[NLRS] Laser communications...

[Doug Reed]

At 10:24 AM 12/27/2002 -0800, Jon wrote:
>Anyone doing any laser communications?

I haven't heard anything recently. Gary WB0LJC and Bob W0AUS both built
laser CW (interrupted beam) systems several years ago. I can't remember if
anyone else built a system or not. Bob has an optical power meter too. I
don't remember if they ever tried the systems over a long distance, more
than a few miles. 

I got samples of a Burr-Brown opto-amp to play with but haven't built
anything with them yet. I went as far as buying a lens and some plastic
pipe fittings that would nest to make a 1.5" diameter receiver tube. Just
tonight I was looking at a $1 5" magnifying mirror and thinking that it
could provide quite a bit of gain for a laser receiver. But it has about a
3 foot focal length.... 

Personally, I like the subcarrier transmitter system over the CW type
system. You can throw a lot of gain after the subcarrier detector. On the
base band CW system, you need to filter the heck out of it just to reduce
the 120 Hz products..... But fast switching the laser at the subcarrier
frequency is a little trickier. But building a voltage limited constant
current source for the diode isn't tough and then you can key it by
shorting across the laser diode to kill the beam. Lots of good info on the
web.....

Why do you ask? You want to start a project to build some? I think it would
be great fun to try building a cloud bounce system. Maybe use one of the
sound card digital modes for modulation, then see how well it worked around
town. The last time I paid any attention on the laser remailer at QTH.net,
there were reports of 20 mile bounces. And even more interesting when it
was a clear air bounce.....

I just got some surplus WORM drives to strip for the lasers. The WORM
drives supposedly have about a 15-25mw infrared laser diode. It would be a
little harder to align and collimate to start with but would eliminate
questions from the neighbors and pilots about shining laser beams..... The
main thing you want to do is expand the beam to maybe 4-6" to reduce the
power density to safe limits. I'd have to dig a bit just to find the 5mw
laser pointers I bought to play with this.....

73, Doug Reed, N0NAS.