LIGHT-EMISSION ERROR PREVENTING CIRCUIT FOR OPTICAL TRANSMITTER

A subscriber-side optical network unit (ONU) has a control LSI (10) for outputting a data signal and a pre-bias signal at fixed intervals, an optical transmitter-receiver (30) for producing optical output in response to these signals, and a light-emission error preventing circuit (20) for preventing light-emission error. First and second light-emission error detecting circuits (21) and (22) output an abnormality detection alarm signal when no rising edge occurs in the data signal and pre-bias signal for a prescribed period of time. An OR element (23), when receiving the abnormality detection alarm signal from at least one of the first and second light-emission error detecting circuits (21) and (22), supplies the optical transmitter-receiver (30) with a shutdown signal and halts the optical output.