| 摘要 |
In the optical head, laser beams are converged by the objective lens and projected on the light-receiving surface of the optical disk. The diverged laser beam is reflected by the light-receiving surface, converged by the objective lens into parallel laser beams, and directed to the light shielding plate. Only the laser beam passing through the region spaced from the optical axis is picked up by the light shielding plate, and part of this laser beam is converged by a convergent lens and projected on photo insensitive and first and second photo sensitive regions of the photo detecting unit. The regions of the photo detecting unit are deviated by DELTA from a back focal point of the convergent lens, and the optical axis of the convergent lens passes through one of the first and second photo sensitive regions. The photo insensitive region has a width d which is not more than twice the larger of absolute values ¦h3( DELTA , delta c)¦ and ¦h3( DELTA , - delta c)¦ of variables given as follows: h3( DELTA , delta c)={-/+ DELTA /m+2m delta c}(yO/F) h3( DELTA , delta c)={-/+ DELTA /m-2m delta c}(yO/F) where +/- delta c is a maximum defocusing allowance with respect to the convergent point of the objective lens, m is a magnification of the beam spot formed on the regions of the photodetector, yO is a radius of an exit pupil of the objective lens, and F is a distance between a principal point of the objective lens and the convergent point of the objective lens.
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