JPS647336A - Focus controller - Google Patents

Abstract

PURPOSE:To attain a stable focus control by using a detection signal from a detection signal from an optical system composed of a spherical lens having a positive refraction, a refractive substance whose end face is arranged in optical axis and an astigmatism lens so as to control the converged light to a recording medium. CONSTITUTION:A radiated laser beam L is reflected in the recording film of an optical disk and the reflected divergent laser beam is converted into a collimated luminous flux by an objective lens 16 at focusing. The beam L reflected in a half prism is made incident on the astigmatism optical system 10. The optical system 10 consists of a spherical lens 17, an astigmatism lens 20 having a negative refraction and a flat or rectangular block shaped refractive substance such as a flat glass 18 arranged between the lenses 17 and 20 and the side end face of the refractive substance 18 is arranged so as to be brought into contact with the optical axis. The photodetector 19 detects two kinds of signals focused at focii F1, F2 and the focus of the converged light to the disk is controlled by using the signal. Thus, stable focus control is attained.