Optical modulation device and bias voltage control method

An optical modulation device that generates a QAM signal whose order is set to be constant, the device comprising:

• an I-component optical modulation unit that modulates a continuous optical signal for an I-component of the QAM signal;
• a Q-component optical modulation unit that modulates a continuous optical signal for a Q-component of the QAM signal;
• a phase shift unit that shifts a phase of an optical signal on an input side and/or an output side for the I-component optical modulation unit and/or the Q-component signal optical modulation unit to adjust a phase difference between an output of the I-component optical modulation unit and an output of the Q-component optical modulation unit after the phase shift to π/2;
• a QAM signal generation unit that multiplexes the output of the I-component optical modulation unit and the output of the Q-component optical modulation unit after the phase shift to generate the QAM signal;
• an I-component data signal output unit that outputs an I-component data signal whose amplitude is controlled to be constant to the I-component optical modulation unit to cause the I-component optical modulation unit to modulate a continuous optical signal using the I-component data signal;
• a Q-component data signal output unit that outputs a Q-component data signal whose amplitude is controlled to be constant to the Q-component optical modulation unit to cause the Q-component optical modulation unit to modulate a continuous optical signal using the Q-component data signal;
• an I-component bias voltage signal output unit that outputs a signal having an I-component bias voltage corresponding to a null point of the I-component optical modulation unit to the I-component optical modulation unit;
• a Q-component bias voltage signal output unit that outputs a signal having a Q-component bias voltage corresponding to a null point of the Q-component optical modulation unit to the Q-component optical modulation unit;
• an I-component bias voltage signal adjustment unit that maximizes or minimizes an intensity of the QAM signal to adjust the I-component bias voltage to an optimum value;
• and
• a Q-component bias voltage signal adjustment unit that maximizes or minimizes the intensity of the QAM signal to adjust the Q-component bias voltage to an optimum value.

A bias voltage control method, comprising:

• I and Q component optical modulation steps of modulating continuous optical signals for I and Q components of a QAM signal;
• a phase shift step of shifting a phase of an optical signal on an input side and/or an output side for the I-component optical modulation step and/or the Q-component signal optical modulation step and adjusting a phase difference between an output of the I-component optical modulation step and an output of the Q-component optical modulation step after the phase shift to π/2;
• and
• a QAM signal generation step of multiplexing the output of the I-component optical modulation step and the output of the Q-component optical modulation step after the phase shift to generate the QAM signal, wherein
• the I-component optical modulation step sequentially includes
• an I-component signal input step of
• inputting an I-component data signal used to modulate a continuous modulation signal and
• inputting a signal having an I-component bias voltage corresponding to a null point of the I-component optical modulation step and
• an I-component bias voltage signal adjustment step of
• determining whether an intensity of the QAM signal at a drift non-occurrence time where no drift occurs in the I-component bias voltage becomes larger or smaller than the intensity of the QAM signal at a drift occurrence time where a drift occurs in the I-component bias voltage,
• adjusting the I-component bias voltage to maximize the intensity of the QAM signal when determining that the intensity of the QAM signal at the drift non-occurrence time becomes larger than the intensity of the QAM signal at the drift occurrence time, and
• adjusting the I-component bias voltage to minimize the intensity of the QAM signal when determining that the intensity of the QAM signal at the drift non-occurrence time becomes smaller than the intensity of the QAM signal at the drift occurrence time, and
• the Q-component optical modulation step sequentially includes
• a Q-component signal input step of
• inputting a Q-component data signal used to modulate a continuous modulation signal and
• inputting a signal having a Q-component bias voltage corresponding to a null point of the Q-component optical modulation step and
• a Q-component bias voltage signal adjustment step of
• determining whether the intensity of the QAM signal at a drift non-occurrence time where no drift occurs in the Q-component bias voltage becomes larger or smaller than the intensity of the QAM signal at a drift occurrence time where a drift occurs in the Q-component bias voltage,
• adjusting the Q-component bias voltage to maximize the intensity of the QAM signal when determining that the intensity of the QAM signal at the drift non-occurrence time becomes larger than the intensity of the QAM signal at the drift occurrence time, and
• adjusting the Q-component bias voltage to minimize the intensity of the QAM signal when determining that the intensity of the QAM signal at the drift non-occurrence time becomes smaller than the intensity of the QAM signal at the drift occurrence time.