| 摘要 |
PURPOSE: To make possible accurate detection of a focus error signal without revising a structure of a photodetector in an optical head device. CONSTITUTION: Luminous flux from a light source 4 are reflected by a half mirror 3, and reflected one side luminous flux are converged on a disk 6 by a condenser lens 5. The luminous flux returned from the disk 6 are limited by a diaphragm 10 through a half mirror 3, and further be made incident on quadripartite detection elements 2a-2d of the photodetector 2 through a lens 1 for generating astigmatism. Spots by the astigmatism occur on the detection element surfaces, and though the spots become linear shapes when out-of-focus is large, since incident luminous flux onto the detection surfaces are limited by the diaphragm 10, no spots bulged out of the detection surfaces. Thus, the precise focus error signal is obtained, and further, no cross talk error occurs in a three beams detector also. |
