[Laser] Re: fundmentals

[Pat Barthelow]

Re polarization rotation... It can be done electronically...

"Back in the Olden Days" ,  I was in school, learning how to use a 
Geodimeter, Model 6.
( 
http://www.gmat.unsw.edu.au/currentstudents/ug/projects/f_pall/html/e12.html 
)
to precision measure long distances in Surveying.   My  ham radio interests 
at the time,  got me nominated by fellow students, to learn the electronic 
innards of the Model 6.   What I found was an RF transmitter, switchable, to 
  4 different, crystal oven stabilized frequencies, (between 10 and 30 mhz)  
whose output was applied to the electrodes of a Kerr Cell modulator.

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

That Model 6, used an ordinary, incandescent light bulb, and an internal 
optical path that went through two polarized filters oriented at 90 
degrees--NO light transmission, unless you rotate, using the Kerr cell,  the 
polarization 90 degrees in the transit between the two filters.  The RF 
applied to the Kerr Cell, a  glass vial, with parallel windows,  containing 
liquid Nitrobenzene,
(explosive, under the right conditions) rotated the incoming polarization, 
producing a sine wave amplitude out the second polarizing filter.

That  sine wave amplitude modulated wave visible light beam went to and from 
a reflector, up to 10 miles away, at night. (Later models use a laser, a 
relatively simple upgrade)  The return beam amplitude information,  was 
mixed with the outgoing beam, and adjustable electronic delays were 
instituted, to create a phase cancellation between the incoming and outgoing 
sine waves.  Phase cancellation was detected, and carefully measured,  using 
a highly damped, center reading  milliameter.

It was very tedious to do by the operator, but after about 15 readings of 
the delay line, (half hour) and another half hour of paper calculations, you 
came up with distance, accurate to about +-10ppm.

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.

Back to the present:
I recently took apart an early model visible laser bar code scanner device.  
The laser beam passed through a small metal encased modulator (the metal can 
was about 1/2" x 1/2" x 3 inches with power and a modulation signal 
terminal,  Two holes in the can allowed passage of a laser beam through the 
modulating crystal, inside.    It was a  precision  crystalline
device, some sort of digital shutter, or perhaps linear modulator.  At the 
local Silicon Valley Flea Markets, these laser scanners were plentiful, and 
cheap; might be useful to ham/laser free space communications...

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

>From: Art <KY1K at verizon.net>
>Reply-To: Free Space LASER Communications <laser at mailman.qth.net>
>To: Free Space LASER Communications <laser at mailman.qth.net>
>Subject: Re: [Laser] Re: fundmentals
>Date: Wed, 14 Feb 2007 09:20:19 -0500
>
>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
>At 02:37 AM 2/14/2007, you wrote:
>>hi James and Glenn
>>i just wanted to point out there IS another modulation
>>other than AM or OOK for light beam carriers. Polarization.
>>You can shift or rotate the polarization of any light beam,
>>be it coherent or not. It would use the principle of the
>>"polarizer-analyzer" setup:
>>
>>http://plc.cwru.edu/tutorial/enhanced/files/lc/light/light.htm
>>
>>One way you could do it would be:
>>At the transmitter you shine the light beam through a
>>polarizing filter. The filter is mounted so that it can be
>>mechanically rotated to change the polarization of the beam
>>through 360 degrees - actually spun via a motor at
>>different rates to encode the message say.. Similar to the
>>motor driven chopper modulations mentioned in here before.
>>
>>Then at the receiver, in front of the photodetector, you
>>have a second polarizing filter that is stationary (the
>>analyzer) to 'decode' the beam. In the end, the beam is
>>converted into AM by the analyzer...(a nice smooth
>>sinewave)
>>at the frequency of the spinning filter of the transmitter.
>>
>>Maybe polarization modulation has an advantage over AM in
>>some cases?? Good quality polarizers can nearly 100% the
>>beam, although some light is lost and absorbed by the
>>filter. Some filters are more efficient than others. While
>>searching through my list archive I noticed James proposed
>>some experiments with polarized light to see what effects
>>it had on a channel a couple years ago. (see march 2005)
>>
>>There are also electronic ways to shift or rotate the
>>polarization of a beam that could work at much faster
>>speeds.