OPTICAL SCANNING DEVICE.

摘要

In an optical scanning device, a light beam (L) is focused astigmatically by an objective lens (O) on a record carrier, e.g. a disk (CD), which reflects it onto a four-quadrant photodetector (PD). The astigmatism is produced by an astigmatic element, e.g., a concave or convex cylindrical lens (ZX) or two mutually perpendicular transparent disks located in the beam path. The light beam (L) can be moved radially by a coarse drive. The objective lens (O) can also be moved radially by a fine drive in a direction (W) which makes an angle alpha to the normals (R) to the data tracks (S). However, the movement of the objective lens (O) controlled by the fine drive through the angle alpha causes imaging defects on the four-quadrant photodector (A, B, C, D), which lead to faulty focusing. To prevent faulty focusing due to imaging defects, a preferred axis of the astigmatic element, e.g., a cylindrical lens or two mutually perpendicular transparent disks, runs parallel to the direction of movement (W) of the objective lens. In the case of a convex cylindrical lens (ZX), the longitudinal axis (LA) is parallel to the direction of motion (W) of the objective lens. In the case of a concave cylindrical lens, the transverse axis is parallel to the direction of motion (W) of the objective lens. In the case of two mutually perpendicular transparent disks, the straight line SG which intersects the two disks is perpendicular to the direction of motion (W) of the objective lens. CD players, video disk players, DRAW disk players, and magneto-optical devices.