| 摘要 |
PURPOSE: To perform signal reading at the front and rear ends of a diffraction element by relatively moving an objective lens for a track irradiated with an optical spot and compensating nonconformity between the track and the optical spot. CONSTITUTION: A substrate 1 is rotated around the axial line 4 at an angular velocityω, and a diffraction element 14 in the continuous shape of a recessed and projected part is provided on the track 15 supported by the substrate. The optical reading device is composed of a light source 5 for generating a light beam 6, objective lens 7 for converging a beam 9 on the point O on the track 15, and photodetectors 100, 101, 110, 111 supported on the reflection side of the substrate 1. The photodetectors 110, 111 are arranged symmetrically against the radius direction of the substrate 1, with its output driving the objective lens 7 through a differential amplifier 102 and a low-pass filter 21 so as to compensate nonconformity between the track and the optical spot. The photodetectors 100, 101 are arranged symmetrically against the moving direction of the track, generating a signal U (t) at the front and rear ends of the diffraction element 14 from a differential amplifier 112.
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