To reduce a parasitic waveform in dimension by providing optical control parts in an intermediate area and a far axial area and interrupting or scattering light to be incident upon these parts.
Spherical aberration of an objective lens 200 is used for compatibility of a recording medium formed with a thin disk 20 and a thick disk 30. Consequently, the incident light upon the intermediate area 222-223 is interrupted or scattered. The incident light upon the far axial area 224-225 is used for the disk 20, for example, when the recording medium is a DVD, while the incident light upon a paraxial area 221 is used for the disk 30, for example, not only when a CD is adopted but also for the disk 20. 1st and 2nd surfaces 211 and 213 for interrupting the passing light in the areas 222-223 and 224-225 are provided on a transparent member 210 positioned on the optical path, and the incident light upon the other areas is transmitted through the member 210. The control surfaces 211 and 213 are provided by forming their total reflection coating layers or interrupting films in specified positions on one surface of the member 210. Consequently, the parasitic waveform is reduced in dimension, and a high precision initial focus error signal can be detected.
SEI HEIYO
SEONG PYONG-YONG
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