IMPROVEMENTS IN, OR RELATING TO, OPTICAL COMMUNICATIONS

摘要

A method of transmitting a plurality n data streams comprises modulating an optical carrier using differential M-ary phase shift key (DMPSK) signalling in which M=2n.. Advantageously the method comprises using differential quaterna ry phase shift keying in which n=2. A particular advantage of the method of the present invention is that since the the data is differentially encoded in th e form of phase changes rather than absolute phase values this enables the modulated optical carrier to be demolutated using direct detection without requiring a phase-locked local optical oscillator.The invention is particularly applicable to WDM communication systems.