JPS647334A - Focus controller - Google Patents

Abstract

PURPOSE:To attain a stable focus control by using a 4-split photodetector so as to detect a light beam from two astigmatism means arranged at an interval along the optical axis and having different focuses and using the detection signal so as to control the focus of the converged light onto a recording medium. CONSTITUTION:The radiated laser beam L is reflected in the recording film of an optical disk 1 and the reflected divergent laser beam is converted into a collimated luminous flux by an objective lens 16. The beam L reflected in a half prism 14 and passing through a cylindrical lens 17 is made incident on astigmatism lenses 18, 20 arranged at an interval along the optical axis of the lens 17 and each side end face of the lenses 18, 20 is arranged so as to be brought into contact with the optical axis of the lens 17. The beam L is converged toward the focus at focusing. The photodetector 19 has the detection area split by a reference line in parallel and perpendicular to the direction of the generator of the lenses 18, 20 and the focus of the converged light to the disk 1 is controlled by using the detected light.