| 摘要 |
PURPOSE:To make a detector small-sized by reducing a distance of a photodetecting part for correcting a radial skew, by using a concave lens for enlarging an image magnification of an image of the detector as a cylindrical concave lens. CONSTITUTION:A concave lens for enlarging an image magnification of an image of a detector 11 is used as a cylindrical concave lens 19, so that the image magnification in the direction where photodetecting parts 15, 16 for correcting a radial skew are placed is not enlarged. Magnification in the lengthwise direction of the cylindrical concave lens 19 is magnification in case when a plano-concave lens does not exist, and when length on a disk 2, length on the detector 11, and focal distances of an objective lens 8 and a collimator lens 6 are denoted as lambda0, lambda1, and f0, f1, respectively, lambda1=(f1/f0).lambda0.K (K is a constant by a position of the detector 11). In this way, magnification of the lengthwise direction of the cylindrical concave lens 19, namely, the direction where the photodetecting parts 15, 16 for correcting a radial skew or a photodetecting part 12 are placed becomes 1/beta of magnification in the direction where photodetecting parts 13, 14 for tracking-servo are placed, intervals of the photodetecting parts 12, 15 and 16 can be made small, and the detector 11 can be made small-sized. |
