Optical: systems and elements – Lens – Including a nonspherical surface
Reexamination Certificate
2002-09-06
2003-10-21
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
Including a nonspherical surface
C369S112230
Reexamination Certificate
active
06636366
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an objective having a high numerical aperture (NA) to realize a large-capacity optical disk, an optical pickup with the objective, an optical disk writer-reader with the objective, and an optical disk reader with the objective.
2. Description of the Related Art
Conventional objectives for compact disks (CDs) have numerical apertures (NAs) of 0.45 to 0.5 and employ laser beams of about 780 nm in wavelength to read and write the CDs. Objectives for digital versatile disks (DVDs) have NAs of about 0.6 and employ laser beams of about 650 nm in wavelength to read and write the DVDs.
To handle high-capacity optical disks, now being developed are next-generation pickups with objectives that have high NAs and operate on short-wavelength beams.
The short wavelength beams may include a blue laser beam of about 400 nm in wavelength.
Examples of objectives having high NAs are reported in the following papers:
(A) Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 978-979, M. Itonaga et al. “Optical Disk System Using High-Numerical Aperture Single Objective and Blue LD”
(B) Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 937-942, I. Ichimura et al. “Optical Disk Recording Using a GaN Blue-Violet Laser Diode”
The paper (A) reports a system employing a single lens having a numerical aperture of 0.7 and the paper (B) a system employing two lens groups having a numerical aperture of 0.85.
Higher numerical apertures result in lowering system margins. To cope with this problem, the systems reported in the above papers further thin a CD transmission layer of 1.2 mm and a DVD transmission layer of 0.6 mm. The paper (A) mentions a thickness of 0.12 mm and the paper (B) a thickness of 0.1 mm. Although the thickness of a transmission layer of an optical disk depends on system margins, it is preferable to be about 0.3 mm or thinner.
The two-lens-group system reported in the paper (B) realizes a greater numerical aperture than the system of the paper (A). The system of the paper (B), however, needs an assembling process of two lens groups, and therefore, is disadvantageous to mass production and increases costs.
According to the paper (B), the two-lens-group system involves a working distance of about 0.13 mm, which is shorter than about 1 mm of a single-lens system in a conventional DVD system. The short working distance increases a risk of colliding with an optical disk, thereby deteriorating the reliability of the system.
Next-generation optical disk systems are required to have single objectives having numerical apertures of 0.7 or above.
It is possible to design lenses having high numerical apertures. Shotaro Yoshida details a method of designing a double-sided aspherical lens having a high numerical aperture in “Study in Aspherical Aplanatic Lens with Particularly Large Aperture Ratio” in Tohoku University Institute of Scientific Measurements Report, March, 1958.
Also, Japanese Patent Laid Open Publication 4-163510 discloses a single objective having a numerical aperture of about 0.6 to 0.8.
A lens having a high numerical aperture can be designed but is not always manufacturable. For actual manufacturing, a designed lens must secure a manufacturing tolerance. In addition, the designed lens must be less affected by wavelength variations or wavelength width of source light, to decrease chromatic aberration.
A critical manufacturing tolerance for a double-sided aspherical lens for an optical disk is a surface-to-surface eccentricity tolerance. In addition to keeping the eccentricity tolerance, the lens must simultaneously satisfy requirements for axial aberration related to perpendicular incident light and off-axis aberration related to oblique incident light.
It is nearly impossible for a lens having a numerical aperture of 0.75 or higher to simultaneously satisfy these requirements.
In a double-sided aspherical lens, off-axis aberration worsens in proportion to an increase in the numerical aperture of the lens even if no consideration is made on the manufacturing tolerance of the lens. If the manufacturing tolerance is considered, the off-axis aberration worsens because the manufacturing tolerance, i.e., the eccentricity tolerance of the lens is securable only by sacrificing the axial aberration and off-axis aberration of the lens.
Although the axial aberration of the lens does not greatly worsen with the consideration of the eccentricity tolerance, the off-axis aberration of the lens greatly worsens when the numerical aperture of the lens is higher than 0.6 and when the eccentricity tolerance is of the micrometer order.
Chromatic aberration is usually put behind the manufacturing tolerance. Namely, the manufacturing tolerance of a lens is considered at first, and then, the shape of the lens is improved as high as possible to minimize the chromatic aberration of the lens.
Many studies have been made on the shapes of double-sided aspherical lenses to improve lens performance. Some of the studies are disclosed in Japanese Patent Laid Open Publications 5-241069 and 4-163510.
The publication 4-163510 discloses a range of lens shapes to ensure good performance. This disclosure mentions nothing about the securing of eccentricity tolerance. A second embodiment of the disclosure explains a lens whose numerical aperture is greater than 0.75 (0.8 for a wavelength of 532 nm). This lens causes a large aberration even on a slight eccentricity. The disclosure mentions nothing about chromatic aberration.
The disclosures cover a wide range of specifications, and therefore, are insufficient to actually design a good lens.
The two-lens-group system mentioned above involves a short working distance, and therefore, greatly increases a risk of colliding with an optical disk when the lens groups employ a higher numerical aperture. Optical disks are generally made of plastic, which unavoidably involves warp. A CD involves a warp of about 0.6 mm and a DVD involves a warp of about 0.3 mm, which is a double improvement from the CD. No further improvement is expected in optical disk warp because the warp depends on disk material. The two-lens-group system has a working distance of 0.13 mm as mentioned above. This working distance may differ depending on lens design but must not be increased greater than 0.2 mm, to make a pickup that employs the two-lens-group system compact. With such a short working distance, the lens system will collide with an optical disk if focus servo runs off due to disturbance, vibration, or defects during a disk write or read operation.
There is another paper (C) Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 1804-1807, G. Hashimoto et al. “Miniature Two-Axis Actuator for High-Data-Transfer-Rate Optical Storage System.” The paper (C) discloses a compact system of two lens groups having a numerical aperture of 0.85 and a focal distance of 0.88 mm. This system may realize a compact actuator or pickup operating at high speed. The system, however, involves a very short working distance of 0.1 mm to increase a risk of collision.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an objective (objective lens or lens) for an optical disk, made of a double-sided aspherical single lens (singlet) having a numerical aperture equal to or greater than 0.75 and capable of minimizing axial aberration, off-axis aberration, surface-to-surface eccentricity aberration, and chromatic aberration. Also provided are an optical pickup, an optical disk writer-reader, and an optical disk reader each employing the objective.
An aspect of the present invention provides an objective for an optical disk, herein the objective has first and second aspherical surfaces, a numerical aperture NA) of the objective is equal to or greater than 0.75 and a radius of curvature R
1
of he vertex of the first surface is defined as follows:
(1
−D
)
A<R
1
<(1
+D
)
A,
A=B/C,
B=
0.85
f
(
n−
1) and
C=n (
0.60866−0.11
t/f
−0.1272
d/f
)(0.83+0.2
NA
)
where n is a refractive index of the lens, f is a foca
Nath Gary M.
Nath & Associates PLLC
Novick Harold L.
Victor Company of Japan , Limited
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