An apparatus and method for controlling bias in an optical modulator is disclosed. The method is particularly applicable to controlling multi-wavelength modulators and wavelength-tunable transmitters. At a calibration stage, a desired optical performance of the modulator is achieved, and an amplitude of a peak-to-peak variation of the output optical signal at a pre-determined amount of dither is stored in a memory as a reference. At operating stage, a controller of the optical modulator adjusts a bias voltage of the modulator until the measured peak-to-peak optical signal variation matches the reference value stored at the calibration stage. For multi-wavelength modulators and tunable transmitters, the calibration is repeated at each wavelength, and corresponding peak-to-peak optical signal variations are stored in the memory.