Focus error detector

摘要

A focus error detector for detecting a signal representing a degree of a focus error of an objective lens with respect to an optical disk includes two focusing lenses each for focusing the reflected light of the optical disk by splitting the reflected light into two beams with respect to the optical axis and two photo-detectors for receiving the respectively focused beams, which simplifies necessary components for a beam size method. The two focusing lenses whose focal lengths are different from each other does not necessitate use of a beam splitter, thereby contributing to reduction of necessary components and optimizing the arrangement of the components. Also, the operational stability is secured by removing the sensitivity to the tilt and shift of an optical axis.