CONTROL METHOD FOR OPTICAL MODULATION DEVICE

PROBLEM TO BE SOLVED: To provide a control method for an optical modulation device that has decreased in the absolute value of a bias voltage and that has remarkably improved in reliability.SOLUTION: An optical modulation device comprises a substrate, an optical waveguide formed on the substrate, and an electrode for applying a voltage, and the optical waveguide includes a mutual action optical waveguide that varies in refractive index by applying a bias voltage to the electrode. A control method is for the optical modulation device, in which optical output from the optical modulation device is formed from a periodic function that has a periodicity of 2 V (V represents a half wavelength voltage) with regard to a bias voltage, and the absolute value of the bias voltage after a predetermined elapse of time from the start of the optical modulation device is greater than the absolute value of a bias voltage at the start of the operation. In this control method, the bias voltage is varied by an integral multiple of the half wavelength voltage in the direction in which the absolute value of the bias voltage decreases after the elapse of the predetermined time.