JPS647337A - 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 negative lens power and using a detection signal so as to control the focus of the converged light to a recording medium. CONSTITUTION:A radiated laser beam L is reflected in the recording film of an optical disk 1 and the reflected divergent laser beam is focused by an objective lens 16. The beam L reflected in a half prism 14 is made incident on astigmatism lenses 18, 20 having a negative lens power arranged in the optical path of the beam L. The lenses 18, 20 are arranged at an interval along the optical axis direction (Z direction) of the lens 16 and a side end face of the lenses 18, 20 is arranged so as to be brought into contact with the optical axis of the lens 16. The light beam from the lenses 18, 20 is detected by using a photodetector 19 whose detection area is split by 4. The focus of the converged light to the disk 1 is controlled by using two kinds of detection signals focused in the focii F1, F2 obtained in this way. Thus, the stable focus control is attained.