Servo unit for optical type information reading device

摘要

A servo unit in an optical information reading device having a lens for focussing an irradiation light beam on the record surface of a disk. A movable mirror positions the focal point of the irradiation light beam formed on the track of the disk. A cylindrical lens is arranged so that the central axis of the cylinder forms a 45 DEG angle with the track direction of the disk to allow a light beam from the record surface to pass through the cylindrical lens. A focussing light receiver having four separate light receiving units is arranged on two orthogonal straight lines. The focussing light receiver receives a focus controlling light beam passed through the cylindrical lens at a distance twice as long as the focal distance of the cylindrical lens. A pair of tracking light receiving elements are provided on both sides of the focussing light receiver such that the light receiving surfaces are flush with the light receiving surface of the focussing light receiver and arranged linearly to receive a pair of tracking controlling light beams. The focal point of the irradiation light beam formed by said focussing lens is positioned on the record surface in response to an output of the focussing light receiver. The movable mirror is adjusted so that the focal point is positioned on the track in response to outputs of the pair of tracking light receiving elements and the focussing light receiver is arranged so that one of the two straight lines coincides with the central line of the cylinder of the cylindrical lens.