Patent ID: 7342865

Claim:
An objective lens for an optical recording/reproducing device which records/reproduces information to/from a recording medium utilizing holography, said objective lens converging a collimated beam in the vicinity of a recording surface of the recording medium to record/reproduce the information, said objective lens comprising: a single lens element having an incident surface to which the collimated beam enters and an exit surface being opposite to said incident surface; said incident surface and said exit surface being configured to be rotationally symmetrical aspherical surfaces; at least within an effective diameter of said single lens element, said incident surface and said exit surface being symmetrical with respect to a plane perpendicular to an optical axis of said single lens element and having the same shape; said single lens element having positive refractive power; wherein the rotationally symmetrical aspherical surface of each of said two refractive surfaces is expressed by a formula (1) below: F ⁡ ( h ) = h 2 r 1 + 1 - ( 1 + K ) ⁢ ( 1 r ) 2 ⁢ h 2 + A 4 ⁢ h 4 + A 6 ⁢ h 6 + A 8 ⁢ h 8 + A 10 ⁢ h 10 ( 1 ) where F(h) is a sag amount (i.e., a distance from a plane, which is tangential to the rotationally symmetrical aspherical surface at the optical axis, to the rotationally symmetrical aspherical surface at a position whose height with respect to the optical axis is h), r is a radius of curvature of the rotationally symmetrical aspherical surface at the optical axis, K is a conical coefficient, and A 4 -A 10 are fourth, sixth, eighth, and tenth order aspherical coefficients, respectively, wherein a first derivative of the sag amount F(h) with respect to h satisfies a condition (2) and a second derivative of the sag amount F(h) with respect to h satisfies a condition (3): −0.35 ≦dF ( h )/ dh ≦+0.35 (2) +0.3 ≦d 2 F ( h )/ d 2 h ≦+1.3 (3).