[Laser] Voice communication using 940nm IR LED in full daylight
Chuck Hast
wchast at gmail.com
Sun May 1 09:19:12 EDT 2011
Stuart,
Back a few years ago I was working on building low cost FSO links. What
was available was hideously expensive, and I was trying to build something
to offer that would have a more reasonable cost/link. I never got to finish it
because I lost my job and had to dedicate my free time to generating income
just to keep going.
Anyhow, the first thing I did was go out and look for information on the
atmospheric optical window. There was quite a lot but most of the graphs
were very broad including both visible (which is quite good really) and IR.
Here is what I settled on, the band of interest was from about 640nm to
about 740nm. At that point there is a very high absorption area, jumping
to about 760 up to about 790, you will find another nice window, then you
will have to jump to about 830 for the next one which goes up to about
880. (Reference: Free Space Optics: Enabling Optical Connectivity in
Today's Networks, Willebrand and Ghuman) The reference book is a good
one.
Most of the absorption is from water vapour, and they are little peaks, but
if you try to slide in between them your source may not be stable enough
to avoid drifting into one of the absorption lines, so best to go above or
below that narrow set of lines in 700nm. The upper end of the above is
contolled by the fact that as the signal source moves from red to ambar
you start to get a lot of dispersion, just compare a green laser to a red
one at night, the green one looks like a "light sword". The red one is just
visible and you need to be looking down the beam axis just off the beam
to really see it.
Here I also found a book review on Google Books so happens they have
the chapter that has a pretty good graph of visible and short IR, the book
is:
Optical wireless communications: IR for wireless connectivity
By Roberto Ramirez-Iniguez, Sevia M. Idrus, Ziran Sun:
http://books.google.com/books?id=pHPuEGZcv-YC&pg=PA33&lpg=PA33&dq=atmospheric+transmission+window&source=bl&ots=tcEe4gxmjj&sig=gxHOMQRhMkSBJFmYmjGoFbvvpG8&hl=en&ei=GVG9Tcq0BsuB0QHyn8jeBQ&sa=X&oi=book_result&ct=result&resnum=3&ved=0CCUQ6AEwAjgU#v=onepage&q=atmospheric%20transmission%20window&f=false
If I recall correctly, along with using red lasers and high power red LED's the
write lasers in CD writers fall right in the proper place for one of those 700nm
windows (need to check it because they are not all at the same wave length)
And of course the DVD write lasers are also in the correct band (650nm). The
issue with the red visible devices is that they are visible and if you
are setting
up a link it is best that it not be visible to keep the prying peoples
attention off
of it.
My main interest was in using FSO links to recover from downed microwave
and mm wave lost links due to wind damage from a tropical storm. Generally
right after a hurricane even you will have nice clear air as the storm tends to
pull the water into it and leave a path of nice clean dry air behind
it, and that
is exactly when you need those deploy able links to work while the other stuff
is being recovered.
Here is a PDF I found when I did a google of "free space optical atmosphere
window":
lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol7003/70031M.pdf
This one has some VERY good graphs in it.
Hope that this is of some help. There is a lot of info on visible and near IR
absorption out there but you have to dig through at lot of very broad graphs,
of course you can also take a hint from the commercial FSO people and
go to 1500nm. But you are getting into optics that are not so easy to find
and I think that the signal source and reciver are also quite a bit more
expensive.
--
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."
---
Home web site:
www.wchast.com
ZoneMinder Demo and test site
www.wchast.com/zm
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