Optical reception apparatus and monitor signal generating method

1. An optical reception apparatus comprising:

a local oscillator outputting local oscillation light having a prescribed wavelength;

an optical mixer receiving the local oscillation light and a multiplexed optical signal in which optical signals being different in wavelength from each other are multiplexed, and selectively outputting an optical signal corresponding to the wavelength of the local oscillation light from the multiplexed optical signal;

a photoelectric converter converting the optical signal output from the optical mixer to an electric signal;

a variable gain amplifier amplifying the electric signal converted by the photoelectric converter, to generate an output signal whose output amplitude is amplified to a certain level;

a gain control signal generating circuit generating a gain control signal for controlling a gain of the variable gain amplifier; and

a monitor signal generating unit generating, using the gain control signal, a monitor signal corresponding to power of the optical signal output from the optical mixer, wherein

when the variable gain amplifier is configured such that an amplification factor of the variable gain amplifier increases as a signal voltage of the gain control signal increases, the signal voltage of the gain control signal increases as the power of the optical signal decreases, or

when the variable gain amplifier is configured such that an amplification factor of the variable gain amplifier increases as a signal voltage of the gain control signal decreases, the signal voltage of the gain control signal decreases as the power of the optical signal decreases.

2. The optical reception apparatus according to claim 1 , wherein the gain control signal generating circuit generates the gain control signal based on an amplitude voltage of the output signal output from the variable gain amplifier and a preset target voltage.

3. The optical reception apparatus according to claim 2 , wherein the gain control signal generating circuit generates a gain control signal with which the amplitude voltage of the output signal output from the variable gain amplifier and the target voltage become equal to each other.

4. The optical reception apparatus according to claim 1 , further comprising a transimpedance amplifier converting the electric signal converted by the photoelectric converter from a current signal to a voltage signal.

5. The optical reception apparatus according to claim 1 , further comprising an optical hybrid circuit including the optical mixer, the optical hybrid circuit receiving the multiplexed optical signal and the local oscillation light, and separating the multiplexed optical signal into an in-phase component and a quadrature component, to output four optical signals.

6. The optical reception apparatus according to claim 1 , wherein

the optical reception apparatus has, as the optical mixer, a first optical mixer and a second optical mixer, and

in the multiplexed optical signal, first polarized light and second polarized light being orthogonal to each other are further multiplexed,

the optical reception apparatus further comprising:

a polarization beam splitter separating the multiplexed optical signal into the first polarized light and the second polarized light;

a first optical hybrid circuit including the first optical mixer, the first optical hybrid circuit receiving the first polarized light and the local oscillation light, and separating the first polarized light into an in-phase component and a quadrature component; and

a second optical hybrid circuit including the second optical mixer, the second optical hybrid circuit receiving the second polarized light and the local oscillation light, and separating the second polarized light into an in-phase component and a quadrature component.

7. The optical reception apparatus according to claim 6 , wherein

the first optical hybrid circuit outputs two optical signals included in the in-phase component of the first polarized light as first differential signals, and outputs two optical signals included in the quadrature component of the first polarized light as second differential signals, and

the second optical hybrid circuit outputs two optical signals included in the in-phase component of the second polarized light as third differential signals, and outputs two optical signals included in the quadrature component of the second polarized light as fourth differential signals.

8. The optical reception apparatus according to claim 7 , wherein

the optical reception apparatus includes a plurality of the photoelectric converters, a plurality of the variable gain amplifiers, and a plurality of the gain control signal generating circuits,

the photoelectric converters respectively convert the first to fourth differential signals output from the first and second optical hybrid circuits to electric signals,

the variable gain amplifiers respectively amplify the electric signals corresponding to the first to fourth differential signals output from the photoelectric converters, to generate output signals whose output amplitude is amplified to a certain level,

the gain control signal generating circuits respectively generate gain control signals for controlling gains of the variable gain amplifiers, and

the monitor signal generating unit generates a monitor signal corresponding to power of at least one type of differential signals out of the first to fourth differential signals.

9. The optical reception apparatus according to claim 1 , further comprising a local oscillation light control unit controlling power of the local oscillation light in accordance with the monitor signal.

10. The optical reception apparatus according to claim 9 , wherein the local oscillation light control unit controls the power of the local oscillation light such that a value of the monitor signal attains a prescribed value.

11. The optical reception apparatus according to claim 1 , further comprising:

a multiplexed optical signal adjusting unit adjusting power of the multiplexed optical signal; and

a multiplexed optical signal control unit controlling the multiplexed optical signal adjusting unit in accordance with the monitor signal.

12. The optical reception apparatus according to claim 11 , wherein the multiplexed optical signal control unit controls the power of the multiplexed optical signal such that the value of the monitor signal attains a prescribed value.

13. A monitor signal generating method for generating a monitor signal corresponding to power of an optical signal received by an optical reception apparatus, the method comprising:

causing a multiplexed optical signal in which optical signals being different in wavelength from each other are multiplexed and local oscillation light having a prescribed wavelength to interfere with each other, to extract an optical signal corresponding to the wavelength of the local oscillation light from the multiplexed optical signal;

converting the extracted optical signal into an electric signal;

amplifying the electric signal using a variable gain amplifier, to generate an output signal whose output amplitude is amplified to a certain level;

generating a gain control signal for controlling a gain of the variable gain amplifier; and

generating a monitor signal corresponding to the power of the optical signal using the gain control signal, wherein

when the variable pain amplifier is configured such that an amplification factor of the variable pain amplifier increases as a signal voltage of the gain control signal increases, the signal voltage of the pain control signal increases as the power of the optical signal decreases, or

when the variable gain amplifier is configured such that an amplification factor of the variable gain amplifier increases as a signal voltage of the pain control signal decreases, the signal voltage of the pain control signal decreases as the power of the optical signal decreases.