APPARATUS FOR READING INFORMATION MEMORIZED INTO RADIATION BEAM REFLECTION RECORD STAR 02 LIKE TRACK PATTERN

摘要

A source 10 of radiation such as a laser supplies a writing beam 18 and a reading beam 16 fora record disc 46. An objective lens assembly 38 is adjustable in position to focus the beams at the plane of the disc 46. A photodetector array 50 senses the recorded information and focusing errors. An astigmatic lens 48 disposed in the optical path produces an astigmatic image of the reading beam and the photodetector array 50 comprises a central photodetector for sensing the recorded information, and a quadrant photodetector array surrounding the central photodetector and including separate sectors disposed along orthogonal axes for sensing relative differences of the astigmatic image along the orthogonal axes to provide focus error correction signals to the objective lens assembly 38. Additional flanking photodetectors may sense first order beams produced by a grating 34 for tracking control via a tilting mirror 42. In a modification the quadrant array is disposed from the central photodetector and senses a first order beam.