| 摘要 |
PROBLEM TO BE SOLVED: To generate an alarm even when an output level is reduced or non- modulated light is outputted by the fault of a driving circuit, an external modulator and a light amplifier, etc. SOLUTION: Modulated light outputted from the back surface of a light emitting element 1 is made incident on a photodetector 3 as monitor light, the photodetector 3 outputs a photoelectric current corresponding to the intensity of the monitor light and the photoelectric current is voltage converted by a current-voltage converter 4 and outputted to a clock extraction circuit 5. The clock extraction circuit 5 extracts clock components corresponding to a transmission speed from signals inputted from the current-voltage converter 4 and an alarming circuit 6 detects clocks inputted from the clock extraction circuit 5, judges the abnormality of optical signals when the detected value becomes smaller than a prescribed value and originates the alarm. |
