OPTICAL MODULATOR AND METHOD OF ENCODING COMMUNICATIONS TRAFFIC IN A MULTILEVEL MODULATION FORMAT

摘要

An optical modulator (10) comprising: an optical splitter (12) arranged to receive an optical carrier signal (14) and split it into sub-signals (16); modulation assemblies (18), each comprising a binary optical modulator (22) arranged to apply a binary phase shift keyed, BPSK, modulation to a respective optical sub-signal to form a BPSK modulated optical sub-signal. A plurality of the modulation assemblies (20) comprise a phase-shifter (24), each arranged to apply a respective phase shift to the respective BPSK modulated optical sub-signal in dependence on a pre-selected multilevel modulation format; encoding apparatus (26) arranged to receive communications traffic bits (28), to map each bit into a respective symbol, and to generate and transmit a respective drive signal to the optical modulator of a modulation assembly, each drive signal arranged to cause the optical modulator to apply said BPSK modulation, to encode the symbol onto the respective optical carrier sub-signal; an output (30); and a combiner (32) arranged to receive each BPSK modulated optical sub-signal and to deliver each sub-signal to the output, to form an output optical signal (34) having said multilevel modulation format.