US 6,982,838 B2
Objective lens for optical pick-up
Koichi Maruyama, Tokyo (Japan); and Daisuke Koreeda, Saitama-ken (Japan)
Assigned to PENTAX Corporation, Tokyo (Japan)
Filed on Jun. 03, 2004, as Appl. No. 10/859,140.
Claims priority of application No. 2003-159309 (JP), filed on Jun. 04, 2003.
Prior Publication US 2004/0246873 A1, Dec. 09, 2004
Int. Cl. G02B 3/02 (2006.01); G02B 13/18 (2006.01); G02B 3/08 (2006.01); G11B 7/00 (2006.01); G11B 7/135 (2006.01)
U.S. Cl. 359—719 13 Claims
OG exemplary drawing
 
1. An objective lens for an optical pick-up which is used to record data to and/or to reproduce data from at least three types of optical discs by selectively using one of at least three light beams having different wavelengths, the at least three types of optical discs having at least two different thicknesses of cover layers,
when a thickness of a first optical disc of the at least three types of optical discs to be used for recording data and/or reproducing data using a first light beam of the at least three light beams having a wavelength shortest of all the wavelengths of the at least three light beams is represented by t1, a thickness of a second optical disc of the at least three types of optical discs to be used for recording data and/or reproducing data using a second light beam of the at least three light beams having a wavelength longer than that of the first light beam is represented by t2, and a thickness of a third optical disc of the at least three types of optical discs to be used for recording data and/or reproducing data using a third light beam of the at least three light beams having a wavelength longest of all of the wavelengths of the at least three light beams is represented by t3, t1 being substantially equal to 0.6 mm, t2 being substantially equal to 0.6 mm, and t3 being substantially equal to 1.2 mm,
when a numerical aperture required for recording data to and/or reproducing data from the first optical disc is represented by NA1, a numerical aperture required for recording data to and/or reproducing data from the second optical disc is represented by NA2, and a numerical aperture required for recording data to and/or reproducing data from the third optical disc is represented by NA3, a relationship NA1≧NA2>NA3 being satisfied,
when the first and second optical discs are used, substantially collimated light beams of the first and second light beams being incident on the objective lens, respectively,
when the third optical disc is used, a diverging beam of the third light beam being incident on the objective lens,
given that magnification and a focal length of the objective lens are respectively represented by M1 and f1 when the first optical disc is used, the magnification and the focal length of the objective lens are respectively represented by M2 and f2 when the second optical disc is used, and the magnification and the focal length of the objective lens are respectively represented by M3 and f3 when the third optical disc is used, following conditions (1), (2) and (3) being satisfied:
−0.02<f1×M1<0.02  (1)
−0.02<f2×M2<0.02  (2)
−0.29<f3×M3<−0.19  (3),
at least one of lens surfaces of the objective lens comprising a diffracting structure having a first region for converging the third light beam on a data recording layer of the third optical disc, the diffracting structure within the first region being configured such that a diffraction order at which diffraction efficiency of the first light beam is maximized is a second order and that diffraction orders at which diffraction efficiency of the second and third light beams are maximized are first orders.