| 摘要 |
PROBLEM TO BE SOLVED: To prevent the destruction of a light-receiving element for light excessive input in a light reception circuit. SOLUTION: In a light input protection circuit 2 inserted between a light direct amplifier 1 and an optic/electric conversion circuit 10, an optical coupler 3 and a voltage control-type optical attenuator 9 are connected to a light input part in series and is connected to the optic/electric conversion circuit 10. The light-receiving element 4 converting the light signal branched in the optical coupler 3 into electricity, a current/voltage converter 5, a comparator 6 comparing the voltage of the branched light signal and reference voltage, a driving circuit 7 driving the voltage control-type optical attenuator 9 and a time constant circuit 8 executing timewise control are connected in series. The on-time of the voltage control-type optical attenuator 9 is delay-controlled through the time constant circuit 8. |
