Patent ID: 8638486

Claim:
An optical modulator comprising: a first modulation waveguide that performs phase modulation based on a first modulation signal, which is an electric signal generated based on a code; a second modulation waveguide that performs phase modulation based on a second modulation signal, which is an electric signal paired with the first modulation signal as differential signals; a demultiplexer that splits an input light into a light input to the first modulation waveguide and a light input to the second modulation waveguide; a first phase adjustment waveguide that changes a phase of a light phase-modulated by the first modulation waveguide; a second phase adjustment waveguide that changes a phase of a light phase-modulated by the second modulation waveguide; a multiplexer that combines a light output from the first phase adjustment waveguide with a light output from the second phase adjustment waveguide, and outputs a combined light as a phase modulated light; an applied voltage controller that controls voltages of the first modulation signal and the second modulation signal; and a phase-adjustment bias controller that controls phase amounts changed by the first phase adjustment waveguide and the second phase adjustment waveguide, wherein the applied voltage controller controls the voltages of the first modulation signal and the second modulation signal based on characteristics indicating a relation of amplitudes between input voltages and output lights of the first modulation waveguide and the second modulation waveguide so that a result of adding an amplitude of an output light from the first modulation waveguide to an amplitude of an output light from the second modulation waveguide in a case the output light from the first modulation waveguide and the output light from the second modulation waveguide equally have a predetermined phase is equal to a result of adding an amplitude of the output light from the first modulation waveguide to an amplitude of the output light from the second modulation waveguide in a case the output light from the first modulation waveguide and the output light from the second modulation waveguide have different predetermined phases, and the phase-adjustment bias controller controls phase amounts changed by the first phase adjustment waveguide and the second phase adjustment waveguide so as to cancel phase errors in desired values, the phase errors resulting from control of the applied voltage controller.