| 摘要 |
PROBLEM TO BE SOLVED: To limit a current to flow to a light receiving element by separating an average optical input level expressed by a DC component in an electric signal from the light receiving element and lowering a bias voltage from a voltage supply means when the average input level is higher than a prescribed level. SOLUTION: At DC separation part 19 of protection circuit 17, the DC voltage of electric signal is separated and the signal level of separated DC component corresponds to the average optical input level and is inputted to an amplifier circuit 21. The average optical input level inputted to the amplifier circuit 21 is amplified and outputted to the base of transistor 23. When the average optical input level higher than the prescribed level is inputted to the base of transistor 23, corresponding to the average optical input level, a current flows between a collector and an emitter. As a result, an output voltage from a driving circuit regulation circuit 15 to a driving circuit 13 is lowered, and the driving circuit 13 lowers the bias voltage of avalanche photodiode(APD) 3 and lowers a current to flow to the APD 3. |
