| 摘要 |
PROBLEM TO BE SOLVED: To make it possible to stop output light even if a signal light source is abnormal and besides, even if a modulator driving signal is abnormal by monitoring the modulator driving signal and detecting DC bias abnormality and mark rate abnormality. SOLUTION: A monitor terminal output waveform is fetched from a Mach- Zehunder type external modulator 15 to a monitor circuit 1. A mark rate detecting circuit 2 detects the abnormality by comparing the mark rate of the modulator driving waveform in the monitor output with the prescribed threshold value. A bias detecting circuit 3 detects the abnormality by detecting the DC bias component in the monitor output by the monitor circuit 1. The output of the mark rate detecting circuit 2, the output of the bias detecting circuit 3 and the information on the presence or absence of the output light by the conventional photodetector 18 are inputted via a three-input OR circuit 4 to a shut-down circuit 20. Then, if the abnormality detection is executed by any thereof, the shut-down circuit 20 detects the same and executes the control to shut off the light output of a semiconductor laser 14 via an APC 19. |
