[Laser] Re: fundmentals

[Art]

WOW, +/- 10 ppm is almost unbelievable.

I remember reading something similar once, possibly about the same 
hardware. But, the author had a PC program and a programmable 
calculator program to go with it.

At the time I wasn't sure it involved a Kerr Cell, and didn't know it 
worked by polarization.

It was a long time ago and I never looked into it further. Not sure 
about the nitrobenzine though-sounds a little shaky::>

Art



At 12:11 PM 2/14/2007, you wrote:
>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...


You are describing an AO modulator, a highly useful device if one is 
interested in very high speed digital communications.

Hope you bought one or 2 of them, AO modulators are very expensive to 
buy and virtually un-repairable-so once they go-they are gone forever.

They can be had on ebay, but many are shot and DOA.

Art