| 摘要 |
PURPOSE:To realize the detection of a focus error signal against an optical disc by providing three, at least, sets of optical systems constituted with a condensing optical system and a detecting optical system consisting of a light guide layer and the diffraction grating of an non-equal space curve and taking a difference signal between light quantities of reflecting light beams propagating on the light guide layer. CONSTITUTION:Since light spots 50a, 50b and 50c are shifted in order by every approximately 10mum in the normal direction of an optical disc plane, the outputs (a) and (c) of optical detectors 11a and 11c represent the same fluctuation as an optical detector 11b, however, their peak positions being shifted symmetrically. Therefore, the difference (a-c) between the outputs (a) and (c) becomes zero when the recording plane of an optical disc 30 is positioned on the light spot 50b, and a focus error signal that takes a positive or a negative value according to the displacement direction of the disc is obtained. Also, an information signal recorded on the optical disc 30 can be reproduced by the output of the optical detector 11b. |
