Patent ID: 8611199

Claim:
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 either one of the optical function surfaces is divided into a first region that includes a rotational symmetry axis and through which the first and second incident light beams that substantially contribute to spot formation pass, and a second region that is a ring-shaped region surrounding the first region and through which only the first incident light beam that substantially contributes to spot formation passes; the first region is provided with a first diffraction structure; the second region is provided with a second diffraction structure, the second diffraction structure being different from the first diffraction structure; and the objective lens element satisfies the following conditions: DO 11 ×DO 12>0 (1); and DO 21 ×DO 22<0 (2), where: DO 11 is the single diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ 1 that are diffracted by the first diffraction structure on the first region; DO 21 is the single diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ 2 that are diffracted by the first diffraction structure on the first region; DO 12 is the single diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ 1 that are diffracted by the second diffraction structure on the second region; and DO 22 is the single diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ 2 that are diffracted by the second diffraction structure on the second region.