Patent ID: 7848211
Filing Date: 2010-12-07
Classification: G11B

Abstract:
1. An objective lens apparatus, capable of focusing a laser light on a first optical record medium, a second optical record medium, and a third optical record medium, each being disc shaped; the first optical record medium having a first cover layer which has a first thickness, the second optical record medium having a second cover layer which has a second thickness larger than the first thickness, the third optical record medium having a third cover layer which has a third thickness larger than the second thickness, the laser light entering each of the cover layers, comprising: a first objective lens which has a first numerical aperture and which is capable of focusing the laser light on the first optical record medium; a second objective lens which has a second numerical aperture smaller than the first numerical aperture and which is capable of focusing the laser light on the second optical record medium; a third objective lens which has a third numerical aperture smaller than the second numerical aperture and which is capable of focusing the laser light on the third optical record medium; and a lens holder which holds the first objective lens, the second objective lens, and the third objective lens; wherein the first objective lens has a first focal length and a first lens principal point, wherein the second objective lens has a second focal length and a second lens principal point, wherein the third objective lens has a third focal length and a third lens principal point, and wherein the lens holder holds the second objective lens such that the second lens principal point is placed at a position in a focus direction apart from the first lens principal point of the first objective lens by a distance of which a difference between the first focal length and the second focal length and an optical path length in air corresponding to a difference between the first thickness and the second thickness are added, and holds the third objective lens such that the third lens principal point is placed at a position in a focus direction apart from the first lens principal point by a distance of which a difference between the first focal length and the third focal length and an optical path length in air corresponding to a difference between the first thickness and the third thickness are added.