Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-07-12
2002-02-19
Tran, Thang V. (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S112080, C369S112250, C369S044230, C359S719000
Reexamination Certificate
active
06349083
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus to record or reproduce recording information in an optical information recording medium and to an objective lens and a pickup apparatus for use in the apparatus. Further, the present innovation relates to a near field type optical disk recording reproducing apparatus in which a distance between an objective lens and a recording surface of an optical disk is smaller than an order of a wavelength of light and to an objective lens and a pickup apparatus for use in the apparatus. In particular, the present invention relates to an recording reproducing apparatus in which NA (numerical aperture) is larger and a high density recording is conducted.
In a high density recording reproducing apparatus in which an optical disk is used as an optical information recording medium, an apparatus using a near field type optical system is suggested.
In such an objective lens in the near field type optical system, U.S. Pat. No. 5,125,750 discloses an optical system in which two pieces of separated lenses having a uniform refractive index are used, in particular, a hemisphere lens is used as a lens at the optical disk side.
In this optical system, since a precise optical system having a larger NA is constructed with the two pieces of lenses, many manufacturing processes are needed for assembling and adjustment. Further, a minute size lens is produced by polishing. As a result, the cost for this optical system becomes very high.
Further, in a recording reproducing apparatus for various optical information recording medium such as a compact disk (CD), a digital video disk (DVD), a mini disk (MD), and a magneto-optics disk (MO), due to the reason that the cost may be reduced, an objective lens made of plastic is used. Also, as an objective lens for use in a near field type optical system, there may be a possibility that a plastic lens is used.
However, although the objective lens made of plastic may be produced at low cost, there may be a problem that a focusing point is shifted due to a change in temperature.
SUMMARY OF THE INVENTION
With a simple structure, the present invention is to obtain an optical system for recording or reproducing a high density optical information recording medium, wherein the optical system has an excellent image forming performance as a near field type optical system and can be easily assembled and adjusted.
Further, an object of the present innovation is to provide an objective lens for use in an optical pickup device, wherein the objective lens is made of plastic material and a shift of a focusing point due to a change in temperature is small, in particular, to provide an objective lens for use in an optical pickup apparatus, wherein a shift of a focusing point due to a change in temperature is kept within a depth of focus even if a numerical aperture at the image side is larger than 0.5.
The above object may be attained by the following structures.
(1) A near field type optical disk recording/reproducing apparatus, comprises
a light source;
an objective single lens to converse light from the light source to a focusing point;
an optical disk; and
a rotating device to rotate the optical disk;
wherein the objective lens has a first surface at the light source side and a second surface at the focusing point side, and the first surface is a curved surface on which a diffraction surface is provided.
(2) In the near field type optical disk recording reproducing apparatus of (1), the first surface is an aspherical surface.
(3) In the near field type optical disk recording reproducing apparatus of (1), the second surface is a flat surface.
(4) In the near field type optical disk recording reproducing apparatus of (1), the diffraction surface is notched in a form of a ring.
(5) In the near field type optical disk recording reproducing apparatus of (1), the following formula is satisfied,
(1−1/
n
)<
R
0
/d<
(1+1/
n
)
wherein n is a refractive index of the objective lens, R
0
is a paraxial curvature radius of the objective lens, and d is a thickness of the objective lens on the optical axis.
(6) In the near field type optical disk recording reproducing apparatus of (1), the following formula is satisfied,
(1−1
)<
R
0
/(
n·f
)<(1+1
)
wherein n is a refractive index of the objective lens, R
0
is a paraxial curvature radius of the objective lens, and f is a focal length of the objective lens.
(7) In the near field type optical disk recording reproducing apparatus of (1), the following formula is satisfied,
0.3×(1−3
/&ngr;d
)<(
n−
1)/
R
0
<1/
f
wherein n is a refractive index of the objective lens, R
0
is a paraxial curvature radius of the objective lens, f is a focal length of the objective lens, and &ngr;d is Abbe's number of the objective lens.
(8) In the near field type optical disk recording reproducing apparatus of (1), the following formula is satisfied,
0.4×(1−3
/&ngr;d
)<(
n−
1)/R
0
<1.2×(1−3
/&ngr;d
)
wherein n is a refractive index of the objective lens, R
0
is a paraxial curvature radius of the objective lens, f is a focal length of the objective lens, and &ngr;d is Abbe's number of the objective lens.
(9) In the near field type optical disk recording reproducing apparatus of (1), the following formula is satisfied,
&ngr;
d
<40
wherein &ngr;d is Abbe's number of the objective lens.
(10) In the near field type optical disk recording reproducing apparatus of (1), further comprising a collimator lens, chromatic aberration of the objective lens is corrected excessively and chromatic aberration of the collimator lens is corrected insufficiently, whereby chromatic aberration of the entire optical system is corrected properly.
(11) A pickup apparatus for use in a near field type optical disk recording reproducing apparatus, comprises
a light source; and
an objective single lens to converse light from the light source to a focusing point;
wherein the objective lens has a first surface at the light source side and a second surface at the focusing point side, and the first surface is a curved surface on which a diffraction surface is provided.
(12) An objective lens for use in a near field type optical disk recording reproducing apparatus, wherein the objective single lens converses light from a light source to a focusing point, comprises
a first surface at the light source side, and
a second surface at the focusing point side, wherein the first surface is a curved surface on which a diffraction surface is provided.
(13) In a near field type optical disk optical system, the optical system is characterized in that an objective lens is a single lens, its light source side is a curved surface, a ring of a diffracting lens is notched on the curved surface.
(14) In the optical system of (13), the curved surface is an aspherical surface.
(15) In the optical system of (13), the focusing point side of the objective lens is a flat surface.
(16) In the optical system of (13), the following formula is satisfied:
(1−1
)<
R
0
/d<
(1+1
)
wherein the curved surface is an aspherical surface, its paraxial curvature radius R is R
0
a thickness of the single lens on the optical axis is d, and a refractive index is n.
As stated above, in a near field type optical disk optical system, the optical system of the present invention comprises an objective lens which is a single lens whose light source side is a curved surface, preferably, an aspherical surface. It may be preferable that a ring of a diffracting lens is notched on the curved surface. It may be preferable that a surface at the focusing point side is a flat surface. It may be preferable that the objective lens is shaped in a flat convex lens as a whole.
If the objective lens of the near field type optical disk optical system is formed by a general flat convex lens which does not includes a diffracting lens on its convex surface, and if an aspherical surface is used as the convex lens, sp
Kiriki Toshihiko
Ota Kohei
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Konica Corporation
Tran Thang V.
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