[Laser] coherence and scintillation

[Chuck Hast]

On 8/29/07, C. Turner <turner at ussc.com> wrote:
> 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.)
>
Clint,
Have you tried multiple laser transmitters, this appears to be one of the
ways that this issue is handled. It appears that 4 of them spaced around
the receiver lens opening is one way to handle it. A separation of about
8 inches between the lasers appears to be a good starting point. I am
building a test data link using 4 of them one above the telescope opening
and one below and one on each side.

-- 
Chuck Hast  -- KP4DJT --
To paraphrase my flight instructor;
"the only dumb question is the one you DID NOT ask resulting in my going
out and having to identify your bits and pieces in the midst of torn
and twisted metal."