Extremely sensitive laser diode direct detection optical communication system for intersatellite links

Abstract

A semiconductor laser diode direct detection optical communication system suitable for use in an intersatellite communication link was constructed, and its performance was measured. The system used Q = 4 pulse position modulation (PPM) to transmit binary source data at a rate of 25 Mbit/s and contained a low-noise silicon avalanche photodiode (APD) as a photodetector. Use of the conventional Gaussian model for the output of the low-noise APD (RCA C30902S) was shown to be inappropriate under low-background radiation levels because its use caused a severe underestimate of the optical APD gain and an overestimate of a system bit error rate. A numerical procedure was developed that made use of the Conradi distribution for the APD output which gave good agreement with the results of the experimental measurements of system performance. A slot clock recovery subsystem was also provided in the receiver. It consisted of a nonlinearity which is easy to implement and a narrow bandpass filter centered at the slot clock frequency. Use of the recovered timing signal showed no penalty in the minimal input optical power required to maintain a specified error rate. The system achieved a bit error rate of 10−6 at a received signal level corresponding to an average of 60 detected photons per transmitted information bit (120 photons per PPM symbol).