The New Photonic Communications

The New Photonic Communications

A directed beam of light from a 3 Watt red Luxeon LED at a distance of almost fifteen miles, easily stands out against the lights of Salt Lake City, Utah.

Since the beginning of wireless, Amateur Radio operators have shown an insatiable curiosity to explore and populate the high frontiers of the electromagnetic spectrum. Many years ago, hams were relegated to the “shortwaves,” thought to be worthless, and they discovered that those frequencies allowed for worldwide contacts. A few years later, hams colonized the VHF and UHF frequencies and found them to be ideal for reliable local communication.

This technological wanderlust of ours may be happening again, perhaps encouraged by the Federal Communications Commission in the US, which, along with many other members of the International Telecommunications Union, has opened the frequencies above 300GHz to licensed Amateur Radio use. Small groups of hams, some in Australia, New Zealand, Tasmania and France, as well as here in the US, are experimenting with lightwaves as a communications medium.

There is some historical precedent for this. The first “wireless” electronic communication of the human voice was done in 1880, not by radio, but over Alexander Graham Bell’s “Photophone.” This device used a mirror vibrating in accord with the sender’s speech to modulate a beam of sunlight, which was detected at the other end by a selenium cell attached to a battery and an earphone.

Obviously, Bell’s invention wouldn’t help much if someone had to make a call at night, but the recent blossoming of Light Emitting Diode (LED) technology is enabling hams today to shoot a beam of concentrated light over many miles of “line of sight” territory. Because the thin beam of laser light is degraded by scintillation (“twinkling”) when propagated through a lot of atmosphere, LEDs are actually a superior transmitting tool.

Sensitive photodiode circuits behind inexpensive Fresnel lens concentrators or off-theshelf telescopes serve to receive the light and retrieve the message. How sensitive can they be? Yves Garnier, F1AVY, detected harmonics of power line frequencies in terrestrial street lighting reflected from the surface of the moon during a recent lunar eclipse. Rye Gewalt, N9LCJ, reported that output “spikes” from a photodiode receiver board he was working on were the result of the board seeing lightning strikes from a thunderstorm many miles away. Neither of these events was producing enough light to be visible to the naked eye.