Objective lens element

Abstract

An objective lens element which has excellent compatibility with optical discs having different base material thicknesses is provided. An objective lens element 141 has optically functional surfaces on an incident side and an exit side. Either one of the optically functional surfaces is divided into an inner part 131 B including a rotational symmetry axis and an outer part 131 F which is a ring-shaped region surrounding the inner part 131 B. On the inner part 131 B, a plurality of discontinuous steps are provided. The plurality of steps change in height in the same direction from the optical axis toward the outer part, and each of the steps causes a constant optical path difference longer than the wavelength λ 1 to the first incident light beam 61 and causes a constant optical path difference shorter than the wavelength λ 2 to the second incident light beam 62.

What is claimed is: 
     
         1 . 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 on the inner part, and 
 the plurality of steps change in height in the same direction from the optical axis toward the outer part, and each of the steps causes a constant optical path difference longer than the wavelength λ 1  to the first incident light beam and causes a constant optical path difference shorter than the wavelength λ 2  to the second incident light beam. 
 
     
     
         2 . The objective lens element according to  claim 1 , wherein each of the steps provides an optical path difference of about 1.25 wavelengths to light of the wavelength λ 1  and provides an optical path difference of about 0.75 wavelength to light of the wavelength λ 2 . 
     
     
         3 . The objective lens element according to  claim 1 , wherein the outer part has an aspherical shape in which no steps are formed. 
     
     
         4 . The objective lens element according to  claim 1 , wherein
 a diffraction grating having a sawtooth shape is formed on the outer part, and   the sawtooth shape has such a height that an optical path difference of about 1 wavelength is provided to light of the wavelength λ 1 .   
     
     
         5 . The objective lens element according to  claim 1 , wherein steps each having such a height that an optical path difference which is substantially an integral multiple of wavelength λ 1  is provided to light of the wavelength λ 1  are formed on the outer part. 
     
     
         6 . The objective lens element according to  claim 5 , wherein each step on the outermost portion of the outer part has such a height that an optical path difference which is substantially N times (N is an integer other than multiples of 5) of wavelength λ 1  is provided to the light of the wavelength λ 1 .