[Laser] polarization modulation

[Tim Toast]

To avoid getting the other topic off too far, i made
another one if it gets any further discussion. "Coffee
grinder polarization modulation fundamentals" hi. 
Maybe the atmosphere only mangles the polarization as much
as it does the amplitude, but it makes sense that it would
be effected as is phase and frequency.

A mechanical rotating polaroid filter at TX and matching
stationary analyzer at the RX is the simplest setup maybe.
But here are some electrical modulators that could be used
for pure polarization modulation experiments:

Kerr cell: 
http://www.elec.gla.ac.uk/groups/opto/Kerr.html
http://en.wikipedia.org/wiki/Kerr_effect#AC_Kerr_effect
- scary toxic liquids and HV or RF required. Remove the
analyzer and use it in the receiver.

Modified Kerr cells: 
- distilled water or other liquids may work at reduced
efficiency, remove analyzer from cell (or keep for AM
modulation)
Apparently most solids and liquids have a small Kerr
effect, with nitrobenzene being the most efficient. One
spin i had on the solid-block-of-glass original was; Using
a stack of clean microscope slides, like the common 1" x 3"
x 1 mm types, and use strips of aluminum foil interleaved
with them on the edges to provide electrical contacts. zap
it with HV pulses or HVDC to create polarization pulses or
dc shifts in the polarization. If this is not insulated
enough from the high voltage required, it may only spark or
'Flash' like a flash tube. Any homemade glass device would
probably require very high voltages to shift the
polarization enough. You would need 90 degrees of shift to
fully modulate the beam, but you might only get a few
degrees.. maybe not enough to be worth all the trouble.

Faraday rotation:
http://www.mmresearch.com/articles/article3/
http://en.wikipedia.org/wiki/Faraday_effect
Have any transparent magnetic materials? Lead-glass (flint)
apparently has enough metal in it to work, even though i
don't think lead is very magnetic.. Iron glass? Ferrite rod
glass? cheap plastic wrap?? who knows. :) 
terbium gallium garnet (TGG) is the best optical material,
but i'm fresh out.. also it's only capable of about 45
degrees of rotation at best. How about a liquid solution of
finely powdered metal? etc.. Time to become alchemists
again.. Again as with the Kerr cell, remove the analyzer or
you will just produce AM.

Pockels effect:
http://en.wikipedia.org/wiki/Pockels_cell

QMR effect:
http://en.wikipedia.org/wiki/QMR_effect

magneto-optic effect:
http://en.wikipedia.org/wiki/Magneto-optic_effect

Zeeman effect:
http://en.wikipedia.org/wiki/Zeeman_effect
may be a form of true FM for laser beams? small shifts
though.

acousto-optic effect:
http://en.wikipedia.org/wiki/Acousto-optic_modulator
- might be modified for pure polarization modulation??

polarization in general:
http://en.wikipedia.org/wiki/Polarization

``````````````````````````````````````````````
>
>I'm not sure polarization is useful in free space,
especially at 
medium and 
>long ranges. I think the atmosphere destroys the
polarization and 
shifts it 
>wildly at random rates.
>
>Perhaps it's more usable at shorter distances.
>
>Maybe someone has tried this recently. There was a
discussion on this 
list 
>many years ago about polarization filters, but I don't
have the full 
>archive here.
>
>Regards,
>
>Art


>
>See Kerr Cell Stuff:
>http://www.elec.gla.ac.uk/groups/opto/Kerr.html
>http://en.wikipedia.org/wiki/Kerr_effect#AC_Kerr_effect
>http://americanhistory.si.edu/collections/surveying/object.cfm?recordnumber=758694
>http://home.earthlink.net/~jimlux/hv/eo.htm
>http://www.star.le.ac.uk/~rw/courses/lect4313_fig57.jpg
>
>I remember seeing "noise" on the meter, and wondering if
that was 
>due to random polarization changes, or simply amplitude
changes due 
>the variable path losses.
>
>So,  I think it is probable that polarization of visible
light beams 
>is maintained at least to a large degree during their
travel through 
>free space.
>

>73, DX, de Pat AA6EG aa6eg at hotmail.com;
>Skype: Sparky599
>Moon or Bust!--Jamesburg Gang Rides Again!




 
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