Patent ID: 7076176

Claim:
An optical signal receiver comprising: an opto-electric converter for converting an optical signal transmitted from an opposed transmitter into an electric signal; a reproduction circuit for reproducing a data signal from an output of said opto-electric converter; a fixed signal generation circuit for generating a fixed signal; a switch for selectively outputting either the data signal reproduced by said reproduction circuit or the fixed signal generated by said fixed signal generation circuit; and a control circuit for detecting an abnormal state of optical signal transmission and controlling said switch, said control circuit being adapted to output the fixed signal from said switch, when it detects the abnormal state while outputting the data signal from said switch, wherein the optical signal includes a main signal and an auxiliary signal giving a DC component level and said control circuit comprises means for detecting an AC component level of the optical signal, means for detecting the DC component level of the optical signal, means for comparing the detected AC component level and the detected DC component level with respective predetermined threshold values, and a clock extraction circuit for extracting a clock component from the electric signal and detecting a synchronized or unsynchronized state, said control circuit being adapted to determine that an abnormal communication state has occurred when it detects at least a state of the AC component level falling under the threshold level, a state of the DC component level falling under the threshold level and/or an unsynchronized state.