Optical: systems and elements – Compound lens system – With image recorder
Reexamination Certificate
2002-02-14
2003-11-18
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Compound lens system
With image recorder
C359S719000
Reexamination Certificate
active
06650469
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Korean Patent Application No. 2001-45219 filed on Jul. 26, 2001, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an objective lens formed of a single lens and having a high NA (numerical aperture) which has an improved structure to overcome limitations of a conventional objective lens formed of a single lens, and an optical pickup apparatus adopting the same.
2. Description of the Related Art
To increase density in recording information on a recording medium, the size of a light spot formed on the recording medium to record and/or reproduce data needs to be reduced. The size of a light spot is proportional to a wavelength &lgr; of light in use, as shown in Mathematical Formula 1, and inversely proportional to the size of NA of an objective lens. Thus, as the NA of the objective lens increases, the recording density increases.
Mathematical Formula 1
Size of light spot∝&lgr;/NA
However, due to limitations in manufacturing, it is difficult to provide an objective lens formed of a single lens having a high NA, for example, 0.7 or more, while satisfying an allowance condition of optical aberration (when slope angle<55° and optical path difference (OPD)=0.03&lgr; rms, decenter>4 &mgr;m and tilt>2 arcmin). This is because of a restricted material of a lens and because a spherical shape of the lens is designed such that it is inappropriate for injection. That is, an objective lens having a short focal length and a high NA and formed of a single lens has a large curvature of a spherical surface so that an angle of a tangent at an effective surface becomes 55° or more. This slope angle makes the manufacturing of a mold difficult and decreases a degree of allowance in decentering between lens surfaces during manufacturing of a lens, thus lowering production yield.
Thus, an objective lens having a high NA and which is easy to manufacture and to secure allowance in assembly is needed. Also, as the NA increases, since an effect by wavefront aberration according to a thickness of a recording medium increases, the thickness of a recording medium, that is, the thickness of a protective layer to protect a recording surface, is required to be thin.
As is well known in the field to which the present invention pertains, DVDs (digital versatile disc) have a protective layer having a thickness of 0.6 mm to protect a recording surface of a recording medium. When a protective layer is thick, as in a DVD, a working distance (WD) between an objective lens and a recording medium must be sufficient. Accordingly, the thickness of a protective layer restricts an increase of NA of the objective lens needed to miniaturize the objective lens formed of a single lens and to provide high density recording. In order to manufacture an objective lens having a small diameter and a sufficient WD to prevent the objective lens from contacting a recording medium, a curvature of a spherical surface increases while allowances in eccentricity and angle of view between surfaces of the lens decreases, so that productivity deteriorates.
An objective lens formed of two lenses, as illustrated in
FIG. 1
, has been suggested by Japanese Patent Publication No. 10-123410 to satisfy a need for the thickness of a thin protective layer and overcome the limitations on the objective lens formed of a single lens.
Referring to
FIG. 1
, a conventional objective lens
10
includes a first lens
11
to condense incident light and a second lens
13
disposed between the first lens
11
and the recording medium
1
to increase an NA of the objective lens
10
. When a red wavelength, for example, light having a wavelength of 635 nm, is used to secure assembly allowance, curvature is assigned to four divided lens surfaces so that a high NA of 0.7 or more can be achieved.
The objective lens
10
includes at least one aspheric surface and uses a low dispersion material to reduce chromatism. An NA of 0.7 or more can be achieved by using a low dispersion material exhibiting an Abbe's number of 40 or more in line d and an NA of 0.8 or more can be achieved by using a low dispersion material exhibiting an Abbe's number of 60 or more.
Given that the focal length of the first lens
11
disposed at a position where light is incident is f1 and the synthesized focal length of two lenses
11
and
13
, that is, the overall focal length of the objective lens
10
, is f, the objective lens
10
provides that 1.7<f1/f<2.5.
Also, when a thickness T of a transparent protective layer to protect a recording surface with respect to NA satisfies conditions of Inequalities 2 below, the objective lens
10
enables correction of aberration.
Inequalities 2
When 0.7<NA<0.8, T≦0.32 mm;
When 0.8≦NA≦0.9, T≦0.20 mm; and
When 0.9≦NA, T≦0.11 mm.
Also, when a refractive index of a lens having a relatively large curvature of a lens surface of two lenses
11
and
13
is n1 and a reflective index of a lens having a relatively small curvature is n2, the objective lens
10
satisfies n1>n2.
Also, when the diameter and the working distance of an incident light beam are BW and WD′, respectively, the objective lens
10
provides that 1.0≦BW<4.5 and 0.05≦WD. The BW and WD′ satisfy conditions of Inequalities 3 below with respect to the NA of the objective lens
10
.
Inequalities 3
When 0.7<NA<0.8,
WD′≦
0.25676
BW+
0.039189;
When 0.8<NA<0.9,
WD′≦
0.14054
BW−
0.064865; and
When 0.9<NA,
WD′≦
0.096429
BW−
0.244640.
The conventional objective lens
10
can realize a high NA of 0.8 or more with respect to a recording medium having a protective layer thinner than that of a DVD. Also, the objective lens
10
, which is formed of two lenses
11
and
13
, can satisfy the condition of a thin protective layer and simultaneously overcome the limitation on the objective lens formed of a single lens.
However, the conventional objective lens
10
formed of two lenses needs a holder to maintain a distance between the two lenses
11
and
13
, that is, an air gap, to be constant. Thus, since the conventional objective lens
10
has a high NA, allowances in distance and angle between the two lenses
11
and
13
are too small during assembly of the holder, so that production yield is deteriorated. Furthermore, since an actuator to control focus by driving the two lenses
11
and
13
being integrated is needed, a recording/reproducing apparatus is complicated and large.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an objective lens having a high NA and formed of a single lens which has an improved structure so that the limitations on the conventional objective lens formed of a single lens and the problems of the conventional objective lens formed of two lenses are overcome, and an optical pickup apparatus adopting the objective lens.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing and other objects of the present invention are achieved by providing an objective lens formed of a single lens, wherein a working distance of the lens is less than a thickness at the center of the lens.
The foregoing and other objects of the present invention may also be achieved by providing an objective lens formed of a single lens, wherein a thickness at the center of the lens is 0.8 times or more an effective diameter of the lens.
The foregoing and other objects of the present invention may also be achieved by providing an optical pickup apparatus comprising a light source to generate and emit a light beam having a predetermined wavelength, an objective lens to focus the light beam emitted from the light source to form a light spot on a re
Jung Seung-tae
Kim Dae-sik
Lee Chul-woo
Dang Hung Xuan
Martinez Joseph
Samsung Electronics Co,. Ltd.
Staas & Halsey , LLP
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