Abstract: An apparatus for scanning the information plane of an optical record carrier with a radiation beam from a diode laser. The reflected radiation from the information plane is separated from the scanning beam and split into two sub-beams by a composite diffraction grating which also functions to focus the respective sub-beams on respective pairs of photodetectors. The composite grating consists of two sub-gratings which adjoin along a common bounding line, the sub-gratings having varying gratings periods and curved grating strips. Movement of the composite grating in the direction of the bounding line changes the axial imaging distance of the sub-beams from the grating, permitting adjustment of such distance to compensate for changes in the axial distance between the diode laser and the photodetector. The diode laser and photodetectors can therefore be assembled together as a single integrated unit.

Abstract: An apparatus for reading and/or recording a trackwise arranged optical structure in a record carrier including a tracking-error detection system with a beam-splitting element and four detectors. The first detector and the second detector cooperate with a first of two sub-beams formed by the beam-splitting element, and the third detector and the fourth detector cooperate with the second sub-beam. It is demonstrated that for larger phase depths of the information details a very suitable tracking signal is obtained by adding the difference between the signals from the first and the second detector to the difference between the signals from the third and the fourth detector.