Patent ID: 6124988
Filing Date: 2000-09-26
Classification: G11B

Abstract:
An objective lens with a first numerical aperture, comprising:a refracting surface that is in rotatory symmetry about an optical axis,wherein a part of the refracting surface is displaced along the optical axis as an annular concave portion centered on the optical axis, andwherein the annular concave portion has values r.sub.1 and r.sub.2 that reduce RMS aberration V when VdV/d(A.sub.21)=0; dV/d(.DELTA.1)=0; and dV/dC=0, the RMS aberration V satisfying: ##EQU5## and wherein r represents a radius from the optical axis, A.sub.63 represents a fifth order spherical aberration coefficient, A.sub.42 represents a third order spherical aberration coefficient, A.sub.21 represents a defocus aberration coefficient, C represents a wavefront offset, .DELTA.1 represents .DELTA.h/(n-1), .DELTA.h represents a depth of the annular concave portion, n represents a refractive index of the objective lens, r.sub.1 represents NA.sub.in /NA.sub.o, r.sub.2 represents NA.sub.out /NA.sub.o, NA.sub.o represents a diameter of a region having a second numerical aperture corresponding to an effective numerical aperture smaller than the first numerical aperture, NA.sub.in represents a diameter of a region formed by an inner edge of the annular portion centered on the optical axis, and NA.sub.out represents a diameter of a region formed by an outer edge of the annular portion centered on the optical axis.