Optical: systems and elements – Lens – Including a nonspherical surface
Reexamination Certificate
2002-04-08
2004-06-01
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
Including a nonspherical surface
C359S718000, C369S112230
Reexamination Certificate
active
06744568
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an objective lens for recording or reproducing information on or from an optical information recording medium, in particular, an objective lens with less aberration, a method for correcting a manufacturing error thereof, and an optical pick-up device using the objective lens.
BACKGROUND OF THE INVENTION
Light-utilizing technology has various characteristics such as high speed owing to high frequency, and capability of spacial information processing, phase processing, etc. Therefore, such technology has been under research and development, and used in practical applications in a variety of fields including communication, instrumentation, processing, and the like.
In the light-utilizing technology, a high-precision objective lens has been used as means for focusing an optical beam which is emitted from a light source and irradiated onto a recording medium. In recent years, the demand for an image recording device, etc. utilizing light has been especially increasing, and technology for realizing mass-storage recording is of increasing importance.
In order to record mass optical information, it is necessary to improve recording media so as to achieve high recording density, and what is more, to form a smaller beam spot, that is, to focus a beam spot sufficiently by an objective lens. In this manner, more information can be recorded by forming a smaller beam spot.
Besides, the diameter of a beam spot is proportional to the wavelength of light emitted from a light source to be used, and inversely proportional to the NA (Numerical Aperture) of an objective lens. Consequently, in order to form a smaller beam spot, it is necessary to shorten the wavelength of the light emitted from a light source to be used, or to increase the NA of the objective lens so as to obtain a higher NA.
As for the wavelength of light, a blue laser diode and a blue or green SHG (Second Harmonic Generation) laser are being developed, and shorter wavelength is being realized.
Meanwhile, as for the NAs of objective lenses, for example, the NA of a CD (Compact Disc) is 0.45, and the NA of a DVD (Digital Versatile Disc) is 0.6. In this manner, the DVD, having a higher NA than the CD, has achieved higher density of information to be recorded.
Besides, an optical pick-up device which uses a 2-group lens (a pair of lenses) so as to realize an objective lens having a further higher NA is disclosed in Japanese Unexamined Patent Publication No. 10-123410 (Tokukaihei 10-123410, published on May 15, 1998). In this optical pick-up device, an objective lens is made up of a 2-group lens in which a pair of lenses are combined using a lens barrel or the like. With this arrangement, the NA of the objective lens can be set at 0.85, achieving a higher NA.
However, when combining these two lenses in the above arrangement, an interval between the lenses, positioning of these lenses should be carried out accurately considering tilts of the respective lenses with respect to an optical axis, and a deviation of centers of the lenses, etc. Therefore, at the time of positioning, it is necessary to adjust the positions of the lenses while actually emitting a laser beam and checking the focusing condition of a beam spot and an aberration actually caused. Consequently, the positioning process is complicated.
In addition, resin is required to fix the 2-group lens. However, resin does not have sufficient heat and moisture resistance properties, and changes over time. In this manner, resin may have characteristic changes, and thus has low reliability.
Further, the 2-group lens weighs heavier as it includes the lens barrel, etc. Generally, focus control and tracking control are carried out by an actuator, by moving an objective lens in a direction parallel or vertical to the optical axis. Here, if the objective lens is heavy in weight, the objective lens cannot be driven at a high speed, making the information recording/reproducing speed slow.
On the other hand, an optical pick-up device using a single lens as an objective lens so as to achieve a higher NA is disclosed in Japanese Unexamined Patent Publication No. 9-311271 (Tokukaihei 9-311271, published on Dec. 2, 1997). As shown in
FIG. 37
, the optical pick-up device includes an objective lens in which a diffraction lens
103
and an aspherical lens
102
are integrated, a convex skew plate
104
, and a concave skew plate
105
.
In the foregoing optical pick-up device, a laser beam emitted from a light source is irradiated via the objective lens onto a disk
101
, and thus information is recorded and reproduced on and from the disk
101
.
With this structure, a chromatic aberration can be corrected by using the objective lens in which the diffraction lens
103
and the aspherical lens
102
are integrated. Besides, a comatic aberration can be corrected by moving the convex skew plate
104
and the concave skew plate
105
respectively in opposite directions.
In this manner, since a single lens is used as the objective lens of the optical pick-up device shown in
FIG. 37
, positioning process, resin for fixing lenses, etc. are not required.
However, the objective lens shown in
FIG. 37
is made up by integrating the diffraction lens
103
and the aspherical lens
102
. Therefore, when diffraction efficiency is reduced due to an error caused in a process for manufacturing the objective lens, lens transmittance of a laser beam (light) is decreased, resulting in a decline in light utilization efficiency.
Besides, since the interval of a diffraction grating of the diffraction lens
103
is very narrow, it is difficult to manufacture the diffraction lens
103
. Using such a diffraction lens
103
involves an increase in manufacturing cost and characteristic changes depending on a polarization direction. Consequently, it is difficult to further downsize the objective lens provided with the diffraction lens
103
.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an objective lens which is downsized, has less aberration, and can correct the aberration easily, without a decline in light utilization efficiency; a method for correcting a manufacturing error thereof; and an optical pick-up device using the objective lens.
To achieve the foregoing object, an objective lens of the present invention is an objective lens having a numerical aperture not less than 0.75, and the objective lens is structured such that it is a single-lens objective lens both surfaces of which are aspherical, and is made of a material satisfying:
1.75
<n
, and 35
<v,
where n is refractive index for at least one of used wavelengths, and v is an Abbe number for a d-line.
According to the foregoing structure, a single-lens objective lens is used as the objective lens, and, for example, a diffraction element, etc. are not required. Therefore, efficiency to utilize light (laser beam) passing through the objective lens can be improved. In addition, manufacture of the objective lens can be facilitated, and further, miniaturization of the objective lens can be realized.
Besides, since the numerical aperture NA of the objective lens is not less than 0.75, for example, it becomes possible to form a smaller beam spot incident on a recording medium, realizing higher recording density in the recording medium.
Generally, a chromatic aberration is caused since the refractive index of a material constituting the objective lens varies depending on the wavelength of light. Due to the chromatic aberration, the position or the size of an image differs depending on the wavelength.
However, by setting as 35<v, the chromatic aberration can be lessened, and such an arrangement can prevent the phenomenon that an image point is shifted and a beam spot is out of focus due to a slight change in the wavelength.
Besides, by setting as 1.75<n, the curvature of a first surface of the objective lens can be reduced, facilitating the manufacture of the objective lens.
An optical pick-up device of the present invention is structured so as to include:
a light s
Iwaki Tetsuo
Kitamura Kazuya
Kurata Yukio
Yamaguchi Takeshi
Choi William
Nixon & Vanderhye PC
Sharp Kabushiki Kaisha
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