| 摘要 |
A light outputted from a laser diode is intensity-modulated by an optical modulator in accordance with a modulation signal in which a sine wave signal outputted from a low frequency oscillator is superposed on a data signal by a drive amplifier. The output light from the optical modulator is branched by an optical coupler and converted into an electric signal by a light receiving element. A frequency component of the electric signal corresponding to the sine wave signal is extracted by a band pass filter. The extracted electric signal is amplified by a variable gain amplifier with a gain. A phase difference between the amplified electric signal and the sine wave signal is determined by a phase detector and a bias voltage controlled based on the phase difference is supplied from a DC amplifier to the optical modulator. The gain of the variable gain amplifier is changed such that the peak value of the extracted electric signal falls within an allowable voltage range of the phase detector. The operation point can be kept at a stable point even if displacement of the operation point is caused due to temperature change and degradation as time.
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