OPTICAL SIGNAL INPUT DISCONNECTION DETECTOR

PURPOSE:To correctly detect the disconnection of a non-return-to-zero(NRZ) optical signal input by comparing an output signal from a differential circuit for forming an RZ waveform from the output signal of a photoelectric conversion circuit and for outputting the shaped signal with a previously determined threshold to decide the 'disconnection' of the optical signal input. CONSTITUTION:An NRZ optical signal is converted into an NRZ electric signal by the photoelectric conversion circuit 1 and its RZ waveform is shaped by the differential circuit 2. A clock whose phase is different from the output signal of the circuit 2 only by a half wavelength is extracted from a clock extraction circuit 3 and an optical input disconnection deciding circuit 7 compares the output signal of the circuit 2 with the threshold by the timing of the clock to decide the 'disconnection' of the optical signal input. Thus, the disconnection of the NRZ optical signal input can correctly be detected.