| 摘要 |
PURPOSE:To attain stable focus control by using a detection signal from two astigmatism means arranged on the same position on the optical axis and having different focii 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 at focusing by an objective lens 16. The beam L reflected in the half prism 14 and passing through a cylindrical lens 17 is made incident inastigmatism lenses 18, 20 arranged in the optical path of the beam L. The lenses 18, 20 are arranged on the same position on the optical axis of the lens 17 and each side end face of the lenses 18, 20 is arranged in contact with each other on the optical axis of the lens 17. The lenses 18, 20 have different focii and the beam L is converged toward focii F1, F2 at focusing. The photodetectors 19 detect two kinds of signals and the focus of the converged light to the disk 1 is controlled by using the signal to attain stable focus control. |
