Abstract: A bifocal crystal optic lens made of a uniaxial crystal comprising two surfaces having a same axis, at least one of the two surfaces being a spherical surface or an aspherical surface. An optical system for an optical pickup device includes; a crystal optic lens consisting of a first lens body made of a transparent uniaxial crystal and a second lens body made of a transparent optical material selected from a second uniaxial crystal and an optical isotropic material, the first and second lens bodies joining via a spherically or aspherically curved interface; and an objective lens aligned with the crystal optic lens in a same optical axis of an incident ray. This optical system for the optical pickup device is capable of making simultaneously two focal points in the same axis, so as to simplify preferably the structure of the compatible video optical disc player for CD and SD to minimize the size thereof, thereby providing a relatively low cost for the manufacture thereof.

Abstract: A method and apparatus for determining an optical disc type. A light beam is directed at an optical disc, and a focal point of the light beam moves in a focus direction with respect to the optical disc. An intensity of a reflection of the light beam from the optical disc is detected during the movement of the focal point of the light beam. A time interval between occurrences of a plurality of peaks of the detected intensity of the reflected light beam is detected, and the optical disc type is determined according to the detected time interval.

Abstract: An apparatus for reading optically recorded data from a disk in which diffracted light beams, which otherwise would be applied to light-detecting elements and which are of higher order than the positive and negative first-order diffracted light beams, are eliminated and only a zeroth-order diffracted light beam is applied to the light-detecting elements. Thereby, a tilt control signal is produced whereby a tilt servo operation can be carried out with a high accuracy. Several different structural arrangements are disclosed for achieving the desired blocking of the higher-than-zeroth-order diffracted light beams.

Abstract: An optical information pickup apparatus for use in an optical disc reader in which a regenerated information signal, a focus servo signal and a tracking servo signal are produced with a single illuminating beam and a single optical system. The light sensing element is divided into four light sensing surfaces. An inner pair of the light sensing surfaces are symmetrically disposed along a line forming a 45.degree. angle with a line on the surface of the detector corresponding to the tracking direction on the recorded disc. The third and fourth light sensing surfaces surround the first and second light sensing surfaces and are divided along the line corresponding to the tracking direction. Signals are combined from the light sensing surfaces to produce the regenerated information signal, the focus servo signal and the tracking servo signal.

Abstract: An optical pickup device to be used in a system for optically playing back recorded information comprises a diffraction grating or a hologram lens instead of a prism for directing a read-out light beam emitted by a light source to the surface of the recording medium and separating a reflection beam reflected by the surface of a recording medium from the read-out light beam. By the elimination of the prism, the size of the pickup device is substantially reduced and the production cost of the device is also reduced.