Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
1995-11-29
2002-08-13
Hindi, Nabil (Department: 2516)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
Reexamination Certificate
active
06434108
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an optical disc on or from which information signals are recorded/reproduced on irradiation of a laser light beam, and an optical disc substrate used therefor.
RELATED ART
Among known optical discs capable of recording/reproducing information signals by the user, there are a phase-change type optical disc, having a layer of a phase-change type material as a recording layer, and a magneto-optical disc having a thin film of a rare earth metal-transition metal alloy as a recording layer.
Specifically, such optical disc is configured so that a dielectric layer, a reflective layer and a protective layer are formed along with the recording layer on a transparent substrate for controlling optical properties and for assuring durability.
On such optical disc, the optical properties of minute-sized domains of the recording layer are changed by irradiation of a laser light for generating a number of pits, and information signals are recorded as a pattern of these pits. Such recording is carried out along numerous guide grooves of different diameters formed on substantially concentric circles on a major surface of the transparent substrate. That is, tracking servo of the laser spots is carried out along the guide grooves as indices in order to permit recording/reproduction to occur at correct positions on the disc.
Referring to
FIG. 1
, showing the cross-section of the transparent substrate, the guide grooves
50
are formed at a preset pitch on a major surface of a transparent substrate
51
. The flat surface within the guide groove
50
is termed a groove or a bottom surface
52
, while the flat surface corresponding to the hillock between two neighboring guide grooves
50
is termed a land or a top surface
53
. The in formation signals are usually recorded on the bottom surface
52
or on the top surface
53
. The former recording system is termed an in-groove recording system, while the latter recording system is termed an on-land recording system.
An inclined surface
54
between the bottom surface
52
and the top surface
53
has an inclination angle of 40 to 60° and a width e of approximately 0.1 &mgr;m. This inclined surface operates as a heat-insulating surface and thus operates for prohibiting the spreading apart of the pit.
That is, the transparent substrate
51
has the bottom surfaces
52
representing the flat surface portion in the groove
50
, the top surface
53
representing the flat surface of the hillock and the inclined surface
54
between the bottom surface
52
and the top surface
53
. With the in-groove recording system, a width a of the bottom surface
52
represents the recording track width, and a total width (e+b+e) of the inclined surface
54
, top surface
53
and the inclined surface
54
between the neighboring bottom surfaces
52
represents a width c of a heat insulating region, as shown in FIG.
1
. On the other hand, with the on-land recording system, a width b of the top surface
53
represents the recording track width, and a total width (e+a+e) of the inclined surface
54
, bottom surface
52
and the inclined surface
54
between the neighboring top surfaces
53
represents a width of a heat insulating region, as shown in FIG.
1
.
With the optical disc, as with other recording media, there is a demand for a still higher recording density for information signals. The recording density on the optical disc may be classed into that along the length of the tracks and that in a direction perpendicular to the track. The recording density along the track length can be raised by reducing the diameter of the laser light beam spot for shortening the pit length. On the other hand, the recording density in the direction perpendicular to the track can be raised by reducing the track pitch.
However, if, with the above-described in-groove recording system or on-land recording system, the track pitch is narrowed to a value lesser than the value currently employed, such as 0.7 &mgr;m or less, interference from pits formed on a neighboring recording track is liable to be incurred, as a result of which cross-talk exceeding a prescribed value of −26 dB, may be produced in the playback signal.
FIGS. 2A
,
2
B schematically show an optical disc on which recording pits, which are amorphous pits, are formed in accordance with the in-groove recording system.
FIG. 2A
is a cross-sectional view of a transparent substrate, and
FIG. 2B
is a top plan view of the disc with the playback light being shown converged on the groove
52
. The width a of the bottom surface
52
and the width b of the top surface
53
are both 0.25 &mgr;m, and the width e of the inclined surface
54
is 0.1 &mgr;m while the track pitch p is 0.7 &mgr;m. The playback laser light has a wavelength of 680 nm, while the numerical aperture NA of an objective lens is 0.6 so that the spot diameter is 1.38 &mgr;m. The Gaussian distribution of the light intensity of the playback light spot is shown superimposed on the plan view of the optical disc.
If pits are recorded with a narrow track pitch of 0.7 &mgr;m in accordance with the in-groove recording system, a portion of a pit M formed on a bottom surface
52
neighboring to an other bottom surface
52
being reproduced is intruded into a playback light spot SP thus producing interference. In addition, since the spatial frequency in the direction perpendicular to the track approaches to a limit of resolution, the tracking error signal amplitude is significantly lowered thus rendering it difficult to perform stable tracking servo, thus resulting in cross-talk.
With the on-land recording system, cross-talk is increased if the track pitch
2
is narrowed to a value of the order of 0.7 &mgr;m, for basically the same reason.
Recently, a land-and-groove recording system of recording on both the top surface and on the bottom surface of the groove has been proposed as a recording system of suppressing interference from neighboring tracks.
FIGS. 3A and 3B
schematically show an optical disc on which pits have been formed in accordance with the land-and-groove recording system.
FIG. 3A
is a cross-sectional view of a transparent substrate, and
FIG. 3B
is a top plan view of the disc with the playback light being shown converged on the top surface
53
. The width a of the bottom surface
52
and the width b of the top surface
53
are both 0.6 &mgr;m, and the width e of the inclined surface
54
is 0.1 &mgr;m while the track pitch p is 0.7 &mgr;m. Since both the bottom surface
52
and the top surface
53
are used as recording tracks, the inclined surface
54
disposed therebetween acts as a heat insulating region. As in
FIG. 2B
, the Gaussian distribution of the light intensity of the playback light spot is shown superimposed on the plan view of the optical disc.
With the land-and-groove recording system, a height difference d between the bottom surface
52
and the top surface
53
and a playback light wavelength &lgr; within the substrate are controlled for satisfying the relation d=&lgr;/6, with the reflectivity of the recording pit M being e.g., 0%.
With the land-and-groove recording system, since the neighboring recording tracks are the bottom surface
52
and the top surface
53
having a different height level, a tacking error signal is produced with a larger amplitude, such that stable tracking servo may be achieved. On the other hand, since it is the reflectivity of the recording pit M and the height difference d between the bottom surface
52
and the top surface
53
that are controlled, reproduction of the top surface
53
is less liable to be affected by interference from the pit M recorded on the neighboring bottom surface
52
. The result is that intrusion of the neighboring pit M into the playback light spot SP to the extent as shown in
FIG. 3B
raises no particular problem, such that the tracking density approximately twice that with the conventional recording system may be achieved.
However, with the land-and groove recording system, the problem of cross-talk
Fukumoto Atsushi
Kasami Yutaka
Yasuda Koichi
Hindi Nabil
Sonnenschein Nath & Rosenthal
Sony Corporation
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