| 摘要 |
PURPOSE:To execute focus control with a high precision by controlling the gain of a driving amplifier in an actuator driving an objective lens by means of an output signal from an oscillation wavelength detecting circuit which operates an output signal from a four-split photo-detector. CONSTITUTION:Light coming from a semiconductor laser 1 is condensed on a disk 8 as a light-condensing spot by an objective lens 19. A reflection-type diffraction grating lens 18 is provided on an optical path in the range from the laser 1 to the lens 19, and the reflected light 4 separated by a lens 18 is detected by the four-split photodetector 6. Output signals from respective light receiving areas in the detector 6 are supplied to a focus error signal detecting circuit 17 and the detected error signals are supplied to a driving circuit 21 to drive the actuator 11 which drives the lens 19. The output signals from respective light receiving areas in the detector 6 are supplied to the oscillation wavelength detecting circuit 22. The gain of the driving amplifier in the circuit 21 is controlled by the output signal from the circuit 22. Thus, focus control can be executed with a high precision.
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