Performance measurements of a diode laser optical communications link with Q = 4 PPM signaling

The theoretical and measured performance characteris-tics of a direct detection optical communication system are reported. The system consisted of a semiconductor diode laser, silicon avalanche photodiode detector, and used Q=4 optical pulse position modulation signaling to transmit binary source data at a rate of 25 megabits/sec. Operation at a receiver bit error probability of 10 -6 was demonstrated at received signal energies corresponding to an average of 160 detected photons per PPM symbol, or 80 photons per source bit.