Free space laser communication

Fenzir said:

Just reading that made me bust out my 1mw red laser. Still love it.

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While the beam shimmered with atmospheric turbulence, voice communications were clear and steady with only a gentle hint of flutter, thanks to the use of FM modulation.

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Here's how the setup works. At the transmitting end, the laser is powered by a 35 kHz oscillator which simply turns the beam on and off 35,000 time/second. Otherwise, this is a "stock" pointer laser in every respect, and no external lenses are used. A microphone FM modulates the 35 kHz oscillator with 5 kHz of deviation. At the receiving end, the incoming red laser light hits a plastic Fresnel lens (the center of a flat-sheet magnifier for enlarging small print, available in stationary stores) and the sheet focuses the light onto a photo detector. Out of the photo detector comes the 35 kHz FM modulated signal. That signal is mixed with a 145.000 MHz local oscillator, producing a signal on 145.035 MHz. That signal is run through coax to a ham radio HT that is tuned to 145.035 MHz, the HT serving as the FM demodulator/receiver.

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