Patent ID: 8004955

Claim:
A coupling lens used in an optical information recording/reproducing device for recording information to and/or reproducing information from an optical disc, the coupling lens comprising a first surface and a second surface, wherein the coupling lens is configured to have a value Z which satisfies a following condition (1): −0.80 ≦Z≦ 0.40 (1), wherein the value Z in the condition (1) is obtained from a following equation (E1): Z = ⅆ ⅆ n ⁢ ( L ′ ) L ′ = ( A - B ) ( D - E ) - ( F - G ) ( H - I ) ⁢ ⁢ where , ⁢ A = t n 2 ⁢ ⁢ B = tL n 2 ⁢ r 1 ⁢ ⁢ D = - t n ⁢ ⁢ E = ( 1 - 1 n ) ⁢ tL r 1 - L ⁢ ⁢ F = t n 2 ⁢ r 2 ⁢ ⁢ G = - ( r 2 - r 1 ) + ( 1 - 1 n 2 ) ⁢ t r 1 ⁢ r 2 ⁢ L ⁢ ⁢ H = 1 - ( 1 n - 1 ) ⁢ t r 2 ⁢ ⁢ I = - ( r 2 - r 1 ) ⁢ ( n - 1 ) + ( n + 1 n - 2 ) ⁢ t r 1 ⁢ r 2 ⁢ L ( E1 ) r1 denotes a curvature radius of the first surface the coupling lens, r2 denotes a curvature radius of the second surface of the coupling lens, n denotes a design refractive index of the coupling lens, L denotes a working distance defined for an object point, L′ denotes an working distance defined for an image point, and t denotes a center thickness of the coupling lens at a central axis of the coupling lens.