[Laser] Re: coherence and scintillation

[Clint Turner]

Hello Chuck,

Chuck Hast Wrote:
 > 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.

I have not tried the multiple Laser approach, nor really considered it - 
mainly because it really didn't occur to me to attempt it.  Based on the 
works of others (various NASA documents, the works of other researchers, 
and the results and opinions by others on this and other groups) plus 
practical mechanical considerations, I have concentrated largely on 
emitters using large apertures rather than using multiple 
smaller-aperture emitters.

The use of multiple Lasers (or any multiplicity of emitters) conjures, 
in my mind, the benefits of spatial diversity on RF paths in addition to 
those sort of aperture gains experienced in both the radio and optical 
world:  There's even that oft-cited - yet difficult to quantify - "more 
then 3dB" of improvement that is reported when when the performance of 
two yagis is compared to just one.

Being that the transmitting aperture for the main comparison test was 
that of an 8" reflector telescope, I believe that this is close to the 
maximum size that one could reasonably accommodate in typical amateur 
experiments that are both practical and affordable when dealing with 
Lasers and, in many ways, it effectively simulates the use of several 
Lasers by virtue of its large aperture.  A larger telescope becomes not 
only difficult to transport (the logistics of my 8" Celestron 
more-or-less preclude it being backpacked to the top of a rugged 
mountain) but there is an increasing reluctance to attempt such a feat 
that is in proportion to the cost of the optics in the first place.

Nevertheless, I do find intriguing the idea of several small-aperture 
Lasers being used in tandem:  I couldn't divine from your email if these 
were more-or-less Laser-pointer type (e.g. narrow beam diameter) 
emitters, or a set of ganged collimated emitters of some unspecified 
size.  An obvious advantage of multiple smaller emitters is that 
different spacings could be employed to synthesize a very large 
aperture:  I suppose that it could be argued that the use of an 8" 
reflector is simply not large enough to span multiple "cells" of the air 
that are largely responsible for the scintillation to significantly 
reduce that particular problem, and being able to space multiple 
emitters over a larger distance would be of benefit.

Another thing also occurs to me:  Being that semiconductor Lasers tend 
to vary somewhat in operating wavelength from unit-to-unit, one 
advantage that might occur from having multiple Lasers ganged together 
in a mechanical array is that some frequency/wavelength diversity also 
results - perhaps serendipitously or by design - and this couldn't help 
but minimize potential problems associated with narrow-spectrum light 
sources as it is pretty much impossible for wavefront phase cancellation 
- a large contributor to scintillation on a coherent beam - to occur 
amongst any of the free-running Lasers.

In this latter case, it seems perfectly reasonable that one could, in 
fact, reduce scintillation through the use of disparate (albeit 
closely-spaced) wavelengths by using multiple Lasers - along with the 
resulting spatial diversity at the transmit end.  It also seems to me 
that many of these problems can be handily avoided by using noncoherent 
(but fairly narrowband) light sources to begin with - something that 
allows practical use of inexpensive, less-accurate transmit and receive 
optics, whether they be "conventional" or Fresnel lenses.

Other experiments that we've done recently (over the same 15 mile path) 
do indicate that spatial diversity through the use of a pair of 
receivers using large-aperture lenses (>120 square inches) do reduce the 
severity of scintillation, either from coherent or noncoherent light 
sources - but this was hardly an unexpected result.  It is hoped that, 
in the next few weeks, further testing will be done to quantify and 
further document these effects.

73,

Clint
KA7OEI