| 摘要 |
PURPOSE:To preclude wavelength deterioration due to wavelength dispersion by shifting an operation point by a half cycle on an operation characteristic curve with the use of an operation point switching signal. CONSTITUTION:The waveform of the input signal of an MZ optical modulator 6A includes a superposed low frequency signal. The modulator 6 is driven with the signal to obtain an amplitude-modulated output light signal. When a driving voltage is set to the upper half cycle of the input/output characteristic of the modulator 6A while there is no operation point drift, the low frequency signal component of the input signal is modulated in mutually opposite-phase relation. No frequency component is therefore detected in the output of a photoelectric converting circuit 18. If an operation drift is generated, the low frequency component of the input signal is modulated in in-phase relation and the output light signal is 180 deg. different. A DC signal with a level corresponding to the operation point drift is obtained in the output of a phase detecting circuit 20. A bias control circuit 22 controls the bias of the modulator 6A so that the output light signal contains no frequency component, and corrects the operation point drift to hold the operation point. |
