[Laser] Re: fundmentals

[Pat Barthelow]

>From: Art <KY1K at verizon.net> Said:

>WOW, +/- 10 ppm is almost unbelievable.

Yes, Art, it was noteworthy at the time.  The instrument error was in two 
parts; a fixed error, often about 8mm to 1 cm,  plus/minus a  1 or 2  ppm 
error. This meant that very high accuracy measurement could not be made on 
short lines.  The application was almost always for large scale Geodetic 
measurements, for fault movement studies, etc, so the fixed error became 
proportionally small.

Special tricks could be employed to get the error as low as possible.  
Accurate Baro data, (temp and moisture) were taken (sometimes at both ends 
of the line), at the time of measurement, and we students (in the late 60's 
early 70s) heard stories about the USC&GS flying a surveyed line with 
aircraft and recording thermometers while measurements were being taken.  
The met. data yielded a very accurate air index of refraction, which 
determined the actual speed of light, and ultimate accuracy of long lines.

Now, GPS and total survey stations have profoundly changed surveying, as 
much as the change between Morse Code and Broadband, in telecommunications.

Calculators and computers were profoundly changing then, also.  I used a 
Curta "coffee grinder" calculator, ( 
http://www.oldcalculatormuseum.com/curta2.html ) and 8 place trig table 
books, and logarithm books  to do the work.  THEN, the HP 35 calculator came 
out..  MANNA from HEAVEN for surveyors.  Now, I just read that Curtas, works 
of mechanical art, fetch $1700 now, they were about $180 back then.  HP 
pocket calculators were $400 in 1971  are collector's items but equiv power 
calculators (with trig functions)  now are about $10 at the local Wal-Mart.

73,  Pat, AA6EG  aa6eg at hotmail.com


>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
>
>
>
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