Communication device including nested modulator and bias control method for nested modulator

A bias control method in a transmitter including:

• a nested modulator of Mach-Zehnder configuration which includes first and second sub modulators and a phase shifter that provides a predetermined phase difference between outputs of the first and second sub modulators;
• and
• a controller configured to control the nested modulator, the bias control method comprising
• a) modulating a double pulse including two consecutive light pulses by phase modulation and intensity modulation according to transmission information, wherein the phase modulation is performed on a relative phase between the two consecutive light pulses and the intensity modulation is performed on either one of the two consecutive light pulses to turn off it in intensity, wherein the double pulse thus modulated is transmitted to a receiver of an optical communication system;
• b) controlling bias voltages applied respectively to the first and second sub modulators based on the transmission information and reception information at the receiver so that a first error rate on the intensity modulation is minimized;
• and
• c) controlling a bias voltage applied to the phase shifter based on the transmission information and the reception information so that a second error rate on the phase modulation is minimized.

An optical communication system comprising a transmitter and a receiver,

• the transmitter comprising:
• a nested modulator of Mach-Zehnder configuration including:
• first and second sub modulators;
• and a phase shifter that provides a predetermined phase difference between outputs of the first and second sub modulators;
• and
• a controller configured to control the nested modulator by:
• modulating a double pulse including two consecutive light pulses by phase modulation and intensity modulation according to transmission information, wherein the phase modulation is performed on a relative phase between the two consecutive light pulses and the intensity modulation is performed on either one of the two consecutive light pulses to turn off it in intensity;
• and transmitting the double pulse thus modulated to the receiver;
• the receiver comprising:
• a receiving unit configured to receive the double pulse subjected to intensity and phase modulation to obtain reception information;
• and
• an error calculator that calculates a first error rate on the intensity modulation and a second error rate on the phase modulation based on a first part of the transmission information and a second part of the reception information, wherein the first part and the second part are notified between the transmitter and the receiver,
• wherein the controller of the transmitter controls bias voltages applied respectively to the first and second sub modulators so that the first error rate is minimized, and controls a bias voltage applied to the phase shifter so that the second error rate is minimized.