An automatic bias controller for an optical modulator is provided. The automatic bias controller comprises a driver for providing an electrical data signal to the modulator and a bias voltage source for providing a bias voltage to the modulator. A microprocessor provides a low frequency digital modulation signal, which is converted to an analogue modulation signal by a digital to analogue converter. The analogue modulation signal is applied to the bias voltage source (so as to modulate the bias voltage) or to the driver (so as to modulate the amplitude of the data signal). Intensity detectors for detecting the intensity of light emitted by the modulator are provided, and an analogue to digital converter converts the output of the intensity detectors to a digital intensity signal which is passed to the microprocessor. The digital intensity signal is analysed, and the bias voltage source instructed to adjust the bias voltage on the basis of the analysed signal. The digital modulation signal is preferably a rectangular wave or time multiplexed series of periods in which the bias voltage and data signal amplitude are varied.