Modulation control

A transmitter for modulating an optical signal using a quadrature phase shift key for use in an optical wavelength division multiplex (WDM) optical communications system, the transmitter comprising a laser for producing an optical signal of a selected wavelength, said signal is split by a splitter, each part of said split signal being applied to a respective phase modulator of the transmitter, each of said phase modulators is adapted to modulate a phase of the signal in dependence on a respective drive voltage, the phase of the output of at least one modulator being shiftable at least in part by a phase shifter of the transmitter, the split signals being recombined by an optical recombiner to form an optical phase shift key output, wherein the output power is monitored by a detector of the transmitter which generates a detector signal, the detector signal being adapted to drive a feedback arrangement of the transmitter to control electrodes of one of the phase modulator and the phase shifter of the transmitter.