Optical receiver

1. An optical receiver comprising:

an optical filter that passes signal light to be received from wavelength-multiplexed signal light;

a light source that outputs local oscillation light;

a 90-degree hybrid circuit that causes the local oscillation light output from the light source to interfere with the signal light passed through the optical filter to output interference signal light;

a converter that converts the interference signal light into an electrical data signal;

a spectrum detector that detects a frequency spectrum of the electrical data signal based on the electrical data signal; and

a controller that controls a center frequency of a passband of the optical filter based on a shape of the frequency spectrum.

2. The optical receiver as claimed in claim 1 , wherein

the controller includes a determinator that compares the shape of the frequency spectrum at a current center frequency of the optical filter with the shape of the frequency spectrum at an existing center frequency of the optical filter, and determines whether to further control the center frequency of the optical filter based on a comparison result.

3. The optical receiver as claimed in claim 1 , wherein

the controller includes a difference detector that detects a difference between powers of respective frequency elements separated by a predetermined frequency in a positive direction and a negative direction from the center frequency of the electrical data signal based on the shape of the frequency spectrum, and a frequency controller that controls the center frequency of the passband of the optical filter so as to reduce the difference.

4. The optical receiver as claimed in claim 3 , wherein

the controller determines a control direction of the center frequency of the passband of the optical filter based on a comparison result between the difference and a threshold value.

5. The optical receiver as claimed in claim 4 , wherein

the controller maintains the center frequency of the passband when the difference is equal to or less than the threshold value.

6. The optical receiver as claimed in claim 1 , wherein

an optical amplifier that amplifies the signal light passed through the optical filter is provided on a front stage of the 90-degree hybrid circuit, and an amplifier that amplifies the electrical data signal is provided on a rear stage of the converter.

7. The optical receiver as claimed in claim 1 , wherein

the controller acquires a frequency offset generated between the signal light and the local oscillation light, and causes the light source to output the local oscillation light at an output frequency that makes the center frequency of a signal characteristic of the electrical data signal zero based on the acquired frequency offset.

8. The optical receiver as claimed in claim 3 , wherein

the predetermined frequency is a frequency obtained by multiplying a half of a transmission signal band of the signal light by a constant of 1 or less.

9. The optical receiver as claimed in claim 3 , wherein

the controller identifies a cutting factor of a signal characteristic of the electrical data signal based on whether or not the difference changes.

10. The optical receiver as claimed in claim 9 , wherein

when the difference does not change, the controller identifies the cutting factor as a transmission line, stores the difference, and adjusts the optical filter in such a direction that the stored difference disappears.

11. The optical receiver as claimed in claim 9 , wherein

when the difference changes, the controller identifies the cutting factor as the optical filter and adjusts the optical filter in such a direction that the difference disappears.