| 摘要 |
A modulator arrangement adapted to use a differential quadrature phase shift key (DQPSK) for use in an optical wavelength division multiplex (WDM) optical communications system comprising at least one precoder 16, which is adapted to generate drive voltages for first and second modulators 6, 8 (see Fig. 1) in dependence upon first and second data streams. The respective drive voltages for the first and second modulators are fed back to the precoder inputs with a delay related to the line speed of the data stream. The length of the delay may correspond to n bits, where n is proportional to the data stream speed. In one embodiment the data stream speed is 20 Gsymbols/sec, wherein n is equivalent to 4 bits or 200 ps. The modulator arrangement may comprise 2xn data streams at 1/n symbol rate and may comprise n precoders arranged in parallel. Specifically the modulator arrangement may comprise four parallel precoders, the outputs of which are fed to one of two respective 4:1 multiplexers 40, 41, the output of each precoder also being fed via one bit delays to its respective inputs, wherein the output of the multiplexers is used to drive the modulators, where the bit rate is proportional to 1/4 of the symbol rate, n being equal to 4.
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