Of all the electromagnetic spectrum the part most studied in the course of human history is light. The main reason has been the early availability of powerful light sources ('Let there be light', Day 1) supplemented five days later by a marvellously effective broadband light detector. Ever since, the subject has been a playground of philosophers and physicists, of theoreticians and of men of practical bent. The amount of knowledge accumulated by the 1950s was formidable. By that time there were light detectors superior to the eye, man-made light sources which were powerful enough to compete with the Sun, and there were means to modulate the intensity of light.
So one could simply use the principles of the photophone: take a light source, modulate it, send it some distance and demodulate it. The new thing would be to shield the beam, e.g. put it in a pipe so that nobody could interfere with it, and make the modulation digital, e.g. turn the light source on and off by using a fast shutter. Such a simple arrangement would be an efficient way to communicate, provided the light source could be modulated fast enough. If light pulses of 1 nanosecond (1 billionth of a second) duration can be produced, that would make possible 15000 high-quality telephone conversations (remember, one channel needs 64000bits/s) by using some guiding structure, e.g. a shiny metal pipe in which light could bounce off the walls. Research on roughly these lines started in 1958 under the direction of Alec Reeves.
Meanwhile some physicists started to think about optical sources in a novel way.
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