Patent ID: 7366362

Claim:
A QPSK light modulator comprising: a light source for outputting a light having a constant wavelength; a first optical demultiplexer into which the light from the light source is inputted, and which branches the light in two directions; a first optical phase modulator into which one of the branched lights is inputted; a second optical phase modulator into which the other of the branched lights is inputted; a π/2 phase shift section provided at a front stage or a rear stage of either the first optical phase modulator or the second optical phase modulator; an encoder that outputs an I signal and a Q signal which are inputted into the first optical phase modulator and the second optical phase modulator through drivers respectively; an optical multiplexer for multiplexing outputs of the first optical phase modulator and the second optical phase modulator to output a multiplexed and modulated light; a second optical demultiplexer provided at a rear stage of the optical multiplexer; and a correcting section for correcting an amplitude unbalance in an optical signal generated in a modulating system from the first optical demultiplexer to the second optical demultiplexer, by using one of the outputs of the second optical demultiplexer, wherein the correcting section includes: a first signal adder for adding a first controlling signal to the I signal; a second signal adder for adding a second controlling signal to the Q signal; a negative feedback section for feeding back the one of the outputs from the second optical demultiplexer to either the first optical phase modulator or the second optical phase modulator; and an amplitude varying section for varying an amplitude of an output signal from the negative feedback section, wherein the signal from the amplitude varying section is added to either the first signal adder or the second signal adder.