Sjouke Burry wrote:
Thanks for the reply. But IR at 500m? Is it feasible?
<snip>
I certainly haven't tried it, but I can suggest some thoughts
to consider about the idea. The sun travels across the sky
and you probably cannot select your direction from xmtr to
rcvr. So there is probably some "worst case" situation with
the sun in the sky that you can't do a lot about. But you
can do some things, I think.
One of them is to figure out ways to limit the bandwidth in
optical wavelength and modulation frequency and limit the
field of view of the receiver, since you have line of sight.
Your source might be broadband (xenon strobe?) but you can
limit the received optical wavelength to some very narrow
band using optical filters. This will exclude a lot of light
from the sun. What you will then need to do is somehow make
the transmitter emit enough against that background to at
least make a PLL function, which can certainly "find lock" at
night time and perhaps hold it during the day. Which is the
next thing -- using a precision modulation frequency on the
transmitter ... as precise as possible ... and use as narrow
as possible acceptance filtering on the receiver. The sun's
energy will be spread out (perhaps there are some low energy
frequencies at whatever wavelength you limit yourself to) and
the narrower you can make this the less energy from the sun
you need to deal with during the day. And of course there
are optical means (long tube, telescope, baffling, etc) that
you can do with the receiver, as well. These can be more
restrictive in one plane than another (vertical vs
horizontal, for example.)
Perhaps a combination of these could get you there?
Then there may be some problem with human dangers from your
emitters. I don't know.
Just thinking out loud about this:
(1) Laser emitters, because of their narrower bandwidth (and
especially gas lasers instead of diode versions), would be
preferred because they can benefit better from thin film
narrow band optical filters as well as provide much better
point-to-point directivity (lower divergence) and in the end
reject more of the background illumination from the sun. (You
don't need the higher modulation bandwidth, necessarily, but
lasers will probably allow you more options there, too.) In
short, I'd probably first look into the selection of
appropriate laser diode emitters (cheaper than gas), but
wavelength may be a factor in making choices.
(2) Weather, rain, fog, smog, some wild animal standing in
the beam for all I know, may affect things aversely. I've no
idea here.. but you need to think about it.
(3) Collimated emitter is wise together with the use of a
very narrow field of view for the receiver. (Keep in mind
reflections that bang around and wise use of baffling and
absorptive coatings on the baffling parts -- at whatever
wavelength you choose to rely upon.)
(4) The sun path, given your two locations, may inevitably
overwhelm whatever you do with narrow receiver FOV, etc. So
you may not be able to avoid disruption at certain times.
(5) Avalanche detectors are more sensitive than PIN, I
gather, so that may help with a transimpedance amp over
distance. But solar radiation may saturate the system, so
this brings back in making an amplifier front end that
attenuates DC signal (and anything 'out of band') as much as
possible. I've read that InGaAs APDs operating at 1.55
micron might help, versus those operating in the usual 780 to
850 nm, which are probably more influenced by solar
illumination when taking into account variations in the
background levels. I don't know, though.
(6) Modulation and limiting acceptance at the receiver is
likely very important to achieve final success and lock out
still more of the solar illumination.
I've also probably failed to think of something important.
All this seems to suggest RF. Too many different facets to
worry about and get right with IR. But you did say "line of
sight" so maybe it is doable??
Oh, well. Interesting to at least consider for a moment.
Jon
