| 摘要 |
PROBLEM TO BE SOLVED: To maintain a signal-to-noise ratio to the maxim even when both the photodetective power and the temperature of APD(avalanche photo-diode) of an optical receiver change. SOLUTION: After the output signal S1 of APD is amplified, it is divided into a signal component S3 and a noise component S4 by filters 3 and 4 to be respectively detected by wave detectors 5 and 6. A divider 7 calculates a signal-to-noise voltage S7 from the result. This signal-to-noise voltage S7 is in the same phase with the output S13 of an oscillator 13 when a bias voltage S14 is lower than the optimum voltage and is in the opposite mode when a bias voltage S14 is higher than an optimum voltage. The product of the voltage S7 and the output S13 of the oscillator 13 is calculated by a multiplier 8, and a voltage obtained by amplifying a voltage S9 obtained by detecting the output 8 of the oscillator 13 by a wave detector 9 and proportionally converting it by a DC/DC converter 12 is superimposed with the output S13 of the oscillator 13 and inputted to the bias voltage terminal 1a of APD 1. |
