[Laser] NLOS Optical Comms

[Andrew T. Flowers, K0SM]

Andy,
Remember that the atmosphere gets thiner faster when you point the laser 
straight up--unless you have clouds or a nice marine layer to provide a 
scattering volume you may not see anything from 1/4mi away--keep in mind 
that the signal is probably there, but you may need a very long 
integration window to detect it at any safe or unsafe power level.  The 
experiments I have been doing here have had data transmission rates 
equivilent to a few bytes/min.  That may not be what you had in mind.  
Now, you might consider an omnidirectional light or some kind--It could 
be a laser diode rigged in some way to radiate 360 degrees toward the 
horizon.  The receiver would just need to have a big enough lens to 
gather the light and probably filtered to prevent saturation from 
sunlight.  In terms of practicallity, RF works much better for this kind 
of stuff!

I have a friend of mine who did his undergrad up at Durham.  In fact, 
I'm using a few slides of the cathedral for an Art History presentation 
in a few weeks.

Andy K0SM/2


Kerry Banke wrote:

>
>
> 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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