| 摘要 |
PURPOSE:To set an accurate focusing position of an objective lens on an information disk for irradiation of a laser beam by providing the line sensors at the positions corresponding to the front and the back of the focus point of the laser light reflected by the information disk and to obtain the difference between detection outputs of luminous fluxes of both line sensors. CONSTITUTION:Line sensors 21 and 22 are set equidistantly from the front and back positions of an image point obtained when an information disk 18 is set at the focal point of an objective lens 17. The laser beam delivered from a laser light source 11 condenses microspots on the disk 18, and the reflected light of the disk 18 is returned by a beam splitter 14 and irradiated simultaneously to the sensors 21 and 22 by a half mirror 19 and a mirror 20 respectively. These sensors perform the scan with a frequency higher than the band of the focus error signal of a scanning circuit 23 to detect a signal pulse. The number of detectors of those line sensors to which the light is irradiated by pulse number/ voltage converting circuits 26 and 27 is detected through comparators 24 and 25. The outputs of these detectors are applied to a differential amplifier 28 to extract the focus error signal. Then the lens 17 is made to follow the disk 18. |
