A modulated light wave demodulator includes an external cavity type laser having a 4-light wave mixing optical phase conjugate element for inverting the spatial phase of an incident light and a light mixing detector. At least part of a modulated light wave, carrying information to be transmitted, is input as a probe light into the optical phase conjugate element and reacts with a laser light wave to produce a phase conjugate light wave, and the phase conjugate light wave and the laser light wave are subjected to a light mixing detection by a light mixing detector to obtain a demodulated light wave. Alternatively, the intensity of the phase conjugate light wave emitted from the optical phase conjugate element is detected and the modulated light wave and the laser light wave are subjected to a light mixing detection by the light mixing detector when the detected intensity of the phase conjugate light wave becomes maximum, so as to thereby obtain a demodulated light wave.
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