A method of, and an apparatus for, operating an optical modulator (8) are described. The modulator is of a type having an optical attenuation versus voltage response which is cyclic and which includes an input (34) to which the application of a bias voltage (v bias ) moves the modulator's response along the voltage axis to maintain the modulator's response at a desired voltage position. The apparatus comprises a bias voltage generator (32) for generating a bias voltage (v bias ) which comprises the sum of a first (v fixed ) and a second (v var ) voltage component. The first component (v fixed ) is fixed during normal operation and is selected to correspond to the expected bias voltage required for maintaining the modulator's response at a desired voltage position. The apparatus further comprises means (28, 30) for detecting the position of the modulator's response and adjusting the second voltage component (v var ) to maintain the response at the desired voltage position. Additionally the apparatus includes means (42) for detecting when the bias voltage (v bias ) approaches the limit of the available bias voltage range; means (36) for adjusting the first voltage component (v fixed ) until it is equal to the sum of the bias voltage and a voltage corresponding to one or more half cycles of the modulator's response; and means (38) for setting the second voltage component (v var ) to zero.