US Patent 7327961 Differential encoder for an optical DQPSK modulator

A modulator arrangement adapted to use a differential quadrature phase shift key for use in an optical wavelength division multiplex (WDM) optical communications system, comprising a precoder (16), which is adapted to generate drive voltages for first and second phrase modulators (6, 8) in dependence upon first and second data streams. The respective drive voltages for the first and second modulators (6, 8) are fed back to the precoder inputs with a delay related to the line speed of the data system, wherein the length of the delay corresponds to n bits.