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Electronic Design

Bell Optical Maser Transmits Light 25 Miles

An optical maser has been successfully used in long-distance communication experiments by the Bell Telephone Laboratories with pulses of coherent light transmitted over a 25-mile line-of-sight path between laboratories at Holmdel and Murray Hill, N.J. Red light generated by a "brute force" ruby maser at Holmdel was visible to the naked eye at Murray Hill and was detected electrically by photomultiplier tubes. The beam angle of 0.1 deg illuminated a circle of only 200-ft diameter at Murray Hill. According to BTL scientists, a searchlight of equal directivity would require a 500-ft mirror.

The maser is similar to the type recently disclosed by the Hughes Research Laboratories. Chromium ions in a ruby cylinder are raised to a higher energy level by intense optical pumping (the so-called "brute force" approach). Their return to the base energy level generates light of a lower frequency than that of the pumping light. Although this process resembles the natural fluorescent actions in ruby, maser output is ideally highly monochromatic, coherent, and directional. These properties would make the optical maser extremely attractive as a communications device and a number of companies are actively pursuing maser development with an eye on optical radar and space communication uses.

At the time of the Hughes disclosure, many scientists doubted that a ruby maser could achieve the coherence and monochromaticity theoretically possible. In the Bell and Hughes masers, a ruby cylinder, silver-plated at both ends, acts as a Fabry-Perot interferometer to reinforce light output along the cylinder axis. (Electronic Design, Oct. 12, 1960, p. 4)

Here's one of the first news articles on lasers, although Bell Labs was still using "optical maser," rather than "laser," to describe the device.

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