| 摘要 |
PROBLEM TO BE SOLVED: To prevent the chances of obstructing a luminous flux by an optical system even though there exists the optical axis deviation caused by the tilt of a medium by setting the pupil diameter of an objective lens and a detection optical system to specific values employing the relationships among the incident luminous flux diameter to the objective lens, the optical path lengths between a diffraction grating and the pupil of the lens and the pupil of the system, the focal distance of the lens and a diffraction maximum angle of the diffraction grating. SOLUTION: Let W be the luminous flux diameter after passing through a collimetering lens 2 by a diaphragm 3, L1 and L2 be the optical path lengths between a diffraction grating 4 and an objective lens 7 and the lens 7 and a condensing lens 9, respectively, f be the focal distance of the lens 7, θ be the maximum angle between the effective luminous flux, that is divided by a diffraction grating 4, with respect to an optical axis, Ao be the diameter of the diaphragm of an objective diaphragm 6 and Ac be the pupil diameter of the lens 9. Then, settings are made to satisfy the following relationships, i.e., Ao>W+2L1.tanθ+0.1214.f and Ac>W+2L1.tanθ+0.1214.f+2L2.tanθ. Thus, the adverse effects to track error signals and read signals, which caused by the optical axis deviation made by the tilt of a recording medium 8, are eliminated. |
