Patent ID: 6563099
Filing Date: 2003-05-13
Classification: G11B

Abstract:
A light information reproducing apparatus for reproducing information from an optical disk having a first optical information medium layer and a second optical information medium layer therein, comprising:a light source; light irradiation means for converging a light from said light source and irradiating said optical disk therewith; light extracting means for extracting a part of a returned light from either of said first or second optical information medium layer of said optical disk that passes a predetermined spatial region therein; a four-element photodetector including first and second light detecting elements placed between third and fourth light detecting elements for detecting the part of the returned light that is extracted by said light extracting means, wherein said first and second light detecting elements are arranged such that, when one of said first and second optical information medium layers is irradiated by said light irradiation means being in focus thereon, outputs of said first and second light detecting elements caused by detecting the part of the returned light from the other one of said first and second optical information medium layers are approximately equal, said third light detecting element which is located next to said first light detecting element such that, when said first optical information medium layer is irradiated by said light irradiation means being in focus thereon, outputs of said first and third light detecting elements caused by detecting the part of the returned light from said second optical information medium layer is approximately equal, and said fourth light detecting element which is located next to said second light detecting element such that, when said second optical information medium layer of said optical disk is irradiated by said light irradiation means being in focus thereon, outputs of said second and fourth light detecting elements caused by detecting the part of the returned light from said first optical information medium layer is approximately equal; and focus error signal generating means for generating a focus error signal by taking a difference between a sum of the outputs of said first and fourth light detecting elements and a sum of the outputs of said second and third detecting elements.