| 摘要 |
The present invention relates to an optical reader for a two dimensional storage disc, comprising means (21, 22, 14, 15) for generating a plurality of laser beams and projecting said beams onto a rotating disc, means (24, 25, 26) for detecting the beams after being diffracted by the disc, and means (24, 27, 28) for determining a focus error signal (29) based on one of said plurality of beams. The plurality of beams comprises an array of beams having a first polarization, and a dedicated central focus tracking beam having a second polarization, orthogonal to the first polarization. The beams may be generated by a polarization dependent diffraction element. Or a beam having one polarization may be diffracted into a plurality of beams and recombined with a beam having another polarization. ® KIPO & WIPO 2007
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