Abstract: A power source control apparatus has a constant current circuit and plural laser diodes. A first switching circuit is turned off, while a second switching element is turned on, before a laser beam is output from the laser diodes. Then, there is formed a circulating circuit for supplying electricity to a current controlling switching element and a reactor. At this time, a fixed constant current is supplied to the reactor, while kept at a command value. When a laser output command signal is generated, the first switching element is turned on, while the second switching element is turned off. Then, there is formed a circuit for supplying electricity to the current controlling switching element, reactor and laser diodes. At this time, influence of inductance of the reactor can be ignored. Thus, a constant current pulse with a steep rise and fall is supplied to the laser diodes.

Abstract: Disclosed is a laser diode protecting circuit adapted to prevent a laser diode from producing an excessive emission when the laser diode is driven at low temperature, thereby assuring that the laser diode will not be damaged or degraded in terms of its characteristic. When the laser diode is started at low temperature, a laser diode protecting circuit has a power monitor circuit for monitoring backward power of the laser diode and a laser diode current limiting circuit for limiting the laser diode current when the backward power becomes equal to the set power. When the laser diode temperature subsequently rises and the backward power falls below the set power, an automatic current control circuit performs automatic current control in such a manner that the laser diode current attains a set current value.