Optical head comprising a diffraction grating for directing two or more diffracted beams to optical detectors

摘要

For use in combination with an optical source (43) for generating a coherent beam along a main optical axis, an optical head device comprises a diffraction grating (75) having a plurality of grating regions which are responsive to the coherent beam for directing a zeroth-order diffracted beam to a focussing lens (44) and are responsive to an optical beam reflected from an optical recording medium (41) for directing sidewards diffracted beams to an optical detector assembly (45). An effective area of a diameter of about 5 mm is sufficient for the diffraction grating (75). Each sidewards diffracted beam may form an angle of 20° with the main optical axis. The optical detector assembly (45) is used in detecting focussing and tracking errors for the recording medium (41) and can be used in reproducing optical information from the recording medium (41). A six-partitioned optical detector assembly (45) is preferred when the grating regions are four in number. Either a four-partitioned or a three-partitioned optical detector assembly (45) is preferred when the grating regions are two in number. Depending on the grating regions, the optical detector assembly (45) preferably has two optical detector elements (71, 72) which are placed on different levels relative to the diffraction grating.