Patent ID: 8903239

Claim:
An optical modulator, comprising: a modulator including an optical waveguide provided in a semiconductor substrate having an electro-optical effect and an electrode to apply an electric field depending on a bias voltage and a modulation signal to the optical waveguide; a driver circuit to generate a modulation signal in accordance with an input signal; a superimposer to superimpose a reference signal on the bias voltage, the reference signal having lower frequency than the modulation signal; and a controller to control a bias voltage in a modulation direction of the modulator and an orthogonal direction orthogonal to the modulation direction based on the frequency component of the reference signal extracted from a modulated optical signal generated by the modulator, wherein the optical waveguide includes first and second optical waveguides forming a Mach-Zehnder interferometer, the electrode includes first and second electrodes to apply an electric field to the first and second optical waveguides, respectively, the modulation signal and a first bias voltage are provided for the first electrode, a reversed signal of the modulation signal and a second bias voltage are provided for the second electrode, the controller includes a first bias controller to control a bias voltage in the orthogonal direction, when the superimposer respectively superimposes the reference signals of same phase on the first bias voltage and the second bias voltage, the first bias controller controls the bias voltage in the orthogonal direction based on the frequency component of the reference signal extracted from the modulated optical signal.