System for enhanced astigmatic focus signal detection

摘要

An auto-focus system for a DVD reader or other optical access unit which reads and/or writes information from/to at least two information layers on an optical storage medium such as a DVD or DVR. The auto-focus system includes an astigmatic lens. A confocal aperture is placed at the first focal line image generated by the astigmatic lens from light rays reflected from the addressed information layer. The placement of the confocal aperture allows extraneous out-of-focus light rays reflected from non-addressed information layers to be blocked from reaching the photo detector array which then creates a focus error signal (FES). The FES drives a servo assembly connected to an access head of the optical access unit, bringing the access head into focus. In some embodiments, the confocal aperture is "bow-tie" shaped, allowing for simplified assembly and easing in-plane rotational alignment.