Optical: systems and elements – Lens – Including a nonspherical surface
Reexamination Certificate
2001-11-29
2003-04-22
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Lens
Including a nonspherical surface
C359S661000
Reexamination Certificate
active
06552858
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an objective lens system used for an optical disk apparatus, an optical microscope, an image device, etc. It also relates to an optical disk apparatus incorporating such an objective lens system.
2. Description of the Related Art
One way to improve the data storing density in an optical disk is to make smaller the diameter of the beam spot produced on the recording region of the disk. In order to provide a small beam spot, use may be made of an objective lens system having a large numerical aperture (NA). However, as the NA is greater (more than 0.8 for example), the positional deviation of the lens relative to the rotating disk becomes less tolerable for achieving proper data-writing or data-reading operations. Thus, when the NA is great, it is necessary to adjust the orientation of the objective lens more accurately than when the NA is relatively small, so that the lens can follow the tilt of the disk properly.
A great NA may be achievable by combining more than one lens, as shown in FIG.
11
. The illustrated objective lens system
90
includes two lenses (first and second lenses)
90
A,
90
B aligned vertically, to provide a great overall NA of the system. To address the tilting problem of the disk, the system
90
employs a floating slider design in which the second lens
90
B is carried on a slider
91
. The illustrated optical disk apparatus is categorized as a “surface recording type,” in which the slider
91
is held in facing relation to the recording layer (d) of the disk D. The slider
91
is provided with a coil
92
used for performing data-recording by magnetic field modulation. In the illustrated optical disk apparatus, a spherical lens is incorporated in the slider
91
.
When the working distance is extremely short, the spherical lens may preferably be configured in a manner as shown in
FIG. 12A
or
12
B. The spherical lens
90
B′ shown in
FIG. 12A
is of the so-called “hemisphere type”, wherein the lens is formed into an exact half of a sphere. The lens
90
B′ has a hemispherical surface
90
a
′ and a circular flat surface
90
b
′ upon which the center O of the sphere is located. The thickness d of the lens
90
B′ is equal to the radius of curvature of the hemispherical surface
90
a′.
The spherical lens
90
B″ of
FIG. 12B
, on the other hand, is of the so-called “supersphere type”, wherein a flat cutting surface
90
b
″ is located below the center O of the sphere. The lens
90
B″ has a superspherical surface
90
a
″ whose radius of curvature (r) is smaller than the thickness (d) of the lens
90
B″ (d=r+r
, where “n” is the refractive index of the lens
90
B″).
In comparison with the lens
90
B″ of the supersphere type, the lens
90
B′ of the hemisphere type is advantageous in having greater tolerance for the thickness of the slider
91
and for the thickness of the lens
90
B′ itself. Also, the lens
90
B′ is advantageously used even when there occurs a rather great deviation from the prescribed working distance. This is because, in the hemisphere type, light beams strike upon the surface
90
a
′ at right angles, the surface
90
a
′ does not have a power, and the focal point f′ comes at the center O of the flat surface
90
b
′. In the hemisphere type lens
90
B′, however, the length of a light passage may significantly vary when the lens
90
B′ shifts in position. This is because the first lens
90
A cooperating with the hemisphere type lens
90
B′ has a greater numerical aperture than when the supersphere type lens
90
B″ is used. Thus, the objective lens system
90
as a whole may fail to function properly when the two lenses
90
A,
90
B′ are offset from each other even by a relatively slight amount. Accordingly, the tracking range may become unacceptably narrow. To compensate for this drawback, use may be made of an additional tracking mechanism. However, this scheme is disadvantageous since the disk apparatus incorporating such an additional unit is more expensive than otherwise.
When use is made of the supersphere type lens
90
B″, on the other hand, the objective lens system
90
composed of lenses
90
A,
90
B″ is less vulnerable to the offset of the two lenses, whereby a wide tracking range is provided. This is due to the fact that the supersphere type lens
90
B″ has power, and the NA of the first lens
90
A can be made small. However, since the lens
90
B has power, the objective lens system is more vulnerable to the thickness variation of the slider
91
or spherical lens
90
B″. Also, the objective lens system may fail to function properly when there is a deviation from the prescribed working distance. Thus, to utilize the superspherical lens
90
B″, the tolerance of thickness for the slider
91
and the lens
90
B″ should be small. Unfavorably, this may lead to an increase in manufacturing cost.
SUMMARY OF THE INVENTION
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide an objective lens system which can enjoy the advantages of both the hemisphere type and the supersphere type without suffering from their drawbacks.
Another object of the present invention is to provide an optical disk apparatus incorporating such an object lens system.
According to a first aspect of the present invention, there is provided an objective lens system that includes: a first lens provided with a first surface for allowing entry of light and with a second surface opposite to the first surface; and a second lens provided with a spherical third surface and with a flat fourth surface opposite to the third surface. The third surface faces the second surface and has a radius of curvature r
0
. The second lens has a thickness d
0
and a refractive index n
0
. The thickness d
0
, the radius of curvature r
0
and the refractive index n
0
are determined so as to satisfy a relation: d
0
>r
0
>n
0
d
0
.
With such an arrangement, the second lens of the present invention can function as a hybrid that combines the conventional hemisphere and supersphere types. As a result, the second lens of the present invention can not only alleviate the shortcomings of the conventional hemisphere and supersphere lenses, but also enjoy the advantages of both of the lenses.
Preferably, the thickness d
0
, the radius of curvature r
0
and the refractive index n
0
may further satisfy the following relation: d0−r0<r0−n0d0.
According to a second aspect of the present invention, there is provided an optical disk apparatus that includes: a light source; and an objective lens system. The objective lens system includes at least one lens which is provided with a spherical surface having a radius of curvature r
0
. The lens has a thickness d
0
and a refractive index n
0
. The thickness, the radius of curvature and the refractive index satisfy the following relation: d
0
>r
0
>n
0
d
0
.
According to a third aspect of the present invention, there is provided an optical disk apparatus that includes: a light source; an aspherical first lens which light emitted from the light source enters; a second lens that includes a spherical surface and a flat surface opposite to the spherical surface, the spherical surface facing the first lens and having a radius of curvature r
0
; and a storage disk facing the flat surface. The second lens is provided with both a spherical lens element including the spherical surface and a transparent base including the flat surface. The lens element has a thickness d
1
and a refractive index n
1
, while the base has a thickness d
2
and a refractive index n
2
. The radius of curvature r
0
, the thicknesses d
1
and d
2
, and the refractive indexes n
1
and n
2
satisfy the following relation: d
1
+d
2
>r>{n
1
/(n
1
+1)}d
1
+{n
2
/(n
2
+
Matsumoto Tsuyoshi
Uno Kazushi
Choi William
Dang Hung Xuan
Fujitsu Limited
Greer Burns & Crain Ltd.
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