[Laser] Ramsey Kit range

[Andrew T. Flowers, K0SM]

This reminds me,

It seems there have been several folks who have put together these 
ramsey kits.  In fact, Dave and I have had the pleasure of using these 
kits in a radio contest or two.  Does someone want to find a way to "hot 
rod" these things to get a little better performance out of them?  It 
seems to me that someone could come up with a "front end" that beats the 
heck out of the phototrasistor that comes with it, keping in mind that 
it needs to have a respose up to over 100KHz or so (it's 18KHz PWM, and 
you need to resolve the waveform fairly accurately I think).  It's 
pretty clear to me that most of the light is getting thrown away due to 
the teeny-tiny active area of those devices.  If someone wants to look 
at this I can show you a schematic what the RX looks like.  It might be 
a worth doing since many folks seem to be getting interested in laser 
comm stuff through these kits...and it's also in the a "ham spirit" to 
figure out how to make a good thing better....

Andy
K0SM/2, Rochester NY

David D. Rea wrote:

>On Sun, 2003-11-09 at 15:20, TWOSIG@aol.com wrote:
>
>>On the other hand, the area of beam, or the power in it, may not be descibed 
>>by a mathematical relation to the square of the distance.  For "short 
>>disatances" the intensity of the received beam may be linearly proportional to these 
>>"short" distance numbers.  If that is the case then the power on the detector 
>>would be proportional to the area of the lens, so that the useful range is 
>>factored by the square of the diameter ratio of the lens to the detector.
>>
>>Both ways of thinking about range seem to be oversimplification of the "real" 
>>world, but is either a practical estimating tool?
>>
>
>Hi James-
>
>You're fairly close to on-target with your theory. The received power
>will fall off with the square of the distance, but this only occurs in
>freespace. Keep in mind that you've got atmospheric nasties to deal with
>as well - after you get above 1 Km or so, you'll start noticing the
>effects of humidity and purturbations in the air; i.e. you'll lose power
>due to absorption and dispersion of water mollecules, and you'll see a
>"shimmering" effect as the beam traverses different thermal planes
>between the transmitter and the receiver.
>
>There has been quite a bit of work done in this subject, as I found out
>when writing a paper on laser communication during an undergrad
>independent study. You can get as deep in the math as you want; there's
>no shortage of folks out there who have written PhD theses on this
>stuff...
>
>To add a layer of complexity, remember that no lens is perfect, either.
>If you're dealing with a nice AR multi-coated glass lens, maybe you'll
>throw away 5% of your signal (if you're lucky). But if you're using a
>less expensive (but MUCH larger area) fresnel lens, plan on tossing
>about 50% of the inbound light back toward the receiver. This is where
>you compensate for reflection with sheer lens size...
>
>Anyway - hope this helps a bit...
>
>73 de Dave K2THZ
>
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