Method and apparatus for focus and tracking in an optical disk system

Abstract

Method and apparatus useful in monitoring and adjusting focus and tracking of a read/write beam in a data disk-storage system are provided. In one embodiment, the reflected beam is split into a plus first order, astigmatically focused first beam and a minus first order astigmatically focused second beam. The two beams produce images which change in a mirror symmetric fashion in response to changes in the focus or tracking but in a parallel fashion in response to changes in the optical axis. Signals from detectors of the two beams are combined to negate the influence of optical axis movement or other optical faults. In another embodiment, a beam reflected from the disk is split into a first beam converging on an image plane and a second beam defocused at the image plane. The second beam has a relatively large diameter at the image plane and is relatively insensitive to optical axis deviations. Phase relationships of the second beam are used to determine if the beam impinging of the disk is focused. By combining signals from the first and second beams, the resultant signal is insensitive to optical axis errors and indicates the magnitude and direction of focus correction needed. The second beam is also used in monitoring or measuring tracking of the read/write beam. In both embodiments, the beam splitting is preferably produced by a zone plate.