Patent ID: 6873588
Filing Date: 2005-03-29
Classification: G02B,G11B

Abstract:
1. An object lens for converging a light beam from a light source onto each of at least two types of optical information recording media having different thicknesses and being made of a transparent plate, said object lens comprising: an inner region near a center axis of the light beam and an outer region far from the center axis, said outer region having a plane optimized to converge the light beam transmitting to said outer region onto a first optical information recording medium among the optical information recording media, and said inner region having a plane optimized to converge the light beam transmitting to said inner region onto another optical information recording medium having a larger thickness than the first optical information recording medium; wherein a phase of the light beam transmitting to an innermost portion in the plane of said outer region is shifted relative to that of the light beam transmitting to an outermost portion in the plane of said inner region, and wherein when the light beam is converged on the first optical information recording medium, wave-front aberration satisfies a condition such that a total amplitude of aberration ≧20 mλ (rms) and a fifth spherical aberration ≦20 mλ (rms).