Optical transmitter, communication apparatus, and method of controlling bias voltage of electro-optic modulator

An optical transmitter comprising:

• an electro-optic modulator;
• a monitor circuit that monitors output light of the electro-optic modulator;
• and
• a processor that controls a bias voltage of the electro-optic modulator using a monitoring result of the monitor circuit,
• wherein the processor superimposes a first dither signal with a first frequency and a second dither signal with a second frequency different from the first frequency, onto one bias voltage in a time sharing manner, calculates a first control error based on a first component oscillating at the first frequency and a second control error based on a second component oscillating at the second frequency based on the monitoring result, and determines a control value for controlling the bias voltage using the first control error and the second control error, and
• wherein the monitor circuit has a photodetector that detects a portion of the output light of the electro-optic modulator, a first filter that extracts the first component oscillating at the first frequency from an output of the photodetector, and a second filter that extracts the second component oscillating at the second frequency from the output of the photodetector, and
• wherein the processor synchronizes switching between the first dither signal and the second dither signal with switching between the first filter and the second filter.

A communication apparatus comprising:

• an optical transceiver;
• and
• an electrical or electronic device provided to operate the optical transceiver,
• wherein the optical transceiver includes an electro-optic modulator, a monitor circuit that monitors output light of the electro-optic modulator, and a processor that controls a bias voltage of the electro-optic modulator using a monitoring result of the monitor circuit,
• wherein the processor superimposes a first dither signal of a first frequency and a second dither signal of a second frequency, which is different from the first frequency, onto the bias voltage in a time sharing manner, calculates a first control error based on a first component oscillating at the first frequency and a second control error based on a second component oscillating at the second frequency from the monitoring result, and determines a control value for controlling the bias voltage using the first control error and the second control error,
• wherein the monitor circuit has a photodetector that detects a portion of the output light of the electro-optic modulator, a first filter that extracts the first component oscillating at the first frequency from an output of the photodetector, and a second filter that extracts the second component oscillating at the second frequency from the output of the photodetector, and
• wherein the processor synchronizes switching between the first dither signal and the second dither signal with switching between the first filter and the second filter.

A method of controlling a bias voltage of an electro-optic modulator, comprising:

• superimposing a first dither signal with a first frequency and a second dither signal with a second frequency different from the first frequency onto the bias voltage of the electro-optic modulator in a time sharing manner;
• monitoring an output light of the electro-optic modulator;
• calculating a first control error based on a first component oscillating at the first frequency, and a second control error based on a second component oscillating at the second frequency from a monitoring result;
• and
• determining a control value for controlling the bias voltage using the first control error and the second control error,
• wherein the monitoring of the output light of the electro-optic modulator includes extracting the first component from the output light of the electro-optic modulator by a first filter, and extracting the second component from the output light of the electro-optic modulator by a second filter, and
• switching between the first dither signal and the second dither signal is synchronized with switching between the first filter and the second filter.

A communication apparatus comprising:

• an optical transceiver;
• and
• an electrical or electronic device provided to operate the optical transceiver,
• wherein the optical transceiver includes an electro-optic modulator, a monitor circuit that monitors output light of the electro-optic modulator, and a processor that controls a bias voltage of the electro-optic modulator using a monitoring result of the monitor circuit, and
• wherein the processor superimposes a first dither signal of a first frequency and a second dither signal of a second frequency, which is different from the first frequency, onto the bias voltage in a time sharing manner, calculates a first control error based on a first component oscillating at the first frequency and a second control error based on a second component oscillating at the second frequency from the monitoring result, and determines a control value for controlling the bias voltage using the first control error and the second control error,
• wherein the electro-optic modulator is a Mach-Zehnder modulator in which a first child modulator and a second child modulator are nested to form a parent modulator, and
• wherein the processor superimposes the first dither signal onto each of a first bias for the first child modulator, a second bias for the second child modulator, and a third bias for the parent modulator in a first section of a bias control cycle to detect the first component from the output light of the electro-optic modulator, and superimposes the second dither signal onto each of the first bias, the second bias, and the third bias in a second section of the bias control cycle to detect the second component from the output light of the electro-optic modulator.

A method of controlling a bias voltage of an electro-optic modulator, the electro-optic modulator being a Mach-Zehnder modulator in which a first child modulator and a second child modulator are nested to form a parent modulator, comprising:

• superimposing a first dither signal with a first frequency and a second dither signal with a second frequency different from the first frequency onto the bias voltage of the electro-optic modulator in a time sharing manner;
• monitoring an output light of the electro-optic modulator;
• calculating a first control error based on a first component oscillating at the first frequency, and a second control error based on a second component oscillating at the second frequency from a monitoring result;
• and
• determining a control value for controlling the bias voltage using the first control error and the second control error,
• wherein the first dither signal is superimposed onto each of a first bias for the first child modulator, a second bias for the second child modulator, and a third bias for the parent modulator in a first section of a bias control cycle to detect the first component from the output light of the electro-optic modulator, and the second dither signal is superimposed onto each of the first bias, the second bias, and the third bias in a second section of the bias control cycle to detect the second component from the output light of the electro-optic modulator.