| 摘要 |
PROBLEM TO BE SOLVED: To detect a tracking error signal and/or land and groove discrimination signal without being affected by defocusing. SOLUTION: The photodetector 10 which receives the light spot groups of the reflected light beams from an optical disk has photodetecting sections 103, 104 and 105 for receiving the light spots for detecting the respective signals among the light spot groups by trisecting the light spots in a prescribed direction. The respective signals described above are obtained by making computation using the detection signals of the respective divided segments of the divided light spots. In such a case, the deterioration in the signal characteristics by the influence of defocusing is prevented by setting ϕ of necessary and sufficient magnitude in such a manner that the value of 2×λ×β/NA/ϕ attains a prescribed permissible set value (for example, 0.3) or below with respect to the wavelength λof the light beams, the numerical aperture NA of the objective lens, the magnification β in the dividing direction of the focal length on the optical recording medium and the photodetecting means side and the spot diameter ϕin the dividing direction of the light spots. |
