Operation of optical modulators

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 (vbias) 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 (vbias) which comprises the sum of a first (vfixed) and a second (vvar) voltage component. The first component (vfixed) 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 (vvar) to maintain the response at the desired voltage position. Additionally the apparatus includes means (42) for detecting when the bias voltage (vbias) approaches the limit of the available bias voltage range; means (36) for adjusting the first voltage component (vfixed) 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 (vvar) to zero.