JPS647335A - Focus controller - Google Patents

Abstract

PURPOSE:To attain a stable focus control by using a detection signal from two astigmatism means supported slidably in the optical axis direction and focused onto different positions 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 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 14 and passing through a cylindrical lens 17 is made incident on astigmatism lenses 18, 20 arranged in the optical path of the beam L and focused onto a different position. 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 lenses 17, 18 and 20 are fixed to a base 22 and movable in the direction of the optical axis. Thus, the adjustment of the optical system is facilitated and the detection signal of the photodetector 19 is used to stabilize the focus control of the converged light to the disk 1.