[Laser] IR experiments, thanks for info

[Chuck Hast]

On Tue, May 10, 2011 at 05:24, I <stuart.wisher at talktalk.net> wrote:
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> Thanks to all for the information regarding atmospheric attenuation around 850 t0 950nm. I have had time now to study it all and follow the links several of you kindly provided. It is still not an easy thing to determine what may be best practically but I left out some important information in my request that has a bearing on the matter. The IR LEDs are not narrowband sources but have a bandwidth of about 30nm at the half power point. This means that narrow absorption bands may not play such an important part since they would not notch out much of the radiation from the LED.
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> I started using Osram 850nm LEDs SFH4230 and recently changed to SFH4231 which are at 940nm and more powerful. I note these are no longer manufactured but Osram have introduced a new IR LED SFH4235 which is almost double power again and radiates around 860nm. I will have to determine whether it makes a good photodiode once I have taken out the protection diode (which is, incidentally, a visible red LED).
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> Due to the large signal margin above noise on the recent 6.3km field test we will continue to develop the gear using the 940nm LEDs since we have them, but it will not be long before I am tempted to try the new 860nm LEDs and perhaps run a comparison test when the signal gets weaker. Our next attempt will be a big jump to 30km, and we have convenient 60km and 90km paths to try, that we used in developing the Transverter and transceiver gear. The big problem here is that we have nothing to see at the far end so we will continue the use of the powerful red beacon for alignment. Using the transceiver should ease matters as once receive is lined up, then transmit is automatically lined up also.
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> I and the team are currently building up completely separate Transceive heads for IR so as not to disturb the visible red transceiver that we have used to set the current UK record, we want to save that gear for later in the year when the dark evenings return. Meanwhile, we plan some fun with IR communication in the summer evenings. These heads are plug for plug compatible with the visible red ones and just go straight into the transverter, we could even compare 625nm red with IR in the future.
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This last piece is of interest to me, to have a 625-650 nm link a 860nm link
and a 940nm link side by side signalling over a long period of time and doing
a actual comparison would be most certainlly interesting, particularly over one
of the longer paths.

I recall a tv station in a location which during the dry season is dry
but during
the rainy season it is very wet, they were sold on the idea of a K band studio
to transmitter link of 21Km end to end going from about 1500m ASL to about
3500m ASL. The video was great until the heavy thunderstorms kicked in
then they could not get enough K goo to even saturate the RX end during
a bad rain storm.  The old C band link started looking real good at that point.

If they had of tested they would have found out that K band was not a so-
lution for their location. What no one told them was a good part of the water
in such storms remains aloft, so on a normal point to point link it probably
would have worked but since one end was well UP inside or even above the
cloud layer, all bets were off.

We have all seen the graphs, you guys have the chance to actually put
some interesting numbers to some of those graphs.

-- 

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."
---
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