| 摘要 |
The present invention relates to an optical scanning device for reading and/or writing on a plurality of tracks on an optical storage medium (1), said scanning device comprising optical means (17) for focusing a plurality of beams, after being reflected from said medium, onto an observation plane (22), and for introducing astigmatism into at least one of said reflected beams, and a photo-detector (18) comprising a plurality of detector segments, arranged in said observation plane (22) to receive said at least one astigmatic reflected beam. The scanning device further comprises means (19) for generating a focus error signal (FES) by combining signals produced by said detector segments, means (19) for generating a central aperture signal by adding signals from all the detector segments, and means (19, 21) for determining when said central aperture signal exceeds a predefined threshold, indicating a useful range of said focus error signal, and, when this is the case, adjusting the focus of said objective lens (15) based on said focus error signal. According to this design, the CA-signal ensures that tracking is only based on the focus error signal in a range in which tracking can be based on it, thereby ensuring satisfactory closed loop tracking.
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