Patent ID: 7990832
Filing Date: 2011-08-02
Classification: G11B

Abstract:
1. An optical pickup comprising: a first light source radiating a light beam of a first wavelength; a second light source radiating a light beam of a second wavelength; a third light source radiating a light beam of a third wavelength; light receiving means for receiving the light beams from the first to third light sources, reflected from a plurality of optical recording mediums having different protective substrate thicknesses, and for converting the received light beams into electrical signals, the optical pickup radiating light beams of different wavelengths and different numerical apertures to the individual optical recording mediums; and an imaging unit composed of an objective lens and a set of diffraction means and adapted for condensing the light beams from the first to third light sources on the optical recording mediums, wherein, the set of diffraction means further comprises: first diffraction means which gives the transmission diffraction efficiency for order-one diffracted light or order-minus-one diffracted light higher than that for light of the other orders in case the light beams of the first, second and third wavelengths are transmitted therethrough; and second diffraction means which gives the transmission diffraction efficiency for order-zero diffracted light higher than that for light of the other orders in case the light beam of the shortest wavelength and the light beam of the longest wavelength out of the first, second and third wavelengths are transmitted therethrough, and the transmission diffraction efficiency for order-one diffracted light or order-minus-one diffracted light higher than that for diffracted light of the other orders in case the remaining light beam is transmitted therethrough, wherein one of the diffraction means in the set of diffraction means is provided to the objective lens and the other diffraction means of the set is provided to an optical plate member secured to the objective lens so that the first and second diffraction means face each other, wherein the first diffractive means provided to the objective lens having the NA, the pitch at the outermost rim, and the groove depth being 0.6, approximately 10 μm, and approximately 0.95 μm, respectively, and wherein the second diffractive means provided to the optical plate having the NA, the pitch at the outermost rim, and the groove depth being 0.6, approximately 90 μm, and approximately 0.68 μm, respectively.