Phase modulation apparatus

A phase modulation apparatus comprising:

• a light source which outputs continuous light having a constant optical phase;
• two phase modulators which modulate the respective optical phases of the two continuous lights, obtained by branching an output of the light source, based on two electrical data signals;
• a π/2 phase shifter which shifts the optical phase of an output of one of the phase modulators by π/2;
• a multiplexer which combines an output of the π/2 phase shifter and an output of the other phase modulator;
• an intensity modulator which intensity-modulates an output of the multiplexer with an electrical clock signal synchronized with the electrical data signal;
• phase shifters which are disposed in two of two electrical paths through which the electrical data signals propagate to the phase modulators and an electrical path through which the electrical clock signal propagates to the intensity modulator and shift the phase of the electrical data signals or the phase of the electrical clock signal;
• and
• a phase control circuit which controls, based on the phase of the electrical data signal or the phase of the electrical clock signal in the electrical path in which the phase shifter is not disposed, the phase of the electrical data signal or the phase of the electrical clock signal in the electrical path in which the phase shifter is disposed so that the intensity of the optical output from the intensity modulator is maximum.