Patent ID: 8760989

Claim:
An objective lens element which has optically functional surfaces on an incident side and an exit side, which 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 which 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 a refractive surface which deflects the first and second incident light beams by refractive power all over the surface, the at least either one of the optical function surfaces being divided into an inner part which 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 which 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, and the at least either one of the optical function surfaces having a plurality of discontinuous steps, each of the plurality of steps causes an optical path difference which is an integral multiple of the wavelength λ 1 , to the first incident light beam at a predetermined environmental temperature, and when an environmental temperature changes, tendency of spherical aberration change caused by change in shape of the refractive surface is opposite to tendency of spherical aberration change caused by change in optical path difference provided by the steps, the change in optical path difference being caused by based on change in refractive index.