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An objective lens unit for an optical pickup used for recording and/or reproducing information signals from an optical disc. The objective lens unit includes a resin layer ( 21 ) having, sequentially from an object side, a first surface S 1 as an aspherical surface and a second surface S 2 as an a

An objective lens unit for an optical pickup used for recording and/or reproducing information signals from an optical disc. The objective lens unit includes a resin layer ( 21 ) having, sequentially from an object side, a first surface S 1 as an aspherical surface and a second surface S 2 as an aspherical surface, at least one of the first surface and the second surface including a diffractive surface, and a lens of glass ( 22 ) having the second surface S 2 as an aspherical surface and a third surface S 3 as an aspherical surface. The lens unit is corrected for the chromatic aberration on an image surface on the optical axis with respect to the light of a reference wavelength not larger than 420 nm within several nm of the reference wavelength, with the numerical aperture of the lens unit being not less than 0.8. This objective lens unit is able to converge the laser light close to the limit of diffraction on the image surface.

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1. An objective lens unit for an optical pickup comprising:a resin layer having, sequentially from an object side, a first surface as an aspherical surface and a second surface as an aspherical surface, at least one of the first surface and the second surface including a diffractive surface; anda le

1. An objective lens unit for an optical pickup comprising:a resin layer having, sequentially from an object side, a first surface as an aspherical surface and a second surface as an aspherical surface, at least one of the first surface and the second surface including a diffractive surface; anda lens of glass having said second surface as an aspherical surface and a third surface as an aspherical surface;wherein the objective lens unit is corrected for the chromatic aberration on an image surface on the optical axis with respect to the light of a reference wavelength not larger than 420 nm within several nm of said reference wavelength; the numerical aperture of the objective lens unit being not less than 0.8. 2. The objective lens unit for an optical pickup according to claim 1 wherein said first surface includes a diffractive surface. 3. The objective lens unit for an optical pickup according to claim 1 wherein said second surface includes a diffractive surface. 4. The objective lens unit for an optical pickup according to claim 1 wherein the working distance from said third surface to an image point is not less than 0.5 mm. 5. The objective lens unit for an optical pickup according to claim 4 wherein one of said first and second surfaces including the diffractive surfaces has an aspherical coefficient value. 6. The objective lens unit for an optical pickup according to claim 5 wherein said first and second surfaces are of the same base curvature and the same aspherical coefficient. 7. The objective lens unit for an optical pickup according to claim 6 wherein said resin layer is of a phase type, andsaid first surface or the second surface is of the blaze shape. 8. The objective lens unit for an optical pickup according to claim 1 wherein an aperture is formed in the outer rim of said first surface. 9. The objective lens unit for an optical pickup according to claim 1 wherein said diffractive surface has the design degree of orders of not less than two. 10. The objective lens unit for an optical pickup according to claim 1 wherein said resin layer has a thickness not larger than 0.1 mm. 11. The objective lens unit for an optical pickup according to claim 1 wherein said lens of glass has a refractive index of a vitreous material not less than 1.65 for the light with a wavelength not larger than 420 nm. 12. The objective lens unit for an optical pickup according to claim 1 wherein said lens of glass has a meniscus shape. 13. The objective lens unit for an optical pickup according to claim 1 wherein a fine micro-irregular shape of a periodic structure having a period approximately one-half the reference wavelength and an amplitude approximately one-half the reference wavelength is formed on said first surface or said second surface, including said diffractive surface, said fine micro-irregular shape being of a concentric pattern finer than a micro-irregular shape of said diffractive surface. 14. The objective lens unit for an optical pickup according to claim 1 wherein a protective cover having a thickness not larger than 0.3 mm is provided between said third surface and said image surface, anda spherical aberration ascribable to said protective cover is corrected. 15. The objective lens unit for an optical pickup according to claim 1 wherein said objective lens unit further includes a fourth surface having an aspherical surface,said resin layer having at least one of said third and fourth surfaces including another diffractive surface. 16. The objective lens unit for an optical pickup according to claim 15 wherein said third surface includes said another diffractive surface. 17. The objective lens unit for an optical pickup according to claim 15 wherein said fourth surface includes said another diffractive surface. 18. The objective lens unit for an optical pickup according to claim 15 wherein said resin layer is of a phase type, andsaid third surface or the fourth surface is of the blaze shape. 19. An optical pickup comprisi ng:a laser light emitting device for radiating the laser light,an objective lens unit for converging the laser light on a recording layer of an optical recording medium,a light receiving device for receiving the laser light and an optical component for causing the laser light radiated from said laser light emitting device to incident on said objective lens unit and for causing the laser light reflected by the recording layer of said optical recording medium and transmitted through said objective lens unit to incident on said light receiving device, wherein:the objective lens unit includes a resin layer having, sequentially from an object side,a first surface as an aspherical surface and a second surface as an aspherical surface,at least one of the first surface and the second surface including a diffractive surface, anda lens of glass having said second surface as an aspherical surface and a third surface as an aspherical surface,wherein the objective lens unit is corrected for the chromatic aberration on an image surface on the optical axis with respect to the light of a reference wavelength not larger than 420 nm within several nm of said reference wavelength, with the numerical aperture of the objective lens unit being not less than 0.8. 20. The optical pickup according to claim 19 wherein the working distance from said third surface to an image point is not less than 0.5 mm. 21. The optical pickup according to claim 19 wherein a protective cover having a thickness not larger than 0.3 mm is provided between said third surface and said image surface, anda spherical aberration ascribable to said protective cover is corrected. 22. The objective pickup according to claim 19 wherein laser light emitting device emits laser light having a wavelength not larger than 420 nm. 23. The objective pickup according to claim 19 wherein said objective lens unit further includes a fourth surface having an aspherical surface,said resin layer having at least one of said third and fourth surfaces including another diffractive surface. 24. A disc driving device for recording and/or reproducing information on a rotating disc-shaped optical recording medium by an optical pickup movable along the radius of said optical recording medium,wherein said optical pickup includes:a laser light emitting device for radiating the laser light with a wavelength of not larger than 420 nm,an objective lens unit for converging the laser light on a recording layer of an optical recording medium,a light receiving device for receiving the laser light and an optical component for causing the laser light radiated from said laser light emitting device to incident on said objective lens unit and for causing the laser light reflected by the recording layer of said optical recording medium and transmitted through said objective lens unit to incident on said light receiving device,wherein the objective lens unit includes:a resin layer having, sequentially from an object side, a first surface as an aspherical surface and a second surface as an aspherical surface, at least one of the first surface and the second surface including a diffractive surface, anda lens of glass having said second surface as an aspherical surface and a third surface as an aspherical surface,wherein the objective lens unit is corrected for the chromatic aberration on an image surface on the optical axis with respect to the light of a reference wavelength not larger than 420 nm within several nm of said reference wavelength, with the numerical aperture of the objective lens unit being not less than 0.8. 25. The disc driving device according to claim 24 wherein the working distance from said third surface to an image point is not less than 0.5 mm. 26. The disc driving device according to claim 24 wherein a protective cover having a thickness not larger than 0.3 mm is provided between said third surface and said image surface, anda spherical aberration ascribable to said protective cover is corrected. 27 . The disc driving device according to claim 24 wherein said objective lens unit further includes a fourth surface including an aspherical surface,said resin layer having at least one of said third and fourth surfaces including another diffractive surface.