Optical Modulation at High Information Rates

One of the major problems in engineering is that of interfacing between two differing technologies: This problem is particularly severe in laser communications systems where solid state electronics is to be interfaced with optics. It is, perhaps, at this technological interface that the early hopes of approaching the Shannon-Weaver information rates of laser systems have been limited. While electro-optic crystals have been shown to have modulation mechanisms operating in excess of 10 GHz 1 and photodetectors with gain-bandwidth products of the order of 100 GHz have been produced, 2 laser communications systems, so far, have demonstrated information rates significantly less than the capabilities of these components. Significant in the bandwidth limitation of demonstrated laser communications systems have been the demands on solid state circuit performance 3 imposed by the optical system and, in particular, the modulator. In this paper an optical modulation scheme is described in which system variations and subsystem configurations have been adopted in an attempt to relax the severe circuit performance requirements so that system information rate capabilities approach the bandwidth capabilities of the opto-electronic transducers. The system described, using an argon laser, has a demonstrated information rate of 1 Gb/s and some 2607