywhere. The paper also states that the laser tube and cavity are 20 and 30 cm long, respectively. On my samples, they are at least 25 and 35 cm. And, their laser tube appears to not be a Melles Griot 05-LHB-290. So perhaps the original prototype was not identical to the versions later reproduced by Frazier (and others), though it’s quite clear that Frazier copied nearly every aspect of the laser design down to the controller-in-a-scope and its front panel layout and labeling. 😉

More on ISHLs:

• Some additional photos of a similar resonator and other components of a complete system can be found at Professor Keto’s Iodine Stabilized HeNe Laser (University of Texas). Each of the photos has a fairly complete description.
• And another photo and more info from the NIST Museum’s Length – Evolution from Measurement Standard to a Fundamental Constant. It includes a photo of a NIST engineer along with a laser identical to the Frazier 100.
• The National Physical Laboratory (UK) provides both 633 nm and 543.5 nm ISHLs. Search for “iodine”.
• Frazier Instrument and Winters Electro-Optics companies also offering ISHLs.

And multi-wavelength iodine stabilized He-Ne laser have also been built. See: “A Tunable Iodine Stabilized He-Ne Laser at Wavelengths 543 nm, 605 nm, and 612 nm”, J. Hu, T. Ahola, K. Riski, and E. Ikonen, Digest of the 1998 Conference on Precision Electromagnetic Measurements, July 6-10, 1998, IEEE Cat. No. 98CH36254. This one used the tube from a PMS/REO LSTP-1010 5 color tunable HeNe laser with a pair of PieZo Transducers (PZTs) behind the rear mirror (tuning prism) and a lock-in amplifier for feedback control. For these wavelengths, the iodine cell can be outside the cavity, but notice that the red wavelength, 633 nm, is not included. Multi-Wavelength Iodine Stabilized HeNe Laser

The only modification to the laser itself was to add a pair of PZT cylinders between the back of the tuning prism and its mount so that the cavity length could be tuned electronically. The iodine cell and laser power detector are external to the cavity.

What I found curious with this (as well as the NIST laser) is that the laser cavity is way too long to restrict the laser to single longitudinal mode operation as would be required for the system to be useful. The authors of the paper don’t appear to address this, nor have I found it mentioned elsewhere.

So I performed a quick experiment using a REO tunable HeNe laser. As expected, with the power in each wavelength maximized, there are multiple longitudinal modes oscillating. And also as expected, there would be a range of the mode sweep cycle where the output would be pure SLM if either the Wavelength Selector or Transverse adjustment were set so as to reduce output power below a specific value, differing for each wavelength as follows:

   Wavelength      Maximum SLM Power
 -------------------------------------
    632.8 nm             56 µW
    611.9 nm             97 µW
    604.6 nm            169 µW
    594.1 nm            320 µW
    543.5 nm            240 µW

These values are very approximate and don’t necessarily mean that the laser can be tuned over any significant range and remain SLM as is required to be useful to lock to an I2 line – that would require even lower power. The 543.5 nm SLM power may be somewhat higher than 240 µW but that’s as much as my laser wanted to put out at the time. It would appear that 594.1 nm would be a very usable wavelength at higher power, but apparently the authors did not find a suitable I2 absorption transition at that wavelength, or at 632.8 nm either. The latter is rather strange as we know that there are more than a half dozen suitable I2 lines within the normal 632.8 nm gain bandwidth to which the Frazier and NIST lasers can be locked.

The NIST (and presumably Frazier) ISHLs use an OC reflectance of only 93 percent to raise the lasing threshold and force SLM operation. (Common red HeNe lasers of this size typically have an OC reflectance of 99 percent.) This option is not available for the multi-wavelength ISHL since the authors used a stock PMS/REO tunable laser tube which has a relatively high reflectance (much greater than 99 percent) internal OC.

Assuming this analysis with respect to usable SLM power to be correct, it does explain why direct locked ISHLs typically have very low power. To achieve higher power, some companies offer what is known as an “offset-locked iodine stabilized HeNe laser”. With these, a normal SLM HeNe laser with a typical output power of 1 to 2 mW (at 632.8 nm) has its optical frequency phase locked to the lower power ISHL. Implementation is actually easier than it sounds but nonetheless is left as an exercise for the motivated student. 😉

Stabilized He-Ne Lasers at Other Wavelengths

All types of schemes for stabilizing red (633 nm) He-Ne lasers have been developed, but most of those that are commonly used in commercial stabilized He-Ne tubes are based on monitoring of one or both polarized modes in the output or waste beams and locking their position to the neon gain curve. For well behaved so-called “random polarized” 633 nm HeNe laser tubes, adjacent modes are generally orthogonally polarized. So, to assure a single mode (single frequency) output, the tube simply has to be short enough that at the lock position, either one mode or two polarized modes are present. In the latter case, a polarizer at the output can block the unwanted mode.

While it might be assumed that exactly the same approach could be taken for “other color” lasers, this turns out not usually be the case. The principle reason is that the nice behavior that has been counted on to keep the lasers well mannered may not be present. So while the tube will still have a pair of orthogonal axes of polarization, adjacent longitudinal modes will not necessarily be orthogonal and/or even have a consistent relative polarization – they may flip like a banshee.

So, where it is desired to implement a stabilized HeNe laser at other wavelengths (visible or IR), the polarization may be the primary issue, but there are a number of other complications including differences in the neon gain bandwidth and generally much lower power:

1. Orthogonal polarization: For the 633 nm HeNe laser, the Physics has cooperated (or Murphy took a millisecond off) with adjacent modes being orthogonally polarized. Since this is not necessarily true at other wavelengths, the use of a short tube may be required so that only a single mode is permitted at the lock point. For example, to assure that only a single mode can oscillate at 543.5 nm would require a tube less than about 12.5 cm in length, which would have an extremely low output power if it could be made to work at all – probably well under 0.1 mW.
2. Neon gain bandwidth: The width of the inhomogeneously-broadened neon gain curve depends on optical frequency and is roughly equal to [633 nm /(Lasing Wavelength) * 1.6 GHz + 100 MHz] where the addition uses the sum of the squares. For most purposes, Doppler broadening dominates and the added 100 MHz term can be ignored since its contribution will be small. Thus, the length of the tube must be selected based on wavelength to assure that only the desired number of longitudinal modes can oscillate. Of course, this may directly conflict with the need for output power! For example, at 633 nm, a tube with a cavity length of 225 mm (667 MHz mode spacing) will allow at most 3 longitudinal modes to oscillate. At 1,523 nm, the gain bandwidth will less than 1/2 of what it is at 633 nm and may be insufficient for even 2 modes to see enough gain, resulting in the output actually going off during part of mode sweep.However, FWHM or other definition of the gain bandwidth has to be adjusted depending on the actual gain and losses of the tube. For example, the mid-IR 3,391 nm line has a gain over 40 times that of the 633 nm red line, so the lasing threshold will be much lower effectively widening the gain curve. And the gain at 544 nm (green) is roughly 1/20th of that at 633 nm.
3. Power output: The gain and/or efficiency for most of the non-red wavelengths is much lower than for 633 nm. Normally, this can be handled using a longer tube. But that directly conflicts with (1) for the green (543,5 nm), yellow (594.1 nm), and orange (604.6 or 611.9 nm) wavelengths since these tubes need to be shorter than even for red.

Various tricks may be used to stabilize HeNe lasers at other wavelengths but in general, it’s often not as easy! Also see the section: A Stabilized HeNe Laser at 1,523 nm.

Back-Reflections and HeNe Lasers

Back-reflections of a laser’s output directly back to it is inherently destabilizing for most lasers, and in some cases even potentially destructive. Many factors determine what effects back-reflections will have including the type of laser, and optics between the laser cavity (inside the laser or external) and the source of the reflections.

HeNe lasers are particularly sensitive to back-reflections, though no damage is ever likely to occur. However, the instantaneous polarization state and amplitude of the longitudinal modes will be affected. These effects may not be noticeable for common HeNe lasers without using fancy instruments since they occur at nanosecond time scales. For these lasers, average output power from the laser will not be affected but for random-polarized HeNe lasers, the intensity of any portion of the beam passed through optics that affect polarization may fluctuate dramatically.

Suffice it so say that one should avoid back-reflections to HeNe lasers, but especially for stabilized HeNes. Even the reflection from a piece of fresh transparent tape in the beam may cause the laser to lose lock. What happens is that when a mode swaps polarization, the controller will attempt to relock but that may require several seconds or longer. Some lasers may indicate their unhappiness by flashing the READY or LOCK indicators, or in the case of the Spectra-Physics 117A and Melles Griot 05-STP-901, making clicking noises. 🙂

The best way avoid such instabilities is to arrange the setup so that there are no back-reflections. 😉 The HP/Agilent interferometer configurations used for metrology applications shown in Most Common Hewlett Packard/Agilent Interferometers are nearly perfect. With their high quality Polarizing Beam-Splitters (PBSs), there are virtually no back-reflections directly to the laser. The next best solution where this is unavoidable is to add an optical isolator at the output of the laser. A Faraday isolator is best but very expensive. For a beam with a single linear polarization, adding a PBS cube and Quarter Wave Plate (QWP) will redirect any reflections downstream that have not had a polarization change off to the side. In many applications, this is sufficient. But in some cases, two Faraday isolators in series are needed to fully tame the HeNe beast. 😉

Reverse Incremental Efficiency of HeNe laser?

You say: “Huh, what?”. 😉 Until recently, it never occurred to me to even think about how the HeNe lasing process and electrical input might be related other than that the HeNe laser is extremely inefficient. Then someone asked the obvious question: “Does the power input to the laser depend on the output power in the beam?”. With a bit of thought, it should be obvious for there to be some relationship. But even for other types of lasers, this is not something that is often considered. The slope efficiency is an important measurement for any laser, being how the laser output changes as a function of the electrical (or other) input. For example, with a laser diode, all that is needed is to measure the input electrical power and output optical power at two points where lasing is occurring and calculate the ratio of the differences. But this is from input to output. For a HeNe laser, such measurements can be done over a portion of the range where the power supply is stable resulting in a typical value of 0.3 mW/W or 0.3 percent, similar to the pathetic absolute efficiency for the HeNe laser!

But what we want here is the opposite – how the input power is affected by the laser output, which I’ll call the “Reverse Incremental Efficiency” or RIE. In other words, compare the input power with the laser operating normally and with the output suppressed, for example, by misaligning a mirror. For a HeNe laser, would there be a detectable change in input power if this were done? With a normal constant current HeNe laser power supply, the result should be a change in tube voltage. If for want of a better term, the “reverse slope efficiency” were 100 percent, then “spoiling” the beam of a 1 mW laser should result in a reduction of 1 mW in power consumed by the tube.

So I did an experiment using a high-mileage JDS Uniphase 1145P laser head with a Melles Griot 05-LPL-915 power supply set at 6.5 mA. The lasing was spoiled using a tube-type Nylon mirror adjuster pushing on the OC mirror mount to kill lasing in a totally reversible manner. Measurements were made while the laser was warming up and outputting 12 mW and then once fully warmed up and outputting 19 mW. The results were rather intriguing:

    ΔPo       ΔVt      ΔPt       RIE
  ------------------------------------
   12 mW     4.1 V   26.65 mW   45.0%
   19 mW     5.2 V   33.80 mW   56.2%

ΔPo is the output power, ΔVt is the change in tube voltage from 0 mW to ΔPo, and ΔPt is the corresponding change in the tube’s power consumption.

At first, my measurements were made with a DMM with only 4 digits of resolution and it appeared as though the the RIE might be exactly 50 percent, which could have had some cosmic significance. 🙂 But it wasn’t to be. With the full 5 digits of a Fluke 87, while the RIE isn’t far from 50 percent, it isn’t 50.00000000%. Too bad. But what this does say is that the incremental efficiency of getting coherent photons out the front of a HeNe laser once it’s running at the normal voltage and current and outputting near rated power is order of 50 percent, not a miniscule value like that 0.3 percent! Note that the results depend on whether the laser is running at reduced and full power. If this had been some obscure effect of mechanical stress on the discharge voltage, then the change in tube voltage would be about the same at both output powers. And pushing on the mirror mount beyond where lasing ceases has no effect on tube voltage. At least until it breaks off. 🙂

To further confirm that this is a true lasing effect, I repeated the experiment with a Melles Griot 05-LHB-570 one-Brewster laser where lasing could be suppressed simply by poking something in the cavity between the tube and OC mirror:

     ΔPo       ΔVt       ΔPt      RIE
  -------------------------------------
   2.55 mW    0.9 V    5.85 mW   43.6%

Even at this much lower output power, the RIE is still fairly high, though uncertainly is greater due to the much lower power and corresponding change in tube voltage.

Then, I did multiple sample points while a like-new 1145P head was warming up:

    ΔPo       ΔVt      ΔPt       RIE
  ------------------------------------
    6 mW     4.8 V   32.1 mW    21.0%
   10 mW     5.7 V   37.1 mW    27.0%
   14 mW     5.8 V   37.7 mW    37.1%
   17 mW     6.0 V   39.0 mW    43.6%
   19 mW     6.3 V   41.0 mW    46.3%
   21 mW     6.3 V   41.0 mW    51.3%
   24 mW     6.4 V   41.6 mw    58.0%

Just when I thought this was making some sense, these data appear to show an unexpected very non-linear relationship. Most of the voltage change occurs between 0 mW a few mW, and it is then nearly constant, perhaps due to the gain saturating. There is still significant uncertainty as the measured values for both absolute tube voltage and the voltage difference fluctuate over time.

And finally on the same head when fully warmed up with a stable 24 mW of output power undisturbed, with controlled misalignment of the OC mirror to generate a few intermediate values:

    ΔPo       ΔVt      ΔPt       RIE
  ------------------------------------
    1 mW     1.3 V    8.5 mW    12.0%
    6 mW     4.4 V   26.8 mW    21.0%
    9 mW     5.2 V   34.5 mW    26.1%

   24 mW     6.3 V   41.0 mW    58.6%
   "" mW     6.5 V   42.3 mW    56.8%
   "" mW     6.8 V   44.2 mW    54.3%

These are generally similar to the measurements during warmup. The last two full power entries reflect the variation that may be present even when the laser is in thermal equilibrium. Even so, there can be small changes in the longitudinal mode positions and thus relative efficiencies of the lasing lines or something. 🙂

A reference to this phenomenon can be found on page 38 of an old NASA report: An Experimental and Theoretical Investigation of Striations in a HeNe Laser. (If this link should decay, simply search for the title.) I’m sure there are many more in depth studies but locating them is left as an exercise for the student. 🙂

On-line Introductions to HeNe Lasers

There are a number of Web sites with laser information and tutorials.

• One of the best so far is the CORD Laser/Electro-Optics Technology Series, Cord Communications, 324 Kelly Drive, P.O. Box 21206, Waco, Texas 76702-1206.In particular:
  • Module 1-10 Helium-Neon Gas Laser–A Case Study goes into considerable detail on the theory as well as some more practical information related to HeNe lasers.
  • Module 3-1 Power Sources for CW Lasers deals with HeNe laser power supplies.
  • Module 4-2 Gas Laser Power Supplies has more on HeNe laser power supplies (some redundancy with Module 3-1).

  See the section: On-Line Introduction to Lasers for the current status and on-line links to these courses, and additional CORD LEOT modules and other courses relevant to the theory, construction, and power supplies for these and other types of lasers.

• The HeNe Laser Manual by Elden Peterson of Voltex, Inc. has a variety of practical information on HeNe lasers including characteristics and power supply considerations. This is a nice concise treatment of the practical aspects of HeNe lasers and power supplies and recommended for those who would like the “short course” before (or in place of) diving in head-first to the material that follows. 🙂 There is also: “HeNe Lasers: Their Quirks and Quarks” by Keith Schmidt, referenced in this manual, which I haven’t seen but sounds interesting.
• MEOS GmbH was a developer of laser educational materials and equipment (among other things) but now appears to be gone for good. They had the lab/study manuals for their courses on a wide variety of laser related topics. While designed to be used in conjunction with the laboratory apparatus which they sell, these manuals include a great deal of useful information and procedures that can be applied in general.As of Summer 2012, MEOS has stopped development and support of these kits. (In fact, the company doesn’t seem to exist anymore, at least not doing anything remotely related to photonics.) However the creator of the experiments and author of the manuals has been acquired by LD Didactic (Leybold) and is continuing this line, which is represented by Klinger in US. The updated manuals are now available for free download at the Leybold Ld Didactic Web Site.Several modules would be of particular interest for HeNe lasers. Unfortunately, the on-line manuals (in PDF format) have disappeared from the MEOS Web site. But I have found and archived most of MEOS manuals. (The ones above may be slightly different.)
  • EXP01 – Emission and Absorption.
  • EXP03 – Fabry-Perot Resonator
  • EXP06 – HeNe Laser
  • EXP20 – Laser Safety
  • EXP27 – Bar Code Reader
• The Helium-Neon Laser (Department of Physics, Middlebury College) is the lab procedure for setting up a external mirror HeNe laser but also includes some basic information.
• 1979-1980 Metrologic Catalog and Laser Handbook had general information on how a HeNe laser works and details of HeNe laser tube fabrication on pages 34 to 39. The manufacturing process is for Metrologic’s “hard-seal metal ceramic” HeNe laser tubes which never caught on and were discontinued after a few years, but is interesting nontheless.
• Also see the section: General Laser Information and Tutorial Sites for other sites that may be worth visiting.

As with *any* laser, proper precautions must be taken to avoid any possibility of damage to vision. The types of He-Ne lasers mostly dealt with in this document are rated Class II, IIIa, or the low end of IIIb (see the section: Laser Safety Classifications. For most of these, common sense (don’t stare into the beam) and fairly basic precautions suffice since the reflected or scattered light will not cause instantaneous injury and is not a fire hazard.

However, unlike those for laser diodes, He-Ne power supplies utilize high voltage (several kV) and some designs may be potentially lethal. This is particularly true of AC line powered units since the power transformer may be capable of much more current than is actually required by the He-Ne laser tube – especially if it is home built using the transformer from some other piece of equipment (like an old tube type console TV or that utility pole transformer you found along the curb) which may have a much higher current rating.

The high quality capacitors in a typical power supply will hold enough charge to wake you up – for quite a while even after the supply has been switched off and unplugged. Depending on design, there may be up to 10 to 15 kV or more (but on very small capacitors) if the power supply was operated without a He-Ne tube attached or it did not start for some reason. There will likely be a lower voltage – perhaps 1 to 3 kV – on somewhat larger capacitors. Unless significantly oversized, the amount of stored energy isn’t likely to be enough to be lethal but it can still be quite a jolt. The He-Ne tube itself also acts as a small HV capacitor so even touching it should it become disconnected from the power supply may give you a tingle. This probably won’t really hurt you physically but your ego may be bruised if you then drop the tube and it then shatters on the floor!

However, should you be dealing with a much larger He-Ne laser, its power supply is going to be correspondingly more dangerous as well. For example, a 35 mW He-Ne tube typically requires about 8 mA at 5 to 6 kV. That current may not sound like much but the power supply is likely capable of providing much more if you are the destination instead of the laser head (especially if it is a home-made unit using grossly oversized parts)! It doesn’t take much more under the wrong conditions to kill.

After powering off, use a well insulated 1M resistor made from a string of ten 100K, 2 W metal film resistors in a glass or plastic tube to drain the charge – and confirm with a voltmeter before touching anything. (Don’t use carbon resistors as I have seen them behave funny around high voltages. And, don’t use the old screwdriver trick – shorting the output of the power supply directly to ground – as this may damage it internally.)

And only change electrical connections or plug/unplug connectors with power OFF, being aware of the potential for stored charge. In particular, the aluminium cylinder of some HeNe laser heads is the negative return for the tube current via a spring contact inside the rear end-cap. So, pulling off the rear end-cap while the laser is powered will likely make YOU the negative return instead! You will probably then bounce off the ceiling while the laser bounces off the floor, which can easily ruin your entire day in more ways than one. 🙁 🙂 This connection scheme is known to be true for most JDS Uniphase and many Melles Griot laser heads, but may apply to others as well.

Now, for some first-hand experience:

(From: Doug (dulmage@skypoint.com).)

Well, here’s where I embarrass myself, but hopefully save a life…

I’ve worked on medium and large frame lasers since about 1980 (Spectra-Physics 168’s, 171’s, Innova 90’s, 100’s and 200’s – high voltage, high current, no line isolation, multi-kV igniters, etc.). Never in all that time did I ever get hurt other than getting a few retinal burns (that’s bad enough, but at least I never fell across a tube or igniter at startup). Anyway, the one laser that almost did kill me was also the smallest that I ever worked on.

I was doing some testing of AO devices along with some small cylindrical HeNe tubes from Siemens. These little coax tubes had clips for attaching the anode and cathode connections. Well, I was going through a few boxes of these things a day doing various tests. Just slap them on the bench, fire them up, discharge the supplies and then disconnect and try another one. They ran off a 9 VDC power supply.

At the end of one long day, I called it quits early and just shut the laser supply off and left the tube in place as I was just going to put on a new tube in the morning. That next morning, I came and incorrectly assumed that the power supply would have discharged on it own overnight. So, with each hand I stupidly grab one clip each on the laser to disconnect it. YeeHaaaaaaaaa!!!!. I felt like I had been hid across my temples with a two by four. It felt like I swallowed my tongue and then I kind of blacked out. One of the guys came and helped me up, but I was weak in the knees, and very disoriented.

I stumbled around for about 15 minutes and then out of nowhere it was just like I got another shock! This cycle of stuff went on for about 3 hours, then stopped once I got to the hospital. I can’t even remember what they did to me there. Anyway, how embarrassing to almost get killed by a HeNe laser after all that other high power stuff that I did. I think that’s called ‘irony’.

Comments on HeNe Laser Safety Issues

(Portions from: Robert Savas (jondrew@mail.ao.net).)

A 10 mw HeNe laser certainly presents an eye hazard.

According to American National Standard, ANSI Z136.1-1993, table 4 Simplified Method for Selecting Laser Eye Protection for Intrabeam Viewing, protective eyewear with an attenuation factor of 10 (Optical Density 1) is required for a HeNe with a 10 milliwatt output. This assumes an exposure duration of 0.25 to 10 seconds, the time in which they eye would blink or change viewing direction due the uncomfortable illumination level of the laser. Eyeware with an attenuation factor of 10 is roughly comparable to a good pair of sunglasses (this is NOT intended as a rigorous safety analysis, and I take no responsibility for anyone foolish enough to stare at a laser beam under any circumstances). This calculation also assumes the entire 10 milliwatts are contained in a beam small enough to enter a 7 millimeter aperture (the pupil of the eye). Beyond a few meters the beam has spread out enough so that only a small fraction of the total optical power could possible enter the eye.

A helium-neon (henceforth abbreviated HeNe) laser is basically a fancy neon sign with mirrors at both ends. Well, not quite, but really not much more than this at first glance (though the design and manufacturing issues which must be dealt with to achieve the desired beam characteristics, power output, stability, and life span, are non-trivial). The gas fill is a mixture of helium and neon gas at low pressure. A pair of mirrors – one totally reflective (called the High Reflector or HR), the other partially reflective (called the Output Coupler or OC) at the wavelength of the laser’s output – complete the resonator assembly. This is called a Fabry-Perot cavity (if you want to impress your friends). The mirrors may be internal (common on small and inexpensive tubes) or external (on precision high priced lab quality lasers). Electrodes sealed into the tube allow for the passage of high voltage DC current to excite the discharge.

Note that a true laser jock will further abbreviate “HeNe laser” to simply “HeNe”, pronounced: Hee-nee. Their laser jock colleagues and friends then know this really refers to a laser! 🙂 While other types of lasers are sometimes abbreviated in an analogous manner, it is never to the same extent as the HeNe.

I still consider the HeNe laser to be the quintessential laser: An electrically excited gas between a pair of mirrors. It is also the ideal first laser for the experimenter and hobbyist. OK, well, maybe after you get over the excitement of your first laser pointer! 🙂 HeNe’s are simple in principle though complex to manufacture, the beam quality is excellent – better than anything else available at a similar price. When properly powered and reasonable precautions are taken, they are relatively safe if the power output is under 5 mW. And such a laser can be easily used for many applications. With a bare HeNe laser tube, you can even look inside while it is in operation and see what is going on. Well, OK, with just a wee bit of imagination! 🙂 This really isn’t possible with diode or solid state lasers.

I remember doing the glasswork for a 3 foot long HeNe laser (probably based on the design from: “The Amateur Scientist – Helium-Neon Laser”, Scientific American, September 1964, and reprinted in the collection: “Light and Its Uses” [5]). This included joining side tubes for the electrodes and exhaust port, fusing the electrodes themselves to the glass, preparing the main bore (capillary), and cutting the angled Brewster windows (so that external mirrors could be used) on a diamond saw. I do not know if the person building the laser ever got it to work but suspect that he gave up or went on to other projects (which probably were also never finished). And, HeNe lasers are one of the simplest type of lasers to fabricate which produce a visible continuous beam.

Some die-hards still construct their own HeNe lasers from scratch. Once all the glasswork is complete, the tube must be evacuated, baked to drive off surface impurities, backfilled with a specific mixture of helium to neon (typically around 7:1 to 10:1) at a pressure of between 2 and 5 Torr (normal atmospheric pressure is about 760 Torr – 760 mm of mercury), and sealed. The mirrors must then be painstakingly positioned and aligned. Finally, the great moment arrives and the power is applied. You also constructed your high voltage power supply from scratch, correct? With luck, the laser produces a beam and only final adjustments to the mirrors are then required to optimize beam power and stability. Or, more, likely, you are doing all of this while your vacuum pumps are chugging along and you can still play with the gas fill pressure and composition. What can go wrong? All sorts of things can go wrong! With external mirrors, the losses may be too great resulting in insufficient optical gain in the resonant cavity. The gas mixture may be incorrect or become contaminated. Seals might leak. Your power supply may not start the tube, or it may catch fire or blow up. It just may not be your day! And, the lifetime of the laser is likely to end up being only a few hours in any case unless you have access to an ultra-high vacuum pumping and bakeout facility. While getting such a contraption to work would be an extremely rewarding experience, its utility for any sort of real applications would likely be quite limited and require constant fiddling with the adjustments. Nonetheless, if you really want to be able to say you built a laser from the ground up, this is one approach to take! (However, the CO2 and N2 lasers are likely to be much easier for the first-time laser builder.)

However, for most of us, ‘building’ a HeNe laser is like ‘building’ a PC: An inexpensive HeNe tube and power supply are obtained, mounted, and wired together. Optics are added as needed. Power supplies may be home-built as an interesting project but few have the desire, facilities, patience, and determination to construct the actual HeNe tube itself.

The most common internal mirror HeNe laser tubes are between 4.5″ and 14″ (125 mm to 350 mm) in overall length and 3/4″ to 1-1/2″ (19 mm to 37.5 mm) in diameter generating optical power from 0.5 mW to 5 mW. They require no maintenance and no adjustments of any kind during their long lifetime (20,000 hours typical). Both new and surplus tubes of this type – either bare or as part of complete laser heads – are readily available. Slightly smaller tubes (less than 0.5 mW) and much larger tubes (up to approximately 35 mW) are structurally similar (except for size) to these but are not as common.

Much larger HeNe tubes with internal or external mirrors or one of each (more than a *meter* in length!) and capable of generating up to 250 mW of optical power have been available and may turn up on the surplus market as well (but most of these are quite dead by now). The most famous of these (as lasers go) is probably the Spectra-Physics model 125A whose laser head is over 6 feet in length. It was only rated 50 mW (633 nm), but new samples under optimal conditions may have produced more than 200 mW. Even more powerful ones have been built as research projects. I’ve seen photos of a Hughes HeNe laser with a head around 8 feet in length that required a 6 foot rack-mount enclosure for the exciter.

Its output power is unknown, but probably less than that of the SP-125A. The largest single transverse mode (SM, with a TEM00 beam profile) HeNe lasers in current production by a well known manufacturer like Melles Griot are rated at about 35 mW minimum over an expected lifetime of 20,000 hours or more, though new samples may exceed 50 mW. However, HeNe lasers rated up to at least 70 mW SM and 100 mW MM are available. Manufacturers include: CDHC-Optics (China), Spectral Laser (Italy), and PLASMA, JSC (Russia). However, the lifetime over which these specifications apply is not known and may be much shorter.

Highly specialized configurations, such as a triple XYZ axis triangular cavity HeNe laser in a solid glass block for an optical ring laser gyro, also exist but are much much less common. Most HeNe lasers operate CW (Continuous Wave) producing a steady beam at a fixed output power unless the power is switched on and off or modulated (or someone sticks their finger in the beam and blocks it!). (At least they are supposed to when in good operating condition!) However, there are some mode-locked HeNe lasers that output a series of short pulses at a high repetition rate. And, in principle, it is possible to force a HeNe laser with at least one external mirror to “cavity dump” a high power pulse (perhaps 100 times the CW power) a couple of nanoseconds long by diverting the internal beam path with an ultra high speed acousto-optic deflector. But, for the most part, such systems aren’t generally useful for very much outside some esoteric research areas and in any case, you probably won’t find any of these at a local flea market or swap meet, though eBay can’t be ruled out! 🙂

Nearly all HeNe lasers output a single wavelength and it is most often red at 632.8 nm. (This color beam actually appears somewhat orange-red especially compared to many laser pointers using diode lasers at wavelengths between 650 and 670 nm). However, green (543.5 nm), yellow (594.1 nm), orange (604.6 and 611.9 nm), and even IR (1,152, 1.523, and 3,921 nm) HeNe lasers are available. There are a few high performance HeNe lasers that are tunable and very expensive. And, occasionally one comes across laser tubes that output two or more wavelengths simultaneously. Although some tubes are designed this way, it is more likely to be a ‘defect’ resulting from a combination of high gain and insufficiently narrow band optics. Such tubes tend to be unstable with the relative power varying among the multiple wavelengths more or less at random.

Note that the single wavelength described above usually consists of more than one longitudinal mode or lasing line (more on this later). However, some HeNe lasers are designed to produce a highly stable single optical frequency or two closely spaced optical frequencies. These are used in scientific research and metrology (measurement) applications, described in more detail below.

Current major HeNe laser manufacturers include Melles-Griot, JDS Uniphase, and LASOS. This is far fewer than there were only a few years ago. So, you may also find lasers from companies like Aerotech, Hughes, Siemens, and Spectra-Physics that have since gotten out of the HeNe laser business or have been bought out, merged, or changed names. For example, the HeNe laser divisions of Aerotech and Hughes were acquired by Melles Griot; Sieman’s HeNe laser product line is now part of LASOS; and Spectra-Physics which was probably the largest producer of HeNe lasers from the very beginning gradually eliminated all HeNe lasers from its product line over the last few years. HeNe tubes, laser heads, and complete lasers from any of these manufacturers are generally of very high quality and reliability.

HeNe lasers have been found in all kinds of equipment including:

• Consumer: Supermarket checkout UPC and other barcode scanners. early laser printers, early LaserDisc players.
• Advertising/entertainment: Holography, small laser shows.
• Measurement: Optical surveying, interferometric metrology and velocimetry, other non-contact measurement and monitoring, ring laser gyro.
• Construction: Laser level, tunnel boring, alignment of saw mill wood cutting, general surveying.
• Industrial: Automotive and other alignment; parts detection, counting, and positioning; particle counting.
• Biotechnology: Blood cell analysis (cytometry), laser induced fluorescence of everything from whole cells to single DNA bases, laser tweezers, confocal microscopy, Raman spectroscopy, anesthesia and other gas analysis.
• Medical/surgical: Patient positioning systems for diagnostic and treatment machines, alignment of high power CO2 and YAG treatment lasers and pointing beams.

Nowadays, many of these applications are likely to use the much more compact lower (drive) power solid state diode laser. (You can tell if you local ACME supermarket uses a HeNe laser in its checkout scanners by the color of the light – the 632.8 nm wavelength beam from a HeNe laser is noticeably more orange than the 660 or 670 nm deep red from a typical diode laser type.)

Melles Griot (now part of IDEX Optics and Photonics Marketplace. Catalogs used to include several pages describing HeNe laser applications. I know this was present in the 1998 catalog but has since disappeared and I don’t think it is on their Web site.

Also see the section: Some Applications of a 1 mW Helium-Neon Laser for the sorts of things you can do with even a small HeNe laser.

Since a 5 mW laser pointer complete with batteries can conveniently fit on a keychain and generate the same beam power as an AC line operated HeNe laser almost half a meter long, why bother with a HeNe laser at all? There are several reasons:

• For many applications including holography and interferometry, the high quality stable beam of a HeNe laser is unmatched (at least at reasonable cost, perhaps at all) by laser diodes (though this is apparently changing at least for some diode lasers. See the section: Holography Using Cheap Diode Lasers. In particular, the coherence length and monochromicity of even a cheap HeNe laser are excellent and the beam profile is circular and nearly ideal Gaussian TEM00 so that simple spherical optics can be used for beam manipulation. Bare edge emitting laser diodes (the only visible type currently available) on the other hand always produce a wedge shaped beam and have some amount of astigmatism. Correcting this to the equivalent quality of a HeNe laser is difficult and expensive.
• As noted in the chapter: Diode Lasers, it is all too easy to ruin them in the blink of an eye (actually, the time it takes light to travel a few feet). It would not take very long to get frustrated burning out $50 diodes. So, the HeNe laser tube may be a better way to get started. They are harder to damage through carelessness or design errors. Just don’t get the polarity reversed or exceed the tube’s rated current for too long – or drop them on the floor! And, take care around the high voltage!
• Laser diode modules at a wavelength of 635 nm (close to the 632.8 nm wavelength of red HeNe lasers) may still be somewhat more expensive than surplus HeNe tubes with power supplies. However, with the increasing popularity of DVD players and DVDROM drives, this situation probably won’t last long.

However, the market for new HeNe lasers is still in the 100,000 or more units per year. What can you say? If you need a stable, round, astigmatism-free, long lived, visible 1 to 10 mW beam for under $500 (new, remember!), the HeNe laser is still the only choice.