BIAS MONITOR CIRCUIT OF LASER DIODE

PURPOSE:To make it possible to monitor a bias current stably and highly accurately, by providing a circuit, which detects the fluctuation of the differential quantum efficiency of a laser diode and automatically corrects a reference voltage. CONSTITUTION:A bias current is converted into a voltage by a resistor 11. The output, which is amplified by a differential amplifier 13, i.e., the modulation component in the monitored bias voltage VM, is guided to a peak detector 19 through a coupling capacitor 18 and a converted into a DC voltage VP. The difference between said voltage and the third reference voltage VR3 from a third reference voltage terminal 22 is amplified by a differential amplifier 20. The output voltage V3 can be made to agree with an error voltage DELTAV by adjusting the amplification factor G3 of the differential amplifier 20. The first reference voltage VR1 and the output voltage V3 are added by an adder circuit 21, and its output, which is the second reference voltage VR2, is compared with the monitored bias voltage VM. When the monitored bias voltage exceeds the second reference voltage VR2, abnormality is judged by a voltage comparator 14. Warning is issued from a warning output terminal.