Apparatus for detecting and correcting focusing and tracking errors

Abstract

A light beam emitted from a semiconductor laser is converged on a recording surface of an optical disk by a lens. The light beam is intensity-modulated and reflected in accordance with information recorded on the optical disk. The light beam is guided to a photodetector through a light beam diverging unit. The diverging unit is formed of first and second glass plates individually having parallel flat surfaces. A gap is formed between the first and second glass plates such that the glass plates are situated at substantially equal distances from the optical axis. The glass plates are oppositely tilted to the optical axis. The light beam is divided into three light beams by the first and second glass plates, the three light beams including first and second light beams and a third light beam which passes through the gap between the glass plates. The individual light beams separated by the light beam diverging device are guided to the photodetector, which has four detecting regions separated by separation lines. An output from the detecting regions irradiated with the first and second light beams is electrically processed to detect a focusing error, while an output from the detecting region irradiated with the third light beam is electrically processed to detect a tracking error. If a difference is detected between the outputs, in the focusing and tracking operation, correction signals corresponding to individual detection signals are supplied to a shifting device supporting the lens for movement, so that the lens is kept focused.