| 摘要 |
A light source, an objective lens, and an astigmatism generation element producing for light for focus control, and an optical receiver are provided. The astigmatism generation element is interposed between the objective lens and the optical receiver and produces focal points in front of and behind the optical receiver within two mutually orthogonal cross-sectional planes including an optical axis of the reflected light. The astigmatism generation element is a Fresnel mirror that has a plurality of orbicular zones and steps connecting adjacent orbicular zones to each other and that takes the orbicular zones as reflecting mirrors. A depth “d” of the steps is set substantially one-half of a wavelength λ, and a depth d1 of the innermost orbicular zone of the orbicular zones is made larger than the depth d of the steps. The influence of the steps of the Fresnel mirror is minimized, so that a superior servo characteristic can be exhibited. |
