Abstract: There is disclosed a tracking error detecting apparatus for an optical head, which includes a semiconductor laser apparatus, a diffraction grating, a beam splitter, an objective lens and a photo-detector, in which a laser beam from the semiconductor laser apparatus is divided into a main beam and two side beams at both sides of the main beam by the diffraction grating, the three beams are introduced through the beam splitter and the objective lens to an optical recording medium, beams reflected on the optical recording medium are introduced through the objective lens to the beam splitter and reflected thereon, the reflected beams are made incident on the photo-detector and detected outputs of at least two beams of the three beams from the photo-detector are used to produce a tracking error signal corresponding to the tacking state of the main beam on the optical recording medium.

Abstract: A signal recording and reproducing device for use with an opto-magnetic disk having a number of concentric recording tracks. An optical head sweeps a given track at least twice during a time interval equal to a predetermined reproduction time for the track. Erasure signals and recording signals are supplied to the optical head during the first time sweeping and the second time sweeping, respectively, in order to effect signal erasure and recording. At the time of switching from the first time sweeping to the second time sweeping, the address data indicating the end point written on the terminal portion of the previously recorded signal area are sensed, and the recording is effected from the thus sensed recording end point.

Abstract: Focussing error detecting apparatus includes a composite lens formed of a single glass lens and a single plastic lens and a photodetector disposed behind the composite lens in opposed relation thereto. The glass lens has a uniform focal length, the plastic lens is formed of a pair of lens regions divided by a flat interface including an optical axis of light incident on the composite lens, and the pair of lens regions have back focal lengths different from each other and optical axes spaced apart from each other. The photodetector is placed so that its photodetecting face is at a middle point between converged points of light emitted from the pair of lens regions on the optical axis of the incident light when the light incident on the composite lens is focussed.

Abstract: A focussing error detecting apparatus comprises a compound lens and a photodetector, in which the compound lens has first and second lens regions having different back focal lengths separated by a boundary plane which includes the optical axis of light incident on the compound lens. The photodetector has its photodetecting face positioned on the optical axis of the light between the focal points of the two lens regions, and includes first and second photodetecting elements for receiving light passed through the first lens region at its outer and inner sections with respect to the boundary plane, respectively, and third and fourth photodetecting elements for receiving light passed through the second lens region at its outer and inner sections respectively. A focussing error signal is produced from the difference between the sum of signals from the first and fourth photodetecting elements and the sum of signals from the second and third photodetecting elements.

Abstract: In an optical disc player in which a reading light beam is caused to impinge on an optical disc having a record track formed with an alignment of geometric variations such as a plurality of pits provided in response to information to be modulated in intensity by the record track and a photodetector is provided for detecting the reading light beam coming from the optical disc to generate an output varying in response to the variations in intensity of the detected reading light beam; there are provided a first circuit for producing a reproduced information signal having level variations representing the geometric variations and a tracking error signal which may have level variations depending on the size of each segment of the geometric variations on the strength of the output of the photodetector, a second circuit for detecting the duration of a portion corresponding to each segment of the geometric variations of the reproduced information signal or output of the photodetector and extracting selectively the le