Spot Size Focus Error Detection For Multiple Beam Optical Scanning Device

摘要

A device (1) for scanning an optical disc (6), the disc (6) comprising substantially parallel data tracks (10). The device (1) comprises an optical unit for creating track light spots (12) on the data tracks (10) and a plurality of detector light spots on a plurality of signal detectors. The device further comprises a focus error unit for with a beam manipulating element (21, 22, 22a, 92) for providing for at least one of the reflected beams a first optical path (24) to a first segmented detector (8a) and a second optical path (25) to a second segmented detector (8b). The lengths of the first optical path (24) and the second optical path (25) exhibit a difference (Dd), such that an intensity distribution of a first detector light spot on the first segmented detector (8a) is equal to an intensity distribution of a second detector light spot on the second segmented detector (8b) when the at least one track light spot is correctly focussed and that the intensity distribution on the first segmented detector (8a) differs from the intensity distribution on the second segmented detector (8b) when the at least one track light spot is not correctly focussed. The difference (Dd) is substantially less than a quotient of a distance between two adjacent detector light spots (Ds) and a numerical aperture (NA) of the lens (7). A dimension (D1) of a light detecting area of the segmented detectors in a direction of adjacent detector light spots is less than the distance (Ds) between two adjacent detector light spots.