An optical communication system using digital pulse position modulation employs a mode locked laser with a mode locking frequency equal to the time slot frequency of the modulation and means dependent on groups of cosecu- tive digits of the data to be transmitted to select pulses from the laser for transmission. In one example, 4-bit groups from the data for transmission select one out of 20 pulses from the laser thus leaving a guard interval of 4 time slot periods between position modulated pulses.
An encoder and a decoder for converting between digitally encoded data and position modulated pulses are described. These use a counter driven by clock pulses at the time slot frequency to supply inputs alternately to two encoding logic circuits or two multiplexers.