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.