Optical transmitter, method for controlling optical modulator, and controller for optical modulator

1. An optical transmitter comprising:

an optical modulator that is driven by a driving signal;

a controller that

for each of a plurality of reference amplitudes, detects a parameter to adjust the driving signal when an output light from the optical modulator is in a state corresponding to a reference amplitude,

obtains a relationship between the parameter and the amplitude corresponding to the state of the output light based on the detected parameter and the plurality of the reference amplitudes, and

controls an amplitude of the driving signal on the basis of the obtained relationship such that the parameter approaches a target parameter.

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

a light intensity of the output light from the optical modulator fluctuates at a periodic cycle in accordance with a fluctuation of the driving signal;

when the output light from the optical modulator is in a state corresponding to a reference amplitude obtained by multiplying the amplitude of the driving signal by a coefficient equal to or more than zero and equal to or less than one, the controller detects the parameter, and determines the target parameter based on the obtained relationship, the target parameter causing the output light to be in a target state corresponding to an amplitude different from the reference amplitude.

3. The optical transmitter according to claim 2 , further comprising a plurality of the optical modulators, wherein:

the controller detects the parameter and obtains the relationship for each of the plurality of optical modulators, and determines the target state based on the relationship obtained for a first optical modulator among the plurality of optical modulators, determines the target parameter based on the determined target state and the relationship obtained for a second optical modulator among the plurality of optical modulators, and controls the amplitude of the driving signal that drives the second optical modulator such that the parameter for the second optical modulator approaches the determined target parameter.

4. The optical transmitter according to claim 2 , wherein the controller obtains a polynomial representing the relationship on the basis of the plurality of detected parameters.

5. The optical transmitter according to claim 2 , wherein:

the controller controls the driving signal to be a quaternary signal, and determines, when a fluctuation amount of a power of a component having a first frequency among the output light from the optical modulator is minimum among amplitudes of the driving signal in a first range under a state where a bias voltage applied to the optical modulator is oscillated at the first frequency lower than the frequency of the driving signal with respect to a basic bias voltage as the center of the oscillation and the basic bias voltage is varied in a voltage range, that the output light is in a state corresponding to the reference amplitude near to an amplitude obtained by multiplying the amplitude of the driving signal corresponding to the periodic cycle by 3/4 .

6. The optical transmitter according to claim 2 , wherein:

the controller controls the driving signal to be a binary signal, and determines, when a power of a component having a second frequency among the output light from the optical modulator is maximum among amplitudes of the driving signal in a second range under a state where the amplitude of the driving signal is oscillated at the second frequency lower than the frequency of the driving signal, that the output light is in a state corresponding to the reference amplitude obtained by multiplying the amplitude of the driving signal corresponding to the periodic cycle by 1/2 .

7. The optical transmitter according to claim 2 , wherein:

the controller controls the driving signal to be a binary signal, and determines, when at least one of a first condition that a power of a component having a third frequency among the output light from the optical modulator is minimum among amplitudes of the driving signal in a third range and a second condition that a power of a component having twice a third frequency among the output light from the optical modulator is maximum among the amplitudes of the driving signal in the third range is satisfied under a state where the amplitude of the driving signal is oscillated at the third frequency lower than the frequency of the driving signal, that the output light is in a state corresponding to the reference amplitude equal to the amplitude of the driving signal corresponding to the periodic cycle.

8. The optical transmitter according to claim 2 , wherein:

the controller detects two of the parameters at a first time point and a second time point after the first time point, and controls the amplitude of the driving signal on the basis of the detected parameters detected at the first time point and the second time point.

9. The optical transmitter according to claim 1 , wherein

the controller controls the amplitude of the driving signal by controlling at least one of an electric signal generator that generates an electric signal and an amplifier that amplifies the generated electric signal and outputs the amplified electric signal as the driving signal.

10. A method for controlling an optical modulator that is driven by a driving signal, the method comprising:

detecting a parameter to adjust the driving signal when an output light from the optical modulator is in a state corresponding to a reference amplitude for each of a plurality of reference amplitudes,

obtaining a relationship between the parameter and the amplitude corresponding to the state of the output light based on the detected parameter and the plurality of the reference amplitudes, and

controlling an amplitude of the driving signal on the basis of the obtained relationship such that the parameter approaches a target parameter.

11. The method according to claim 10 , wherein:

a light intensity of the output light from the optical modulator fluctuates at a periodic cycle in accordance with a fluctuation of the driving signal;

when the output light from the optical modulator is in a state corresponding to a reference amplitude obtained by multiplying the amplitude of the driving signal by a coefficient equal to or more than zero and equal to or less than one,

detecting the parameter, and

determining the target parameter based on the obtained relationship, the target parameter causing the output light to be in a target state corresponding to an amplitude different from the reference amplitude.

12. The method according to claim 11 , further comprising:

detecting the parameter and obtaining the relationship for each of the plurality of optical modulators; and

determining the target state based on the relationship obtained for a first optical modulator among the plurality of optical modulators, determining the target parameter based on the determined target state and the relationship obtained for a second optical modulator among the plurality of optical modulators, and controlling the amplitude of the driving signal that drives the second optical modulator such that the parameter for the second optical modulator approaches the determined target parameter.

13. A controller controls is configured to control an optical modulator that is driven by a driving signal;

detects a parameter to adjust the driving signal when an output light from the optical modulator is in a state corresponding to a reference amplitude for each of a plurality of reference amplitudes;

obtains a relationship between the parameter and the amplitude corresponding to the state of the output light based on the detected parameter and the plurality of the reference amplitudes; and

controls an amplitude of the driving signal on the basis of the obtained relationship such that the parameter approaches a target parameter.

14. The controller according to claim 13 , wherein:

a light intensity of the output light from the optical modulator fluctuates at a periodic cycle in accordance with a fluctuation of the driving signal;

when the output light from the optical modulator is in a state corresponding to a reference amplitude obtained by multiplying the amplitude of the driving signal by a coefficient equal to or more than zero and equal to or less than one, the controller detects the parameter, and determines the target parameter based on the obtained relationship, the target parameter causing the output light to be in a target state corresponding to an amplitude different from the reference amplitude.

15. The controller according to claim 14 , wherein:

the controller detects the parameter and obtains the relationship for each of the plurality of optical modulators, and determines the target state based on the relationship obtained for a first optical modulator among the plurality of optical modulators, determines the target parameter based on the determined target state and the relationship obtained for a second optical modulator among the plurality of optical modulators, and controls the amplitude of the driving signal that drives the second optical modulator such that the parameter for the second optical modulator approaches the determined target parameter.