Utilization of Optical-Frequency Carriers for Low- and Moderate-Bandwidth Channels

Since the advent of the laser, communication engineers have been intrigued by the promise of fantastic bandwidth capability in opticalfrequency communication systems. As a result, attention has been focused on high-capacity, high-bandwidth considerations. Recent advances in component fabrication--for example, light-emitting diodes (LED's), junction lasers, avalanche photodiode detectors, and low-loss optical fibers--have made it feasible to consider the use of opticalfrequency carrier systems for moderate- and even low-bandwidth channels. Fundamental and practical differences between optical-frequency channels and radio-frequency channels* necessitate a reevaluation of concepts acquired from experience with the latter. To this end, in the following sections we consider four potentially attractive forms of modulation of optical-frequency signals and derive results for the required average received signal power in terms of system requirements and parameters. In Section II we consider a system using analog inten* We use the term "optical frequency" to mean frequencies roughly in the range 10 to 1000 THz and the term "radio frequency" to indicate frequencies below roughly 3 THz. 731