Optical record carrier scanning apparatus with scanning beam focus error detection

Claim 1

An improved apparatus for scanning the information surface of an optical record carrier, which apparatus comprises a source of a beam of radiation, means for focusing such beam into a scanning spot on the information surface which produces reflected radiation therefrom, and a focus-error detection system producing a focus error signal signifying any deviation between the plane of focus of the radiation spot and the information surface, such improvement being characterized in that the focus-error detection system comprises: a first and a second optical diffraction grating arranged parallel to each other in succession in the path of the radiation reflected from the information surface of said record carrier, each grating having a constant strip period, the directions of the grating strips of the two gratings being at a small angle relative to each other so as to form an interference pattern of light and dark bands in radiation emergent from the second grating, such interference pattern having an orientation which changes in accordance with deviations between the plane of focus of said radiation spot and said information surface; and an assembly of radiation detecting elements arranged adjacent the second grating to receive the radiation emergent therefrom, such assembly having coordinate axes oriented at an angle relative to the orientation of said interference pattern at the location of said assembly; whereby changes in the orientation of said interference pattern change the angle thereof relative to said assembly and cause corresponding changes in output signals produced by the detecting elements of said assembly.