| 摘要 |
PURPOSE:To prevent the deterioration of a semiconductor laser excluding excessive current flow in the semiconductor laser, by driving the semiconductor laser with the output signal of an error amplifier after sample-hold applying a timing signal, and prohibiting the timing signal by a protection circuit when the output signal of a light detection circuit is smaller than or equal to a specified value. CONSTITUTION:A photo-diode generates a current proportional to the light output of a semiconductor laser 1, and an error amplifier 4 amplifies the difference between the output voltage VM of an amplifier 3 and a reference voltage Vref. This value is subjected to sample-hold by a timing signal in a sample-hold circuit 5. A driving circuit 6 for semiconductor laser flows a current corresponding with the output signal of the sample-hold circuit 5 into the semiconductor laser 1. Thereby the driving current of the semiconductor laser 1 is stabilized as a value corresponding with the reference voltage Vref, and the light output of the semiconductor laser 1 is controlled so as to be constant. When the light output of the semiconductor laser 1 is less than or equal to a specified value, and the output signal of the amplifier 3 is less than or equal to the threshold level VTH, a gate 7 turns OFF, the timing signal is prohibited, and the sampling operation of the sample-hold circuit 5 is interruped. |
