I just requested this on interlibrary loan:
formatting link
1.html
From the FAQ,
Quoting Tony Siegman:
(From: A. E. Siegman ( snipped-for-privacy@stanford.edu).)
The reason that HeNe lasers can run - more accurately, like to run - in mul tiple axial modes is associated with inhomogeneous line broadening (See sec tion 3.7, pp. 157-175 of my book) and "hole burning" effects (Section 12.2, pp. 462-465 and in more detail in Chapter 30) in the Doppler-broadened las er transitions commonly found in gas lasers (though not so strongly in CO2) and not in solid-state lasers.
The tendency for alternate modes to run in crossed polarizations is a bit m ore complex and has to do with the fact that most simple gas laser transiti ons actually have multiple upper and lower levels which are slightly split by small Zeeman splitting effects. Each transition is thus a superposition of several slightly shifted transitions between upper and lower Zeeman leve ls, with these individual transitions having different polarization selecti on rules (Section 3.3, pp. 135-142, including a very simple example in Fig. 3.7). All the modes basically share or compete for gain from all the trans itions.
The analytical description of laser action then becomes a bit complex - eac h axial mode is trying to extract the most gain from all the subtransitions , while doing its best to suppress all the other modes - but the bottom lin e is that each mode usually comes out best, or suffers the least competitio n with adjacent modes, if adjacent modes are orthogonally polarized.
There were a lot of complex papers on these phenomena in the early days of gas lasers; the laser systems studied were commonly referred to as "Zeeman lasers". I have a note that says a paper by D. Lenstra in Phys. Reports, 19
80, pp. 289-373 provides a lengthy and detailed report on Zeeman lasers. I didn't attempt to cover this in my book because it gets too complex and len gthy and a bit too esoteric for available space and reader interest. The ea rly (and good) book by Sargent, Scully and Lamb has a chapter on the subjec t. You're probably aware that Hewlett Packard developed an in-house HeNe la ser short enough that it oscillated in just two such orthogonally polarized modes, and used (probably still uses) the two frequencies as the base freq uencies for their precision metrology interferometer system for machine too ls, aligning airliner and ship frames, and stuff like that.