[Laser] modulated retroreflectors

[Patrick Barthelow]

Tim said:

 To: laser at mailman.qth.net
 Subject: [Laser] modulated retroreflectors 
This talk about corner cubes got me wondering about passive optical repeaters using modulated corner cubes. Has anyone in here experimented with modulating a retro-reflector either homemade or not?


 

 

 

 

Hmmm.... sweet memories of an earlier life...In the 80s and early 90s, I was involved in motorsports, often using a 10 ghz police radar gun to "spy" on the the competition's speeds at a track.  The gun was also applied to speeds around Laguna Seca Raceway, new track layout, to validate the engineers designs of run off areas and energy absorption devices for errant cars.   The new track layout only  had initial estimates of speeds of various classes of cars, for design inputs for the safety gravel traps and catch fences. I used the Radar Gun to gather some real speed data.  (modulated corner cube story follows) 

 

Curious,  I also built some experiental  radar jammers using the Solfan brand of Gunn diode door openers and burgler alarm motion sensors.   They worked VERY fine, causing the radar gun to read anything you wanted, according to a 555 chip chopping the 10 ghz Solfan carrier at the desired doppler frequency offset, which as I recall was easily in the audio range at 10 ghz for "typical" street or track speeds.  So just switch a resistor to cause the Police radar gun to read, say 45mph.  Doesnt matter what your real speed was, the hot signal from the gunn diode overwhelmed the radar receiver and it just read the modulation freq of the jammer.

 

I proposed to build into some large front bumper mounted,  rally lamps, (rubber covered, of course) ,  a modulated retroreflector corner cube.  Two right angle metal "mirrors", the third side being  a thin  membrane, metallized plastic foil that was modulated with an audio carrier.  Peraps even a small speaker with a flat "cone" mounted at a slight off angle from 90 degrees. 

Build a small audio oscillator driving a tiny speaker to modulate the return wave, and Voila!  You can get the desired 45, or 65 mph, or whatever  to read on the radar gun.  Whether the corner cube signal in the rally lamp housings overwelmed the "Real" reflected signal from car, was never tested.

 

In any event, the more modern Police radars are multi band, and sophisticated, having a "Jammer" detector warning light on the  panel, to alert the Police to look closely at the stopped car.  A Gunn diode oscillator/horn, in the car,  at say, +10 dbm produced a WAY stronger signal than was deliverd by scatter from the car body.  So when the police radar gun "saw" an "S-9  +60db" signal, it turned on the Jammer lamp, and the Police looked VERY closely at the car for jammers.

 

Then.... in the olden days of Geodimeters, Surveyors and Geodisists, used modulated light beams to precisely measure distance.  In the AGA Geodimeter Model 6, now a museum piece, but the hot setup in the late 60s for measuring maybe 10 mile distances to maybe 5 ppm accuracy, they used Kerr Cells to modulate the visible light  beam.  The model 6,  had a glass vial with Nitrobenzene inside, which, when HF modulated between two electrodes, circularly  rotated the polarization of the light beam shining through it, producing a sine wave modulated beam out of the last (crossed) polaroid filter.  The outgong  and return (from glass  retroprisms)  wave phases were measured, by using an adjustable  electronic phasing line, the Geodimeter was "virtually" moved forward or backward, until exact phase cancellation occurred.  Doing this for 3 or 4 precisely known modulating freqencies, allowed precison distance to be determined, accurately,  if you knew the Light velocity, C, based on simultaeous, accurate meterological measurements.  The several modulation frequencies were in the HF range  (something between about 12 mhz and 30 mhz, as I recall) 

 

Earlier Geodimeters, models 3 and 4 actually used precision, adjustable, mechanical mirror arrays  to lengthen or shorten the optical path to achieve phase cancellation.

 

For Geodimeters, and Retroprism based EDM, See:

http://americanhistory.si.edu/collections/surveying/maker.cfm?makerid=1

http://celebrating200years.noaa.gov/distance_tools/geodimeter_6.html

http://celebrating200years.noaa.gov/distance_tools/0411.html

http://www.scienceandsociety.co.uk/results.asp?W=4&F=0001&Step=1

http://www.scg.ulaval.ca/gps-rs/fr/Histo/expo/artefacts.htm#ExpoGMT5

 

 

Best Regards,    73,   
Pat Barthelow  apolloeme at live.com
"Echoes of Apollo"  
A celebration of the Apollo 11 Moon Landing 40th Anniversary,  
http://www.echoesofapollo.com
http://www.parkes.atnf.csiro.au/news_events/apollo11/
http://www.jamesburgdish.org
 
> Date: Wed, 18 Mar 2009 06:54:29 -0700
> From: Tim
 
toasty256 at yahoo.com
> To: laser at mailman.qth.net
> Subject: [Laser] modulated retroreflectors
> 
> 
> This talk about corner cubes got me wondering about passive optical repeaters using modulated corner cubes. Has anyone in here experimented with modulating a retro-reflector either homemade or not?
> 
> While googling the subject (mostly research papers) i saw that some are using an AOM technique to get AM or phase shifts or polarization effects on the beam. While others use a shutter to mechanically do it at lower frequencies. Or something as simple as the old Bell photophone type flexible mirror to modulate a beam with your voice.
> 
> Seeing how touchy the alignment is when making a good retroreflector from scratch, it's easy to see how a very small misalignment of one of the 3 mirrors can effect the whole reflection in a major way.
(snip....)



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