Objective lens for optical pick-up
AssigneeAsahi Kogaku Kogyo Kabushiki Kaisha
InventorKoichi Maruyama
Life of this patent
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Abstract
An objective lens includes a refractive lens having a positive refractive power, and a diffractive lens structure having a plurality of concentric ring-shaped steps that are formed on a lens surface of the refractive lens. The objective lens is a biconvex plastic lens having first and second aspherical surfaces. The diffractive lens structure has such a wavelength dependence that at least two diffracted beams having different wavelengths with the same diffraction order form appropriate wavefronts for at least two kinds of optical discs having cover layers of different thickness, respectively. Further, the diffractive lens structure has a predetermined power at any points in a middle ring area that is concentrically arranged about the optical axis at a middle range of the radius of the refractive lens.
Claims17
An objective lens for an optical pick-up, comprising:
- a refractive lens having a positive refractive power;
- and
- a diffractive lens structure having a plurality of concentric ring-shaped steps that are formed on at least one lens surface of said refractive lens,
- wherein said diffractive lens structure has such a wavelength dependence that at least two diffracted beams having different wavelengths with the same diffraction order form appropriate wavefronts for at least two kinds of optical discs having cover layers of different thickness, respectively,
- wherein said diffractive lens structure has a predetermined power at any points in a middle ring area that is concentrically arranged about the optical axis at a middle range of the radius of said refractive lens, and
- wherein said middle ring area is defined as an area through which a light beam whose NA is 0.20 through 0.38 passes.
An objective lens for an optical pick-up, comprising:
- a refractive lens having a positive refractive power;
- and
- a diffractive lens structure having a plurality of concentric ring-shaped steps that are formed on at least one lens surface of said refractive lens,
- wherein said diffractive lens structure has such a wavelength dependence that at least two diffracted beams having different wavelengths with the same diffraction order form appropriate wavefronts for at least two kinds of optical discs having cover layers of different thickness, respectively,
- wherein said diffractive lens structure has a predetermined power at any points in a middle ring area that is concentrically arranged about the optical axis at a middle range of the radius of said refractive lens, and
- wherein said refractive lens has a high NA exclusive area through which a light beam of a first NA, which is necessary only for an optical disc having a first recording density, passes, and a common area through which a light beam of a second NA, which is lower than said first NA and is necessary and sufficient for an optical disc having a second recording density that is lower than said first recording density, passes, wherein said common area is located at the center of the lens surface and is surrounded by said high NA exclusive area, and wherein said middle ring area is included in said common area.
An objective lens for an optical pick-up, comprising:
- a refractive lens having a positive refractive power;
- and
- a diffractive lens structure having a plurality of concentric ring-shaped steps that are formed on at least one lens surface of said refractive lens,
- wherein said diffractive lens structure has such a wavelength dependence that at least two diffracted beams having different wavelengths with the same diffraction order form appropriate wavefronts for at least two kinds of optical discs having cover layers of different thickness, respectively,
- wherein said diffractive lens structure has a predetermined power at any points in a middle ring area that is concentrically arranged about the optical axis at a middle range of the radius of said refractive lens, and
- wherein said diffractive lens structure has no power in an area that surrounds said middle ring area.
An objective lens for an optical pick-up, comprising:
- a refractive lens having a positive refractive power;
- and
- a diffractive lens structure having a plurality of concentric ring-shaped steps that are formed on at least one lens surface of said refractive lens,
- wherein said diffractive lens structure has such a wavelength dependence that at least two diffracted beams having different wavelengths with the same diffraction order form appropriate wavefronts for at least two kinds of optical discs having cover layers of different thickness, respectively, and
- wherein an optical path difference function Φ(h) is defined such that the value thereof monotonously varies in a middle ring area that is concentrically arranged about the optical axis at a middle range of the radius of said refractive lens, when an additional optical path length added by said diffractive lens structure is expressed by the following optical path difference function:
- Φ(h)=(P2h2+P4h4+P6h6+ . . . )×m×λ
- where P2, P4 and P6 are coefficients of second, forth and sixth orders, h is a height from the optical axis, m is diffraction order and λ is wavelength.
Dependencies are inferred from the claim text.