Calibration method for phase modulator, calibration method for balanced photodetector, and calibration system for phase modulator

A calibration system of a phase modulator whose phase modulation amount is controlled by a control signal, the calibration system comprising:

• a calibration circuitry including:
• a first delay interferometer to be used as a phase calibration reference;
• and a second delay interferometer in which a phase modulator to be calibrated is installed in an optical path,
• a light source configured to input light to the calibration circuitry;
• a photodetector connected to an output of the calibration circuitry;
• and
• a control signal generator configured to generate the control signal,
• wherein the first delay interferometer and the second delay interferometer are connected in cascade as a front stage and a rear stage, respectively,
• a delay time is measured for each of the first delay interferometer and the second delay interferometer, based on a time interval between pulses split from one input pulse, and the delay time is synchronized with a cycle of a transmission clock,
• a phase difference is adjusted for the first delay interferometer so that input continuous-wave laser light produces an intensity-enhancing interference at an output, and
• the control signal is determined with the calibration circuitry.