Re: POLARIZATION

[Hugh Logan <hlogan@ix.netcom.com>]

Herbert H. Gottlieb wrote:
>  Hugh Logan <hlogan@ix.netcom.com> writes:
> > About eight years ago I saw a video presentation about lasers from
> > M.I.T. If my memory is correct, the problem is something like this:
> > Take a single (transverse 00) mode randomly polarized He-Ne laser
> > (without a Brewster window). Observe the output with a Fabry-Perot
> > scanning interferometer. Place a Polaroid filter between the exit
> > window of the laser and the scanning interferometer. As the laser
> > warms up, one can observe successive longitudinal modes of the laser
> > on the scanning interferometer. If my memory is correct, one can
> > observe that the successive laser modes have polarizations that are
> > 90 degrees apart from each other, tuning them in by rotating the
> > Polaroid. The video presentation left one with the question, "Why?"
> > I was able to replicate the demonstration in our lab. I discussed this
> > with several people, some quite expert with lasers, but did not come
> > up
> > with a conclusive answer.
> >
> > Hugh Logan
> This phenomenon is called ELLIPTICAL POLARIZATION. It is often observed
> in HeNe lasers of a milliwatt or less having laser tubes that are rather
> short. Most such lasers produce beams  that switch polarization from one
> favored angle of polarization to another angle that is displaced by 90
> degrees from the first angle. This switching occurs about 20 seconds
> after the laser is first turned on and continues at longer and longer
> intervals as the laser warms up to a steady operating temperature.
>
> Herb Gottlieb from New York City
> ( Where elliptical polarization of short HeNe lasers drove us up a wall
> when it was first observed a few years ago)
Are you suggesting that the major and minor axes of the ellipse are
interchanged as one goes from from one longitudinal mode to the next?
I think that the ellipse in either case would have to be very skinny --
aproaching the degenerate case of linear polarization. Although I was
not thinking of elliptical polarization at the time, my vague
recollection is that any mode could be -- at least approximately --
extinguished by rotating the Polaroid. If it were elliptical, other 
than the degenarate case (or nearly so) of linear polarization, I don't
think the mode could appear to be extinguished by rotating the polaroid.

What I am really interested in is the physical mechanism for producing
this 90 degree change in phase. In my previous discussions, one of my
colleagues suggested that it must have something to do with the laser
medium, but could not go further than this. Others tried to explain it
in terms of the optics of the laser cavity, but were unable to carry
this through.

It was my impression that if only one of the cavity modes fell within
the region where the gain was above the threshold value would one get
lasing. As I recall, the frequency separation of the longitudinal modes
is approximately c/(2*L). The shorter the cavity, the farther apart
these cavity modes would be. I believe we are talking about single mode
lasers. I have always thought that the appearance 
of successive modes during warmup was the result of thermal expansion,
changing the possible longitudinal cavity modes of 
frequency = (n*c)/(2*L) by virtue of a slight change in L. But I 
still don't know why the polarization switches by 90 degrees. I don't 
immediately see anything in the cavity that would favor one 
polarization over the other from one mode to the next. Can this effect
be related to the polarization of light emitted from neon atoms or
groups of neon atoms during the lasing process? or to any asymmetry
between the cavity modes and the gain profile as the cavity modes 
shift?

Puzzled in Concord,
Hugh Logan