[Laser] Laser comm stuff

[Andrew T. Flowers, K0SM]

I haven't seen much activity on the list as of late, but I figured I 
would let folks know about some stuff I have been working on.  First of 
all, I've completed a couple of baseband receivers and transmitters, 
which I'm hoping to get on the air for our upcoming amateur radio 
contests in August and September.  I've put the schematics on my site, 
except for the LM386 audio amplifier.  You can get that straight for the 
datasheet--just do a google search.  There are pictures of the 
containers and how it mounts in a 4" tube with a 4" lens.  There's also 
a tunafish recipe...you'll understand if you see the site.  I'll be 
updating the site as I get stuff completed and get a chance to take a 
pictures of it.

One thing that I found interesting was the AF frequency spectrum of city 
lights.  I made a few recordings of Rochester's lights scattering off 
of  low clouds and calculated the power spectrum using an FFT.  It shows 
that

1) city lights are roughly square-wave modulated at 120 Hz....at least 
here in NA :-)  This comes as no surprise--remember, the light will 
conduct on both the positive and negative sides of the cycle.  

2)Also, I'm assuming the large majority of this light is coming from 
excited gases--He or Na lights. They are roughly square waves with poor 
HF response--I'm extrapolating this from the FFT, as I don't have a good 
way to measure a single light's waveform without clipping in the 
receiver...guess I need a gain control....you can see the first four 
harmonics (120, 240, 360, 480 Hz) are quite strong and audible, but they 
drop off rapidly after that.  I would guess that this is due to slight 
rise and fall times in the streetlights.  

I was using a Clairex CLD-142 Si photodiode without any filtering.  A 
major source of the signal was likely the 850nm peak of excited sodium, 
since that falls right in the heart of the detectors sensitivity.

Anyway, the site is http://mail.rochester.edu/~af006m/index.html and 
follow the "radio" link...475 THz is still RF to me :-)  Enjoy!

Andy, K0SM/2