Optical transmitter and bias control method of optical modulator

1. An optical transmitter comprising:

a drive signal generator configured to generate a drive signal from input data;

an optical modulator configured to generate an optical signal corresponding to the drive signal, intensity of output light of the optical modulator periodically changing with respect to an applied voltage;

a reference signal generator configured to generate a first reference signal of a first frequency and a second reference signal of a second frequency, the first frequency and the second frequency being different from each other;

a superimposer configured to superimpose the first reference signal on the drive signal and superimpose the second reference signal on a bias voltage of the optical modulator;

a converter configured to convert the optical signal into an electric signal;

a detector configured to detect an intensity and a phase of the first reference signal included in the optical signal from the electric signal with a synchronous detection using the first reference signal generated by the reference signal generator and an intensity and a phase of the second reference signal included in the optical signal from the electric signal with a synchronous detection using the second reference signal generated by the reference signal generator, and generate a first monitor signal indicating the detected intensity and phase of the first reference signal and a second monitor signal indicating the detected intensity and phase of the second reference signal; and

a controller configured to control the bias voltage of the optical modulator in a control scheme determined based on the first monitor signal and the second monitor signal.

2. The optical transmitter according to claim 1 , wherein

the controller determines the control scheme for controlling the bias voltage based on the phase indicated by the first monitor signal at the bias voltage where the phase indicated by the second monitor signal changes.

3. The optical transmitter according to claim 1 , wherein

the controller decides whether or not an amplitude of the drive signal is larger than a threshold based on a phase indicated by the first monitor signal at a bias voltage where the phase indicated by the second monitor signal changes, and determines the control scheme for controlling the bias voltage based on a result of the decision.

4. The optical transmitter according to claim 1 , wherein

when the second monitor signal does not change in sweeping the bias voltage, the controller determines the control scheme for controlling the bias voltage based on the first and second monitor signals after changing an amplitude of the drive signal.

5. The optical transmitter according to claim 1 , wherein

the controller determines the control scheme for controlling the bias voltage based on a product of the first and second monitor signals at a bias voltage where a phase indicated by the second monitor signal changes.

6. The optical transmitter according to claim 1 , wherein

the controller determines the control scheme for controlling the bias voltage based on a phase indicated by the second monitor signal at a bias voltage where a phase indicated by the first monitor signal changes.

7. The optical transmitter according to claim 1 , wherein:

the optical modulator includes I and Q arms;

the drive signal generator generates I and Q arm drive signals;

the controller performs a process of determining the control scheme for controlling a bias voltage for an I arm, and a process of determining the control scheme for controlling a bias voltage for a Q arm at different time points.

8. The optical transmitter according to claim 1 , wherein:

the optical modulator includes I and Q arms;

the drive signal generator generates I and Q arm drive signals;

a reference signal superimposed on the I arm drive signal, a reference signal superimposed on a bias voltage of the I arm, a reference signal superimposed on the Q arm drive signal, and a reference signal superimposed on a bias voltage of the Q arm are different in frequency; and

the controller performs in parallel a process of determining a control scheme for controlling a bias voltage on an I arm, and a process of determining a control scheme for controlling a bias voltage on a Q arm.

9. An optical transmitter comprising:

a drive signal generator configured to generate a drive signal from input data;

an optical modulator configured to generate an optical signal corresponding to the drive signal, intensity of output light of the optical modulator periodically changing with respect to an applied voltage;

a superimposer configured to superimpose a first reference signal on the drive signal and superimpose a second reference signal on a bias voltage of the optical modulator;

a detector configured to detect the first reference signal and the second reference signal included in the optical signal, and generate a first monitor signal indicating intensity and a phase of the detected first reference signal and a second monitor signal indicating intensity and a phase of the detected second reference signal; and

a controller configured to control the bias voltage of the optical modulator in a control scheme determined based on the first monitor signal and the second monitor signal, wherein

the controller specifies a bias voltage at which a phase of the second monitor signal changes from a state in which the second monitor signal is in phase with the second reference signal to a state in which the second monitor signal is reversed phase of the second reference signal, and determines the control scheme for controlling the bias voltage according to whether the first monitor signal at the specified bias voltage is in phase with the first reference signal or reversed phase of the first reference signal.

10. An optical transmitter comprising:

a drive signal generator configured to generate a drive signal from input data;

an optical modulator configured to generate an optical signal corresponding to the drive signal, intensity of output light of the optical modulator periodically changing with respect to an applied voltage;

a superimposer configured to superimpose a first reference signal on the drive signal and superimpose a second reference signal on a bias voltage of the optical modulator;

a detector configured to detect the first reference signal and the second reference signal included in the optical signal, and generate a first monitor signal indicating intensity and a phase of the detected first reference signal and a second monitor signal indicating intensity and a phase of the detected second reference signal; and

a controller configured to control the bias voltage of the optical modulator in a control scheme determined based on the first monitor signal and the second monitor signal, wherein

the controller specifies a bias voltage at which a phase of the second monitor signal changes from a state in which the second monitor signal is reversed phase of the second reference signal to a state in which the second monitor signal is in phase with the second reference signal, and determines the control scheme for controlling the bias voltage according to whether the first monitor signal at the specified bias voltage is in phase with the first reference signal or reversed phase of the first reference signal.

11. An optical transmitter comprising:

a drive signal generator configured to generate a drive signal from input data;

an optical modulator configured to generate an optical signal corresponding to the drive signal, intensity of output light of the optical modulator periodically changing with respect to an applied voltage;

a superimposer configured to superimpose a first reference signal on the drive signal and superimpose a second reference signal on a bias voltage of the optical modulator;

a detector configured to detect the first reference signal and the second reference signal included in the optical signal, and generate a first monitor signal indicating intensity and a phase of the detected first reference signal and a second monitor signal indicating intensity and a phase of the detected second reference signal; and

a controller configured to control the bias voltage of the optical modulator in a control scheme determined based on the first monitor signal and the second monitor signal, wherein

the controller is capable of performing a first control scheme in which the bias voltage is increased when the second reference signal is in phase with a frequency component of the second reference signal detected from the optical signal and the bias voltage is reduced when the second reference signal is reversed phase of a frequency component of the second reference signal detected from the optical signal, and is capable of performing a second control scheme in which the bias voltage is reduced when the second reference signal is in phase with a frequency component of the second reference signal detected from the optical signal and the bias voltage is increased when the second reference signal is reversed phase of a frequency component of the second reference signal detected from the optical signal; and

the controller selects one of the first and second control schemes according to the first and second monitor signals.

12. A bias control method of controlling a bias of an optical modulator which has characteristics where intensity of output light periodically changes with respect to an applied voltage, the method comprising:

generating, by using a reference signal generator, a first reference signal of a first frequency and a second reference signal of a second frequency, the first frequency and the second frequency being different from each other;

superimposing the first reference signal on a drive signal generated from input data;

superimposing the second reference signal on a bias voltage of the optical modulator;

converting an optical signal generated by the optical modulator into an electric signal;

detecting an intensity and a phase of the first reference signal included in the optical signal from the electric signal with a synchronous detection using the first reference signal generated by the reference signal generator;

detecting an intensity and a phase of the second reference signal included in the optical signal from the electric signal with a synchronous detection using the second reference signal generated by the reference signal generator;

generating a first monitor signal indicating the detected intensity and phase of the first reference signal and a second monitor signal indicating the detected intensity and phase of the second reference signal; and

controlling a bias voltage of the optical modulator in a control scheme determined based on the first and second monitor signals.