Dynamic information storage or retrieval – Systems having plural physically distinct independent tracks... – Having layered storage medium
Reexamination Certificate
1998-03-25
2001-01-09
Hindi, Nabil (Department: 2753)
Dynamic information storage or retrieval
Systems having plural physically distinct independent tracks...
Having layered storage medium
C054S050000, C054S044300
Reexamination Certificate
active
06172957
ABSTRACT:
This application claims the benefit of Japanese Application Number 9-93247 filed on Mar. 27, 1997, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a disc playback apparatus, and more particularly, to an optical pickup for use in playing back a multi-layer disc and a multi-layer disc playback apparatus for reproducing recorded information from the multi-layer disc.
2. Description of the Related Art
A Digital Video Disc (DVD) is an optical disc capable of storing a large amount of digital information. It is capable of storing digital information, such as animation and computer information, in a disc of the same 12 cm diameter as a compact disc (CD); at a recording density 6-8 times greater than a CD. Various devices are used in recording onto a DVD to achieve such high density recording. For example, the DVD uses a laser having a wavelength of 650 nm or 635 nm, shorter than a wavelength of 780 nm used for a CD, to increase the recording capacity. The numeral aperture, NA, of an objective lens is 0.6, larger than the NA of 0.45 for a CD. As a result, a high density recording of about 5 G byte has been achieved on a single side of the disc.
Because the DVD is of higher density recording than a CD, the spot diameter of a laser beam for reproducing bit information needs to be far smaller than that of the CD. The spot diameter of a laser beam is proportional to the wavelength &lgr; of the laser beam and inversely proportional to the numeral aperture NA of an objective lens. In the DVD, this is realized by using a laser beam having a short wavelength and an objective lens having a large numeral aperture NA.
Further, by using a multi-layer disc composed of a plurality of recording layers instead of a single layer recording disc, a further high-density disc has been realized. In such a multi-layer disc, the focal point of the light beam is set to a desired recording layer for recording or reproduction.
FIG. 6
illustrates light beam behavior due to wave front aberration when data in a multilayer disc is reproduced by a single pickup. An objective lens
602
a
of the pickup illustrates the position thereof when a first recording layer
601
a
of the multi-layer disc
610
is being played back. An objective lens
602
b
illustrates the position thereof when the n-th layer, which is far from the disc substrate surface
601
s
is being played back.
In a pickup optimally designed for focusing a light beam on the first layer, there is no trouble in playing back the first layer. However, if the objective lens approaches the disc substrate surface
601
s
to play back the n-th layer and the light beam is focused on the n-th layer, wave front aberration occurs due to the thickness of layers between the first layer and the n-th layer. As a result, as shown in
FIG. 6
, the diameter of the beam spot irradiated onto the n-th layer is larger than that on the first layer.
In the multi-layer disc, a distance from the recording layer optimally designed for the optical pickup creates a difference in the diameter of the beam spot actually focused on the layer. As a result, wave front aberration (mainly spherical aberration) occurs, and each recording layer has a different wave front aberration. Particularly, a recording layer far from the recording layer optimally designed is more affected by the wave front aberration.
That is, when a light beam is gathered onto a recording layer other than the optimally designed recording layer for the optical pickup, the light beam cannot be focused due to an influence of the aforementioned wave front aberration. Thus, in recording layers other than the optimally designed recording layer, it is necessary to enlarge the pit size or pit interval. Accordingly, the recording density per layer drops. As a result, the recording density of the overall multi-layer disc cannot be raised as much as desired.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to an improved optical pickup that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the invention to provide a multi-layer disc playback apparatus capable of playing back a multi-layer disc having a high recording density per layer, by correcting wave front aberration caused due to a difference in distance between an optimally designed recording layer for an optical pickup and each of the recording layers.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the improved optical pickup and multi-layer disc playback apparatus includes an optical pickup for use in a multi-layer disc playback apparatus for playing back a multi-layer disc having a plurality of recording layers, comprising a wave front aberration correcting means which is disposed in optical path between a light source and an objective lens and corrects wave front aberration of light emitted from the light source in accordance with a recording layer to be played back.
Further, the present invention provides a multi-layer disc playback apparatus for playing back a multi-layer disc having a plurality of recording layers, comprising a selection means for selecting a recording layer to be played back, a reproduction means for reproducing information recorded in the recording layer according to a signal read by the optical pickup, a wave front aberration correcting means which is disposed in optical path between a light source and an objective lens of the optical pickup and corrects wave front aberration of light emitted from the light source, and a control means for controlling the wave front aberration correcting means depending on a selected recording layer.
According to the present invention, when a multi-layer disc is played back, its wave front aberration that occurs due to a difference of distance between the optimally designed recording layer for the optical pickup and each of the recording layers is corrected. Thus, it is possible to play back a multi-layer disc having a higher recording density per layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
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patent: 9-128785 (1997-05-01), None
Hindi Nabil
Morgan & Lewis & Bockius, LLP
Pioneer Electronics Corporation
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