Communication systems

A transmitter transmits signals on multiple channels, and they are wavelength multiplexed according to Coherent WDM. Quality of the received signals is very much enhanced by the transmitter tuning the relative phases of neighbouring channels according to delay characteristics of the optical path. In one example, the transmitter adds a relative delay of 1/4 bit period between each data signal and the beat frequency signal for the odd and even channel groups, to counter the receiver side delays introduced by the AMZI. This corresponds to aligning the transmitter such that the eye crossing corresponds to a minimum of the 80 GHz beat signal. In order to obtain a stable phase relationship between adjacent channels the 80 GHz tone is set to a particular value. Any harmonics or sub-harmonics will similarly vary with phase and therefore can also be used as an error signal for a phase stabilisation circuit. The phase stabilisation circuit in the transmitter therefore can maintain a desired stable optimum phase alignment between neighbouring channels.