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:
However, such damage could be an indication of a trauma that misaligned the mirrors - though this is quite unlikely - see the next paragraph.
Note: if you have a high power (long) tube, mirror alignment may not be correct until the tube warms up and/or external permanent adjusters may be required to stabilize the mirrors. Without these, there may be no, low, or fluctuating power. Very slightly pressing on the mirror mounts - or even on various parts of the tube itself - (with a well insulated tool!) will result in a significant variation in power. There may also be a "This Side Up" label on the tube or head indicating the proper orientation for optimal performance. Parts in the tube droop due to gravity (not the electrons, ions, or photons!). This probably applies mostly to HeNe tubes that are greater than 15 to 20 mW and possibly only some types and condition.
See the section: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes for more information.
Aside from manufacturing defects, one way for such a failure to occur is for a power supply fault to drive grossly increased current through the HeNe tube. It is possible for this to result in an abrupt termination of the discharge inside the bore and an inductive kick and huge voltage spike due to the wiring. With the bore momentarily unavailable, the only other path is for an arc through the glass barrier. Like the failure of a MOSFET gate oxide due to electrostatic discharge, once any breech develops, it does not heal! The addition of a spark gap surge protector sized to break down at just over the specified starting voltage may represent a prudent precaution when driving large expensive higher power HeNe tubes. Figure about 25 KV per inch - though this can vary considerably depending on the shape of the electrodes and environmental conditions.
This is one reason not to use a power supply much larger than needed for your particular HeNe tube. I found out the hard way when while violating my recommendation not to use a microwave oven transformer, this happened with a large (35 mW) HeNe tube due to a wrong connection which bypassed the ballast resistor. It was not pretty :-(. The HeNe tube is now good as a sort of high tech neon sculpture but not much else.
If the tube has a getter electrode (see the section: HeNe Laser Tubes and Laser Heads), check the color of the getter spot on the glass in its vicinity. It should generally be black or silver in appearance. A milky white or red color indicates air leakage - the tube is probably no longer functional. It may be possible to reactivate the getter electrode by heating it by RF induction to drive off more getter material that may be present but (1) this is definitely for the advanced course and (2) the likelihood of helping the HeNe tube at this point is small unless the amount of leakage was very very infinitesimal.
Any source of RF power can be used to determine if a bare tube still has a reasonably low internal pressure (but not if it will lase). However, this approach cannot be used to test enclosed laser heads because it is generally not possible to view the inside of the actual HeNe tube and the (metal) case would prevent RF penetration or create other problems.
CAUTION: Damage may occur to the HeNe tube if the glow continues for more than a couple of seconds. I have not attempted to find out. Damage may also occur to you if your parents find out you were using the family microwave for this purpose :-(.
Note: In case your were wondering, this is not an effective way of exciting the tube to lase - the discharge intensity inside the bore (capillary) where it counts is way too low.
If the color is more toward the pink, lavender, or white, the gas fill may be incorrect or some air may have leaked in. See below.
If you have a spectroscope (see the section: Instant Spectroscope for Viewing Lines in HeNe Discharge), it is easy to see if this is the case as the neon lines in Bright Line Spectra of Helium and Neon will be predominant.
If you can sustain a discharge but it is the wrong color (weak or white/pink color), you may have one of those really old Epoxy sealed tubes that leak and air has leaked in. The tube is probably not worth repairing but might make an interesting wall hanging (power optional).
Tubes with many hours on them may have some brownish or metallic deposits on the inside of the bore but this is often harmless.
See the section: Basic HeNe Power Supply Considerations.
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.
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!
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:
CAUTION: Do not run the HeNe tube with reversed polarity for too long!
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.
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 :-).
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.
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:
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!).
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:
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.
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.
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).
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.
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!).
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.
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).
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.
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.
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!
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.
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!
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 :-(.
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).
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:
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.
Proceed as follows:
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.
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.
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.
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 :-).
In either case, see the section: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes.
The following is best done using a drill press but it is not essential:
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!
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:
This will focus the laser beam to a (diffraction limited) point F1 in front of the lens from which it will then diverge.
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 :-).
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:
__S__
|_____| Rare earth magnet
____________________N_______________________
| |
| HeNe laser head |=====> Polarized HeNe beam
|____________________________________________|
__S__
|_____| Rare earth magnet
N
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.
However, where just the helium (remember how slippery those He atoms can be!) has leaked out, there may be an alternative:
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.