An Optical-Frequency Pulse-Position-Modulation Experiment

Recent advances in low-loss optical fibers, 1 in solid-state photodetectors, 2 and in optical-frequency (O.F.) power sources3,4 have stimulated interest in O.F. communication systems for numerous applications. Pulse-position modulation (PPM) is particularly attractive for O.F. communications because the optical energy source can be operated at a low, message-independent, duty cycle to extend the lifetime of the device, and the technique affords a high noise immunity to the types of noise t h a t dominate in a well-designed optical receiver. By employing a short pulse width and thereby expanding the bandwidth of the transmitted signal, the effect of detector leakage noise (dark current) and some forms of amplifier noise are reduced. This paper describes an experiment performed to evaluate the performance that can be achieved in transmitting a single message channel by means of optical PPM.* Included are descriptions of the following: * In an independent work (Ref. 5), a transmitter and receiver are described for an optical PPM system in which timing information was provided by a reference pulse in each time slot. The signal-to-noise ratio performance of the receiver was not given. 285