1. Field of the Invention
The present invention relates to an optical output control circuit of a semiconductor laser for constantly setting the optical output of the semiconductor laser irrespective of ambient temperature.
2. Description of the Related Art
FIG. 4 shows an optical output control circuit of a conventional semiconductor laser. A reference voltage Vref is applied to the base of a transistor 42 for flowing a modulating electric current to the semiconductor laser 41, and its emitter is connected to the emitter of a transistor 43 having a base to which a pulse signal for modulation is inputted. These two emitters are connected to the collector of a transistor 44, and an operational amplifier 45 is arranged between the base and the emitter of the transistor 44. The voltage of an emitter resistor 46 of the transistor 44 is inputted to the operational amplifier 45. Further, negative feedback is applied to an inversion input terminal (−) of the operational amplifier 45 by a thermistor 47. A non-inversion input terminal (+) of the operational amplifier 45 is connected to the ground by a thermistor 48.
In the above construction, the transistors 42 and 43 are alternately turned on/off by a continuous pulse signal so that the modulating electric current is flowed to the semiconductor laser 41. Further, since the thermistors 47, 48 are connected to the operational amplifier 45, the semiconductor laser 41 is operated such that the modulating electric current flowed to the semiconductor laser 41 is increased correspondingly with a rise in temperature. Thus, the optical output is constantly set even when the semiconductor laser 41 is reduced in efficiency.
Further, one portion of the optical output of the semiconductor laser 41 is detected by a light receiving element 50. The detected voltage is inputted to an optical output control circuit 51. The optical output control circuit 51 operates a transistor 52 for flowing a bias electric current to the semiconductor laser 41 such that the optical output of the semiconductor laser 41 is constant irrespective of the ambient temperature. Namely, if the detected voltage is reduced by a reduction in efficiency of the semiconductor laser 41 caused by the rise in temperature, the optical output control circuit 51 controls the operation of the transistor 52 so as to compensate for this reduction in the detected voltage so that the bias electric current is increased (see Patent Literature 1).
[Patent Literature 1]
Japanese Unexamined Published Patent Application No. H5-259563 (FIG. 1)
The level of the optical output outputted from the semiconductor laser 41 is determined by a sum of the bias electric current and the modulating electric current. Therefore, if the pulse signal for modulation is continuously inputted in the above construction, the modulating electric current and the bias electric current are changed dependently on temperature. Accordingly, the optical output level is controlled by the optical output control circuit 51 such that the optical output level is constant at all times. However, when the pulse signal for modulation is inputted in a so-called burst shape, no modulating electric current is flowed to the semiconductor laser 41 in a period in which no pulse signal for modulation is inputted. Accordingly, no electric current is flowed to the light receiving element 50 so that the optical output control circuit 51 increases the flow electric current to the semiconductor laser 41 to a maximum. As this result, there is a problem of destroying the semiconductor laser 41. Accordingly, no semiconductor laser can be used in the burst mode in the conventional construction.