Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2002-04-18
2004-02-10
Neyzari, Ali (Department: 2655)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S013420, C369S275400, C428S064200, C428S690000
Reexamination Certificate
active
06690626
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical disk that is used for information recording or reproduction.
2. Related Background Art
It has been desired in the area of optical disks to achieve higher-density information recording. A method for realizing such high-density recording has been proposed in the form of a DWDD (domain wall displacement detection) type optical disk.
In the DWDD type optical disk, it is necessary to weaken magnetic coupling between adjacent recording tracks. Therefore, when manufacturing the DWDD type optical disk, prior to recording of information signals, a magnetically separating process for magnetically separating adjacent recording tracks is performed. The magnetically separating process is performed by methods such as those disclosed in JP 6(1994)-290496 A and JP10(1998)-340493 A.
A structure of the DWDD type optical disk and a method of magnetically separating process according to a conventional technique are described as examples with reference to FIG.
5
. In an optical disk shown in
FIG. 5
, a first dielectric layer
502
, a magnetic layer
503
, a second dielectric layer
504
, and a protective coating layer
505
are laminated in this order on a substrate
501
. On a surface of the substrate
501
on a side of the thin film layers, grooves
506
are formed. A portion between two of the grooves
506
adjacent to each other in a radial direction is referred to as a land
507
that is used as a recording/reproducing track. The grooves
506
have a width of, for example, 0.2 &mgr;m, and the lands
507
have a width of 1.4 &mgr;m. The magnetic layer
503
includes at least three magnetic thin films that are used for reproduction employing a DWDD system. In order for the DWDD system to be performed, it is necessary to magnetically separate the lands
507
as the tracks used for recording/reproduction.
The following description is directed to a method of performing the magnetically separating process with respect to this optical disk. In the magnetically separating process, a laser beam
508
to be used for annealing is focused on the groove
506
by an objective lens
509
and allowed to scan along the grooves
506
, so that magnetic coupling between the magnetic layers
503
on the grooves
506
is lost. As a result, in each of the lands
507
, which is a region interposed between the grooves
506
that have been subjected to annealing, both sides of the land
507
is magnetically separated, thereby allowing a DWDD operation to be performed. The laser beam
508
used in this process has, for example, a laser power of 2 mW and a wavelength &lgr; of 780 nm. The objective lens
509
has a NA of 0.5, and a beam spot of about 800 nm in diameter is formed. The beam spot of the laser beam
508
travels at a speed of, for example, 2 m/second.
When the lands
507
interposed between the grooves
506
are used as the recording/reproducing tracks as in the foregoing description, at an innermost or outermost end of a recording/reproducing track region, the groove
506
is provided. That is, even the land
507
positioned at the innermost or outermost end is interposed between the grooves
506
. Accordingly, both sides of each of all land tracks can be magnetically separated by allowing the laser beam
508
for annealing to scan over all the grooves
506
.
In many of recordable type optical disks with tracking guide grooves, an address represented by a track number and a sector number is composed of pit rows of a length of several tens of microns. In regions in which the pit rows are provided, the grooves are interrupted. That is, these pits are provided in flat surfaces without the grooves.
Essentially, an optical disk is designed so that a light beam is incident in a direction perpendicular to a disk surface, whether in recording or in reproduction. When a light beam is incident at an angle (when a tilt is caused), it causes adverse effects such as deterioration in recording sensitivity, reproducing sensitivity, and resolving power, crosstalk, crosserase, or the like.
On the other hand, as the information oriented society advances, optical disks have been requested to achieve higher-density recording, and thus demands for adaptability to a high-NA objective lens and reduction in thickness of a disk substrate have been growing. Further, as wider applications of optical disks are being found, environments in which optical disks are used in portable devices such as a still camera and a video camera have been increasing. These environments, however, cause more tilting between a light beam and a disk, and thus some tilt correction methods have been proposed.
The simplest and most commonly used among such methods is a method shown in FIG.
6
. The method utilizes a phenomenon in which an intensity distribution of reflected light depends on a tilt. A laser beam
508
that is used for recording/reproduction is focused by an objective lens
509
via a beam splitter
601
and irradiated onto an optical disk
501
. However, when the optical disk
501
is not perpendicular to the laser beam
508
, namely, a tilt is caused as shown in the figure, a portion of a reflected light beam
602
reflected from the optical disk
501
is not incident on the objective lens
509
. As a result, a differential output corresponding to the tilt is detected from a differential amplifier
604
of a bisected detector
603
.
However, in this method, it is necessary as a precondition that no diffraction or variations in reflection should be caused in a beam spot when the reflected light beam
602
is reflected from the optical disk
501
. Thus, tilt detection is performed using flat portions (hereinafter, referred to as mirror portions) without grooves on the optical disk
501
.
As shown in
FIG. 5
, when tracks are magnetically separated by allowing a light beam to scan, in each magnetic film positioned between the tracks, magnetic anisotropy is reduced, and a physical and structural change is caused in a central portion. This causes the complex refractive index to be changed, and as a result, reflectance is reduced. Therefore, in a disk having this configuration, when a recording/reproducing beam passes over a track, the reflectance of portions on both sides of the track changes, and thus employing discontinuous grooves and simply providing the mirror portions are not sufficient to enable exact detection of the tilt.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical disk in which, while adjacent recording/reproducing tracks are magnetically separated so that a DWDD operation can be performed, tilting can be detected based on an intensity distribution of reflected light.
An optical disk of the present invention includes a disk-shaped substrate and at least a first dielectric layer, a magnetic layer, and a second dielectric layer that are formed on the substrate. In the substrate, a predetermined region ranging in a radial direction is used as a data region for recording/reproducing data. The data region includes recording/reproducing tracks that are composed of a plurality of discontinuous lands or grooves ranging from an innermost track to an outermost track. The magnetic anisotropy of each of the magnetic layers positioned between the respective recording/reproducing tracks is reduced to a level lower than that of the magnetic anisotropy of the magnetic layers positioned on the recording/reproducing tracks, so that the magnetic layers are magnetically separated only between the recording/reproducing tracks respectively and not magnetically separated in flat portions in which the grooves are interrupted.
According to this configuration, since the magnetically separating region is not provided in the flat portions (mirror portions) in which the recording/reproducing tracks are interrupted, the reflectance is not changed over the flat portions. Accordingly, based on a reflectance held so as to be even, in recording or reproduction, a tilt of a, disk can be detected using light reflected from the mir
Birukawa Masahiro
Hiroki Tomoyuki
Hozumi Yasushi
Merchant & Gould P.C.
Neyzari Ali
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