Objective lens for pickup and light pickup apparatus

Details Objective lens for pickup and light pickup apparatus Objective lens for pickup and light pickup apparatus

2000-09-01

2002-06-25

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

C359S649000, C359S719000, C369S112010, C369S112230, C250S201500

Reexamination Certificate

active

06411442

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an objective lens for pickup in an optical recording apparatus which conducts recording on or reproducing from an information recording medium such as an optical disk, and to a optical pickup apparatus employing the objective lens.
In an optical system of an optical recording/reproducing apparatus employing a medium of an optical disk, there is commonly used an aspherical single objective lens. For achieving high density of recorded information signals, a size of a spot formed on a recording medium by the objective lens has been required to be small, and there have been investigated high NA of an objective lens and utilization of a light source for short wavelength.
Though there has been developed a GaN blue semiconductor laser having an oscillation wavelength of about 400 nm, a wavelength is varied by mode hop or by laser output, and monochromaticity of oscillation wavelength is poor because high-frequency superimposition is used. Therefore, in a light-converging optical system for high density optical disk wherein a GaN blue semiconductor laser is used, it is considered that correction of axial chromatic aberration is necessary.
In an aspherical single objective lens for an optical disk, spherical aberration and coma are corrected by aspherical surfaces. However, when a numerical aperture is large, image height characteristics are deteriorated. When an optical disk is made to be of higher density, deterioration of the image height characteristics becomes an extremely serious problem even if the value of the deterioration is small. In particular, when the numerical aperture is 0.65 or more, the problem is conspicuous. Further, when the numerical aperture is greater, deterioration of eccentricity sensitivity is also a serious problem.
SUMMARY OF THE INVENTION
The invention has been achieved for solving the problem stated above. Namely, with regard to an objective lens for a optical pickup apparatus, an object of the invention is to provide an aspherical single objective lens whose numerical aperture is great and image height characteristics are excellent. In particular, an object of the invention is to provide an objective lens which is suitable to be used for a high density recording/reproducing apparatus wherein a numerical aperture is not less than 0.65, preferably 0.7 or more, and more preferably 0.75 or more, and there is used a laser wherein a wavelength of a light source is as short as about 500 nm.
Further, providing an objective lens which makes eccentricity sensitivity to be excellent is also an object of the invention. Furthermore, providing an objective lens which makes spherical aberration and coma to be excellent is also an object of the invention.
When a thickness of a protective layer (transparent base board) of an information recording medium is small to be 0.2 mm or less, or when there is no protective layer, a working distance can be small. An offer of an objective lens which is suitable to be used in a recording/reproducing apparatus having such small working distance is an object of the invention.
It is further an object of the invention to provide a optical pickup apparatus, an optical information recording medium recording/reproducing apparatus, and an optical information recording medium recording/reproducing method, which employ these objective lenses stated above.
Further, an object is to provide a optical pickup apparatus having an optical system wherein axial chromatic aberration is corrected by the simple structure, in a high density optical recording/reproducing apparatus. In particular, it is an object to provide a optical pickup apparatus wherein a numerical aperture on the part of an information recording medium is 0.65 or more, preferably 0.7 or more, and more preferably 0.75 or more, and shortest wavelength of a light source to be used is as small as 500 nm or less.
The above object can be attained by the following structures.
(1) An objective lens for use in an optical pickup apparatus to record or reproduce information in an optical information recording medium, comprising:
an aspheric surface;
wherein the following conditional formula is satisfied:
 1.1≦
d
1
/
f≦
3
where d
1
represents axial lens thickness and f represents a focal length.
(2) In the objective lens of (1), a numerical aperture of the objective lens is not smaller than 0.65.
(3) In the objective lens of (2), a numerical aperture of the objective lens is not smaller than 0.75.
(4) In the objective lens of (1), the following conditional formula is satisfied:
f/&ngr;d≦
0.060
where &ngr;d represents Abbe's number.
(5) In the objective lens of (1), the following conditional formula is satisfied:
1.40≦
n
where n represents a refractive index at a used wavelength.
(6) In the objective lens of (5), the following conditional formula is satisfied:
1.40≦
n≦
1.85
(7) In the objective lens of (1), the following conditional formula is satisfied:
0.40≦
r
1
/(
n·f
)≦0.70
where r
1
represents a paraxial radius of curvature of one surface of the objective lens.
(8) In the objective lens of (7), r
1
represents a paraxial radius of curvature of the surface of the objective lens at the light source side.
(9) In the objective lens of (1), a used wavelength is not longer than 500 nm.
(10) In the objective lens of (1), the objective lens is an objective lens for use in an optical pickup apparatus to record or reproduce information in an optical information recording medium having a protective layer whose thickness is not larger than 0.2 mm.
(11) In the objective lens of (10), a numerical aperture is not smaller than 0.7.
(12) In the objective lens of (7), the following conditional formula is satisfied:
1.50≦
n
where n represents a refractive index at a used wavelength.
(13) In the objective lens of (1), the objective lens is a plastic lens.
(14) In the objective lens of (1), the objective lens is a glass lens.
(15) In the objective lens of (1), the following conditional formula is satisfied:
1.85≦
n
where n represents a refractive index at a used wavelength.
(16) In the objective lens of (1), the objective lens further comprises a diffracting section.
(17) In the objective lens of (1), the objective lens further comprises a flange section on an outer periphery thereof.
(18) In the objective lens of (1), the flange section comprises a surface extended in a direction perpendicular to an optical axis.
(19) In the objective lens of (1), each of both lens surfaces is an aspherical surface.
(20) An optical pickup apparatus to record or reproduce information in an optical information recording medium, comprises:
a light source to emit light flux;
a converging optical system to condense the light flux emitted from the light source; and
an optical detector to detect reflection light from the optical information recording medium;
wherein the converging optical system comprises an objective lens to condense the light flux on an information recording surface of the optical information recording medium and the objective lens comprises an aspheric surface; and
wherein the following conditional formula is satisfied:
1.1≦
d
1
/f≦
3
where d
1
represents axial lens thickness of the objective lens and f represents a focal length of the objective lens.
(21) In the optical pickup apparatus of (20), a numerical aperture of the objective lens at the optical information recording medium side is not smaller than 0.65.
(22) In the optical pickup apparatus of (29), a numerical aperture of the objective lens at the optical information recording medium side is not smaller than 0.75.
(23) In the optical pickup apparatus of (20), the following conditional formula is satisfied:
f/&ngr;d≦
0.060
where &ngr;d represents Abbe's number of the material of the objective lens.
(24) In the optical pickup apparatus of (20), the following conditional formula is satisfied:
1.40≦
n
where n represents a refractive index of the material of the objective lens at a used wavelength.
(25) In the optical pickup apparatus of (24)

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