| 摘要 |
PURPOSE:To surely detect cut off of an optical signal, by connecting a narrow- band band-pass filter which does not pass base-band signals and modulated pilot signals to the output of a photoelectric converter. CONSTITUTION:A narrow-band band-pass filter 8 for no4ise detection is connected with the output of a photoelectric converter 6. Since the noise in the output 7 of the photoelectric converter 6 sharply increases when optical input signals 5 (the size of which is, for example. S1) are cut (namely, when the optical input is zero), only noise is detected by a narrow-band BPF 8 which does not pass the frequencies of base-band signals and modulated pilot signals and sent to a succeeding detector (not shown in the figure) where the noise is rectified and detected. Thus an input cut off is detected. |
