Patent ID: 6974939
Filing Date: 2005-12-13
Classification: G11B

Abstract:
1. An apparatus for recording and/or reproducing information to and/or from a recording medium having at least two information recording layers that are mutually spaced along an optical axis, comprising: a light source that irradiates the at least two recording layers with a light beam propagating along the optical axis, at least two combined lenses including a first lens and a second lens that are mutually spaced by a distance d along the optical axis and focus said light beam to a spot on the at least two information recording layers, an actuator that changes said distance d to change the spherical aberration of the condensed light beam at said spot on each said information recording layer and a numerical aperture of said at least two combined lenses, wherein the light beam condensed upon each of said at least two information recording layers has a respective minimum spherical aberration for each information recording layer, said at least two combined lenses are designed such that the minimum spherical aberration of the condensed light beam on a specified one of the at least two information recording layers is smaller than the minimum spherical aberration for any other information recording layer, when recording and/or reproducing information to and/or from a target one of the at least two information recording layers, the actuator changes the distance d such that the spherical aberration of the light beam assumes the minimum value for the targeted information recording layer, and a lens shape, a lens thickness, and a refractive index of each of said at least two combined lenses are set such that, with an increasing distance of the targeted information recording layer from said at least two combined lenses, the numerical aperture of these lenses is increased such that a diameter of the light beam condensed on the targeted recording layer is decreased as far as increase of noise due to side lobe falls in an allowable range.