Polarization multiplexed light transmitter and control method thereof

A polarization multiplexed light transmitter which transmits a polarization multiplexed light containing a first optical signal and a second optical signal of which polarization states are mutually orthogonal, comprising:

• a delay regulating unit configured to regulate relative delay times of the first and second optical signals;
• a delay control unit configured to make orthogonal polarization components contained in a monitor light obtained by taking out a part of the polarization multiplexed light to interfere with each other, to generate a polarization interfering light, and configured to judge a delay time between the first and second optical signals based on a change in a state of an electric signal obtained by photo-electrically converting the polarization interfering light, and configured to control a regulation amount in the delay regulating unit so that the judged delay time reaches a predetermined value;
• a polarization separating section that separates an output light from a light source section into two lights having polarization states which are mutually orthogonal;
• a first optical modulating section that modulates one of the lights separated by the polarization separating section to generate the first optical signal;
• a first modulation driving section that generates a first modulation signal for driving the first optical modulating section;
• a second optical modulating section that modulates the other light separated by the polarization separating section to generate the second optical signal;
• a second modulation driving section that generates a second modulation signal for driving the second optical modulating section;
• and
• a polarization combining section that combines the first and second optical signals generated by the first and second optical modulating section into one light,
• wherein the delay regulating unit makes relative delay times of the first and second modulation signals output from the first and second modulation driving sections to be variable, and
• the delay control unit comprises:
• an optical branching section that branches the part of the polarization multiplexed light as the monitor light;
• a polarization interfering section that includes a polarization separating device, and the monitor light branched by the optical branching section is fed to the polarization separating device in a state where polarization directions of the orthogonal polarization components contained in the monitor light are deviated from a direction of a main axis of the polarization separating device, to thereby generate the polarization interfering light;
• a photoelectric conversion section that converts the polarization interfering light generated by the polarization interfering section into the electric signal;
• a signal processing section that eliminates at least a direct current component from the electrical signal converted by the photoelectric conversion section;
• a power measuring section that measures the power of the electric signal passed through the signal processing section;
• and
• a delay control section that judges the delay time between the first and second optical signals based on a change in the power measured by the power measuring section, to control the regulation amount in the delay regulating unit so that the judged delay time reaches the predetermined value.