Controlling a quadrature phase shift key encoder in a wavelength division multiplex (WDM) optical communication system

摘要

A modulator arrangement for modulating an optical signal using a quadrature phase shift key for use in an optical wavelength division multiplex (WDM) optical communications system comprising a laser 2 for producing an optical signal of a selected wavelength, which signal is split by a splitter 4, each part of said split signal being applied to a respective phase modulator 6, 8, each of which phase modulators is adapted to modulate the phase of the signal in dependence on a respective drive voltage. The phase of the output of at least one modulator 6 is shiftable at least in part by a phase shifter 10, the split signals being recombined by an optical recombiner 12 to form an optical phase shift key output, wherein the output power is monitored by a detector 20, the detector signal then being used to drive a feedback arrangement to control electrodes of the phase modulators 6, 8 and/or phase shifter 10. The detector may be a two photon absorption detector. Furthermore the feedback arrangement may comprise an oscillator adapted to provide a pilot frequency which drives a multiplier and a summing amplifier. The quadrature phase shift key may be a differential quadrature phase shift key.