Photophone—an optical telephone receiver

The photophone (name originally given by A.G. Bell) is an optical telephone receiver based on the optoacoustic effect. J. Acoust. Soc. Am., Vol. 58, Suppl. No. 1, Fall 1975.S53 90th Meeting: Theory will be given for a modern version intended to operate on a few mW of power modulated light from an optical fiber. The receiver consists of an absorption cell, a response‐equalizing device such as an air column or diaphragm, a tapered acoustic tube acting as a transformer, and an earpiece similar to a conventional telephone earpiece including a response equalizing device. In an alternative version, the cell is directly coupled to the earpiece without a transformer. The photophone gives a flat (3 dB) response to power modulated light over the telephone voice band 300–3300 Hz. The optoacoustic effect is treated theoretically on the basis of a pseudogas model for solid absorbers in the form of a fine mesh. We find that the sensitivity (the optical power modulation required to produce a sound pressure level SPL =81 dB in a closed volume of 6 cm³, representing the earpiece pressed against the ear) is 2.8, 0.9 and 51 mW for the air‐filled, xenon‐filled, and direct‐coupled photophones, respectively.