Wavelength conversion device and method of performing wavelength conversion

1. A wavelength conversion device comprising:

a wavelength converter configured to multiplex signal light of first polarization and excitation light to perform wavelength conversion to obtain the signal light of the first polarization of a different wavelength in accordance with the excitation light and configured to multiplex signal light of second polarization, which is perpendicular to the first polarization, and the excitation light to perform wavelength conversion to obtain the signal light of the second polarization of a different wavelength in accordance with the excitation light;

a first modulator configured to modulate the signal light of the first polarization before the wavelength conversion to output the signal light of the first polarization that has been modulated and reduce a modulation component in signal light after wavelength conversion to output the signal light after the wavelength conversion;

a second modulator configured to modulate the signal light of the second polarization before the wavelength conversion to output the signal light of the second polarization that has been modulated and reduce the modulation component in the signal light after the wavelength conversion to output the signal light after the wavelength conversion; and

a multiplexer configured to multiplex the signal light of the first polarization after the wavelength conversion output from the first modulator and the signal light of the second polarization after the wavelength conversion output from the second modulator, thereby to output a polarization multiplexed signal.

2. The wavelength conversion device according to claim 1 , wherein

the first modulator modulates the signal light of the first polarization before the wavelength conversion to output the signal light of the first polarization that has been modulated to the wavelength converter and reduces a modulation component in the signal light of the first polarization after the wave length conversion returned from the wavelength converter to output the signal light of the first polarization after the wavelength conversion, and wherein

the second modulator modulates the signal light of the second polarization before the wavelength conversion to output the signal light of the second polarization that has been modulated to the wavelength converter and reduces a modulation component in the signal light of the second polarization after the wave length conversion returned from the wavelength converter to output the signal light of the second polarization after the wavelength conversion.

3. The wavelength conversion device according to claim 1 , wherein

the first modulator modulates the signal light of the first polarization before the wavelength conversion to output the signal light of the first polarization that has been modulated and reduces a modulation component in the signal light of the second polarization after the wavelength conversion to output the signal light of the second polarization after the wavelength conversion, and wherein

the second modulator modulates the signal light of the second polarization before the wavelength conversion to output the signal light of the second polarization that has been modulated and reduces a modulation component in the signal light of the first polarization after the wave length conversion to output the signal light of the first polarization after the wavelength conversion.

4. The wavelength conversion device according to claim 1 , wherein

one modulator out of the first modulator and the second modulator modulates the excitation light before the wavelength conversion to output the excitation light that has been modulated and reduces a modulation component in the excitation light after the wavelength conversion to output the excitation light after the wavelength conversion, and wherein

the wavelength conversion device includes a detector configured to detect the modulation component of the excitation light after the wavelength conversion output from the one modulator.

5. The wavelength conversion device according to claim 4 , further comprising:

a control circuit configured to control the one modulator so as to reduce the modulation component in the signal light after the wavelength conversion in accordance with the modulation component in the excitation light after the wavelength conversion detected by the detector.

6. The wavelength conversion device according to claim 1 , wherein

each of the first modulator and the second modulator modulates the excitation light before the wavelength conversion to output the excitation light that has been modulated and reduces a modulation component in the excitation light after the wavelength conversion to output the excitation light after the wavelength conversion, and wherein

the wavelength conversion device includes

a detector configured to detect the modulation component of the excitation light after the wavelength conversion output from the first modulator and detect the modulation component of the excitation light after the wavelength conversion output from the second modulator,

a control circuit configured to control the first modulator so as to reduce a modulation component in the signal light of the first polarization after the wavelength conversion in accordance with a detection result of the detector and control the second modulator so as to reduce a modulation component in the signal light of the second polarization after the wavelength conversion in accordance with a detection result of the detector.

7. The wavelength conversion device according to claim 1 , wherein

the first modulator modulates the excitation light a polarized state of which is rotated before the wavelength conversion by 45 degrees to output the excitation light that has been modulated to the wavelength converter, wherein

the wavelength converter multiplexes the excitation light that has been modulated and rotated by 45 degrees and the signal light of the first polarization that has been modulated to perform the wavelength conversion to obtain the signal light of the first polarization of a different wavelength and the excitation light in accordance with the excitation light that has been rotated by 45 degrees, wherein

the first modulator reduces a modulation component in the signal light of the first polarization and a modulation component in the excitation light after the wavelength conversion returned from the wavelength converter, wherein

the second modulator modulates the excitation light the polarized state of which is rotated before the wavelength conversion by 45 degrees to output the excitation light that has been modulated to the wavelength converter, wherein

the wavelength converter multiplexes the excitation light that has been modulated and rotated by 45 degrees and the signal light of the second polarization that has been modulated to perform the wavelength conversion to obtain the signal light of the second polarization of a different wavelength and the excitation light in accordance with the excitation light that has been rotated by 45 degrees, wherein

the second modulator reduces a modulation component in the signal light of the second polarization and a modulation component in the excitation light after the wavelength conversion returned from the wavelength converter, and wherein

the wavelength conversion device includes

a detector configured to detect the modulation component in the excitation light after the wavelength conversion output from the first modulator and the modulation component in the excitation light after the wavelength conversion output from the second modulator, and

a control circuit configured to control the first modulator and the second modulator so as to reduce the modulation component in the signal light of the first polarization or the modulation component in the signal light of the second polarization in accordance with a detection result of the detector.

8. The wavelength conversion device according to claim 1 , further comprising:

a detector configured to detect a modulation component in the signal light of the first polarization after the wavelength conversion output from the first modulator; and

a control circuit configured to control the first modulator so as to reduce the modulation component in the signal light of the first polarization after the wavelength conversion in accordance with a detection result of the detector.

9. The wavelength conversion device according to claim 1 , wherein

a first optical path length by which the signal light of the first polarization after the wavelength conversion returns from a port on a wavelength converter side of the first modulator is set to a distance that corresponds to a wavelength half a wavelength shifted from an integral multiple of a wavelength of the signal light of the first polarization that has been modulated, and wherein

a second optical path length by which the signal light of the second polarization after the wavelength conversion returns from a port on a wavelength converter side of the second modulator is set to a distance that corresponds to a wavelength half a wavelength shifted from an integral multiple of a wavelength of the signal light of the second polarization that has been modulated.

10. The wavelength conversion device according to claim 9 , further comprising:

a first optical delay line configured to adjust the first optical path length with a delay amount that corresponds to the wavelength of the signal light of the first polarization that has been modulated by the first modulator; and

a second optical delay line configured to adjust the second optical path length with a delay amount that corresponds to the wavelength of the signal light of the second polarization that has been modulated by the second modulator.

11. The wavelength conversion device according to claim 9 , further comprising:

a first frequency adjustment circuit configured to adjust the first optical path length with a modulation frequency of the first modulator that corresponds to the wavelength of the signal light of the first polarization that has been modulated by the first modulator; and

a second frequency adjustment circuit configured to adjust the second optical path length with a modulation frequency of the second modulator that corresponds to the wavelength of the signal light of the second polarization that has been modulated by the second modulator.

12. A method of performing wavelength conversion, the method comprising:

multiplexing signal light of first polarization and excitation light to perform wavelength conversion to obtain the signal light of the first polarization of a different wavelength in accordance with the excitation light, and multiplexing signal light of second polarization, which is perpendicular to the first polarization, and the excitation light to perform wavelength conversion to obtain the signal light of the second polarization of a different wavelength in accordance with the excitation light;

modulating the signal light of the first polarization before the wavelength conversion to output the signal light of the first polarization that has been modulated, and reducing a modulation component in signal light after wavelength conversion to output the signal light after the wavelength conversion;

modulating the signal light of the second polarization before the wavelength conversion to output the signal light of the second polarization that has been modulated, and reducing the modulation component in the signal light after the wavelength conversion to output the signal light after the wavelength conversion; and

multiplexing the signal light of the first polarization after the wavelength conversion that has been output and the signal light of the second polarization after the wavelength conversion that has been output, thereby outputting a polarization multiplexed signal.