Signal processing device and signal processing method

1. A signal processing device which processes two polarization signals,

the two polarization signals being generated from four digital signals generated through photoelectric conversion and analog-to-digital conversion on four output light components generated by making polarization multiplexed and multi-level modulated signal light interfere with local light using an optical 90° hybrid as signals corresponding to two polarization components of the signal light, the signal processing device comprising:

a first compensation unit which compensates for each of the two polarization signals so as to cancel a first amount of phase rotation which is the amount of phase rotation calculated based on the signal strength of the two polarization signals; and

a second compensation unit which compensates for each of the two polarization signals so as to cancel a second amount of phase rotation which is the amount of phase rotation calculated based on the perturbative component of the two polarization signals,

wherein the first compensation unit includes

a strength calculation unit which calculates the first amount of phase rotation based on the two polarization signals,

a first filter unit which multiplies the first amount of phase rotation by a coefficient for averaging the first amount of phase rotation with time, and

a first phase modulation unit which calculates a first coefficient for cancelling the first amount of phase rotation in the two polarization signals by using the first amount of phase rotation after being processed by the first filter unit, and

the second compensation unit includes

a perturbative component calculation unit which calculates the second amount of phase rotation based on the two polarization signals,

a second filter unit which multiplies the second amount of phase rotation by a coefficient for averaging the second amount of phase rotation with time,

a second phase modulation unit which calculates a second coefficient for cancelling the second amount of phase rotation of a first polarization signal by using the second amount of phase rotation after being processed by the second filter unit, and

a third phase modulation unit which calculates a third coefficient for cancelling the second amount of phase rotation of a second polarization signal by using the complex conjugate of the second amount of phase rotation after being processed by the second filter unit.

2. The signal processing device according to claim 1 , further comprising:

a linear compensation unit which is provided before the first compensation unit and compensates for a linear effect applied to the signal light in a transmission path.

3. The signal processing device according to claim 1 , further comprising:

the optical 90° hybrid;

a photoelectric conversion unit which performs the photoelectric conversion;

an AD conversion unit which performs the analog-to-digital conversion; and

a polarization signal generation unit which generates the two polarization signals from the four digital signals.

4. A signal processing device which is used to process two polarization signals for generating a polarization multiplexed optical signal, the signal processing device comprising:

a first compensation unit which compensates for each of the two polarization signals so as to cancel a first amount of phase rotation which is the amount of phase rotation calculated based on the signal strength of the two polarization signals; and

a second compensation unit which compensates for each of the two polarization signals so as to cancel a second amount of phase rotation which is the amount of phase rotation calculated based on the perturbative component of the two polarization signals,

wherein the first compensation unit includes

a strength calculation unit which calculates the first amount of phase rotation based on the two polarization signals,

a first filter unit which multiplies the first amount of phase rotation by a coefficient for averaging the first amount of phase rotation with time, and

a first phase modulation unit which calculates a first coefficient for cancelling the first amount of phase rotation in the two polarization signals by using the first amount of phase rotation after being processed by the first filter unit, and

the second compensation unit includes

a perturbative component calculation unit which calculates the second amount of phase rotation based on the two polarization signals,

a second filter unit which multiplies the second amount of phase rotation by a coefficient for averaging the second amount of phase rotation with time,

a second phase modulation unit which calculates a second coefficient for cancelling the second amount of phase rotation of a first polarization signal by using the second amount of phase rotation after being processed by the second filter unit, and

a third phase modulation unit which calculates a third coefficient for cancelling the second amount of phase rotation of a second polarization signal by using the complex conjugate of the second amount of phase rotation after being processed by the second filter unit.

5. The signal processing device according to claim 4 , further comprising:

a mapping unit which allocates the plurality of signals to a multi-level signal to generate the two polarization signals;

a DA conversion unit which performs digital-to-analog conversion on each of the two polarization signals after being processed by the first compensation unit and the second compensation unit; and

an electro-optic conversion unit which performs modulation processing using two analog signals generated by the DA conversion unit to generate the optical signal.

6. The signal processing device according to claim 1 ,

wherein the strength calculation unit calculates the first amount of phase rotation θ1 according to Expression (1)

θ 1 =γ(|E x | 2 +|E y | 2 )  (1)

provided, Ex: the strength of the first polarization signal, Ey: the strength of the second polarization signal, and γ: a coefficient.

7. The signal processing device according to claim 6 ,

wherein the perturbative component calculation unit calculates the second amount of phase rotation θ2 according to Expression (2)

θ2=γ2ExE*y  (2)

provided, Ex: the strength of the first polarization signal, Ey: the strength of the second polarization signal, and γ2: a coefficient.

8. The signal processing device according to claim 7 , wherein γ=2 65 2 .

9. A signal processing method which processes two polarization signals,

the two polarization signals being generated from four digital signals generated through photoelectric conversion and analog-to-digital conversion on four output light components generated by making polarization multiplexed and multi-level modulated signal light interfere with local light using an optical 90° hybrid as signals corresponding to two polarization components of the signal light, the signal processing method performing:

first compensation processing for compensating for each of the two polarization signals so as to cancel a first amount of phase rotation which is the amount of phase rotation calculated based on the signal strength of the two polarization signals; and

second compensation processing for compensating for each of the two polarization signals so as to cancel a second amount of phase rotation which is the amount of phase rotation calculated based on the perturbative component of the two polarization signals,

wherein the first compensation processing includes

strength calculation processing for calculating the first amount of phase rotation based on the two polarization signals,

first filter processing for multiplying the first amount of phase rotation by a coefficient for averaging the first amount of phase rotation with time, and

first phase modulation processing for calculating a first coefficient for cancelling the first amount of phase rotation in the two polarization signals by using the first amount of phase rotation after the first filter processing is performed, and

the second compensation processing includes

perturbative component calculation processing for calculating the second amount of phase rotation based on the two polarization signals,

second filter processing for multiplying the second amount of phase rotation by a coefficient for averaging the second amount of phase rotation with time,

second phase modulation processing for calculating a second coefficient for cancelling the second amount of phase rotation of a first polarization signal by using the second amount of phase rotation after the second filter processing is performed, and

third phase modulation processing for calculating a third coefficient for cancelling the second amount of phase rotation of a second polarization signal by using the complex conjugate of the second amount of phase rotation after the second filter processing is performed.

10. A signal processing method which processes two polarization signals for generating a polarization multiplexed optical signal, the signal processing method performing:

first compensation processing for compensating for each of the two polarization signals so as to cancel a first amount of phase rotation which is the amount of phase rotation calculated based on the signal strength of the two polarization signals; and

second compensation processing for compensating for each of the two polarization signals so as to cancel a second amount of phase rotation which is the amount of phase rotation calculated based on the perturbative component of the two polarization signals,

wherein the first compensation processing includes

strength calculation processing for calculating the first amount of phase rotation based on the two polarization signals,

first filter processing for multiplying the first amount of phase rotation by a coefficient for averaging the first amount of phase rotation with time, and

first phase modulation processing for calculating a first coefficient for cancelling the first amount of phase rotation in the two polarization signals by using the first amount of phase rotation after the first filter processing is performed, and

the second compensation processing includes

perturbative component calculation processing for calculating the second amount of phase rotation based on the two polarization signals,

second filter processing for multiplying the second amount of phase rotation by a coefficient for averaging the second amount of phase rotation with time,

second phase modulation processing for calculating a second coefficient for cancelling the second amount of phase rotation of a first polarization signal by using the second amount of phase rotation after the second filter processing is performed, and

third phase modulation processing for calculating a third coefficient for cancelling the second amount of phase rotation of a second polarization signal by using the complex conjugate of the second amount of phase rotation after the second filter processing is performed.