Wideband, Bidirectional Lightguide Communication With An OpticallyPowered Audio Channel

Lightguides connecting remote customer stations to a central switching office would make wideband telecommunications available to large markets. A two-wavelength, bidirectional link utilizing single-strand optical fiber might be economical compared to rechargeable batteries or auxiliary metallic conductors, particularly if used with an optical detector capable of powering the telephone channel during electric utility outage at a remote station. The feasibility of incorporating emergency optical powering into a wideband, two-way optical link is described below. Both wideband and optically powered services can be realized with the GaAlAs photovoltaic diode used in previous demonstrations of 1359 optically powered speech signalling 1 and sound alerting 2 at a remote station. This detector provides efficient photovoltaic conversion 3 along a high-resistance (3-kilohm) load line and short-circuit frequency response extending into the television carrier range. Since their capacitances are at least an order of magnitude larger than those of reversebiased p-i-n diodes of comparable area, these devices must be used with low-input-impedance circuits at large bandwidths. This reduced high-frequency sensitivity is compensated for, at least in part, by the high light levels used in photovoltaic applications.