In a telephone system, the use of optical carrier waves for transmission has an advantage over the use of electrical wires in environments of very high electromagnetic fields. Moreover, optical fibers for transmitting telephone signals are made from relatively plentiful raw materials as compared with the raw materials required for electrical wires (copper, usually). Accordingly, the use of optical fibers for telephone transmission from sender to receiver is an attractive alternative for a telephone communication system. One of the problems associated with such a system is the conversion by a receiver of the incoming optical signal on the fiber into an acoustic signal which is audible by a human ear.
Almost 100 years ago, Alexander Graham Bell invented a completely optical communication system including apparatus which he named "photophone". The system was fairly simple, utilizing a transmitter for converting human voice signal waves into correspondingly power-modulated optical signals. These optical signals were detected by a (remote) receiver for converting the optical signals into audible acoustic signals which were a faithful representation of the original human voice signals. Several of the patents issued on this system include U.S. Pat. No. 235,199, (Dec. 7, 1880) to A. G. Bell; U.S. Pat. No. 235,496 (Dec. 14, 1880) to A. G. Bell and S. Tainter, and U.S. Pat. No. 241,909, (May 24, 1881) to A. G. Bell and S. Tainter. In addition, a paper on this subject was published by A. G. Bell in Philosophical Magazine, Vol. 11 (Series 5), pp. 510-528 (1881), entitled "Upon the Production of Sound by Radiant Energy." Such an optical communication system relied upon a rather intense source of light, which then could be provided only by sunlight, a relatively unreliable source, and upon transmission of the light through the air, a relatively unreliable transmission path. With the advent in recent years of intense optical laser sources and of optical fibers, the possibility of a reliable optical communication system is thus more realistic. Such a system includes at one end a transmitter feeding an optical fiber. The optical fiber would ordinarily bring the optical signal to a repeater which then feeds an amplified optical signal to another optical fiber, ultimately bringing the optical signal to an opto-acoustic receiver. The receiver then converts the optical signal into an audible acoustic signal for delivery to a receiving human ear.
The opto-acoustic receivers proposed in the prior art involved a hollow chamber wich contained an optical absorbing material such as dark-colored cotton-wool or other fibrous materials, spongy metal, or lampblack. The process of absorption of the light signal produced corresponding acoustic waves. At the opposite end of the chamber from which the light entered was attached a hollow cylindrical acoustic wave transmission tube for bringing the acoustic waves to a human ear.