Optical transmitter and control method therefor

An optical transmitter for generating and transmitting a multiphase modulated signal, comprising:

• a light source generating an optical light;
• a splitter splitting the light received from the light source into a plurality of lights;
• a plurality of optical paths each for inputting a respective light split by the splitter;
• a plurality of phase modulator portions each performing phase modulation of a respective light propagating through a corresponding optical path in accordance with a data signal;
• a first bias supply portion supplying bias voltages for regulating an operating point of each of the plurality of phase modulator portions;
• phase shifter portions each imparting a predetermined phase difference into the respective light propagating through corresponding optical paths;
• a second bias supply portion supplying bias voltages for phase variation to a side of the phase shifter portions;
• a coupler coupling the lights output from the corresponding optical paths, the coupler including an optical coupler that includes a plurality of input ports corresponding to the plurality of optical paths and two output ports and utilizes a multi-mode interference;
• a pilot signal superimposition portion superimposing a pilot signal on bias voltages being supplied to at least the side of the phase shifter portions, the pilot signal having a frequency lower than a frequency of a bit rate of the data signal;
• a monitor portion monitoring an output of one of the two output ports of the optical coupler utilizing the multi-mode interference;
• a bias control portion performing a feedback control of at least the second bias supply portion in accordance with a result of monitoring performed by the monitor portion.

A control method for controlling an optical transmitter that generates and transmits a multiphase modulated signal, comprising:

• generating an optical light using a light source;
• splitting the light received from the light source into a plurality of lights;
• providing a plurality of optical paths each for inputting a respective light split by said splitting;
• performing phase modulation of a respective light propagating through a corresponding optical path in accordance with a data signal;
• supplying bias voltages for regulating an operating point of each phase modulation;
• providing phase shifter portions each imparting a predetermined phase difference into the light propagating through the corresponding optical paths;
• supplying bias voltages for phase difference regulation to a side of the phase shifter portions;
• and
• coupling the light output from the corresponding optical paths, superimposing a pilot signal on only one of bias voltages being supplied to a side of at least one of a plurality of phase modulator portions and to the side the phase shifter portions, the pilot signal having a frequency lower than a frequency of a bit rate of a respective data signal;
• monitoring an output of said coupling;
• and
• performing a feedback control of each supplying of said bias voltages in accordance with a result of said monitoring.

A method of controlling an optical transmitter, comprising:

• supplying bias voltages for regulating an operating point of each phase modulation performed on light portions propagating through optical paths based on a data signal;
• imparting a predetermined phase difference into a light among said light portions;
• supplying bias voltages for phase difference regulation of said light portions and coupling the light portions;
• selectively superimposing a pilot signal on only one of said bias voltages, the pilot signal having a frequency lower than a frequency of a bit rate of the data signal;
• and
• performing a feedback control of each supplying of bias voltages in accordance with a result of a monitoring of an output of the coupling.