Coherent optical receiver

A coherent optical receiver comprising:

• a variable optical attenuator that adjusts a power of an input optical signal;
• a polarization beam splitter that generates a first polarization component and a second polarization component that are orthogonal to each other from an optical signal output from the variable optical attenuator;
• a local oscillator that generates local oscillator light of a target wavelength;
• a 90-degree optical hybrid circuit that mixes the first polarization component and the local oscillator light to generate a first optical signal and a second optical signal and mixes the second polarization component and the local oscillator light to generate a third optical signal and a fourth optical signal, a phase of the second optical signal being shifted by 90 degrees from a phase of the first optical signal, and a phase of the fourth optical signal being shifted by 90 degrees from a phase of the third optical signal;
• a photo detector circuit that respectively converts the first through fourth optical signals into first through fourth electric signals;
• first through fourth amplifiers that respectively amplify first through fourth electric signals;
• again controller that controls gains of the first through fourth amplifiers;
• a detector that detects amplitudes of output signals of the first through fourth amplifiers;
• a memory that stores, for each of the first through fourth amplifiers, dependence information that indicates a relation between an input optical power of an optical signal of a specified wavelength and a gain of a corresponding amplifier, the dependence information being obtained when an optical signal of the specified wavelength is input into the coherent optical receiver and an amplitude of the output signal of the corresponding amplifier is controlled to be a specified target amplitude;
• and
• a controller that calculates a first input optical power based on an amplitude of the output signal of the first amplifier, a gain of the first amplifier, and the dependence information corresponding to the first amplifier, calculates a second input optical power based on an amplitude of the output signal of the second amplifier, a gain of the second amplifier, and the dependence information corresponding to the second amplifier, calculates a third input optical power based on an amplitude of the output signal of the third amplifier, a gain of the third amplifier, and the dependence information corresponding to the third amplifier, calculates a fourth input optical power based on an amplitude of the output signal of the fourth amplifier, a gain of the fourth amplifier, and the dependence information corresponding to the fourth amplifier, calculates an input optical power of the target wavelength based on the first through fourth input optical power, and controls an attenuation amount of the variable optical attenuator based on the calculated input optical power, when a wavelength division multiplexed optical signal is input into the coherent optical receiver.