PROTECTIVE DEVICE FOR SEMICONDUCTOR LASER

PURPOSE:To prevent the deterioration or breakdown of a semiconductor laser by bypassing currents flowing through the semiconductor laser when overcurrents flow through the semiconductor laser and an optical output from the semiconductor laser intends to exceed a fixed value. CONSTITUTION:When overcurrents intend to flow through a semiconductor laser 1, an optical output from the laser 1 increases. The impedance of a photodiode 2 drops at the same time, and input voltage to a minus terminal for a comparison amplifier 5 drops. When the input voltage drops to a value lower than the threshold voltage of the amplifier 5, an output from the amplifier reaches an ''H'' level, and an output from an AND circuit 7 is brought to an ''L'' level. An inverter 8 receives the signal of the ''L'' level, an output from the inverter is brought to an ''H'' level, and a thyristor 4 is turned ON. Consequently, most of currents flowing the laser 1 by that time are bypassed to the thyristor 4 side, and currents hardly flow through the laser 1. Accordingly, the laser 1 can be protected from breakdown or deterioration.