The two men might easily have been mistaken for spies. In the damp night they parked their car carefully—down the road from one of the buildings of Lincoln Laboratory, the Lexington, Mass., research center that M.I.T. operates for the U.S. Government. Through the car window they sighted in on the lab with a snooperscope, a World War II device for spotting objects in the dark. And they saw just what they were looking for. "We're in!" exclaimed one of the men with ill-concealed excitement.
The two observers were stealing no secrets; they were checking on their own work. The bright gleam of infra-red light that they had seen through the snooperscope bore out their suspicion that they had stumbled on a new and revolutionary kind of communication device.
The big breakthrough had come as Physicist Robert J. Keyes checked on the properties of a gallium arsenide diode developed by Lincoln Lab Engineer Theodore M. Quist. A less-than-gnat-sized electronic device that generates pure infra-red light when a small direct current is passed through it, the diode turned out to have an extraordinary property: the intensity of the normally invisible infra-red beam could be easily controlled by varying the strength of the current that generates it. Keyes speculated that if his little light beam was visible at any distance, it could be modulated to carry the human voice, or even the more complex frequencies of a television program.
From the Mountain. After that first, promising nighttime test, Keyes and his associates decided to try their diode light at longer range. They set up shop on the top of Mount Wachusett, a modest peak (alt. 2,006 ft.) 34 miles from Lincoln Lab. The first long-distance experiments were not successful, mostly because of hastily assembled equipment. After many months of work, an improved transmitter pointed at Lincoln Laboratory from Mount Wachusett. The tiny gallium arsenide diode, only 0.01 in. in diameter, was placed precisely at the focus of a 5-in. reflecting telescope that concentrated its infra-red light into a tight bundle. On the roof of the lab, the researchers set up their receiver—the reflector of a 5-ft. war-surplus searchlight with a sensitive photocell at its focal point.
One night last winter, the entire experiment was ready. The men on Wachusett turned on the diode and reported their action to Lincoln Lab by telephone. Standing on the lab roof, Physicist M. John Hudson pointed a snooperscope toward the mountain and immediately picked out the bright spot of light that marked the glowing diode. By telephone he told the men on the mountain to begin talking into a microphone and modulating the infrared beam. The response came clearly across the cold night air and was picked up by the lab-top receiver. "I'm starting now." Those words had covered 34 miles, passing over an infra-red beam that carried only .005 watt of energy. It would take 1,500 such diode beams to equal the power used by a single flashlight bulb.
Potential Unlimited. For hours the scientists on Mount Wachusett declaimed joyfully over the remarkable beam. Next they turned on a TV receiver, tuned in a Boston channel and retransmitted the picture to the laboratory by infra-red rays. The results were more than satisfactory.