Optical quaternary phase modulator

摘要

In a quaternary phase modulator including two phase modulators (15A,15B) disposed in parallel and a phase adjuster (16) that adjusts a phase difference when the outputs of the two phase modulators are combined, there are provided a second light (21) source that introduces light propagated in a backward direction, a first controller (25) that controls the bias of the two phase modulators so that the intensity of the backward light is a minimum on the input side of the quaternary phase modulator, and a second controller (27) that controls the bias of the phase adjuster so that a result monitored by a photodiode having a bandwidth not exceeding the bit rate on the output side of the quaternary phase modulator is a minimum.