Optical transmitter and its control method

An optical transmitter comprising:

• an amplitude modulation unit performing amplitude modulation of a main signal with a low-frequency signal having a predetermined frequency;
• an optical modulator receiving an input signal generated by the amplitude modulation unit after the amplitude modulation, and modulating an incoming light in response to the received signal in accordance with a predetermined modulation-characteristic curve to output an optical output signal;
• and
• an operating point control unit applying a predetermined bias voltage to the optical modulator to control a level of the input signal substantially applied to the modulation-characteristic curve of the optical modulator, so that the input signal generated after the amplitude modulation is applied to a minimum portion of the modulation-characteristic curve when the main signal is in OFF state, the operating point control unit comprising a phase comparator which compares a phase of the optical output signal of the optical modulator with a phase of the low-frequency signal, and the bias voltage applied to the optical modulator by the operating point control unit is changed to a different polarity depending on whether the two signals are in phase or 180 degrees out of phase.

A control method of an optical transmitter provided with an optical modulator, comprising:

• performing amplitude modulation of only a one-side amplitude of a main signal with a low-frequency signal having a predetermined frequency;
• causing the optical modulator to receive an input signal generated after the one-side amplitude modulation, and modulate an incoming light in response to the received signal in accordance with a predetermined modulation-characteristic curve to output an optical output signal;
• applying a predetermined bias voltage to the optical modulator to control a level of the input signal substantially applied to the modulation-characteristic curve so that the one-side amplitude of the main signal is applied to a minimum portion of the modulation-characteristic curve of the optical modulator;
• and
• the amplitude modulation superimposes a signal, having the predetermined frequency in phase or 180 degrees out of phase with the low-frequency signal, on the input signal generated after the one-side amplitude modulation, so that an opposite-side amplitude of the main signal is canceled.

A control method of an optical transmitter provided with an optical modulator, comprising:

• performing amplitude modulation of a main signal with a low-frequency signal having a predetermined frequency;
• causing the optical modulator to receive an input signal generated after the amplitude modulation, and modulate an incoming light in response to the received signal in accordance with a predetermined modulation-characteristic curve to output an optical output signal;
• and
• applying a predetermined bias voltage to the optical modulator to control a level of the input signal substantially applied to the modulation-characteristic curve of the optical modulator, so that the input signal generated after the amplitude modulation is applied to a minimum portion of the modulation-characteristic curve when the main signal is in OFF state;
• wherein
• the optical transmitter comprises a phase comparator which compares a phase of the optical output signal of the optical modulator with a phase of the low-frequency signal, and the bias voltage applied to the optical modulator is changed to a different polarity depending on whether the two signals are in phase or 180 degrees out of phase.