Semiconductor laser driving circuit, semiconductor laser device, image recording apparatus, and optical disk apparatus

#The present invention relates to a semiconductor laser driving circuit which can be used in an optical disk apparatus, a laser beam printer, a copying machine, an optical data communication system, various measuring device, or the like, an optical disk apparatus and an image recording apparatus using the semiconductor laser driving circuit.

#A semiconductor laser has many features: easy modulation of light-emission power by control of the driving current, a compact structure, small consumption power, and high electro-photo conversion efficiency. For this reason, the semiconductor laser is popularly used in an optical disk apparatus and an image recording apparatus such as a laser printer, a copying machine, or the like. However, the semiconductor laser has the following characteristics:

#(1) The differential quantum efficiency changes due to changes in temperature and aging.

#FIG. 1 is a circuit diagram showing the arrangement of a semiconductor laser driving circuit according to the first embodiment of the present invention;

#FIG. 2 is a circuit diagram showing the arrangement of a semiconductor laser driving circuit according to the second embodiment of the present invention;

#FIG. 3 is a circuit diagram showing the arrangement is of a semiconductor laser device for a laser beam printer apparatus according to the third embodiment of the present invention;

#The preferred embodiments of the present invention will be described hereinafter with reference to the accompanying drawings.

#Prior to the description of the embodiments of the present invention, FIG. 9 shows the results of simulation of the response waveforms of the monitor current output from a light detector comprising a photodetector (PD) upon turning on/off a conventional semiconductor laser device. When the driving current is not set to be completely "0" but a constant current is supplied during the extinction period in which the semiconductor laser does not emit light, good pulse characteristics can be obtained, as indicated by a response waveform 200, although the extinction ratio lowers. In contrast to this, when the driving current is set to be completely "0", the waveform cannot have a sharp leading edge, the feedback efficiency lowers, and sufficient OFF characteristics cannot be obtained, as indicated by a response waveform 201. In addition, a light-emission delay occurs. In order to improve the OFF characteristics, when a control signal is set at negative level in the extinction state to attain deep modulation, the waveform in the ON state deteriorates, as indicated by a response waveform 202, resulting in an unstable control operation. When control indicated by the response waveform 201 or 202 is made, a light-emission delay occurs, and the edge of the printed image is blurred or shifted in, e.g., a laser printer.

#In the embodiment of the present invention, the final output-stage transistor in a driving circuit for generating a driving current is controlled not to be completely turned off during the extinction period in which the semiconductor laser does not emit light, on the basis of the above-mentioned simulation results. More specifically, a voltage equal to in the vicinity of the operation threshold value voltage is applied across the base-emitter path of an output-stage transistor, which generates a driving current to be supplied to a semiconductor laser, during the extinction period of the semiconductor laser, thereby maintaining a constant driving current and preventing a light-emission delay. Furthermore, not only prevention of such light-emission delay, but also the extinction ratio is improved by setting the driving current to be supplied to a semiconductor laser during the extinction period to be substantially "0".