Patent ID: 7852736
Filing Date: 2010-12-14
Classification: G11B

Abstract:
1. An optical element having an incident plane and an exit plane for first laser light and a plurality of other laser lights, each of which has a different wavelength from that of the first laser light, on an optical path of the first laser light and the other laser lights, in order to share an objective lens for focusing light on a first recording medium comprising a recording layer, which receives the first laser light through a transmission protection layer having a thickness of D 1 and on a plurality of recording media corresponding to the other respective laser lights, the plurality of laser lights including second laser light, corresponding to a second recording medium including a transmission protection layer having a thickness of D an effective diameter of a luminous flux required when the first laser light, the second laser light, and the third laser light enter the objective lens being R said optical element comprising: a first aberration correcting device for correcting aberration caused by a difference between the thickness D a second aberration correcting device for correcting aberration of the objective lens caused by wavelength changes in the first laser light, the second laser light, or the third laser light; and a third aberration correcting device, which is disposed between the effective diameter R wherein at least one of said second and third aberration correcting devices is a diffraction pattern in which a plurality of phase steps are formed in association with each wavelength of the first laser light, the second laser light, and the third laser light; wherein the diffraction pattern is formed, concentrically and zonally; wherein the diffraction pattern includes a first area with an outer diameter associated with the effective diameter R the first area and the third area comprise said second aberration correcting device, and the second area comprises said third aberration correcting device; wherein wavelength ranges of the first laser light, the second laser light, and the third laser light are 400 to 410 nm, 635 to 670 nm, and 780 to 810 nm, respectively; wherein the diffraction pattern is formed to maximize diffraction efficiency of (10×n)th-order diffracted light (n is an integer) of the first laser light in the first area; the diffraction pattern is formed to maximize diffraction efficiency of (5×(2n−1))th-order (n is an integer) of the first laser light in the second area; and the diffraction pattern is formed to maximize diffraction efficiency of (10×n)th-order diffracted light (n is an integer) of the first laser light in the third area; and wherein the first area, the second area and the third area are non-overlapping.