Polarized-wave-multiplexing optical transmitter and control method of polarized-wave-multiplexed optical signal

1. A polarized-wave-multiplexing optical transmitter comprising:

an optical combiner generating a polarized-wave-multiplexed optical signal by polarized-wave-multiplexing a first optical modulation signal and a second optical modulation signal;

an optical power fluctuation portion fluctuating optical power of the first optical modulation signal and the second optical modulation signal periodically so that a sign of each fluctuation is opposite to each other;

a total-optical-power detection portion detecting fluctuation amount of total optical power of the polarized-wave-multiplexed optical signal; and

an optical power controller reducing an optical power difference between the first optical modulation signal and the second optical modulation signal based on detection result of the total-optical-power detection portion.

2. The polarized-wave-multiplexing optical transmitter as claimed in claim 1 , wherein the optical power controller controls the optical power of the first optical modulation signal and the second optical modulation signal so that variation of the fluctuation amount of the total optical power with respect to an average of the total-optical-power is a predetermined value or lower.

3. The polarized-wave-multiplexing optical transmitter as claimed in claim 1 , wherein the fluctuation amount of the total optical power is a fluctuation amount of a single cycle of fluctuation of the optical power of the first optical modulation signal and the second optical modulation signal with use of the optical power fluctuation portion.

4. The polarized-wave-multiplexing optical transmitter as claimed in claim 1 further comprising:

a divider dividing an optical signal incoming from a light source into two polarized-waves crossing at right angle;

an incoming angle setting portion setting incoming angle of the two polarized-waves of an optical signal input into the divider from the light source; and

an optical modulator obtaining the first optical modulation signal and the second optical modulation signal by modulating the two polarized-waves,

wherein the optical power fluctuation portion and/or the optical power controller control the optical power of the first optical modulation signal and the second optical modulation signal by changing the incoming angle of the polarized-waves with use of the incoming angle setting portion.

5. The polarized-wave-multiplexing optical transmitter as claimed in claim 1 further comprising:

a first optical attenuator of which optical attenuation amount is changeable; and

a second optical attenuator of which optical attenuation amount is changeable, wherein the optical power fluctuation portion and/or the optical power controller controls the optical power of the first optical modulation signal and the second optical modulation signal by fluctuating optical attenuation amount of the first optical attenuator and the second optical attenuator.

6. The polarized-wave-multiplexing optical transmitter as claimed in claim 1 , wherein:

the first optical modulation signal and the second optical modulation signal are generated from two optical signals output from different two light sources; and the optical power fluctuation portion and/or the optical power controller controls the optical power of the first optical modulation signal and the second optical modulation signal by fluctuating driving electrical signal of the two light sources.

7. The polarized-wave-multiplexing optical transmitter as claimed in claim 6 , wherein:

the first optical modulation signal and the second optical modulation signal are optical modulation signals obtained from a normal phase signal and a reversed phase signal of a RZ modulation signal of which polarized-wave condition is rotated by 90 degrees from each other; and

the optical power fluctuation portion and/or the optical power controller controls the optical power of the first optical modulation signal and the second optical modulation signal by fluctuating a bias voltage applied to the RZ modulator.

8. A control method of a polarized-wave-multiplexed optical signal comprising:

generating a polarized-wave-multiplexed optical signal by polarized-wave-multiplexing a first optical modulation signal and a second optical modulation signal;

periodically fluctuating optical power of the first optical modulation signal and the second optical modulation signal so that a sign of each fluctuation is opposite to each other;

detecting fluctuation amount of total optical power of the polarized-wave-multiplexed optical signal; and

reducing optical power difference between the first optical modulation signal and the second optical modulation signal based on detection result of the detecting of the fluctuation amount.