Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
1999-11-01
2003-04-29
Young, W. R. (Department: 2655)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C369S109020
Reexamination Certificate
active
06556537
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical recording medium having data recorded in the form of a train of pits, a master for use in production of the optical recording medium having data recorded in the form of a train of pits, and an optical recording/reproducing apparatus to write and/or read data to and/or from an optical recording medium having data recorded in the form of a train of pits.
2. Description of the Related Art
As a typical one of the optical recording media, an optical disc to and/or from which data is optically written and/or read, is well known. The optical disc includes a read-only type having data pre-recorded in the form of a train of pits, a magneto-optic type to and from which data is written and read using the magneto-optical effect, and a phase-change type to and from which data is written and read using a phase change of a recording layer thereof.
For these types of optical discs, it is very important that tracking servo and track seek can stably be done during data write and/or read. Note that the “tracking servo” is to have a light spot track a recording track. The “track seek” is to move the light spot to a desired recording track. For the track seek, it is necessary to count recording tracks the moving light spot has traversed until the light spot arrives at the desired recording track. This is generally called “traverse count”.
Conventionally, the reading resolution of an optical pickup mounted in an optical recording/reproducing apparatus has been improved to attain a high density of recording in the conventional optical discs. For the conventional optical discs, a main measure taken to improve the reading resolution of the optical pickup is to use a laser light of a shorter wavelength &lgr; for reading data or an objective lens having a larger numerical aperture (NA) for focusing the laser light onto an optical disc.
The wavelength &lgr; of a laser light used to read data, numerical aperture (NA) of objective lens and track pitch will be shown in Table 1 concerning CD, MD, MD Data
2
, DVD+RW and DVD-ROM as examples of tie optical disc.
TABLE 1
Laser light
wavelength
Numerical
Track pitch
Recording
in nm
aperture NA
in &mgr;m
density
CD, MD
780
0.45
1600
Low
MD Data2
650
0.52
950
↑
DVD + RW
650
0.60
800
↓
DVD − ROM
650
0.60
740
High
As shown in Table 1, the conventional optical discs use a laser light of a shorter wavelength &lgr; or an objective lens having a larger NA to implement a narrow track, whereby a high recording density is attained.
Even with the narrow track for the conventional optical discs, however, the spatial frequency of the track pitch is limited to the order of a half to two/thirds of the cut-off frequency of the optical pickup of the optical recording/reproducing apparatus. Note that the “cut-off frequency” is a frequency at which the reproduced signal amplitude is approximately zero. On the assumption that a laser light used to read data has a wavelength &lgr; and an objective lens used to focus the laser light on the optical disc has a numerical aperture NA, the cut off frequency is 2NA/&lgr;.
The reason why the spatial frequency of the tack pitch is limited to a half to two/thirds of the cut-off frequency is that signals necessary for tracking servo and track seek should have sufficient levels, respectively, for stability of the tracking servo and track seek.
More specifically, a signal indicative of a difference between outputs from two photodetectors disposed symmetrically with respect to the track center of an optical disc (so-called “push-pull” signal) and a sign al indicative of a sum of the outputs from the two photodetectors (so-called “cross-track” signal) are used for the tracking servo and track seek.
Referring now to
FIG. 1
, there is illustrated how to produce a push-pull signal and cross-track signal. As shown, the push-pull signal is produced by detecting a reflected light from an optical disc by two photodetectors A and B disposed symmetrically with respect to the track center of an optical disc and taking a difference (A−B) between the two outputs from the two photodetectors A and B. The cross-track signal is produced by taking a sum (A+B) of the outputs from the two photodetectors A and B.
More particularly, assume here that two photodetectors are symmetrically with respect to the track center of an optical disc to detect a reflected light from the optical disc during data write and/or read from the optical disc. On the assumption that a quantity of light detected by one of the two photodetectors is A and that detected by the other photodetector is B. A signal indicative of a difference between the quantities A and B is a push-pull signal which is expressed as A+B, while a signal indicative of a sum of the quantities A and B is a cross-track signal which is expressed as A−B. Assume that the push-pull signal has an amplitude C, cross-track signal has an amplitude D and the value of the cross-track signal at a mirror surface portion of the optical disc where no trains of pits are formed is Mmax. Then, the push-pull signal amplitude ratio is expressed by C/Mmax, while the cross-track signal amplitude ratio is expressed by D/Mmax.
For MD Data
2
and DVD+RW , for example,the push-pull method is adopted for the tracking servo. Namely, the push-pull signal is used for the tracking servo. For a high stability of the tracking servo, the push-pull signal amplitude ratio has to be about 0.10 or more. For the track seek, cross-track signal is used for the traverse count. The cross-track amplitude ratio has to be about 0.05 or more. Further, in case data has been prerecorded as a train of pits on the optical disc, the pit modulation of the shortest pit has to be about 0.08 or more for normal reading of the data.
Thus, in MD data
2
and DVD+RW, the spatial frequency of the track pitch is limited to a range of a half to two/thirds of the cut-off frequency for the push-pull signal amplitude ratio to be 0.10 or more, cross-track signal amplitude ratio to be 0.05 or more and for the pit modulation of the shortest pit to be 0.08 or more.
For CD and MD, the three-spot method is adopted for the tracking servo. Namely, the cross-track signal is used for the tracking servo. For a high stability of the tracking servo, the cross-track signal amplitude ratio has to be about 0.10 or more. For the track seek, the push-pull signal is used for the traverse count. For a stable track seek, the push-pull signal amplitude ratio has to be about 0.05 or more. Further, in case data has been pre-recorded as a train of pits on the optical disc, the pit modulation of the shortest pit should be about 0.08 or more for normal readings of the data.
Thus, in CD and MD, the spatial frequency of the track pitch is limited to within a range of a half to two/thirds of the cut-off frequency for the cross-track signal amplitude ratio to be 0.10 or more, push-pull signal amplitude ratio to be 0.05 or more and for the pit modulation of the shortest pit to be 0.08 or more.
However, the optical recording media are required to have a higher recording density. Namely, the optical discs are required to write data with a higher recording density. For a higher recording density of the optical recording media, the track pitch should be made smaller, for example. However, the conventional optical recording media are disadvantageous in that if the track pitch is made too smaller, signals necessary for the tracking servo and track seek cannot be produced to have sufficient level, respectively, so that the tracking servo and track, seek cannot be done stably.
Assume that a laser light used in reading data has a wavelength &lgr; of about 650 nm and an objective lens used has a numerical aperture NA of about 0.52. Then, in a conventional optical disc adapted for a tacking servo by the push-pull method, when it has a track pitch of 0.95 &mgr;m, the push-pull signal amplitude ratio will be about 0.14, which assures a sufficient signal amplitude, so that th
Sony Corporation
Young W. R.
LandOfFree
Optical recording medium having two pit trains of mutually... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical recording medium having two pit trains of mutually..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical recording medium having two pit trains of mutually... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3005217