| 摘要 |
An objective lens having no more that two lens elements with refractive power is disclosed for reading or writing at high density on a recording medium. The first lens element, in order from a light source side, is biconvex and includes at least one aspheric surface so that it is individually well-corrected for spherical aberration. The second lens element is of a meniscus shape with its concave surface on the recording medium side of the objective lens. If the objective lens is used without a cover glass at the recording medium, the convex surface of the second lens element is made to have an aplanatic surface, which is a surface that is capable of changing the convergence or divergence of a cone of rays without introducing any spherical aberration, coma or astigmatism. Whether or not there is a cover glass at the recording medium, the following condition is satisfiedwhereR4 is the radius of curvature of the surface on the optical recording side of the second lens element L2, andBF is the back focal length of objective lens.The surface of the second lens element on the optical recording medium side is preferably made to be normal to rays converging to/diverging from the focus of the objective lens, thereby minimizing aberrations. If there is no cover glass, this is accomplished by making R4 equal BF. In addition, the following condition is preferably satisfiedwhereF1 is the focal length of the first lens element, andF is the focal length of the objective lens.
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