Communicating with lasers, Pages (1) (2) (3) (4)

 

by Robert F. Gonsett, W6VR, as edited by Tom Farley (continued)

 

Observers at the PARC Field Day site are blown away by the brilliance of the half milliwatt red laser beam coming from Boucher Hill 12.7 miles away. "Ball of fire" and "mountaintop aflame" were two of the descriptions given. 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.

 

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.

 

"Hey, we got a jammer!" Someone held up a red LED bicycle safety lamp near the laser beam receiver at the PARC Field Day site, interfering with the desired signal from Palomar Mountain and creating a loud buzz in the receiver. All the hams present - each too familiar with radio jamming - burst out in laughter. Jamming had reached the optical spectrum! (next page -->)

 

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