JPS6421410A - Objective lens for optical pickup - Google Patents

Abstract

PURPOSE:To manufacture a lens with a low cost by applying glass press working, by forming a first plane in an aspherical plane and a second plane in aspherical plane, and satisfying a specific condition. CONSTITUTION:The first plane to which a parallel ray is made incident is formed in the aspherical plane, and the second plane confronting with an optical disk is formed in the spherical plane. Also, it is assumed that the conditions are set as 0.35<(N/2){(N<2>-1)/N<2>}(r1/F)<0.39 (where, N: refractive index of lens medium in wavelength of incident light, F: focal distance, r1: radius of curvature of inference inscribed sphere at apex of aspherical plane), and as 0.3<D/F<0.7 (where, D: central thickness size of lens), and the range of the number of apertures are set as 0.5<NA<0.53. Also, the desirable aspherical shape of the first plane is expressed in equation I by setting an optical axis as an x-axis and the radius direction of the lens as y-axis, and also, at the meridian plane of a rectangular coordinate system where the apex of the aspherical plane is set as an origin. In the equation I, r1 represents the radius of curvature of the in-reference inscribed sphere at the apex of the aspherical plane, and (k) is a circular cone constant, and (d), (e), (f), and (g) are expansion coefficients. In such a way, manufacturing can be facilitated.