Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
2000-11-07
2002-08-20
Edun, Muhammad (Department: 2653)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C369S275100, C369S047100, C369S053100
Reexamination Certificate
active
06438098
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a writable and rewritable optical recording medium, a tracking method for the medium and an optical recording/reproducing apparatus for recording and reproducing information on/from the optical recording medium.
Recently, writable and rewritable optical discs, which are used as storage means for personal computers and as package media for music and video information, have been developed to achieve higher recording density.
Each optical disc has a writable and rewritable area and a pre-pit area previously formed as a pit thereon for storing information that must not be erased.
Portions other than the pre-pit area are writable areas in which “grooves (track)” and “lands (not track)” are formed.
A typical structure of a conventional optical disc is shown in
FIGS. 1 and 2
.
FIG. 1
is a plan view and
FIG. 2
is a perspective, partly in cross-section, of the optical disc.
In
FIG. 2
, G denotes a groove, L denotes a land and PP denotes a preformed pit (hereinafter referred to as pre-pit). A laser light beam
3
is collected by an objective lens
2
and illuminates the recording surface of the disc through a substrate
1
. The grooves G are nearer to the objective lens
2
than the lands L. The lands L, grooves G and pre-pits PP are coated with a recording layer (not shown) made of magneto-optical material or phase-change material or photosensitive dye material. In the shown case, record marks M are recorded in grooves. This is because marks recorded in grooves can achieve higher quality of reproduced signal than marks recorded on lands between the grooves.
The following is an example of a method for optimally selecting a depth of the groove G and a depth of the pre-pit PP to be formed on an optical disk.
The example represents the experimental results made on optical discs, which have the same track pitch (inter-groove distance) of 0.74 microns but differ from each other by their groove depth Dg and pre-pit depth Dp, by using an optical system composed of a laser emitting a light beam of a wavelength &lgr;=650 nm and an objective lens NA0.6. The groove G and the pre-pit PP are of 0.35 microns in width. The recording layer made of phase-change material InAgSbTe was applied. The recording and reproducing were carried out by rotating each disc at a linear velocity of 3.5 m per second.
FIG. 3
shows amplitudes of reproduced signals obtained, respectively, from marks recorded in grooves of different depths Dg and from pre-pits having different depths Dp.
More specifically, a number of optical discs having different groove depths Dg and different pre-pit depths Dp were subjected to measurements of amplitudes of reproduced signals obtained from 0.3 micron long marks recorded in grooves G and amplitudes of reproduced signals obtained from 0.3 micron long pre-pit.
The measurement results shown in
FIG. 3
indicate that the marks recorded in shallower grooves have larger amplitude of reproduced signals, i.e., better S/N ratios. This means that it is preferable to decrease the depth Dg of groves G to improve the S/N ratio of reproduced signals of marks thereof. This offers a great advantage in particular for discs of higher recording density.
On the other hand, tracking of a light beam focused on groove G is needed to achieve orderly recording information in the form of marks M in the grooves G and precisely reproducing the information. For this reason, the depth of the grooves G must be decided in view of an amplitude characteristic of a signal reproduced from the mark and an amplitude characteristic of tracking signal (i.e., a push-pull signal) obtained on the basis of an average intensity distribution of light components reflected in a direction perpendicular to a direction of the grooves G.
FIG. 4
is a graph showing the dependence of amplitudes of push-pull signals obtained from grooves G and from pre-pits PP upon groove depth Dg and pre-pit depth Dp respectively.
In the graph, &lgr; denotes a wavelength of a light beam and n denotes a refractive index of a substrate of an optical recording medium.
As seen from the graph, the maximal amplitude of the push-pull signal can be obtained when the groove depth Dg or the pre-pit depth Dp is equal to &lgr;/(8n). This means that the grooves G having the depth Dg of &lgr;/(8n) are desired for obtaining push-pull signals being large enough to achieve the precise tracking. However, in view of the amplitude of the reproduced signal obtained from a mark, it is preferable to select the groove depth Dg being smaller than &lgr;/(8n). For example, a depth value indicated by A in
FIGS. 3 and 4
is about 20 nm at &lgr;=650 nm and n=1.5 and it is preferable to obtain a large push-pull signal as well as an improved S/N ratio of the reproduced signal of the mark.
On the other hand, it is found from the relationship between the pre-pit depths Dp and amplitudes of reproduced pre-pit signals (
FIG. 3
) that the amplitude of the reproduced prepit signal can achieve a maximal value at the pre-pit depth Dp of &lgr;/(4n) and decreases as the pre-pit depth Dp decreases. Hence, a depth value B (about 100 nm in
FIGS. 3 and 4
) may be selected as the pre-pit depth Dp. This selection, however, may be accompanied by decreasing the amplitude of the push-pull signal at the pre-pit depth of about &lgr;/(4n) as shown in FIG.
4
. Namely, it is difficult to increase both the amplitude of the reproduced pre-pit signal and the amplitude of the pre-pit push-pull signal.
In other words, it is difficult to use the push-pull signals for tracking in the pre-pit areas. Therefore, the use of differential phase detection (DPD) method, which is different from the push-pull method by its detection principle, is desirable for tracking in the pre-pit area. This method obtains information necessary for tracking by detecting a change of a refraction pattern of light beam illuminating the surface of the optical recording medium (optical disk) and reflected therefrom, or by detecting the differential phase of the refraction pattern change.
FIG. 5
shows the relationship between the depths Dp of pre-pits and the amplitudes of DPD signals obtained from the pre-pits.
As shown in
FIG. 5
, the DPD method is suited to tracking in the pre-pit areas since it can obtain a large-amplitude tracking signal from pre-pits having the depth Dp of about &lgr;/(4n), at which the tracking signal obtained by the pushpull method has a very small amplitude.
Returning to
FIG. 1
, problems involved in an optical recording medium having grooves and pre-pits will be discussed.
FIG. 1
is a plan view of an optical recording medium constructed of grooves G having the depth Dg of A and pre-pits PP having the depth Dp of B.
The combination of the selected grooves G having the depth Dg=A with the selected pre-pits PP having the depth Dp=B requires switching from the tracking method for the groove areas to the tracking method for the pre-pit areas and vice versa. Namely, the push-pull method is applied to a groove area while the DPD method is applied to a pre-pit area. Otherwise, effective tracking signals cannot be obtained.
The switching of tracking methods is desirable to be carried out within a very nallow area designated for this purpose. The reason is as follows: If the switching operation timing was shifted out of the tracking mode switching area, mismatched tracking, e.g., the DPD tracking would be conducted in a shallow groove area or the push-pull tracking would be conducted in a deep pre-pit area until the switching operation is accomplished. Consequently, the correct tracking control could not be realized.
The tracking mode switching area exists between the groove area and the pre-pit area. This area has a very short length of several microns. When the optical recording medium is rotated at a linear velocity of 1 to several meters per second, light beam passes this area in only a several microseconds. In other words, the tracking mode switching must be done for several microseconds.
Although only the tracking mo
Nakajima Junsaku
Nomura Masaru
Takeuchi Hitoshi
Edun Muhammad
Sharp Kabushiki Kaisha
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