Patent ID: 7304934

Claim:
An optical head apparatus having a common objective lens through which first laser beams emitted from a first laser beam source are condensed onto the recording surface of a first optical data storage medium, and second laser beams of a wavelengths shorter than said first laser beams that are emitted from a second laser beam source are condensed onto the recording surface of a second optical data storage medium being covered by a transparent protective layer which is thinner than said first optical data storage medium; wherein said refracting surface of said objective lens is divided into a center end refracting surface region around the optical axis thereof and an outer circumferential refracting surface region surrounding the outside of said center end refracting surface region; multiple microscopic concentric notches are provided throughout said center end refracting surface region, thereby providing a center end diffraction grating; for reproduction of data on said first optical data storage medium utilizing said first laser beam source are selected diffracted beams obtained via said center end refracting surface region; for reproduction of data on said second optical data storage medium utilizing said second laser source are selected beams that passed through said outer circumferential refracting surface region and diffracted beams obtained via said center end refracting surface region; where the height H of said notches of said center end diffraction grating is expressed by the following equation: h 2 <H <( h 1 +h 2 )/2 wherein h 1 =λ 1 /(n 1 −1) h 2 =λ 2 /(n 2 −1) wherein H is the height of said notches of said center end diffraction grating; n 1 is the refractive index at said center end refracting surface region for said first laser beams of a wavelength λ 1 that come therein; n 2 is the refractive index at said center end refracting surface region for said second laser beams of a wavelength λ 2 that come therein.