Optical transmitter and control method for optical transmitter

1. An optical transmitter comprising:

a modulator configured to generate an optical signal indicating transmission data;

a square law detector configured to detect an intensity of the optical signal using a photodetector and output first intensity data indicating the detected intensity; and

a processor configured to

calculate, based on the transmission data, an electric field of the optical signal generated by the modulator by using parameters pertaining to a state of the modulator,

calculate second intensity data indicating the intensity of the optical signal based on the calculated electric field,

update the parameters so as to reduce a difference between the first intensity data and the second intensity data, and

control the state of the modulator based on the parameters.

2. The optical transmitter according to claim 1 , wherein

the processor controls the state of the modulator based on the parameters when the difference is smaller than a specified threshold.

3. The optical transmitter according to claim 1 , wherein

the modulator is provided with an IQ modulator including a first Mach-Zehnder modulator, a second Mach-Zehnder modulator, and a phase adder that adjusts a phase difference between the first Mach-Zehnder modulator and the second Mach-Zehnder modulator,

the parameters include a first parameter indicating a deviation of a bias voltage of the first Mach-Zehnder modulator, and

the processor controls the bias voltage of the first Mach-Zehnder modulator based on a value of the first parameter obtained when the difference is smaller than the threshold.

4. The optical transmitter according to claim 3 , wherein

the parameters include a second parameter indicating a deviation of the phase difference between the first Mach-Zehnder modulator and the second Mach-Zehnder modulator, and

the processor controls a bias voltage applied to the phase adder based on a value of the second parameter obtained when the difference is smaller than the threshold.

5. The optical transmitter according to claim 1 , wherein

the processor averages the second intensity data through low-pass filter processing, and

the processor updates the first parameter based on a difference between the first intensity data and the averaged second intensity data.

6. The optical transmitter according to claim 1 , wherein

the modulator generates a first optical signal and a second optical signal from the transmission data,

a combiner that combines the first optical signal and the second optical signal to generate a polarization multiplexed optical signal is provided on an output side of the modulator,

the processor calculates an electric field of the first optical signal and an electric field of the second optical signal based on the transmission data,

the processor performs rotational processing for the electric field of the first optical signal and the electric field of the second optical signal in a process between the calculation of the electric fields and the calculation of the second intensity data, and

the processor controls an amount of rotation of the rotational processing based on the difference between the first intensity data and the second intensity data.

7. The optical transmitter according to claim 1 , wherein

the processor has a low-pass filter function that averages the second intensity data,

the processor does not average the second intensity data when the square law detector generates the first intensity data at a first sampling rate, and

the process or averages the second intensity data when the square law detector generates the first intensity data at a second sampling rate that is lower than the first sampling rate.

8. A method for controlling an optical transmitter including a modulator that generates an optical signal indicating transmission data, the method comprising:

generating first intensity data indicating an intensity of the optical signal by using a photodetector;

calculating, based on the transmission data, an electric field of the optical signal generated by the modulator by using parameters pertaining to a state of the modulator;

calculating second intensity data indicating the intensity of the optical signal based on the electric field;

updating the parameters so as to reduce a difference between the first intensity data and the second intensity data; and

controlling the state of the modulator based on the parameters.

9. An optical transceiver that includes an optical transmitter and an optical receiver, wherein

the optical transmitter includes:

a modulator configured to generate an optical signal indicating transmission data;

a square law detector configured to detect an intensity of the optical signal using a photodetector and output first intensity data indicating the detected intensity; and

a processor configured to

calculate, based on the transmission data, an electric field of the optical signal generated by the modulator by using parameters pertaining to a state of the modulator,

calculate second intensity data indicating the intensity of the optical signal based on the calculated electric field,

update the parameters so as to reduce a difference between the first intensity data and the second intensity data, and

control the state of the modulator based on the parameters.