Abstract: A volume holographic memory for recording three-dimensional distribution of interference fringes generated owing to interference between signal light and reference light is provided, wherein the volume holographic memory is formed into a cylindrical shape. Also an optical information recording/reproducing apparatus is provided which incorporates an interference-fringe generating unit for generating interference fringes formed owing to interference between signal light and reference light; a storage unit for storing three-dimensional distribution of the interference fringes generated by the interference-fringe generating unit; a detecting unit for detecting diffracted light obtainable from the storage unit by irradiating the storage unit with reference light; and a reproducing unit for reproducing signal light from diffracted light detected by the detecting unit, wherein the storage unit is constituted by a volume holographic memory having a cylindrical shape.

Abstract: A plurality of tracks are spirally arranged in a group on an optical disc. The tracks in the group are simultaneously radiated by laser beams. Adjacent tracks in the group are disposed at a radial distance d1, and adjacent spots of the laser beams on the tracks are disposed at a radial distance ds which is smaller than the distance d1.

Abstract: A focus error detector comprises an inner group of light detector elements formed in a circular area about a focus point of an illumination beam for generating a first focus error signal. A second group of light detector elements formed in an annular-shaped area around the circular area for generating a second focus error signal, an amplifier for amplifying the second error signal, and an adder for combining the first and second focus error signals to generate a composite focus error signal.

Abstract: In a focus servo apparatus for an optical type information read-out system having a light source, a condenser lens for focusing light from the light source on a recording surface of a recording medium and for focusing light reflected by the surface and containing the information recorded on the surface, a cylindrical lens for passing the reflected light, a light receiving element responsive to the reflected light from the cylindrical lens for providing an output and a driving device responsive to the output for regulating the position of the condenser lens relative to the surface, the cylindrical lens has its main, or primary, principal point substantially at the focusing point of the condenser lens for the reflected light and the physical axis of the cylindrical lens makes an angle of about 45.degree. with respect to the moving direction of the recording medium.

Abstract: A beam splitter employed in an optical disc player comprises a pair of triangular prisms which are combined and adhered together to form an integral rectangular prism. The thus formed beam splitter is positioned on a metal chip whose surface is substantially rectangular having adjacent two sides projecting upward. The adjacent lower surfaces of the beam splitter fit the upward projecting sides of the metal chip and an adhesion material is coated only on the half triangular region defined by the sides and the combining line of the prisms to adhere the beam splitter to the metal chip.