[Laser] NLOS Optical Comms

[Kerry Banke]

Andy -  I'm right in the middle of conducting some crude tests of that 
specific setup. The approach I'm using may be wrong for this type of 
operation as I'm beaming a 4" expanded 1 Watt 910 nm laser vertically and 
receiving it with a 4" glass lens with 8" focal length. The laser is being 
modulated at 230 Hz. I'm using the Spectran audio analyzer software on a 
laptop to detect the signal.  I don't have a quantitative measurement of my 
receiver sensitivity but it will easily hear  the scintillation of most any 
star
visible to the unaided eye. This system is only functional at night 
currently. I recently purchased narrow band optical filters but somehow 
there is an obvious mismatch with the laser wavelength which I need to sort 
out.  With this setup I have easily detected the laser beam going  over  a 
non-line-of-sight  location three miles away with a signal level suitable 
for low data rate communications ( which is what I am hoping to do in the 
next few evenings). This is with the laser pointed directly towards in 
azimuth  and above the horizon of   receiving location by  about 2 degrees 
in elevation.  The receiver was also pointed slightly above the horizon in 
the direction of the laser.

With this same setup I have difficulty detecting the vertically pointed 
laser at 1/4 mile in clear sky.   I know others on this list have done 
wider beam vertical transmissions with what may appear to be better 
results.  All I can provide is my current experience with narrow beams at 
near IR. In your own words I say it's a non-starter considering my results 
at low frequency, night time and clear skies.

  -Kerry Banke N6IZW -

At 08:26 AM 11/15/2004, you wrote:
>Dear All,
>
>I was hoping i could pick your collective brains regarding Laser Comms. 
>I'm a post-doc student at the University of Durham in the UK and I have 
>been asked to look into the possibility of non-line-of-sight free-space 
>optical comms. My supervisor hopes that a signal can be detected from a 
>laser pointing straight upward by receivers located within a mile or so 
>radius of the transmitter- so far so good, but he also wants the system to 
>operate in daylight, clouds or no clouds, fog or no fog and at high 
>frequency! My initial reaction is that this is a non-starter- what do you 
>think?!
>
>Regards,
>
>Andy Maiden.
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