Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
1999-03-09
2002-06-18
Neyzari, Ali (Department: 2752)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C369S275100
Reexamination Certificate
active
06407979
ABSTRACT:
FIELD OF THE INVENTION
This invention is generally related to a high density optical disk and a method of high density recording on an optical disk. Specifically, it relates to an optical disk which records on both lands and grooves with a reduced track pitch and the recording method.
BACKGROUND OF THE INVENTION
For high speed data processing, high density optical disks have attracted attention. With the ISO standardization of 5.25-inch and 3.5-inch disks for optical and phase change schemes for overwriting, these disks can be expected to have even more widespread use in the future. Recently, the DVD (digital video disk) standardization such as SD standardization is about to be finalized. This standardization is expected to accelerate the use of optical disks in the area of multi-media.
In these optical disks, grooves and lands are used to guide a laser spot emitted from a pickup of a read/write system to the data. These grooves and lands are formed in a spiral from the center of the medium to its outer circumference. These grooves are called guide grooves. More specifically, as defined in the ISO standard, the recessed portions, which are more distant, as viewed from the pickup, are referred to as lands, and the raised portions closer to the pickup, are referred to as grooves.
Using a land recording method, the groove widths of from about 0.3 &mgr;m to about 0.6 &mgr;m, and the groove depths of about &lgr;/8(n) to about &lgr;/4(n) wherein &lgr; is the wavelength of a laser beam used during input and output operations, and n is a refractive index of substrate. Although the standard track pitch is 1.6 &mgr;m, narrower track pitches are used to increase the density of the recorded data.
When using an optical pick up having an object lens of about 0.5 to about 0.6 aperture numerical (NA), the impact of the narrow track pitch on the undesirable simultaneous reading of data from adjacent tracks (hereafter referred to as the optical crosstalk) becomes critical and the tracking error signal is also negatively affected for accurate tracking.
For the optical disks currently on the market, the width W of substrate surface grooves is defined as W=(W
top
+W
bottom
)/2, where W
top
is the width at the top of the groove, and W
bottom
the width at the bottom. The groove depth is defined as the height of the substrate surface groove from the bottom of the groove to the top (step difference).
Various methods have been tried in an effort to increase the recording density of such optical disks, such as making the laser beam spot smaller by reducing the wavelength of the light source so as to read data at a higher recording density. There is a limit, however, in reducing the spot size due to the wavelength of semiconductor laser that can be used for a light source. In general, short wavelength laser has problems in forming a desirable beam shape as well as in an insufficient output level.
Other efforts to provide a higher recorded data density resulted in the proposal of a technology called magnetically induced super-resolution (MSR). This technology is capable of reading high-density recorded data using a currently available light source wavelength and a spot size. The MSR technology makes use of the temperature distribution of the recording medium inside an area under the light spot (due to a combination of the heating of the medium by the laser and the rotating motion of the medium) to mask a portion of the signal on the medium falling under the light spot so that it will not be detected as the read signal. This masking makes the effective aperture area from which a signal is read smaller than the laser spot size, thus making it possible to read higher density data.
The Front Aperture Detection (FAD) scheme, which is one way of implementing the MSR technology, will be briefly described, in reference to FIG.
2
. FIG.
2
(
a
) is a plan view of a FAD-type MSR disk
31
, and FIG.
2
(
b
) is a cross-sectional view of Section I—I of FIG.
2
(
a
). The FAD optical disk
31
has three magnetic layers: a recording layer
32
, made of TbFeCo; a cutoff layer
33
, made of TbFe; and a readout layer
34
, made of GdFeCo. The signal is read from the readout layer
34
. In the initial state as shown in FIG.
2
(
b
), because coupling force is readily exchanged between the adjacent layers, the orientations of the magnetic field follow the magnetization of the recording layer
32
, which stores the data as indicated by recording marks
38
. During a read operation, an external magnetic field Hr is applied. When a relative position is established between a readout light spot
35
and a medium
31
, as shown in FIG.
2
(
a
), a temperature differential is created between a front low temperature area
36
and a rear high temperature area
37
under the light spot
35
.
When the high temperature area
37
reaches the Curie temperature at which its magnetization is obliterated of the cutoff layer
33
, the coupling of readout layer
34
's magnetization to recording layer
32
(through cutoff layer
33
) is diminished, causing the magnetization of readout layer
34
to invert to align with the magnetization of the external magnetic field Hr. In FIG.
2
(
b
), the magnetized direction of location A is inverted.
In the high temperature area
37
, the magnetization of readout layer
34
always exhibits a constant state regardless of the presence of a recording mark
38
and is a mask without contributing to the readout signal. On the other hand, the signal is detected only in a low temperature area
36
where its recorded state is maintained. Thus, the low temperature area
36
serves as an effective signal detection aperture of the laser spot. This enables the system to only read recorded mark
38
a
in the area
36
. In this FAD technique, the masking conditions are determined by the Curie temperature of the cutoff layer
33
. Thus the MSR disks are manufactured fairly easily by controlling the composition of this cutoff layer. Techniques other than FAD have been proposed for the MSR disks. These include Rear Aperture Detection (RAD) and Center Aperture Detection (CAD), in which the high temperature portion of the spot is the aperture, and the remaining spot area is masked. RAD and CAD MSR disks have a smaller aperture because it is the higher temperature portion of the spot (the temperature distribution resulting from illumination by the readout laser) that is an aperture, and the lower temperature portion that is a mask. In contrast to this, in MSR disks using the FAD technique, the crescent shape of the aperture (low temperature area
36
within readout spot
35
) renders it impossible to prevent leakage of signals from adjacent tracks.
The land-groove recording method has been also proposed for a high density recording. In contrast to the method in which data is recorded on either lands or grooves, the land-groove method increases the recording density to a half track pitch from a full track pitch by recording data on both lands and grooves. For example, if the center-to-center distance between adjacent lands (or grooves) and the next adjacent lands (or grooves) is 1.4 &mgr;m, the proposed land-groove technique increases the data capacity, by reducing the track pitch to 0.7 &mgr;m.
In this technique, an appropriate groove depth substantially reduces optical crosstalk or simultaneous data read from adjacent grooves (or lands). In addition, the center-to-center distance between lands (or grooves) of 1.4 &mgr;m provides a sufficient space required to maintain for a tracking error signal.
However, crosserase or heat crosstalk is observed in recording or erasing information on tracks when the temperature of the adjacent tracks rises due to the heat from a laser beam. The higher temperature erases information on the adjacent tracks. Optical disks and the phase change schemes use thermal recording technique. In these optical disks, the shorter the distance between adjacent tracks, the more likely heat travels into adjacent tracks. Accordingly, it is inevitable for the crosserase problems to arise.
Hayashi Masatoshi
Matsumoto Hiroyuki
Morita Seiji
Neyzari Ali
Nikon Corporation
Woodcock & Washburn LLP
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