| 摘要 |
PURPOSE:To attain the optimization and ease of design by not using a special optical instrument as a knife edge so as to simplify the optical system and increasing considerably the degree of freedom of the design. CONSTITUTION:An emitted light from a semiconductor laser 8 is reflected on a polarized separating plane 5a of a beam splitter 5 via a collimate lens 7 and reflected on a disc surface through a lambda/4 plate and an objective 3. The reflected light passes through the lens 3 and the lambda/4 plate 4, and the polarized plane of the reflected light is rotated by 90 deg. around an optical axis 2 to the polarized face of the irradiated laser light from the laser 8. Thus, the reflected light passing through the lambda/4 plate 4 passes through the polarized separating plane 5a as it is, the luminous flux of the upper half is formed on a focus F6 of a condenser lens 6 through the lens 6 and an output Va of a photodetector 10 is zero. The lower half luminous flux of the optical axis 2 passes through a parallel flat plate 13, where it is refracted and the image is formed on a focus F13 on the optical axis and the relation of Vb 0 is obtained in this case. The relation of relative position between the focus of the objective 3 and the disc face 1 is recognized from the value of Va-Vb in this way. |
