Polarization Oscillation in a HeNe Laser?

While playing with a HeNe laser, some polarizers and quarter wave plates, and a photometer at work today, I noticed that the polarization of the output beam is not stable.  

The intensity of light output of the beam is constant at 1.25mW, however the primary direction of polarization seems to be oscillating with a period on the order of a minute--the intensity of light passing through a polarizer varies by roughly 6X (from 140uW to 900uW).  

My question is, is this a known 'feature' of HeNe lasers?  And can anyone explain it?  

Dunno whats with the periodic changing, but if its not polarized, the polarization is liable to be pretty random. You can get polarized tubes with a brewster's window in them though. That way there's gain in only one plane of polarization and you can get a (roughly) linearly polarized beam.

Yeah, it's not polarized as it's lacking a Brewster window, so I would expect random polarization.  As it is, it's partially polarized with the direction of partial polarization "rocking" back and forth or perhaps rotating--I didn't have time to look at it long enough to explore it more deeply.  Even a static partial polarization wouldn't seem too unusual to me.  But an oscillating polarization?

I have had a laser that had a polarization drift before, it was not a HeNe but a diode laser. The diode output didnt actually change but the beam went through several windows at various angles before getting to the frequency stabilization system (which used polarization of the beam as way to stabilize it) and one of the windows was leading to a polarization drift that followed a temperature drift, this led to a frequency drift since the laser was actively stabilized.

If you can simultaneously measure temperature right near the laser and also the output of the laser after the polarizer you might be able to see if it is the same type of thing.