| 摘要 |
PURPOSE:To reduce the number of parts, and to miniaturize a device by receiving reflected light from a disk of + or -1st order diffracted light generated by a diffraction grating with light receiving elements divided into two or more parts and arranged on the far and near sides of the disk separated from an image-forming point by almost equal distances, and detecting complementarily the changes in the beam size accompanied by a focus dislocation, with both light receiving elements. CONSTITUTION:On an optical path between a light source 1 and a disk 6, a diffraction grating 2 from the light source 1, a beam splitter 3, a collimator lens 4, and an objective lens 5 are provided. The device is constituted so that the objective lens 5, the collimator lens 4, and the beam splitter 3 are provided at a part nearer to the disk 6 on the optical path between a light receiving element 7 and the disk 6, and the light receiving element 7 receiving 0-th order light arranged in the neighborhood of the image image-forming point, and light receiving elements divided into at least two or more at the near side and the far side separating by the same distance in an optical axis direction setting the image-forming point as a reference, are arranged, thereby the 1st order diffracted light and the 1st order diffracted light can be received respectively.
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