| 摘要 |
PROBLEM TO BE SOLVED: To surely protect a photodetector and others from a light surge by detecting an input level in front of the photodetector and turning off the power source of photodetector or a preamplifier when the level indicates a light surge. SOLUTION: Part of an input light signal which is branched by an optical coupler 11 is converted by the photodetector 12 into an electric signal, and an input level detecting circuit 13 detects the light input level. This value corresponds to the light input level of the photodetector 71. A power-ON/OFF means 20 is composed of switches 21 and 22 which are controlled with the output of the circuit 13, and turns on and off the connection of a bias power source 31, the photodetector 71, a power source 32 for the preamplifier, and the preamplifier 72. Namely, the switch 21 or 22 is turned off in a state wherein a light surge is detected or predicted from the light input level. Further, the switch 21 or 22 is turned on after the light surge recovery, and the element 71 or amplifier 72 can be placed in operation immediately. |
