| 摘要 |
PURPOSE:To constitute a photodetecting circuit low in distortion by detecting a distortion component generated from an avalanche photodiode (APD) and controlling an APD bias so that the distortion component is minimized. CONSTITUTION:An optical signal from an optical fiber F is converted into an electric signal by the APD1. The electric signal is amplified by an amplifier 2 and the amplified signal is inputted to an LPF5 passing a video signal up to fv, a BPF6 for fp and a BPF for 2fp. A pilot signal passing through the BPF6 is converted into a DC signal corresponding to the pilot signal level and a variable gain amplifier 8 is controlled by the DC signal to perform AGC. On the other hand, a secondary harmonic distortion component passed through a band filter 7 is converted into a DC signal corresponding to the distortion level by a detection smoothing part 10 and an APD bias voltage is controlled by a bias control part 3' so that the DC signal is minimized. The variation of a signal output due to the change of the APD bias is compensated by said AGC. |
