Automatic bias controller for electro-optic modulator

A low frequency pilot signal having predetermined positive and negative excursions is applied to an electrical modulation signal input (26) of a Mach-Zehnder electro-optic modulator (18) together with an electrical input signal. A bias voltage VB selected to bias the modulator (18) at a linear point (V pi /2) is also applied to the signal input (26). The optical output signal of the modulator (18) is sensed, and the magnitudes of the positive and negative excursions of the output signal which correspond to the positive and negative excursions of the pilot signal are compared with each other. If the modulator (18) is linearly biased as desired, the excursions will have the same relative relationship as the excursions of the pilot signal. If the modulator (18) has drifted away from the linear bias point (V pi /2), one excursion will have a larger magnitude relative to the other than in the pilot signal, indicating the direction of deviation. The bias voltage VB is automatically adjusted toward the linear bias point (V pi /2). Where the bias voltage range includes a plurality of linear bias points (V pi /2) within the limits thereof, the voltage difference (2V pi ) between two of the linear bias points (V pi /2) is automatically measured at startup. If the modulator (18) drifts into one of the limits, the bias voltage VB is reset to a value spaced from the respective limit by the voltage difference (2V pi ), thereby automatically resetting the bias voltage VB to another linear point (V pi /2) within the range.