| 摘要 |
PROBLEM TO BE SOLVED: To provide, at comparatively low costs, an optical disk apparatus which uses an SSD method as a focus-error detection method. SOLUTION: An optical head device comprises a light source 51, image- formation optical systems 52, 53, 54 which condense radiant light from the light source 51 on the face of an optical information recording medium 55, an image-reformation optical system 56 which recondenses returned light from the optical information recording medium 55, a first photodetector 1 which is arranged and installed before the focus of the image-reformation optical system 56 and a second photodetector 2 which is arranged and installed after the focus. The first photodetector 1 receives nearly the semicircle of converged light by the image-reformation optical system 56, and the second photodetector 2 receives nearly the semicircle of diverged light after convergence by the image-reformation optical system 56. On the basis of the difference between the output of the first photodetector 1 and the output of the second photodetector 2, a focus-error signal by an SSD method is detected. |
