[Laser] coherence and scintillation

[Terry Morris W5TDM]

Clint, Gordon & Ron

Excellent experiment, and very well documented. I have always had a gut 
feeling that the coherence of the Laser was a factor in the scintillation 
problem, but had no data to make a proper determination. I would have liked 
your results better if the optical power of the LED and Laser were a little 
closer. 3 watts for the Lexon LED and ~5mW for the Laser may have clouded 
your results some, but in my opinion maybe only reduced the SNR some. This 
may also have been what reduced the color saturation of the video.

Again, over all a great experiment and documentaion. Sure makes it look like 
the Lexon III is a real  option to a Laser. I put your report page in my 
favorites for the future. I can see a Lexon based transmitter in next 
project.

73
Terry W


>From: "C. Turner" <turner at ussc.com>
>Reply-To: Free Space LASER Communications <laser at mailman.qth.net>
>To: laser at mailman.qth.net
>Subject: [Laser] coherence and scintillation
>Date: Wed, 29 Aug 2007 16:55:15 -0600
>
>Over the past several months, I've done some semi-scientific comparisons of 
>coherent light versus noncoherent light, using the same apertures for both 
>receive and transmit over a 15 mile path.
>
>Some details may be seen here:
>
>http://ka7oei.com/Coherent_versus_noncoherent_test.html
>
>These experimental comparisons were done using a standard, cheap red Laser 
>pointer module (from a $3 Laser level) and a Radio Shack high-brightness 
>red LED, both of which being Pulse-width modulated.  The intent was to 
>determine, empirically, what the difference would be, given that everything 
>was the same - except the spectral width of the light source.
>
>As can be seen from the results, Lasers have certain drawbacks when it 
>comes to the experiencing of scintillatory effects.  It is also worth 
>mentioning that the beamwidth of the laser was *not* really an issue in 
>this case as "beam wander" was a probably a negligible factor in this 
>experiment:  This was seemingly verified by "walking the beam" on the far 
>end to determine the spot size and noting that its cutoff was well beyond 
>the receive aperture, as well as noting that at the beam edges, the rate of 
>beam dropoff was quite dramatic (e.g. two steps further, there's no sign of 
>the beam.)
>
>73,
>
>Clint
>KA7OEI
>
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