Abstract

A direct detection optical communication system for use in an intersatellite link was constructed with an AIGaAs laser diode (Hitach-HL8314E, λ = 833.4 nm) as the transmitter and an avalanche photodiode (RCA C30902S) as the photodetector. The system used Q = 4 pulse position modulation (PPM) signaling and operated at 25 Mbits/s. The PPM slot clock at the receiver was recovered by using a transition detector followed by a phase lock loop (PLL). The PPM word clock was recovered by using a second PLL whose input consisted of back- to-back PPM pulse pairs contained in the received random PPM pulse sequences. A special PLL circuit was devised to phase lock further the word clock with the slot clock. The use of the recovered slot and word clocks at the receiver resulted in no penalty in receiver sensitivity compared to a receiver that used common transmitter/receiver clocks. The measurements of the received bit error probabilities vs detected photons per information bit agreed well with the theoretical computation results. The system achieved a bit error probability of 10⁻⁶ at <60 detected photons per information bit (−63.5 dBm). The receiver was capable of acquiring and maintaining slot and word synchronization for received signal levels of >15 photons per information bit.

© 1988 Optical Society of America