Patent ID: 7778139

Claim:
An objective lens used for an optical information recording/reproducing device for recording information to and/or reproducing information from two types of optical discs including a first optical disc and a second optical disc having a recording density lower than that of the first optical disc, by selectively using one of two types of substantially collimated light beams including first and second light beams, when wavelengths of the first and second light beams are respectively represented by λ 1 (nm) and λ 2 (nm), λ 1 <λ 2 being satisfied, the objective lens comprising: a first optical element; and a second optical element made of material different from that of the first optical element, wherein the first and second optical elements are cemented via a cementing surface, wherein the objective lens is configured to satisfy a condition: 0.006 < { ( nR ⁢ ⁢ 2 - nR ⁢ ⁢ 1 ) - ( nB ⁢ ⁢ 2 - nB ⁢ ⁢ 1 ) } × [ { ( k ⁢ ⁢ 2 r ⁢ ⁢ 2 3 ) + 8 × A ⁢ ⁢ 42 } × f ⁢ ⁢ 1 3 + 2.29 ] < 0.038 ( 1 ) where nB 1 represents a refractive index of the first optical element with respect to the wavelength λ 1 , nB 2 represents a refractive index of the second optical element with respect to the wavelength λ 1 , nR 1 represents a refractive index of the first optical element with respect to the wavelength λ 2 , nR 2 represents a refractive index of the second optical element with respect to the wavelength λ 2 , r 2 represents a radius of curvature of the cementing surface, k 2 represents a conical coefficient of the cementing surface, A 42 represents a fourth order aspherical coefficient of the cementing surface, and f 1 represents a focal length of the objective lens defined when the first laser beam is used.