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Re: POLARIZATION




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)

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