Patent ID: 7526006

Claim:
A laser power control circuit for obtaining a constant laser power by performing negative feedback so that a photoelectric-converted voltage which is obtained by photoelectric-converting a portion of light emitted from a semiconductor laser becomes equal to a reference voltage, said laser power control circuit including: a variable voltage supply connected to the reference voltage; a first switch that selectively outputs either the reference voltage or the photoelectric-converted voltage; a differential amplifier having two input terminals, a first one of the input terminals being connected to the variable voltage supply, and a second one of the input terminals being connected to the first switch; a second switch that selects whether an output of the differential amplifier is transmitted to a laser driving circuit or not; a selector that receives the reference voltage and an output voltage from the differential amplifier, and selects one of the reference voltage and the output voltage from the differential amplifier; and an A/D converter that receives from the selector the selected one of the reference voltage and the output voltage from the differential amplifier, and converts the selected one of the reference voltage and the output voltage from the differential amplifier into a digital signal.