Optical transmitter and modulated optical signal generating method

An optical transmitter comprising:

• an optical modulator including a first modulation unit and a second modulation unit respectively configured to propagate a first optical signal and a second optical signal that are obtained by splitting input light;
• a signal generator configured to generate a first drive signal and a second drive signal that are respectively supplied to the first modulation unit and the second modulation unit;
• a phase controller configured to control a phase difference between the first optical signal and the second optical signal in the optical modulator;
• a converter configured to convert the first drive signal into a first converted signal having a number of bits less than the first drive signal and convert the second drive signal into a second converted signal having a number of bits less than the second drive signal;
• a multiplier configured to multiply an exclusive OR signal between the first converted signal and the second converted signal by an electrical signal representing an output signal of the optical modulator;
• and
• a phase difference detector configured to detect the phase difference based on the output signal of the multiplier, wherein
• the signal generator generates the first drive signal and the second drive signal based on the phase difference detected by the phase difference detector.

An optical transmitter comprising:

• a first optical modulator including a first modulation unit and a second modulation unit respectively configured to propagate a first optical signal and a second optical signal that are obtained by splitting input light;
• a second optical modulator including a third modulation unit and a fourth modulation unit respectively configured to propagate a third optical signal and a fourth optical signal that are obtained by splitting input light;
• a polarization multiplexer configured to generate a polarization multiplexed optical signal from a first modulated optical signal outputted from the first optical modulator and a second modulated optical signal outputted from the second optical modulator;
• a signal generator configured to generate a first drive signal, a second drive signal, a third drive signal, and a fourth drive signal that are respectively supplied to the first modulation unit, the second modulation unit, the third modulation unit, and the fourth modulation unit;
• a phase controller configured to control a phase difference between the first optical signal and the second optical signal in the first optical modulator and control the phase difference between the third optical signal and the fourth optical signal in the second optical modulator;
• and
• a phase difference detector configured to detect the phase difference between the first optical signal and the second optical signal and detect the phase difference between the third optical signal and the fourth optical signal, wherein
• the signal generator generates the first drive signal and the second drive signal based on the phase difference between the first optical signal and the second optical signal detected by the phase difference detector;
• and
• the signal generator generates the third drive signal and the fourth drive signal based on the phase difference between the third optical signal and the fourth optical signal detected by the phase difference detector.

A method of generating a modulated optical signal using an optical modulator including a first modulation unit and a second modulation unit that respectively propagate a first optical signal and a second optical signal obtained by splitting input light, the method comprising:

• supplying a first drive signal and a second drive signal to the first modulation unit and the second modulation unit respectively;
• controlling a phase difference between the first optical signal and the second optical signal in the optical modulator to be a first specified value or a second specified value;
• deciding whether the phase difference is controlled to be either the first specified value or the second specified value;
• and
• correcting the first drive signal and the second drive signal based on whether the phase difference is controlled to be either the first specified value or the second specified value.