| 摘要 |
PURPOSE:To provide one balanced photodetector having a satisfactory S/N by increasing an AM noise effect even when disturbance is applied to the two output fibers of a photocoupler. CONSTITUTION:A photocoupler 3 synthesizes outputs from a local light emitting source 1A and an intensity modulated signal light source 2. Photodetectors 4A and 4B execute heterodyne photodetection to synthesized signal beams 12A and 13A from the photocoupler 3. An amplifier 5A and a variable gain amplifier 5B amplify the outputs of the photodetectors 4A and 4B. Band pass filters 6A and 6B pass only the intensity modulated frequency signals 12D and 13D. A subtracter 7 subtracts the intensity modulated frequency signals 12D and 13D. A control circuit 8 adjusts the gain of the variable gain amplifier 5B and equalizes the amplitudes of the amplified signals 12C and 13C. Band pass filters 6C and 6D pass the amplified signal 12C, AM noise 22C, amplified signal 13C and AM noise 23C. A differential circuit 9 synthesizes the amplified signals 12C and 13C and cancels the AM noise 22C and 23C. |
