Patent ID: 7342865
Filing Date: 2008-03-11
Classification: G11B

Abstract:
1. 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: 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 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):