[Laser] Re: fundmentals

[Tim Toast]

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.



 
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