Patent ID: 7738340

Claim:
An optical disk apparatus carrying out at least information reproduction from among information recording, information reproduction and information deletion on an optical disk having a plurality of recording layers, comprising: a light source; an optical system comprising an objective lens condensing a light beam from the light source on a recording layer selected from the plurality of recording layers of the optical disk, and leading a returning light beam reflected by said selected recording layer to a predetermined position; a photodetector disposed at said predetermined position and receiving the returning light beam; an aberration correcting part selecting any one of a plurality of aberration correction amounts previously set, according to the recording layer selected from the plurality of recording layers, and correcting the spherical aberration based on the aberration correction amount as the selection result; and a processing device carrying out at least the information reproduction from among information recording, information reproduction and information deletion with the use of the output signal of the photodetector; and wherein the optical disk includes a first recording layer and a second recording layer; and said aberration correcting part selects an aberration correction amount with which spherical aberration becomes approximately minimum at a position corresponding to (¼)t, where spherical aberration becomes approximately zero, from the first recording layer toward the second recording layer where t denotes a distance between the first recording layer and the second recording layer, when said selected recording layer is the first recording layer, while the aberration correcting part selects an aberration correction amount with which spherical aberration becomes approximately minimum at a position corresponding to (¾)t, where spherical aberration becomes approximately zero, from the first recording layer toward the second recording layer, when said selected recording layer is the second recording layer; and wherein said objective lens is set in such a manner that wavefront aberration may become approximately minimum at a position corresponding to (½)t from the first recording layer toward the second recording layer when the applied light beam is approximately parallel light; and wherein the following formula provides spherical aberration not more than 0.035 λrms: t≦ 15 n 3 λ/{( n 2 −1)NA 4 } where: n denotes a refractive index; λ denotes a wavelength of the light beam applied; and NA denotes a numerical aperture of the objective lens of the optical disk apparatus, and wherein λ=0.40 μm, n=1.623, NA=0.65, and t=87 μm.