Nonlinear Analysis of a Photovoltaic Optical Telephone Receiver

There is now strong indication 1 ' 2 that optical transmission using lightguides3 and optical cables is technologically approaching a readiness for use in telecommunications. While we are not here suggesting that any extensive application of optical telephones is foreseeable, we have nevertheless found the prospect of limited special applications sufficiently interesting to undertake a study of optical telephone receivers from a device point of view. The receiver is only one of a number of devices, some of which perhaps have not even been invented yet, that would 1569 be required in an optical telephone. In addition, further devices would be required in the electrical-optical interface between the lightguides and the metallic network. The receiver, however, determines one key property of the system, the optical power required at the interface to transmit speech to the human ear with an acceptable volume and quality. We presuppose that an optical telephone receiver is required to convert analog-modulated light power to sound pressure at the ear with no other power available. There are two mechanisms that might be employed to do this using analog modulation: the optoacoustic effect in which sound is directly produced when power-modulated light is absorbed, and the photovoltaic effect in which an intermediate electrical signal is produced which produces sound by way of an electrical earphone. We have previously completed a theoretical 4,5 and experimental 6 study of optoacoustic receivers in which it was necessary to solve a variety of linear acoustical problems to establish the feasibility of the device and to obtain its response.