N6IZW/W6VR Laser QSO
The first step in establishing a laser QSO is determining that a clear optical path exists. Here, Stan (W9FQN) at the Palomar Amateur Radio Club ("PARC") Field Day site in Valley Center, CA, sends a mirror flash to Bob (W6VR) on Boucher Hill at Palomar Mountain. The flash itself is the white dot in the center of the photo. The tiny circle around the dot is a camera artifact. The white stripe to the right of the dot is the reservoir at the Field Day Site.
****Flashing this 12.7 mile optical path confirmed that there were no trees, bushes or buildings in the way - constant problems in optical work. This path would be used again in the evening by N6IZW and W6VR for a laser light QSO, setting a new distance record for laser-voice communications in San Diego County.
Pictured below is one of two full-duplex voice-modulated laser setups constructed by Kerry, N6IZW. Each uses an ordinary 0.5 milliwatt red 'pointer' laser ($2.80) as the transmitter. Because a laser beam is very narrow, the apparatus is aimed with the aid of a rifle telescope (the black cylinder on top). Precise positioning is accomplished with two micrometers mounted on the metal bar below and to the left of the telescope.
****The laser itself is mounted to the white colored block just under the right end of the rifle scope. The laser appears as a small black object on the right side of the white block, and is about the diameter of a fountain pen. ****The distant white colored PVC pipe contains simple voice modulating electronics and a low powered LED good for short range communications and testing (won't communicate many miles, only the laser will do that). The PVC pipe closest to the camera contains the laser beam receiving equipment. Having both sending and receiving equipment at each location allows full duplex communications (being able to talk in both directions at the same time, like a telephone).
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!
This photograph was taken at the PARC Field Day site using the Zero Lux (night vision) feature of a home video camera at 8:48 p.m. Although the sky was dark to the eye, the camera used what little available light was present and produced its night-vision image in white, black and green. ****Note the two bright white circles in the lower right portion of the photo. The smaller circle (red in reality) is the scattered light from the 0.5 milliwatt laser beam used to communicate with Palomar Mountain. The large white blotch is scattered red light from the low powered LED inside the far PVC tube. The LED's beam was too broad and too weak to communicate with Palomar (we blocked the laser beam momentarily just to make sure).
****As an added attraction, a brilliant meteor trail graced the sky over Palomar Mountain seconds before this photo was taken. The two hams on the left gestured to retrace its path. A spectacular night was had by all. ****Photos and text by Bob Gonsett, W6VR