Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-09-08
2002-04-16
Hindi, Nabil (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S112060, C369S112080, C369S112230
Reexamination Certificate
active
06373807
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup for optically recording and reproducing information on and from a record medium, such as an optical disk or the like, and more particularly relates to an optical pickup having a plurality of light sources for emitting a plurality of kinds of light beams whose wave lengths are different from each other, and further relates to an information reproducing apparatus and an information recording apparatus containing this optical pickup.
2. Description of the Related Art
Recently, an optical disk has been developed which can have the same diameter as those of CD and DVD and can further record information at a higher density than that of the DVD (hereafter, this is referred to as “a high dense optical disk”. The DVD is an optical pickup having a record capacity equal to about seven times that of the CD. However, a memory capacity of the high dense optical disk exceeds that of the DVD. In this high dense optical disk, for example, about 15 gigabytes of information can be recorded on only one surface.
In parallel with the development of the high dense optical disk, it is considered to use a blue laser diode as a light source of a light beam to record and reproduce information. An oscillation wave length of this blue laser diode is 430 nm. It is shorter than that of a red laser diode (having an oscillation wave length of 650 nm) which is presently used for the record and reproduction of the DVD and the CD. As for the blue laser diode, for example, a blue semiconductor laser for using InGaN as an activation layer and an SHG laser for using LiNbO
3
as an SHG (Second Harmonic Generation) layer have been put into practical use.
It is desirable to use a blue laser light beam emitted from the blue laser diode, when recording the information on the high dense optical disk and when reproducing the information from the high dense optical disk.
The high dense optical disk is an optical pickup having the appearance similar to those of the CD and the DVD. Thus, it is desirable to record the information on the high dense optical disk and reproduce the information from the high dense optical disk by using a DVD player, a CD player or a CD/DVD compatible player.
On the other hand, an optical disk having an information record surface composed of two or more layers is developed in order to further improve the record capacity of the optical disk. Accordingly, the memory capacity of the optical disk can be increased by two times or more. The DVD having such two-layer structure is being realized.
An intermediate layer for dividing the two-layer information record surface is required to form the two-layer information record surface on the optical disk. Thin film made of gold (Au) is typically used as the intermediate layer. This reason is that in a case of the gold intermediate layer, since a degree of absorbing the red laser light beam is small, a stable laser light beam can be obtained.
However, the gold intermediate layer has the high degree of absorbing the blue laser light beam. Thus, the employment of the blue laser light beam disables the utilization of the gold intermediate layer. Actually, it is known that the employment of a laser light beam having a wave length of about 450 nm or less disables the utilization of the gold intermediate layer. As mentioned above, the blue laser light beam is required in order to record the information on the high dense optical disk or reproduce the information from the high dense optical disk. Hence, in the case of the high dense optical disk having the two-layer structure, it is necessary to use an intermediate layer other than the gold intermediate layer.
Thus, for example, in order to achieve a compatible type memory reproducing apparatus which can record and reproduce on and from both optical disks of the high density optical disk having the two-layer structure and the DVD having the two-layer structure, it is necessary to mount a blue laser light source and a red laser light source within an optical pickup and then switch between them based on the type of optical disk.
Next, an objective lens is required in order to collect the laser light beam onto the optical disk. In order to realize a small optical pickup having the blue laser light source and the red laser light source, it is desirable to collect the blue laser light beam and the red laser light beam through a single objective lens. However, if the two laser light beams are collected through the single objective lens, the dispersion resulting from wave length dependence of a refractive index occurs in each laser light beam. The occurrence of the dispersion causes chromatic aberration in each laser light beam. As a result, there may be a case that the laser light beam can not be excellently collected.
FIG. 1
is a graph showing the relationship between a wave front aberration RMS (a vertical axis) and an angle of an image height (a horizontal axis) in a blue laser light beam and the relationship between a wave front aberration RMS and an angle of an image height in a red laser light beam. Data on the graph is obtained in such a way that optical material typically used because of traditionally small dispersion is used to create an objective lens and then this objective lens is used to collect the red laser light beam and the blue laser light beam in an optimal condition. In addition, the angle of the image height implies an angle between a direction of a perpendicular of the objective lens and an optical axis of an incident laser light beam. The wave front aberration RMS implies a root mean square of an error between an ideal wave front without aberration and an actual wave front.
As shown in
FIG. 1
, when the image height is 0 degree, the chromatic aberration can be sufficiently compensated for both the laser light beams. However, as the image height becomes higher, each chromatic aberration is deteriorated. Actually, in the case of the red laser light beam, when the image height exceeds 0.6 degrees, the wave front aberration RMS exceeds 0.07. This value of 0.07 is a value known as a so-called Marshall limitation. When the wave front aberration exceeds 0.07, the function as the optical pickup can not be carried out any longer.
In addition, when gathering the data on the graph of
FIG. 1
, the blue laser light beam is made to enter the objective lens as collimated light (infinite system (afocal system)), in order to suppress as much as possible the chromatic aberration in each laser light beam. On the other hand, the red laser light beam is made to enter the objective lens as diffusion light or convergence light (finite system (focal system)).
Then, it may be considered to create the objective lens by using material having the smaller dispersion than those of conventional optical materials, for example, fused quartz or fluorite. However, in this case, a material cost is expensive, and a fused temperature is high, which causes the molding for the objective lens to be difficult, and thereby reduces the manufacturing yield.
Moreover, the following two techniques may be considered as the technique of reducing the manufacturing cost, making the molding process easier and ensuring the compatibility between the high dense optical disk and the DVD. The first technique is a technique of using a hologram element, making optical paths of the respective laser light beams different from each other (refracting them so that they are not parallel to each other) and then collecting the respective laser light beams. The second technique is a technique of providing a notch or a groove in a part of a transmission surface of an incident side of the objective lens.
However, although the first technique can perfectly remove the chromatic aberration and also create the optical pickup at a small size and a low cost, it has the problem that the utilization of the hologram causes a loss of a light quantity in each laser light beam to be large. The first technique further has the problem that it is difficult to remove a laser ligh
Kikuchi Ikuya
Ohtaki Sakashi
Hindi Nabil
Nixon & Vanderhye PC
Pioneer Corporation
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