Patent ID: 8767518
Filing Date: 2014-07-01
Classification: G11B

Abstract:
1. An objective lens element that has optical function surfaces on an incident side and an exit side, that converges a first incident light beam of a wavelength λ 1 through a base plate having a thickness t 1 to form a spot, and that converges a second incident light beam of a wavelength λ 2 longer than the wavelength λ 1 through a base plate having a thickness t 2 larger than the thickness t 1 to form a spot, wherein: at least one of the optical function surfaces is divided into an inner part that includes a rotational symmetry axis and through which the first and second incident light beams that substantially contribute to spot formation pass, and an outer part that is a ring-shaped region surrounding the inner part and through which only the first incident light beam that substantially contributes to spot formation passes; a plurality of stair-like diffraction structures is provided on the inner part; a plurality of sawtooth-like diffraction structures is provided on the outer part; and, wherein: the following condition is satisfied: Δd is a difference in level between a first curved surface and a second curved surface, the level difference Δd being disposed in the vicinity of a boundary between the inner part and the outer part, the first curved surface extending at an intermediate level of recesses and projections of the stair-like diffraction structures provided on the inner part, and the second curved surface extending at an intermediate level of recesses and projections of the sawtooth-like diffraction structures provided on the outer part; a is an integer; and n