Optical transmitter, optical transmission apparatus, and method of controlling optical transmitter

1. An optical transmitter comprising:

an optical modulator including a first modulator, a second modulator, and a phase shifter that provides a predetermined optical phase difference between the first modulator and the second modulator;

a light source that makes light enter the optical modulator; and

voltage control circuitry that

detects a distortion of light power characteristics from output light of the optical modulator in a state where no data signal is input to the optical modulator,

adjusts a second bias voltage applied to the second modulator or the phase shifter to correct the distortion when controlling a first bias voltage applied to the first modulator.

2. The optical transmitter according to claim 1 ,

wherein the voltage control circuitry that

sweeps a first bias voltage applied to the first modulator to determine an optimum bias voltage to be set in the first modulator, and

detects the distortion of the light power characteristics in an operation of determining the optimum bias voltage.

3. The optical transmitter according to claim 2 , further comprising:

a pilot signal generator that generates a pilot signal superimposed on the first bias voltage; and

a monitor circuit that monitors output light of the optical modulator to detect a power and a phase of a pilot component included in the output light,

wherein the distortion detector compares a first power value at a first voltage value of the first bias voltage when the pilot component is detected with a second power value of the pilot component detected at a second voltage value away from the first voltage value by integer times of a period of the light power characteristics, to detect the distortion.

4. The optical transmitter according to claim 3 ,

wherein when a difference between the first power value and the second power value is smaller than a first threshold value, the voltage control circuitry compares a third power value at a third voltage value obtained by adding a predetermined value to the first voltage value with a fourth power value at a fourth voltage value obtained by subtracting the predetermined value from the first voltage value, to detect the distortion.

5. The optical transmitter according to claim 3 ,

wherein the voltage control circuitry detects a difference between the first power value and the second power value as an amount of distortion when the difference is equal to or greater than a first threshold value.

6. The optical transmitter according to claim 4 ,

wherein the voltage control circuitry detects a difference between the third power value and the fourth power value as an amount of distortion when the difference is equal to or greater than a second threshold value.

7. The optical transmitter according to claim 2 ,

wherein the voltage control circuitry is configured to determine a correction direction for adjusting the second bias voltage when the distortion is detected, and

adjust the second bias voltage in accordance with the correction direction.

8. The optical transmitter according to claim 7 ,

wherein the voltage control circuitry is configured to compare a fifth power value of the pilot component at a fifth voltage value obtained by shifting the second bias voltage by a predetermined amount in a positive direction, with a sixth power value of the pilot component at a sixth voltage value obtained by shifting the second bias voltage by the predetermined amount in a negative direction to determine, as the correction direction, a direction in which a power value decreases.

9. The optical transmitter according to claim 7 ,

wherein the voltage control circuitry compares a first power value at a first voltage value of the first bias voltage when the pilot component is detected with a fifth power value of the pilot component at a fifth voltage value obtained by shifting the second bias voltage by a predetermined amount in the positive direction to determine, as the correction direction, a direction in which a power value decreases.

10. The optical transmitter according to claim 7 ,

wherein the voltage control circuitry compares a first power value at a first voltage value of the first bias voltage when the pilot component is detected with a sixth power value of the pilot component at a sixth voltage value obtained by shifting the second bias voltage by a predetermined amount in the negative direction to determine, as the correction direction, a direction in which a power value decreases.

11. An optical transmission apparatus comprising:

a plurality of optical transmitters; and

a multiplexer that multiplexes optical signals with different wavelengths, the optical signals being output from the plurality of optical transmitters,

wherein each of the plurality of optical transmitters includes

an optical modulator including a first modulator, a second modulator, and a phase shifter that provides a predetermined optical phase difference between the first modulator and the second modulator,

a light source that makes light enter the optical modulator, and

voltage control circuitry that

detects a distortion of light power characteristics from output light of the optical modulator in a state where no data signal is input to the optical modulator,

adjusts a second bias voltage applied to the second modulator or the phase shifter to correct the distortion when controlling a first bias voltage applied to the first modulator.

12. A method of controlling an optical transmitter, the method comprising:

making light from a light source enter an optical modulator including a first modulator, a second modulator, and a phase shifter that provides a predetermined optical phase difference between the first modulator and the second modulator in a state where no data signal is input;

monitoring output light of the optical modulator to detect a distortion of light power characteristics; and

adjusting a second bias voltage applied to the second modulator or the phase shifter to correct the distortion when controlling a first bias voltage applied to the first modulator.

13. The method of controlling an optical transmitter according to claim 12 , further comprising:

sweeping a first bias voltage applied to the first modulator to determine an optimum bias voltage to be set in the first modulator;

detecting the distortion of the light power characteristics in a process of determining the optimum bias voltage; and

correcting the distortion by adjusting the second bias voltage applied to the second modulator or the phase shifter when the distortion is detected.

14. The method of controlling an optical transmitter according to claim 13 , further comprising:

superimposing a pilot signal on the first bias voltage;

detecting a power and a phase of the pilot component included in the output light of the optical modulator; and

comparing a first power value at a first voltage value of the first bias voltage when the pilot component is detected with a second power value of the pilot component detected at a second voltage value away from the first voltage value by integer times of a period of the light power characteristics, to detect the distortion.

15. The method of controlling an optical transmitter according to claim 14 , further comprising:

when a difference between the first power value and the second power value is smaller than a first threshold value, comparing a third power value at a third voltage value obtained by adding a predetermined value to the first voltage value with a fourth power value at a fourth voltage value obtained by subtracting the predetermined value from the first voltage value, to detect the distortion.

16. The method of controlling an optical transmitter according to claim 14 , further comprising:

detecting a difference between the first power value and the second power value as an amount of distortion when the difference is equal to or greater than a first threshold value.

17. The method of controlling an optical transmitter according to claim 15 , further comprising:

detecting a difference between the third power value and the fourth power value as an amount of distortion when the difference is equal to or greater than a second threshold value.

18. The method of controlling an optical transmitter according to claim 13 , further comprising:

determining a correction direction for adjusting the second bias voltage when the distortion is detected; and

adjusting the second bias voltage in accordance with the correction direction.