Geht das???? (Und... Was bringt das??)
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Unrandomizing the Polarization of a Randomly Polarized HeNe Tube
The best option where a polarized beam is required is to start with a HeNe laser that produces a polarized beam! However, for the experimenter, there is at least one alternative - magnets to the rescue!
I have found that placing powerful magnets alongside a random polarized tube will result in a highly linearly polarized beam. While this may be common knowledge at the Afternoon Teas attended by laser physicists (assuming they drink tea), it certainly isn't something found in popular books on lasers.
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:
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|_____| Rare earth magnet
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| HeNe laser head |=====> Polarized HeNe beam
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|_____| Rare earth magnet
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Use of Magnets to Generate Polarized HeNe Laser Beam shows acceptable locations for one pair of magnets along side a typical 1 mW HeNe tube. This placement was found to be effective but possibly not totally optimal - experimentation may be required.
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 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 (current dropped by about 10 percent with the magnets using an unregulated power supply) 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).
Where the capillary of the plasma tube is exposed as with many older lasers, and the magnets can be placed in close proximity to the bore, their strength can be much lower. Some commercial lasers (like the Spectra-Physics model 132) offered a polarization option which adds a magnet assembly alongside the tube. However, I doubt that this is done commercially with any modern HeNe tubes with coaxial gas reservoirs.
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. Die Seite, auf der das zu finden ist, lautet: Sam's Laser FAQ