Optical disc substrate

Details Optical disc substrate Optical disc substrate

2000-03-27

2002-06-25

Psitos, Aristotelis M. (Department: 2651)

Dynamic information storage or retrieval

Storage medium structure

Optical track structure

Reexamination Certificate

active

06411593

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Korean Application No. 99-10272, filed Mar. 25, 1999, in the Korean Patent Office, the disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a substrate of an optical disc with lands and grooves, and more particularly, to an optical disc substrate with deep grooves having a depth of &lgr;/4n to &lgr;2n, where &lgr; is the wavelength of a laser beam emitted from an optical pickup to record/read index from the optical disc and n is the refractive index of the optical disc substrate.
2. Description of the Related Art
Optical discs are information recording media adopted by a disc player, which writes and/or reads information in a non-contact manner. The need for a high recording density at a limited data recording region has provided a suggestion for an optical disc substrate which allows data writing on both its grooves and lands.
FIG. 1
is a schematic view of an existing optical disc substrate adopting a land-and-groove recording method. As shown in
FIG. 1
, an optical disc substrate 1 comprises a plurality of tracks spirally formed from the center to the periphery of an optical disc, alternately forming a plurality of grooves 3 having a predetermined depth and a plurality of lands 5 having the same level as the surface of the optical disc substrate.
In particular, the format book for a 2.6-gigabyte DVD-RAM suggests a ratio of the land width and the groove width be approximately 50:50. A land-and-groove recording method applied to such an optical disc having the above configuration is advantageous in that a difference in height between lands and grooves reduces crosstalk, which is noise generated from adjacent tracks, and writing on both lands and grooves increases the recording density. Another advantage of the land-and-groove recording method is a larger amplitude of a pushpull signal compared to a recording method which allows writing on only either lands or grooves. This larger amplitude of the push-pull signal is because an optical track pitch which causes the push-pull signal is half as small as a data track pitch.
For a high recording density in optical discs having the above configuration, the track pitch (TP) must be reduced. In this case, the size of a write beam spot must be reduced to keep writing and reading characteristics. However, as the recording capability of optical DVDs increases, a relative track pitch with respect to the write beam spot size decreases, which is shown in Table 1, causing “cross erase” which refers to erasure of signals on adjacent tracks, and thus limiting the increase in recording density.
TABLE 1
Type of Recording Media
2.6 GB
4.7 GB
15 GB
18 GB
Items
DVD-RAM
DVD-RAM
HD-DVD
HD-DVD
Wavelength of laser
650
650
400
400
beam (nm)
Numerical aperture
0.6
0.6
0.6
0.65
(NA)
Track pitch (&mgr;m)
0.74
0.615
0.34
0.30
Ratio of track pitch
0.68
0.57
0.51
0.49
to beam spot size
The cause of cross erase can be summarized into two factors. One is thermal absorption of the write beam by adjacent tracks, and the other is thermal transfer to adjacent tracks during writing. The thermal transfer between recording layers in an optical disc, which causes a temperature increase in the optical disc can be avoided by spacing adjacent tracks further apart.
In this way, an optical disc with deep grooves has been proposed. In a case such as an optical disc, the groove depth is larger than the groove depth Gd, &lgr;/6n, where &lgr; is the wavelength of a laser beam of an optical pickup and n is the refractive index of an optical disc substrate, of a general optical disc, which elongates the thermal conductive distance and in turn suppresses the occurrence of both cross erase and crosstalk. However, the problem with deep-groove optical discs is the phase reversion of a tracking error signal, the so called “push-pull signal,” at a depth below that of a predetermined depth.
FIG. 2
shows the push-pull ratio (PPR), divided push-pull ratio (DPP) and on-track ratio (OTR) for a 4.7-gigabyte DVD-RAM with respect to the depth of grooves.
Phase reversal of the push-pull signal means that the deep grooves are tracked according to the tracking conditions for lands of a general optical disc as shown in FIG.
1
. Since the tracking conditions for lands and grooves cannot be the same, the DVD-RAM format described with reference to
FIG. 1
delimits the depth of grooves to be less than or equal to &lgr;/4n, where &lgr; is the wavelength of a laser beam for an optical pickup to record/read data on/from the optical disc and n is the refractive index of an optical disk substrate, which permits the same phase of push-pull signals for lands and grooves.
The crosstalk signal and push-pull signal are influenced by a slant angle &thgr; of the grooves. As shown in
FIG. 1
, the slant angle &thgr; of grooves refers to the angle between a top surface of the lands and the groove sidewalls (or also described as a lateral extension of the lands or the grooves and an extension of the groove sidewalls).
FIG. 3
illustrates the crosstalk and the push-pull signal for an optical pickup adopting an objective lens having a numerical aperture (NA) of 0.6 with respect to the groove depth when the slant angle &thgr; of grooves is 60° and 80°. In
FIG. 3
, A and B indicate the crosstalk signals at the slant angle &thgr; of grooves of 60° and 80°, respectively. C and D indicate the push-pull signals at the slant angle &thgr; of grooves of 60° and 80°, respectively.
For the result of
FIG. 3
, the groove depth has been normalized based on the wavelength (&lgr;) of the incident laser beam and the refractive index (n) of the disc substrate. In
FIG. 3
, the horizontal dashed line at the push-pull signal of 0 indicates an optical groove depth of &lgr;/4n. Thus, it can be concluded that the optical groove depth for a predetermined wavelength of a laser beam varies depending on the slant angle &thgr; of grooves even at the same physical groove depth (the physical groove depth is an absolute depth without taking into account the refractive index n and the wavelength &lgr; of the optical disc substrate, and the optical groove depth is the depth taking n and &lgr; into account). Also, as previously mentioned, the phase of the push-pull signal reverses around the optical groove depth of &lgr;/4n. Also, the push-pull signal of deep grooves varies depending on the slant angle &thgr; of grooves.
FIG. 3
shows that when the physical depth of the grooves is small, the effect of the slant angle &thgr; of the grooves on the variation of the push-pull signal and the crosstalk signal is negligible, compared to that of the groove depth. However, as the physical depth of grooves increases, variations of the push-pull signal and crosstalk signal with respect to the slant angle &thgr; of grooves, i.e., at 60° and 80°, increase.
SUMMARY OF THE INVENTION
To solve the above problems, it is an object of the present invention to provide an optical disc substrate with deep grooves having a depth of &lgr;/4n or more, where &lgr; is the wavelength of a laser beam from an optical pickup and n is the refractive index of a substrate, which results in improved crosstalk and cross erase characteristics.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The objects of the present invention is achieved by an optical disc substrate for land-and-groove recording, comprising: a plurality of deep grooves having a predetermined depth, individual ones of the deep grooves having sidewalls slanted at an angle of &thgr;; and a plurality of lands having the same level as the surface of the optical disc substrate, wherein a depth D of each of the grooves for minimum crosstalk is determined by the following mathematical relation [1]
D=0.4022−0.4574×A+0.6458×A
2
  [1]
where

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