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Abstract

A QPSK modulator comprising: two phase modulators implemented in parallel for outputting the light phase-modulated with an information signal; a phase shifter for shifting the phase of the light phase-modulated with the first phase modulator of the two phase modulators and for outputting the phase-shifted light; and a combiner for combining output light of the phase shifter and output light of the second phase modulator, in which a drive signal generated by multiplexing a signal of a first and second frequencies and the information signal is inputted into the first and second phase modulators, and in which the QPSK modulator feeds back a detected amount to a voltage applied to the phase shifter so that the phase shift amount may be .pi./2, the detected amount of signals having the frequency of the difference between or the sum of the first and second frequency which are extracted from the modulated light.

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First Claim