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    HeNe Laser Testing, Adjustment, Repair

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    HeNe Tube Testing, Identifying Connections, Determining Output Power

    How Can I Tell if My Tube is Good?

    If your HeNe tube starts and lases but is somewhat unstable, see the section: Unstable or Flickering HeNe Tube.

    A variety of faults can result in a HeNe tube not working properly. However, where the tube starts (there is a stable glow discharge and it is the correct color - see below), there are a couple of possibilities that are not due to a bad tube:

    Several types of problems can prevent a HeNe tube from lasing properly or make it hard to start:

    Unstable or Flickering HeNe Tube

    Where your HeNe tube starts and lases normally but is unstable, flickering or going out and then restarting whenever the power line voltage dips slightly or for no apparent reason, the problem may be power supply or HeNe tube related. A different power supply or slight adjustments or modifications may make your HeNe tube happy, at least temporarily. However, where the HeNe tube is an inexpensive vanilla flavored variety, replacement may be the easiest solution if it turns out to be marginal :-).

    Cyclical Variations in Output Beam Power

    A HeNe laser that is in good condition will produce an output beam that is quite stable and will have no visible (at least by eye) variations in output power. However, sometimes you will find a laser that exhibits periodic variations in output intensity even where the discharge is perfectly stable. This is much more likely with an old, high mileage, or very long HeNe tube. The basic mechanism is longitudinal mode hopping due to thermal expansion of the resonator - the distance between the mirrors is gradually changing which results in favoring different modes. The cause is usually low resonator gain. Possible reasons for this include: These result in fewer longitudinal modes having sufficient gain to sustain the lasing process. As the resonator length changes, these lines move with respect to the gain curve of the lasing medium. Where there is cyclic variation in output power, only a very few lines are of sufficient gain to sustain lasing and then only when they are near the peak of the gain curve. The tube stops lasing entirely when there are no lines with sufficient gain to sustain oscillation. See the section: Longitudinal Modes of Operation.

    A similar sort of varying intensity behavior will result if a polarizing filter is placed in the output beam of a randomly polarized HeNe tube or a HeNe tube that is supposed to be linearly polarized but isn't working properly because its internal Brewster plate has fallen off or its polarizing magnets have weakened or are mispositioned. However, in this case, what happens is that as the laser switches between longitudinal modes and/or the mirror alignment shifts ever so slightly, the polarization angle and thus the output intensity of the beam may change significantly. This is perfectly normal for a randomly polarized tube but indicates a problem with one that is supposed to be linearly polarized. See the section: Unrandomizing the Polarization of a Randomly Polarized HeNe Tube.

    Note that if the discharge is actually going on and off, the cause is entirely different - an incompatibility with the power supply, incorrect ballast resistor, low line voltage, etc. See the section: Unstable or Flickering HeNe Tube.

    (From: Daniel Lang (dbl@anemos.caltech.edu)).

    The typical HeNe laser's linewidth is wide enough for 2 or 3 longitudinal modes to oscillate simultaneously. As the laser warms up, the cavity expands, causing the modes to decrease in frequency. When a mode gets too low with respect to the He-Ne linewidth, it goes out and after a bit, a new one appears on the high side of the linewidth. This typically has a period of 3 to 10 seconds. I suspect that an old laser that is doing this is down to 1 or 2 modes due to reduced gain and may be approaching 0 or 1 mode, causing a visible intensity modulation.

    I noted a similar problem when using a HeNe for Laser Doppler Velocimetry. In this case we were seeing a low level intensity modulation that would start at approximately 60 Khz, sweep through zero and back to 60 Khz and then disappear for several seconds before starting again. The entire cycle repeated in approximately 5 to 10 seconds. The longitudinal mode spacing for our laser was 385 MHz. The >0 to 60 KHz only appeared when the laser was operating in 3 modes. The frequency difference between modes 1 & 2 was not quite the same as the difference between modes 2 & 3 except when exactly symmetrical (amplitude of mode 1 = amplitude of mode 3). We were seeing the difference of the differences! The longer interval free of intensity modulation occurred when only 2 modes were oscillating.

    Reasons for Short HeNe Tube Lifetime

    As noted, sealed HeNe should last many thousands of hours with no noticeable degradation in performance.

    If you are experiencing excessively short life (e.g., a month instead of years), the first things to check are operating current and polarity. See the section: Making Measurements on HeNe Laser Power Supplies. Of course, if you omitted the ballast resistor, life will likely be very short :-(.

    If the HeNe tube and power supply are mismatched, one can damage the other. For example, running a 1 mW HeNe tube on a power supply designed for a 35 mW HeNe tube may not only result in too high a current by design (e.g., 8 mA instead of 3 mA) but may also result in much higher current if the compliance range of the power supply is exceeded (i.e., the voltage across the HeNe tube is much lower than the power supply can handle). Conversely, attempting to power a 5 mW HeNe tube using the power supply from a barcode scanner (designed for a .5 to 1 mW HeNe tube) will likely result in a blown power supply. Just because the Alden connectors mate and/or the tube lights up doesn't imply anything about compatibility! Also note that maximum optical output occurs at the optimum operating current - too high or too low and it goes down.

    New and even used HeNe tubes and power supplies from reputable surplus dealers will generally last a long time if not abused. But, much of what you get at swap meets and hamfests has been pulled from equipment for one reason or another. So, the problems you are experiencing may have nothing to do with your setup!

    Determining Electrical Characteristics of Unmarked HeNe Tube or Laser Head

    So, you found this fabulous HeNe laser in the dumpster and would like to power it. There are no markings of any kind - not even the manufacturer is known. If it is a laser head, the tube itself may be labeled - if you can get to it nondestructively. Where even the tube is unmarked, start by narrowing down the range of expected electrical characteristics: The rest is experimentation. You will need an HeNe power supply capable of handling a tube with the worst case voltage and current based on its size. Make sure you include a 75 K ohm ballast resistor of adequate wattage (10 W will be sufficient for anything up to 10 mA). A laser head will usually have an internal ballast resistor. Make sure the polarity is correct - see the section: Identifying Connections to Unmarked HeNe Tube or Laser Head. Once you get the tube to light, adjust the current for maximum beam intensity. Running at slightly higher than optimal current won't do any immediate damage but shouldn't be allowed to continue for too long. Then, label the HeNe tube or laser head with your findings so you will know how to deal with it the next time you pull it out of the cabinet :-).

    Identifying Connections to Unmarked HeNe Tube or Laser Head

    Where you can visually inspect the wiring, this is trivial as we all know that the large aluminum 'can' electrode is the cathode (negative) terminal.

    Or, if the connector is the standard male 'Alden' type, the shorter (narrower) side goes to the anode (positive) and the longer (fatter) side goes to the cathode (negative). When such a connector is present, there is also usually a ballast resistor (typically about 75K ohms) built into the HeNe tube assembly or laser head between the Alden's positive terminal and the anode.

                     Anode (+) ==|________|  |---_______
                             _____________|  |  |_______ HV Cable   
              Cathode (-) ==|_____________|__|---
    However, suppose the whole thing is sealed and all we have are some dangling wires or an unusual unmarked connector? Here are some guidelines. Try to obtain agreement on several of the following tests as no single one is necessarily a guarantee of correct identification:

    How Can I Determine Output Power

    This would be easy with a laser power meter. However, most of us are not so fortunate as to own such an instrument. See the section: What Makes a Laser Power Meter So Expensive?. There are two aspects of this same problem: Note: There will often be a CDRH safety sticker (usually yellow or white) on the HeNe tube or laser head. The wattage listed on this sticker is NOT a reliable indication of output power. It is an upper bound and may be much higher than either the rated or actual output power. For example, a .5 mW laser will likely have a safety sticker value of 1 mW; a 1 or 2 mW laser will show 5 mW; and a 12 mW laser may show 15 or 25 mW. Some unscrupulous or careless HeNe laser or tube resellers will list this as the power output of the device - buyer beware! Few people can or will check this. If it sounds to good to be true, it probably is :-(.
    1. There are a few ways of determining the tube's specified output power:

      • Some manufacturers code the (usually minimum) output power into the model number. For example, Aerotech tubes and laser heads have a model number that is of the form: XYZ where X is the model designation, Y is the output power in mW (e.g., 2 = 2 mW, 05 = .5 mW, etc.) and Z is either R or P denoting a random or linearly polarized beam respectively.

      • Match the model number of the tube or laser head to the manufacturer's catalog listing. This may be easier said than done since many surplus tubes either don't have a model number printed on them or are old enough (but still perfectly good) so that the model is no longer listed in a current catalog. The manufacturer (if they still exist) will know and contacting them may be worth the effort. However, don't expect an overly enthusiastic response if you are asking about a 10 year old $20 HeNe tube! Any information so obtained may not be accurate either.

      • Attempt to compare the physical dimensions with those of tubes with known output power. This is not very reliable as the output power of a tube of identical diameter and length can easily vary by a factor of two or more by design or just due to sample-to-sample variations (at the time of manufacture, tubes are selected and sold based on their actual output power but they may appear to be physically identical).

      • Try to locate an indication on the tube itself of *measured* power output. Very often, the actual power output determined at the time of manufacture will be hand written or printed somewhere on the tube. This may be on the glass or metal shield (if used) or one of the ends. It may be on the outside of a laser head on the manufacturer's specification sticker (not the safety sticker, see above) or concealed inside. Examine every nook and cranny and the tube's secrets may very well be revealed!

    2. It is much tougher to determine if the output of your HeNe tube is actually correct without a calibrated laser power meter. However, comparisons can be made.

      Note: If this is a high power really long HeNe tube (e.g., 15 mW or more), anything approaching rated power may not be present until it has warmed up (possibly as long as 15 to 30 minutes). In addition, for these and/or unconventional HeNe tubes used in high quality lasers, there may be other physical factors affecting power output including mirror micro-adjustments, need for IR line suppressing or discharge stabilization magnets, mounting, and even orientation (like: This Side Up!). See the section: How Can I Tell if My Tube is Good?. However, none of these should be a factor for small common inexpensive HeNe tubes.

      • First, confirm that you are supplying the proper operating current. The output beam power will be maximum at the proper current - lower on either side. A power supply with a broken regulator could be producing greatly increased current which will result in much reduced output (and excessive heating, sputtering, and shortened tube life - and probably won't do the power supply much good either). At 2 to 3 times the rated current, there may be no beam at all!

      • If you have access to a working HeNe laser or a new HeNe tube with known output, this is best as the wavelength will be the same. But, keep in mind that a 4:1 ratio of beam intensity represents a perceived brightness ratio that is closer to 2:1.

      • If you have a (diode) laser pointer of known power and wavelength, it can be used. However, this gets to be complicated if the wavelength AND power differ (as is likely with many laser pointers (670 nm) at this time. In addition, power levels for laser pointers are maximums and the actual power is not generally known.

      Estimating relative power works better on your finger or palm in the raw beam than on a white card unless the beam is first spread out using a lens. The beam spreads in the reddish finger or palm tissue and the difference is much more apparent. (Don't worry, you won't even be able to detect a 5 mW HeNe beam on your flesh from the any heating effect but don't do this with a 20 W argon laser!).

      Both the perceived brightness AND the size of the spot will vary with HeNe beam power. After a little practice, estimating the output power will become second nature - sort of like recipe measurements: "just use a pinch of salt in the stew!". However, if you have a collection of neutral density filters, you can use these to match brightnesses which may be just a bit more precise! The laser power meter would be even better :-).

    3. Back to HeNe Laser Testing, Adjustment, Repair Sub-Table of Contents.

      Cleaning HeNe Optics, Problems with Mirror Alignment

      Cleaning HeNe Laser Optics

      Fortunately, this is almost a non-issue for sealed HeNe tubes as there is only one accessible surface that matters and it is only in the output beam - not part of the laser resonator. Thus, no amount of gunk or dirk on its surface can affect laser action in any detectable way. However, if it is not clean, the output beam may become diffused or distorted. And, there may be external optical components like lenses, mirrors, and prisms that need to be cleaned as well.

      Those who maintain lasers professionally will insist on the use of laboratory (gas chromatograph or spectroscopic) grade methanol and acetone. For small sealed HeNe laser tubes and their optics, this really isn't necessary. The type of isopropyl alcohol sold in drug stores - designated medicinal 91% - is quite acceptable.

      Lens tissue is best, Q-tips (cotton swabs) will work. They should be wet but not dripping. Be gentle - the glass and particularly the anti-reflection coating on the output mirror surface (and other optics) is soft. Wipe in one direction only - don't rub. Also, do not dip the tissue or swab back into the bottle of alcohol after cleaning the optics as this may contaminate it. The alcohol should be all you need in most cases but some materials will respond better to acetone or just plain water.

      CAUTION: Don't overdo it - optical components may be bonded or mounted using adhesives that are soluble in alcohol or acetone (but probably not water). Too much and the whole thing could become unglued :-(. In addition, any plastic optics may be totally ruined by even momentary contact with strong solvents.

      Checking and Correcting Mirror alignment of Internal Mirror Laser Tubes

      Note: While written specifically with small HeNe tubes in mind, the techniques described in the following sections apply equally well to HeNe, Ar/Kr ion, and other sealed laser tubes which have internal mirrors on compliant mounts (those attached to the end-caps via narrow sections of ductile metal tubing).

      Precise mirror alignment is critical to proper functioning of HeNe tubes and lasers in general. For a HeNe tube, the mirrors must be aligned (parallel to each other and perpendicular to the tube bore) to a pointing accuracy better than one part in 1/10th of the ratio of bore diameter to resonator length to achieve optimal performance.

      For a typical HeNe tube, this is one part in 2,500. If the alignment is off by one part in 1,000 (1 miiliradian or 1 mR), there wil1l likely be no output at all. You won't fix this by trial and error! Spherical mirrors may have a somewhat wider range where a beam will be produced but still require precise alignment to achieve optimal performance.

      Where a HeNe tube produces a weak or low quality beam or doesn't lase at all and no other faults have been identified (such as improper operating current, or problems with the gas fill), mirror misalignment is quite possible though it does take effort to mess these up as the mirror mount tube(s) must actually be bent. However, dropping a HeNe tube or using it for a hammer could just accomplish this!

      Note: For really long high power HeNe tubes (e.g., above 15 mW or so), see the comments in the section: How Can I Tell if My Tube is Good?. Your tube may need to warm up or it may require external adjusters permanently installed or you may have it mounted incorrectly. DO NOT attempt to remedy the mirror alignment problems by physically bending the mounts if gently rocking the mirrors (see below) results in any beam. Your likelihood of success is about the same as winning the State Lottery Super Seven. This may not be needed in any case as there may be nothing wrong with the tube!

      There are two types of situations:

      • The tube produces an output beam but its power is less than expected or seems to be distorted.

      • The tube appears to work in all other respects but there is no output beam.
      The procedures described below are simplified versions of those that can be used for testing and adjusting of mirror alignment on many types of lasers (including HeNe and Ar/Kr ion lasers with external mirrors) and are ideal for use with internal mirror argon ion (blue/green) tubes because a readily available red HeNe laser can be used for testing and adjustment. Here, they have been adapted specifically for use with small sealed HeNe tubes.

      Note: It is assumed that your problem HeNe tube has each of its mirror mounts separated from the end-cap/electrode assembly by a restricted area that is not obstructed. If this is NOT the case (at one or both ends), there may already be a mirror adjusting device permanently attached to the tube and it will have to be used (unless it is removed) rather than the tools described below. In its favor, fine adjustment with such a device is more precise (though it will be less convenient for 'rocking the mirror') and alignment problems are less likely in the first place (unless someone was mucking with the screws!).

      Minor Problems with Mirror Alignment

      A beam which is much less intense than expected or distorted (not circular with a reasonably smooth gaussian profile) may be due to bad mirror alignment. Checking mirror alignment as long as the laser produces some sort of beam is easy and very low risk. Correcting it may be possible as well.

      If there is no beam at all but no evidence of bent mirror mounts or other visible damage, this technique may also be used with care to see if one of the mirrors is SLIGHTLY misaligned. However, if gentle rocking of the mirror mount does not result in a beam (see below), DO NOT attempt to actually bend the mount since there is no way of knowing in which direction the correction (if any) is needed. See the section: Major Problems with Mirror Alignment.

      Proceed as follows:

      1. A (homemade) tool for rocking (and possibly adjusting) the mirror mount is needed. See the section: Means of Adjusting HeNe Tube Mirrors.

      2. Mount the HeNe tube in such a way that the ends are free and clear so the adjuster(s) can be used without interference. For example, tape the tube into a wooden V-block clamped to your workbench.

      3. Start with the cathode end (arbitrary choice) and power it up the HeNe tube at the optimal operating current. If you are using a plate or tube type adjustment tool, take care to assure that it doesn't extend beyond the reduced diameter section of tubing and only applies force to the metal of the mirror mount, not the mirror or its frit seal!

        For long HeNe tubes (perhaps, 15 mW or more), allow the setup to warm up and stabilize for at least 20 minutes before checking or attempting any adjustment of mirror alignment.

      4. GENTLY rock the tool back and forth as you watch the beam's reflection from a white surface. Do this in X (horizontally) and then in Y (vertically). Go easy! It doesn't take much force to change alignment through the entire range that matters - perhaps a few ounces at most. Don't get carried away and actually bend the mount at this time - or break the seal :-(.

        What you should see is the beam power (brightness) pass through a maximum and then diminish on either side of this point. Testing is best done with a laser power meter but one of your eyeballs (or both of them) will work well enough for most purposes.

        • If the maximum is at the relaxed position in both axes (you can try the 45 degree ones as well to be sure), mirror alignment is correct (or at least close enough that the chance of being able to improve it without using more sophisticated adjustment equipment and a laser power meter is vanishingly small.)

        • If the maximum (or any beam at all) is off to one side, you can CAREFULLY try bending the mount permanently to attempt to correct it. First, determine the angle of the tool that results in the least force being needed to correct the alignment.

          CAUTION: The mirror mount is ultimately attached to the glass envelope of the tube. The glass-metal seal may not be that strong. Don't get to carried away! With care this adjustment should be possible - barely :-).

          Note: Where the maximum intensity results with the mirror very slightly deflected, it is possible that the mirror alignment at the opposite end of the tube is actually to blame and you are simply compensating for its pointing error. Thus, it is better to check the mirrors at both ends of the tube before attempting to adjust either of them. However, the only way to be sure is to look at both maximum beam power AND the shape of the beam (it should have a circular cross section when both mirrors are precisely parallel to each other and perpendicular to the bore of the tube).

          • Increase the force gradually until you have a feel of how much it takes to actually deform the mirror mount. Even a significant pointing error will only require a nearly microscopic correction. The change in mirror mount angle that you need to achieve is likely to be a fraction of a mR - less than 1 part in 1,000! Not easy.

          • Approach the desired deflection in small increments and overshoot just enough so that the mirror mount springs back to the optimal position. Avoid repeatedly bending it back-and-forth or you will eventually be using the HeNe tube as a high-tech wall hanging :-(.

          • Once optical output is maximum and this point is *roughly* centered when testing by rocking the mirror mount, pat yourself on the back and consider it as good as it gets. Don't push your luck!

        • In the case where there was no beam at all, if GENTLE rocking doesn't result in a beam at any position, DON'T press your luck at this end! The misalignment may be too great for this approach or the problem may be with the mirror at the other end of the HeNe tube!

      5. Gently remove the tool without applying excessive force to the mirror mount.

      6. Repeat this procedure for the anode end of the tube - just be careful not to touch the high voltage!

      Alignment should now be optimal. Confirm by rechecking it at the cathode and anode ends and making any very *slight* adjustments that may be needed.

      PERFORM ANY ADJUSTMENTS ONLY AT YOUR OWN RISK! Checking the alignment by gently rocking the mirror(s) is safe and effective. However, actually bending the metal is much more difficult and likely to result in death to your HeNe tube. The required pointing accuracy of much less than 1 mR is not much to fool with! If the brightness change that is bothering you is just barely perceptible or you just *think* that it may not be perfectly centered, LEAVE THE MIRROR ALIGNMENT ALONE! Plexiglas or wood plates (even with any inserts) and plastic tubes are really too soft for precise control beyond the elastic limit (i.e., when actually bending the metal permanently). Your control will be poor and you will be much more likely to bend the mirror mount far off to one side never to work again or break it off completely. The lever type adjusters can be more precise but may result in excessive stress to the mounts if used to make more than very small adjustments since it applies an unbalanced force spreading the mirror mount and end-cap apart.

      Means of Adjusting HeNe Tube Mirrors

      Unlike the mirror mounts on high quality lab or industrial lasers, those on inexpensive sealed HeNe tubes are integral to the metal end-caps and not normally considered user adjustable. There is usually a restricted region separating the mirror mount itself from the end-cap and actual deformation or bending is required to alter mirror alignment. As expected, there is no easy way to do this without the assistance of some sort of (homemade) tool - and repeated flexing of the metal tubing beyond its elastic limit WILL result in eventual failure of the seal.

      You cannot just grab the mirror mount in your hand and deform them as though your are Superman (unless you are) since additional leverage and finer control is needed (not to mention the several KV that may be present at one end of the HeNe tube end at least!).

      • For testing that the mirrors are aligned optimally, these tools will enable you to 'rock the mirrors' while watching for maximum output beam brightness.

      • For actually adjusting the mirror alignment, these tools will enable the mirror mounts to be with reasonable accuracy.

        Here are some suggestions for easily fabricated tools which will permit fairly precise movement of the mirror mounts. The plate and tube type are best for 'rocking the mirror' to check alignment without changing it. The lever type may be more precise for making final adjustments since it applies force at the exact place that it is needed.

      • Plate type - Obtain a plate of rigid nonconductive material such as Plexiglas about 4 inches square and 3/8ths of an inch thick (high quality hardwood plywood will also work but is not as sturdy). Use a micrometer or caliper to measure the diameter of the adjustable portion of mirror mount. Drill a hole of this diameter in the center of the plate (preferably using a drill press) with a sharp drill bit of the proper type for the material you are using. You want a snug fit but not one that is so tight that installing and removing the tool may deform the mount. If the exhaust tube interferes with the tool, drill a small clearance hole for it.

        A more robust enhancement is to obtain or machine a metal sleeve that just fits over the mirror mount and glue this into a press-fit hole in the insulating board (rather than just using a bare hole).

      • Tube type - This adjuster can be constructed from a piece of rigid plastic tubing that just fits over the mirror mount. A six inch length will provide enough of a lever to easily 'rock' the mirror and even bend the mirror mount if needed.

        It probably won't even be necessary to remove the HeNe tube from its case to use this tube type tool and it may be your only option if the HeNe tube is permanently glued inside a laser head barrel. But then, how could its mirror alignment have gotten messed up in the first place?

        Note: If testing or adjusting at the output end of the HeNe tube, the visibility of the beam may be impaired by this type tool. In this case, you should either use the plate-type tool or watch the weak beam usually visible from the opposite end of the HeNe tube (remove any opaque coating that may be present).

        I have actually used a tool of this type (actually, a female Alden HV connector!) and succeeded in correcting the alignment of a small HeNe tube which had no output beam at all.

      • Lever type - Another way of adjusting the mirrors without constructing any fancy tools is to use a piece of metal in the narrowed region between the mirror mount and HeNe tube end-cap to VERY slightly spread them apart on one side. I have actually used a LARGE straight blade screwdriver for this purpose (but with great care so as not to go overboard since there is a lot of leverage and the required displacement is microscopic).

        • The advantage of this approach is that it is possible to achieve quite fine adjustments by a kind of gentle repetitive pressing while watching the beam because you are applying force at the precise location where the metal must deform.

        • The disadvantage of this approach is that you can only move in one direction without changing the position of the tool or rotating the HeNe tube. Thus, it is not as convenient as the plate or tube type tools for checking alignment by rocking the mirror mount.

        • I don't know how far you can push your luck with this technique!

      • Thumbscrew type - It is also possible to construct mirror mount adjustment assemblies operated by thumbscrews to correct or optimize the pointing accuracy. For a small misalignment, this would avoid the risks of actually trying to bend the mounts since the range of motion would still be within the elastic limits of the metal. The adjusters would be firmly attached (glued or clamped) to the HeNe tube end-caps or bolted to a rigid baseplate. (However, in the latter case, expansion of the HeNe tube as it warms up will complicate matters.) They could be left permanently in place applying the proper force to the mirror mounts to maintain mirror alignment and always providing the option of making fine adjustments at any time if needed.

        I leave details of this approach as an exercise for the student :-).

        However, the use of such drastic measures is probably gross overkill for use with these small inexpensive HeNe tubes.

      • Set-screw type - These consist of a close fitting metal sleeve with three tapered point set-screws in tapped holes pressing against the edges of the mirror mount and tube end-cap at the restricted region thus applying force to very slightly spread them apart. Some HeNe and other laser tubes come with these permanently installed.

        This approach is good for final tweaking of mirror alignment but not really convenient for testing.

        CAUTION: Use an insulated tool (hex driver) for adjustment!

      Of course, a nearly infinite number of variations on all of these themes are possible.

      CAUTION: For all of the tools, make sure that, pressure is ONLY applied to the tube of the mirror mount beyond the narrow section - not the part attached to the body of the HeNe tube or the glass or even frit seal of the mirror itself. It is especially important to avoid applying any pressure to the mirror glass (which is quite soft) or the glass frit (glue, glass 'solder') holding the mirror in place which is even softer. On some HeNe tubes, there is just a thin ring of this material and it can be easily fractured. I have done it, hisssss :-(.

      CAUTION: DO NOT use a metal (conductive) material for the tool as the mirror mounts connect directly to the high voltage power supply!

      Providing two such tools - for both the cathode and anode ends of the HeNe tube, may simplify some of the alignment procedures. This will also be required if the diameters of the mirror mounts at each end of the tube are not the same.

      Alignment jigs are used in the factory during tube manufacture but these are made from strong rigid components so that even the smallest adjustment of the thumbscrews actually gets transmitted precisely to the mirror mount. Anything as complex as this is overkill for checking mirror alignment but might be desirable to permit fine tuning while the laser is operating.

      Major Problems with Mirror Alignment

      Where the mirror mounts are obviously bent or damaged, or if the techniques described in the section: Minor Problems with Mirror Alignment don't result in any beam, the HeNe tube starts normally, and other problems have been ruled out, further testing must be done 'off-line' - not powered. This requires a second (working) laser or special optics. Without one of these, anything you do will be hit or miss (mostly miss) as the mirrors must be nearly perfectly aligned before there will be any output beam at all. What is more likely to happen is that you will end up breaking the seal from repeated bending of the mirror mount.

      If only one mirror is actually misaligned, you can use the procedures from the section: Minor Problems with Mirror Alignment to identify the error (by rocking the mirror and looking for a beam with power on) and then carefully tweaking its alignment.

      However, if the mirrors at both ends of the tube are messed up, the chances of success are quite slim - especially for those high power expensive HeNe tubes. Getting close won't be good enough since rocking either mirror by itself will never result in any beam.

      Unless your baby is a high power and/or expensive HeNe tube, it may not be worth the effort to attempt the procedures described below. While testing and/or correcting major mirror alignment may represent an irresistible challenge, the cost in terms of time, materials, and frustration could prove to be substantial.

      As if this isn't enough, if one (or both) of the mirrors on your HeNe tube are not planar (often concave at the high reflector end), or there is an internal Brewster plate or etalon, even more care will be required in equipment setup and subsequent steps may be complicated at that end at least.

      In addition, the mirror plates on some lasers have faces which are ground with some wedge - so their surfaces are NOT precisely parallel. This prevents any light reflected off of the outer surface from bouncing back into the resonant cavity and interfering with the lasing (which might result in some instability or ghost beams). Alignment is complicated for a mirror where wedge is present due to non-parallel reflections and slight refraction through the mirror. I don't believe you will find these in common HeNe tubes but yours may be the exception.

      The longer the HeNe tube, the worse it gets!

      I would suggest that if the tube is valuable enough to warrant the expense, see if one of the HeNe laser manufacturers or laser system refurbishers will perform the alignment for you. The ratio of their probability of success compared to your probability of success will approach infinity. OK, perhaps not quite infinity. It probably won't be significantly greater than the ratio of the mass of the Sun to that of a typical electron. :-). I have no idea if this is a viable option or what it might cost.

      Having said that, if you are still determined to proceed, alignment is best done with a working narrow beam laser (i.e., HeNe, argon ion, etc.).

      If you do not have a working laser to use for this purpose, various plans for construction of laser mirror aligners using simple optics and readily available materials are provided in: "Light and Its Uses" [5]). However, some of these are for wide bore tubes and may not work well with the .5 to 1.5 mm bores of typical modern HeNe tubes.

      If you have another functioning HeNe laser or tube (you can use the power supply for the one you will be adjusting since it will not be needed until the mirrors are roughly aligned), or possibly even a collimated diode laser or laser pointer) it may be possible to use it as an alignment laser to adjust the mirrors. A low power (i.e., .5 to 1 mW) laser is adequate and preferred since it will be safer as well.

      Plan on spending a lot of time on this. Therefore, select a location to work where you can spread out and won't be disturbed for hours. The kitchen table is probably not appropriate!

      1. Provide a mounting so that both lasers can be arranged precisely in line with each other and separated by at least 1 tube length (the further apart they are, the more sensitive will be the test to pointing accuracy). This is likely harder done than said but is THE most important step as anything you do to the mirror mounts will depend on the absolute precision of this setup. Unfortunately, without an optical bench, this may be very difficult to achieve. However, a length of 2"x4" U or box (or larger) extruded aluminum stock should provide the necessary rigidity.

        • The Tube-Under-Test (TUT) should be mounted on a pair of V-blocks. (Line the V-blocks with tape to protect the TUT from scratches if they are made out of metal.)

        • If the non-output mirror of the TUT has an opaque coating, the paint or tape will need to be carefully removed.

        • The alignment laser (A-Laser) needs to be mounted so that it can be positioned precisely on-axis with respect to the TUT. The best way to do this is with a pair of X-Y vernier positioners of the type used for this purpose in optics research labs. An alternative (which is what I have done) is to adapt some surplus microscope mechanical X-Y stage assemblies bolted to a rigid base provided with clamps to hold the alignment laser head. Make sure that the settings can be locked in position so they don't drift due to the weight of the A-Laser. You may need to be resourceful and improvise! See the section: Simple Adjustable Optics Platform for details of one very basic approach which is quite adequate for checking laser mirror alignment.

        • Drill or punch a 1 mm diameter hole in the center of a white card. Fasten this card to the front of the A-Laser so that its beam passes through the center of the hole. Henceforth, I will call this the 'AL-Bezel'.

      2. Provide a means of adjusting the mirror mounts as described in the section: Means of Adjusting HeNe Tube Mirrors.

        Note: If a mirror mount on the TUT is very visibly bent (and this is not just compensating for a mirror that was accidentally fritted in place at an angle), it should be straightened as best as possible (by eye) before the procedure below is attempted. Otherwise, initial alignment between the A-Laser and the TUT will have too much error or be impossible to achieve at all. To check for this damage, rotate the TUT on the V-blocks and watch the surface of each mirror. If *significant* wobble in its angle is evident, it should be corrected now by CAREFULLY bending the mount. At least, if you screw up and break the seal, at least you won't have wasted any additional time and effort :-(.

      3. Provide a mounting for both lasers so that their optical axes are perfectly in line and the output end of the A-Laser and the closer end of the TUT are separated by about one TUT-tube length (the further apart the better - to a point - as alignment sensitivity increases but the apparatus becomes more and more unwieldy). Make sure there is clearance for your mirror adjusting tool at the mirror mount of the TUT facing the A-Laser.

        The adjustable (dual X-Y) mount for the A-Laser and V-blocks for the TUT should be securely clamped or screwed to a rigid surface so that their relationship cannot accidentally shift by more than the diameter of a fat hydrogen atom :-).

        The following three steps, (4) through (6), will need to be repeated for the cathode and anode ends of the TUT. As an arbitrary choice, start with the cathode end of the TUT facing the A-Laser. Observe any "This side up" labels on the TUT (probably only for some large HeNe tubes).

      4. Alignment of the A-Laser and TUT must be absolutely precise. A fraction of a mm is significant. Take your time. You have all week. Here are two ways of achieving this:

        • If the wavelengths of the two lasers differ significantly, there will be enough transmitted light so that the beam from the A-Laser is able to pass through both mirrors of the TUT and project a bright spot on a screen placed beyond the TUT. An ideal situation exists where a red (632.8 nm) HeNe laser is being used to test a green (543.5 nm) HeNe tube or vice-versa.

          The reason for this behavior is that the dichroic mirrors used in these HeNe tubes have a reflectivity which peaks at the laser wavelength. As the wavelength moves away from this, they transmit more and more light. For example, if you sight down an unpowered red HeNe tube, it will appear blue-green and quite transparent indicating that blue-green light is passed with little attenuation but red light is being reflected or blocked. (Actually, orange and possibly yellow light is also reflected well by these mirrors as shown by their typical goldish appearance.)

          However, this approach cannot be used if the wavelengths of the two lasers are the same or even fairly close since the reflectivity of the two mirrors will be a maximum and very little light will be transmitted. This will be the case when attempting to check one red (632.8 nm) HeNe laser with another (which is probably what you are doing, right?) or even with a 670 nm diode laser pointer.

          Proceed as follows:

          • Place the TUT in position on the V-blocks. Fasten it down so it cannot move - even if you should need to apply force to adjust the mirror.

          • Position a white card to act as a screen just beyond the far end of the TUT.

          • Power up the A-Laser. Go back and forth between the X-Y adjustments at each end of the A-Laser mount to get the beam from the A-Laser to pass as cleanly as possible through the bore of the TUT. Watch for where the spot falls on the front of the TUT's mirror and for a clean spot exiting the other end of the TUT that is projected onto your screen.

          • Alignment of the A-Laser with respect to the TUT is optimal when the brightness of the light of the A-Laser exiting the other end of the TUT is maximum and the presense of off-axis circles or arcs are minimized (the pattern is symmetric).

            Note: Except for a very short TUT, it is likely that the A-Laser's beam would be wider than the bore of the TUT at the far end at least. Make sure you are optimizing the central peak of the beam of the A-Laser by checking on all sides to make sure. Just getting a beam out the other end is not enough.

        • An alternative approach is to constuct an external bore sight - a sort of laser capillary simulator - to align the two tubes. With care, this can be at least as accurate as the previous method and will work equally well regardless of the relative wavelengths of the A-Laser and TUT.

          Proceed as follows:

          • Place the TUT in position on the V-blocks. You will need to remove and replace the TUT a couple of times so don't fasten it down yet.

          • Put matching marks on the TUT and the V-blocks so that the TUT can be easily removed and replaced without changing its position or rotational orientation. (Even very slight unavoidable manufacturing errors in the centering of the capillary in the TUT will be enough to cause problems if the orientation can not be exactly duplicated.)

          • Fabricate a pair of Bore Sight Mounts (BSMs) - wood blocks or metal angle brackets each having a 1/2" diameter hole approximately centered on the TUT axis.

          • Fabricate a pair of Bore Sight Cards (BSCs) - just some 1" x 1" pieces of thin cardboard (more white cards!) with a center hole approximately the diameter of the TUT's bore that can be fastened onto the BSMs with adhesive tape.

          • Position the BSMs so that the BSCs will be almost touching the mirrors at each end of the TUT (but take care not to scratch them). Fasten the BSMs to the base of the V-block assembly with adhesive tape.

          • For each end of the TUT, place a BSC against the BSM and fine tune its position so that its hole is precisely centered on the TUT's bore. Do this (using a dental mirror if necessary) by looking through the hole in the BSC and down the length of the TUT's capillary. Fasten the BSC in position with adhesive tape and then double check that it is still perfectly centered - an error of .1 mm matters!

            Note: For HeNe tubes with an internal angled Brewster plate or etalon, there will be a slight shift in the apparent position of the bore at that end due to refraction. However, the hole must be lined up with the physical location of the bore, not its (shifted) image.

          • Temporarily remove the TUT from the V-blocks without disturbing the BSMs and their associated BSCs.

          • The holes in the two BSCs now form a bore sight assembly which exactly matches the position of the ends of the TUT's actual bore. Use the A-Laser's X-Y adjusters to precisely center its beam on the holes of BOTH BSCs. If the A-Laser's beam is narrower than the hole(s) in the BSCs, hold a translucent screen against the BSC so you can see exactly where the beam falls. Once the centering at both ends is perfect, lock the X-Y adjusters in position and then double check that nothing has moved.

          • Remove the the BSMs and replace the TUT in the proper orientation on the V-blocks. Fasten it down so it cannot move - even if you should need to apply force to adjust the mirror.

      5. With everything in position and the A-Laser powered up, there should be a reflection of the A-Laser's beam back onto the AL-Bezel from the mirror of the TUT. If the mirror alignment is perfect, the main reflection will be entirely within the hole in the AL-Bezel with just a symmetric halo (with some dancing interference fringes showing) due to multiple reflections between the A-Laser and TUT's mirrors. In the case of a non-planar mirror, this may be somewhat spread out (but I already warned you about dealing with these).

        Note: There will actually be two sets of reflections from the two surfaces of the mirror glass of the TUT. The one from the inner surface - which is probably much stronger - is the relevant one but both should coincide when alignment is correct. Where the front mirror of the A-Laser is non-planer, the secondary reflections may be spread out somewhat but the more important primary reflection will be unaffected.

        • CAUTION: If the alignment looks good, DON'T TOUCH IT! Maybe the real problem is at the other end of the TUT or something else!

          If the reflections are off to one side, FIRST CHECK THAT YOUR SETUP HAS NOT SHIFTED POSITION. GO BACK AND DOUBLE CHECK YOUR A-LASER and TUT ALIGNMENT! For slight errors, problems with the setup are more likely than problems with the TUT's mirror alignment.

        • If you are positively sure beyond any shadow of a doubt that imperfect TUT mirror alignment is the cause of your not-centered reflections, you can attempt to use your tool to CAREFULLY adjust the mirror mount until the reflections are perfectly centered.

          Again, double check that the critical alignment of the two lasers hasn't shifted before messing with the mirrors!

          CAUTION: The mirror mount is ultimately attached to the glass envelope of the tube. The glass-metal seal may not be that strong. Don't get to carried away! With care this adjustment should be possible - barely :-).

          • Increase the force gradually until you have a feel of how much it takes to actually deform the mirror mount. Even a significant pointing error will only require a small correction. And, the final accuracy needs to be a fraction of a mR - much less than 1 part in 1,000! Not easy.

          • Approach the desired deflection in small increments and overshoot just enough so that the mirror mount springs back to the optimal position. Avoid repeatedly bending it back-and-forth or you will eventually be using the TUT as a high-tech wall hanging :-(.

          • As alignment for the mirror you are adjusting approaches perfection, you will see multiple spots from multiple reflections between the output mirror of the alignment laser and the mirror of the TUT as the light bounces back and forth between them. When perfect, there will likely be a halo of dancing interference fringes surrounding the hole as well.

      6. If you adjusted the mirror, go back and check the alignment of the A-Laser and TUT to assure that it is still perfect. A significant change in the angle of the mirror could affect the apparent location of the bore of the TUT used for alignment in step (4). In this case you will need to correct the alignment and then repeat steps (5) and (6). Relax - this process will converge so you won't be stuck in an infinite loop forever!

      7. Turn the TUT around and repeat steps (4) to (6) for its other mirror.
      If mirror alignment was your problem (and for larger tubes, if you believe in minor miracles!), the TUT should hopefully now produce at least some output beam when powered up.

      • If it produces even a very weak beam, all you need to do is to perform final mirror alignment.

      • If there is no evidence of a beam at all, one or both mirrors may still be too far out of alignment. Go back and repeat steps (4) to (6) for both ends of the TUT. If there is still no output, with luck, only one mirror is now at fault and can be identified and corrected with the TUT powered up.

      In either case, see the section: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes.

      Simple Adjustable Optics Platform

      For simple alignment checks, an adjustable platform can be constructed in about 10 minutes from common materials. You will need:
      • A piece of Plexiglass, hardwood plywood, or even high density particle board, about 6" x 12" x 3/4" (or whatever size your laser or other optical system will fit on with room to spare).

      • Three, tapped (totally through) metal spacers about 3/4" long. A thread size of 8-32 or 10-32 is adequate though more threads/inch would be even better.

      • Three, 2" (or longer) machine screws to fit the tapped spacers.

        The following is best done using a drill press but it is not essential:

      • Drill holes so the tapped spacers will be a snug fit in the board 1/2" from the corners at one end of the board and at the center of the other end, 1/2" from the edge.

      • Press the spacers into the holes. If they are too loose, secure them with 5-minute Epoxy (how else to guarantee a total assembly time of less than 10 minutes!).

      • Round off the ends of the machine screws using a file or grinding wheel so they will each contact the surface upon which this assembly will rest at a single point.

      • Install the machine screws into the tapped spacers so they poke out of the bottom of the platform.
      There you have it! This will permit a laser or other optical system secured to this platform to be adjusted easily for height, pitch, and yaw. If you need more of a height adjustment range, sit the platform on something else.

      While this isn't quite as precise as one milled out of a solid block of high strength (aircraft quality) aluminum alloy using anti-backlash spring-loaded micrometer adjustment screws, it will suffice for many purposes and costs next to nothing!

    4. Back to HeNe Laser Testing, Adjustment, Repair Sub-Table of Contents.

      Improving Collimation, Controlling Polarization

      Improving the Collimation of a HeNe Laser with a Beam Expander

      The following applies to any laser which outputs a substantially parallel beam but is written specifically for HeNe lasers. Collimation of laser diodes require a slightly different approach - see the section: Beam Characteristics of Laser Diodes.

      Although the divergence of a HeNe laser is already pretty good without any additional optics, the rather narrow beam as it exits from the tube does result in a typical divergence between 1 to 2.5 mR (half of total angle of beam). 1 mR is equivalent to an increase in beam diameter of 2 mm per meter.

      As noted in the section: HeNe Laser Tubes and Laser Heads, beam divergence is inversely proportional to the beam diameter. Thus, it can be reduced even further by passing the beam through beam expander consisting of a pair of positive lenses - one to focus the beam to a point and the second to collimate the resulting diverging beam. Though the beam will start out wider, it will diverge at a proportionally reduced rate.

      A small telescope can be used in reverse to implement a beam expander to collimate a laser beam and will be much easier to deal with than individual lenses. (This is how laser beams are bounced off the moon but the telescopes aren't so small.) Using a telescope is by far the easiest approach in terms of mounting - you only need to worry about position and alignment of two components - the laser tube and telescope. The ratio of original to expanded beam will be equal to the magnifying power of the telescope. Even a cheap 6X spotting scope will reduce divergence six-fold.

      If you want to use discrete optics:

      • The focusing lens should have a short focal length (F1) such as a microscope or telescope eyepiece (e.g., F1 of 10 mm) or low power microscope objective (e.g., 10X). Note: the objective lens from a dead CD player has an ideal focal length - about 4 mm - but is aspheric and would probably not be that great but give it a try!

        This will focus the laser beam to a (diffraction limited) point F1 in front of the lens from which it will then diverge.

      • The collimating lens should be a large diameter medium focal length (e.g., 15 mm D2, 100 mm F2) lens placed F2 from the focus of the first lens.
      For optimal results, the ratio of collimating lens diameter to focal length (D2/F2) should greater than or equal the ratio of HeNe beam diameter to focusing lens focal length (D1/F1). This will ensure that all the light is captured by the collimating lens.

      The beam will be wider initially but will retain its diameter over much longer distances. For the example, above, the exit beam diameter will be about 10 mm resulting in nearly a 10 fold reduction in divergence.

      Adjust the lens spacing to obtain best collimation. A resulting divergence of less than 1 mm per 10 meters or more should be possible with decent quality lenses - not old Coke bottle bottoms or plastic eyeglasses that have been used for skate boards :-).

      Unrandomizing the Polarization of a Randomly Polarized HeNe Tube

      Sealed HeNe tubes with internal mirrors which are linearly polarized usually incorporate an internal Brewster plate to suppress the unwanted polarization orientation - and these HeNe tubes are more expensive. Where mirrors are external, the Brewster windows on the plasma tube accomplish this function and the output of these is always linearly polarized.

      Common inexpensive sealed HeNe tubes produce a beam that is either randomly polarized or slowly varying (as the tube heats). The presence of either of these characteristics makes such a laser unsuitable for many experiments and applications. (These tubes are normally designated as 'random polarized' which translates as: "The manufacturer has no idea of what the polarization characteristics will be at any given time".)

      Where the polarization is truly random, a polarizing filter in the beam path will produce a linearly polarized beam at the expense of at least one half the output power (that which is blocked because its polarization orientation is wrong and because of losses in the filter). However, where the polarization orientation of the laser is slowly changing, this approach will result in unacceptable varying output intensity from the polarizing filter. Additional optics including polarizing beam splitters, mirrors, and combiners can produce a stable polarized beam but these are complex and expensive.

      I have found that placing powerful magnets alongside a random polarized tube will result in a highly linearly polarized beam.

      A type of magnet that works quite well has a strength of several thousand gauss. The ones I used came from the voice coil positioner of a moderate size hard disk drive. They are rare earth magnets with dimensions of about 1.25" x 2.5" x .375" with the broad faces being the N and S poles. The amount of polarization is most pronounced by placing one of the broad faces of the magnet against the tube near its mid-point. Some adjustment may be needed to optimize the effect. I do not know how much magnetic field strength is needed but even moving this magnet 1/4" away from the tube surface greatly reduced the ratio of light intensity in the two orthogonal polarization axes.

      CAUTION: These types of magnets are very powerful. In addition to erasing your credit cards and other magnetic media, they will tend to crush, smash, or shatter anything (including flesh or your HeNe tube) between them and/or between them and a ferrous metal. Some portions of a HeNe tube or laser head may contain parts made from iron or steel. These rare earth magnets also tend to be quite brittle. In addition, the violent uncontrolled movement may place you and a HV terminal in the same space at the same time as well! Take care.

      With the magnet's N or S pole placed on the side of the tube, the result was a vertically polarized beam. By rotating a polarizing filter in the beam path, beam intensity could be varied from nearly totally blocked to nearly totally transmitted and the polarization orientation followed the magnet as it was rotated around the tube.

      The control wasn't perfect - a small amount of light with a slowly varying polarization did sneak through. However, it was significantly less than 1 percent of total beam power for these particular tube and magnet combinations (I have tried this with 2 different tubes with similar results). The constant portion of the residual beam may have just been a result of the imperfect nature of the polarizing filter.

      By using two similar magnets - one on either side of the tube with N and S poles facing each other (mounted on an aluminum U-channel for support and so they would not crush the tube), the variation in residual beam intensity was virtually eliminated. I do not know if this effect was due to the increased magnetic field or its more homogeneous and symmetric nature. This was also used successfully with an enclosed HeNe laser head:

                             |_____| Rare earth magnet
           |                                            |
           |             HeNe laser head                |=====> Polarized HeNe beam
                             |_____| Rare earth magnet
      As far as I could tell, with this dual magnet configuration, the output beam characteristics were similar to those of a polarized HeNe tube. However, additional and/or more powerful magnets might be necessary with other tubes.

      Output power did not appear to be affected - in fact, it may have increased slightly (or perhaps it was my imagination but see the section: Magnets in High power or Precision HeNe Laser Heads). A polarizing filter would nearly totally block the beam at one orientation and have minimal effect 90 degrees away from this.

      I do not know about the stability or reliability of this scheme or whether it is ever used in commercial HeNe lasers. But, the only other effects seem to be to increase the required input starting/operating voltage and/or magnitude of the negative resistance of the tube slightly and possibly to shift to point of maximum beam power to a higher tube current (5 mA instead of 4 mA for one tube - but this could have just been my imagination as well).

      Since it is possible to control the polarization orientation with permanent magnets, the next step would be do this with electromagnets. This would permit polarization to be dynamically controlled. Adding a fixed polarizer would provide intensity modulation without any connection to the power supply or expensive electro-optic devices. Hopefully, by using multiple sets of coils distributed along the side of the HeNe tube, a lower field strength would be adequate. Liquid helium cooled superconducting electromagnets would definitely add to the cost of the project :-). Perhaps, someday, I will try this out.

    5. Back to HeNe Laser Testing, Adjustment, Repair Sub-Table of Contents.

      Rejuvenating HeNe Tubes

      The major HeNe laser manufacturers and laser repair companies may offer regassing services for larger more expensive HeNe tubes. Whether the cost of such am operation can be justified is another matter. For a high quality research laser it probably makes sense. For small sealed HeNe tubes, low cost replacements are readily available.

      However, where just the helium (remember how slippery those He atoms can be!) has leaked out, there may be an alternative:

      "I have two large green HeNe tubes and and a 1 micron IR HeNe that are dead from obvious low helium pressure (spectrum from grating shows only weak He lines) has anyone had any success with putting tubes in a pressure chamber filled with Helium so it diffuses the other way?"

      HeNe tubes which do not lase well or at all due to loss of helium can sometimes be rejuvenated by soaking them in helium at normal atmospheric pressure for a few days or weeks.

      However, there could be other causes like misaligned mirrors or excessive tube current (due to a defective power supply). Check for these possibilities first and confirm loss of helium with a spectroscope if possible.

      The point to realize is that it is the partial pressure of each gas that matters. Neon is a relatively large atom and does not diffuse through the tube at any rate that matters. However, helium is able to diffuse even when the pressure difference is small. Even for a HeNe tube at 2 Torr (1/380th of normal atmospheric pressure), the partial pressure of helium in the tube is much much greater than its partial pressure in the normal atmosphere. So, helium leaks out even though the total pressure outside is several hundred times greater. Conversely, soaking a HeNe tube in helium at 1 atmosphere will allow helium to diffuse into the tube at several hundred times the rate at which it had been leaking out. Thus, only a few days of this treatment may be needed if the problem is low helium pressure.

      This hardly seems worthwhile for a $25 1 mW HeNe tube but it is something to keep in mind for other more substantial types.

      (From: Mark W. Lund (lundm@xray.byu.edu)).

      I have rejuvenated HeNes with low helium pressure. Since the partial pressure of 1 atmosphere helium is much higher than inside the tube you don't really need to use high pressure, or even increased temperature. I just put them in a garbage bag and blasted some helium into it from time to time. The length of time necessary in my case was a few days, but depending on the glass type, thickness, and sealing method this may vary. It would be good to test the power every couple of days so you don't overshoot too much.

      One warning, helium has a lower dielectric strength than air, so don't try to operate the laser in helium, it may arc over.

      (From: Philip Ciddor (pec@dap.csiro.au)).

      My information is very old, but may be helpful. Early 2 mW red tubes had about 2 torr of He, so soaking in 760 torr (1 atmosphere) of He for 1 day per year of life roughly restored the initial He pressure, since diffusion rate is proportional to pressure difference. I have no data on the gas mix in current green or IR tubes, but if you can find it, similar scaling may be feasible.

      (From: Sam).

      I cannot overemphasize the importance of carefully monitoring the amount of helium that has diffused back into the HeNe tube (by removing it from the bag of He and testing with a spectroscope periodically and for a laser beam) - once its pressure goes to high, the only (non-invasive) way of lowering it is to wait a few years :-).

      CAUTION: In addition to not attempting to operate the HeNe tube itself in a helium atmosphere, there may even be problems with He diffusing into power supply components or ballast resistors and lingering there. So, if possible, remove the HeNe tube from its laser head or system enclosure for the helium soak.

    6. Back to Sam's Laser FAQ Table of Contents.
    7. Back to HeNe Laser Testing, Adjustment, Repair Sub-Table of Contents.
    8. Forward to HeNe Laser Power Supplies.