Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1998-11-05
2003-04-22
Korzuch, William (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S044140, C369S094000, C369S044370
Reexamination Certificate
active
06552990
ABSTRACT:
TECHNICAL FIELD
The present invention relates to optical pickup devices and, more specifically, to an optical pickup device performing recording and/or reproducing for a digital video disk (a DVD) and a compact disk (a CD).
TECHNICAL BACKGROUND
An optical disk with a thickness of about 1.2 mm, such as a CD-ROM (Compact Disk-Read Only Memory), has been provided for reading information using a semiconductor laser. In this type of optical disk, by performing focusing servo control and tracking servo control for an objective lens for pickup, a pit string of a signal recording surface is irradiated with laser beam to reproduce a signal. Recently, such optical disks are increasingly becoming higher in density for recording an animation lasting for a long period of time.
For example, a DVD standard has been proposed for recording information of 4.7 G bytes on one side of the optical disk having a diameter of 12 cm, which is the same as that of the CD-ROM. A transparent substrate of the DVD has a thickness of about 0.6 mm. A single DVD, which has two such transparent substrates applied to each other back to back, can record information of 9.4 G bytes. Further, as a write-once optical disk having the same diameter, thickness of substrate and recording density as those for the CD-ROM, there exists a CD-R (Compact Disk-Recordable).
Since three different types of optical disks, including the DVD, the CD-ROM and the CD-R, would exist in the same apparatus in the near future, a device capable of compatibly reproducing these three different types of optical disks is required. The DVD and the CD-ROM or the CD-R, however, cannot be reproduced by a single optical pickup device because of the difference in thicknesses of the transparent substrates.
Then, in Japanese Patent Laying-Open No. 5-303766, an apparatus has been proposed which allows reproduction of a high density optical disk having a thin transparent substrate with a thickness of 0.6 mm and a standard density optical disk having a standard transparent substrate with a thickness of 1.2 mm by using a single optical pickup device. In the apparatus, an objective lens with a numerical aperture of 0.6 is employed which has been designed to reproduce the high density optical disk by a laser beam with a short wavelength. When the standard density optical disk is to be reproduced, an aspherical optical device is inserted on the side of a light source of the objective lens which is provided with an aperture for shielding the periphery of laser beam to decrease an effective numerical aperture of the objective lens.
Moreover, to change the effective numerical aperture of an objective lens selectively shielding the periphery of laser beam emitted from a semiconductor laser to collect laser beam, an apparatus has been disclosed in Japanese Patent Laying-Open No. 8-321065 which is provided with a liquid crystal selectively rotating the plane of polarization of laser beam and a polarizing plate allowing transmission of only laser beam which is polarized in a specific direction, and which can compatibly reproduce optical disks having substrates with different thicknesses. While the apparatus can compatibly reproduce the DVD and the CD-ROM having substrates of different thicknesses, it cannot reproduce the CD-R as a laser beam with a wavelength of 635 nm is employed. The reason is as follows.
FIG. 1
is a diagram showing a relation between a pit depth and intensity of reflected light for every laser beam with a different wavelength. As shown in
FIG. 1
, when the laser beam with a wavelength of 635 nm is employed, the intensity of reflected light is the highest with the pit depth of about 105 nm. On the other hand, when laser beam with a wavelength of 780 nm is employed, the intensity of reflected light is the highest with the pit depth of about 125 nm. In the case of the CD-R, reflectance significantly changes with the wavelength of laser beam as organic dye is used for a recording film and a sufficient intensity of reflected light cannot be obtained using a single-wavelength laser with a wavelength of 635 nm. Thus, the CD-R cannot be suitably reproduced. Therefore, a two wavelengths to beam laser is required for the optical pickup device capable of compatibly reproducing the DVD and the CD-R or the CD-ROM. When laser beam with a wavelength of 430 nm would be employed due to the trend of shorter wavelength in the near future, such a two wavelengths to beam laser would be even more highly required.
Therefore, it is an object of the present invention to provide an optical pickup device capable of performing recording and/or reproducing for optical disks having substrates with different thicknesses by using a laser beam with two different wavelengths.
DISCLOSURE OF THE INVENTION
According to the present invention, an optical pickup device performing recording and/or reproducing for a first optical disk having a first transparent substrate and a second optical disk having a second transparent substrate with a thickness which is smaller than that of the first transparent substrate includes an objective lens, a laser beam generating means and an optical device. The objective lens is arranged opposite to the first or second optical disk. The laser beam generating means selectively generates a first laser beam with a first wavelength and a second laser beam with a second wavelength which is different from the first wavelength. The optical device includes: a central region arranged between the objective lens and the laser beam generating means, allowing transmission of the first laser beam without any change and increasing the diameter of the second laser beam by diffraction; and a peripheral region allowing transmission of the first laser beam without any change and substantially shielding the second laser beam by diffraction or absorption.
Preferably, a hologram is formed in the central region of the optical device.
More preferably, the hologram includes a plurality of annular convex portions each having four steps and coaxially formed, where a height h
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of each step is determined in accordance with the following expressions (1) to (5).
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&A
Ichiura Shuichi
Kajiyama Seiji
Kanou Yasuyuki
Tsuchiya Yoichi
Yamada Masato
Armstrong Westerman & Hattori, LLP
Chu Kim-Kwok
Korzuch William
Sanyo Electric Co,. Ltd.
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