Patent ID: 8160456

Claim:
A bias controller for an optical modulator, comprising: a biasable component that is operable, when appropriately biased by an applied bias voltage, to configure the modulator to operate at quadrature; a generating arrangement for generating power signals indicative of the optical output power of the modulator during modulation; a processor operably connected to the generating arrangement and the biasable component and arranged to receive the power signals from the generating arrangement and to control the bias voltage applied to the biasable component; and a data store for said modulator having stored therein a plurality of values for peak optical power and respective predetermined values for target optical power for quadrature; wherein the processor is configured to perform the following: vary the bias voltage applied to the biasable component and to determine from power signals received from said generating arrangement a peak optical output power for the modulator, determine in substantial dependence upon the peak optical power only, a target optical power for quadrature by reference to the data store, monitor the optical output power during modulation by the modulator, and adjust the bias voltage to bring the modulator back to quadrature if the optical output power should be determined to be different from the target optical power.