Abstract: A bias controller for an optical modulator. The modulator includes a bias electrode that is operable when appropriately biased by an applied bias voltage to configure the modulator to operate at quadrature, and the bias controller includes: a generating arrangement for generating power signals indicative of the optical output power of the modulator; and a processor operably connected to the generating arrangement and said bias electrode and arranged to receive said power signals from said generating arrangement and to control the bias voltage applied to said bias electrode.