Patent Application
20080279087
The present invention relates to a method for multi-level recording and reproduction of information by information-coding pits at multiple recording levels.
Lately, the optical memory industries are expanding, including CDs and DVDs for read-only, write-once recording mediums employing a thin film metal or a pigment type recording material, rewritable recording mediums employing a magneto-optical material or a phase-changeable material. The application fields are spreading to consumer uses and external memories of computers. Research and development is being made for a higher density of recording. For making finer the optical spot for recording and reproduction of information, the wavelength of the light source is being changed from a red region (650 nm) to bluish violet region (405 nm); and the numerical aperture of the objective lens is being increased from 0.6-0.65 to 0.85. On the other hand, techniques of multi-level recording and reproduction are proposed for more effective recording and reproduction without changing the light spot size.
For example, the inventors of the present invention disclosed, in Japanese Patent Application Laid-Open No. H5-128530, a recording method which records information at multiple levels by utilizing the width in the track-direction of the information-coding pit and a positional shift degree of the information-coding pit in the track direction relative to the reproduction light spot; and disclosed also a reproduction method which reproduces the multi-level information recorded in the information-coding pits by comparison of the detected signal from the light spot with preliminarily learned detection signals.
In Meeting of ISOM2003, which is an international society of research of optical disks, a report is presented on eight-level recording and reproduction by employing a bluish violet light source (405 nm), an optical system of NA 0.65, and an optical disc of a track pitch of 0.46 μm having information-coding pit recording domains having a width of 0.26 μm in the track direction (Write-once Discs for Multi-Level Optical Recording: Preprint Fr-Po-04).
The eight-levels of the information-coding pit can be provided, for example, as shown in
In the above recording, the information-coding pits are recorded at random. A light beam is projected in a spot at wavelength of 405 nm with a numerical aperture of 0.65, and the quantity of reflected light is measured by a photodetector. The reproduced signals are sampled from the multi-level information-coding pits at a timing when the light spot is brought to the center C of the width W in the track direction of the information-coding pit recording domain. Thereby the reproduced signals for the respective levels distribute as shown in
The reproduced signal value of the respective levels has a distribution owing to influences of the adjacent information-coding pits (inter-symbol interference). When the distribution of a reproduced signal overlaps the adjacent levels as shown in
The report presented in ISOM2003 Meeting discloses a system for separate detection of a reproduced signal in which a series of an objective pit and preceding and following pits of known information-coding pit values are read and memorized (learned), and the reproduced signal from the objective information-coding pit is detected by reference to the memorized information-coding pit values (correlated). This system achieves a recording density of about 16 Gbit/inch2.
With a light source of bluish purple (405 nm) and an optical system of NA 0.85 to minimize the light spot size, and with the multi-level system disclosed in ISOM2003 Meeting, a high recording density of about 30 Gbit/inch2 is estimated to be achieved from light spot size ratio.
However, the multi-level system disclosed in ISOM2003 Meeting employs eight levels of pits including seven information-coding pits having the widths changed stepwise in the track direction and one no-information-coding pit. When the light spot is made smaller in the system of bluish violet light force (405 nm) and NA 0.85, the minimum pit width will be as small as 25 nm. Such a fine information-coding pit cannot readily be recorded on an optical disk.
Further, in information reproduction by the multi-level system disclosed in the ISOM2003 Meeting, the reproduction is conducted by reference to the learnt information for the purpose of eliminating the influence of the preceding and following information-coding pits (inter-symbol interference). This system requires a complicated reproducing system because the difference in the width in the track direction of the information-coding pits cannot be detected separately by reference to one fixed threshold value disadvantageously. Furthermore, in the multi-level system disclosed in the ISOM2003 Meeting, the information-coding pits are respectively recorded with the pit center positioned at the center of the information-coding pit recording domain.
The present invention is made in view of such a technical background. The present invention intends to provide a method for recording and reproducing optical information at a high recording density at multiple levels without complicating the reproducing system and an apparatus therefor.
According to an aspect of the present invention, there is provided a method for recording and reproducing information at multiple levels, comprising steps of:
The reproduced signal is preferably detected at a time when the center of the reproducing light spot has come to the center of the width in the track direction of the information-coding pit.
Another aspect of the present invention, there is provided an apparatus for recording and reproducing information in multiple levels, comprising:
Best modes of the present invention are explained below by reference to drawings.
In
Spindle motor 3 is controlled by spindle motor controller 9 and rotates optical disk 4. Optical disk 4 is an optical information recording medium mountable onto and demountable from optical information recording-reproducing apparatus 1.
Optical head 5 records and reproduces information optically on optical disk 4. Optical head 5 which employs, for example, a light source of a wavelength λ of 405 nm and an objective lens of a numerical aperture 0.85, will give a light spot of a size 1.2(λ/NA), about 0.57 μm. The track pitch is designed to be 0.32 μm. Optical head control circuit 6 controls the position of the light spot projected from optical head 5 and controls auto-tracking, seeking, and auto-focusing.
In
When this constitution of the optical disk is employed in a conventional method shown in
Eight levels can be designed with the above three widths of the pits, for example, as below.
As the erasable recording medium, magnetooptical materials are also useful other than the aforementioned phase-changeable recording medium. With the magnetooptical recording medium, a magnetic field not shown in the drawing is also applied together with the light beam in the optical information recording-reproducing apparatus to change the shape of the information-coding pit to form plural reproduction levels of information-coding pits. Otherwise, a recording medium for read-only can be used. Organic materials or metal films are also useful as the recording material. With such a recording material, in projection of a light beam onto a recording domain, by adjusting the light quantity for recording and erasing and the timing therefor, the shape of the information-coding pit can be changed to form plural reproduction levels of information-coding pits. Also with a recording medium of read-only, information-coding pits can be formed in a projection-depression shape called phase pits on a substrate: multi-level of recording can be made by modifying the area or optical depth of the phase pits.
For increasing the memory capacity, the domain for recording the information-coding pit should be made smaller. With the smaller domain, two or three information-coding pits in the recording domains come to be covered by the reproduction beam spot as shown in
In
In this embodiment, the size of the light spot is about 0.57 μm, and the width of the recording domain is 200 nm (0.2 μm). In this dimensional scale, the areal information recording density can be improved by a factor of about 1.5 in comparison with conventional systems (e.g., areal-information density of 19.5 Gbit/inch2 in 17-PP modulation and 2T=139 nm).
Three signals of 4(W/16), 8(W/16), and 12(W/16) selectively taken out from the distributions of reproduced signals shown in
The information-coding pits recorded on optical disk 4 are read out with a light spot. The reflected light from optical disk 4 is received by a photodetector in optical head 5. The signal detected by the photodetector is sampled at the timings when the center C of the optical spot passes the positions: the center position of the information-coding pit recording domain (this detected signal being referred to as S(center)), the position A advanced from the center by 2(W/16) (this detected signal being referred to as S(front)), and the position D delayed from the center by 2(W/16) (this detected signal being referred to as S(back)).
The width in the track direction of the recorded information-coding pit is judged from any of the detected signals of S(center), S(front), and S(back) at positions C, A and D respectively by reference to fixed thresholds. Specifically,
On the other hand, the position of the information-coding pit, namely the position at domain center, the front side of the domain center, or the back side of the domain center, is detected by the intensity of S(front) and S(back) at positions A and D, and the difference thereof.
In
Curves 17, 18, and 19 show respectively a change curve of reproduced signal with scanning movement of the light spot: curve 17 for information-coding pit 14 recorded before the domain center, curve 18 for an information-coding pit 15 recorded at the domain center; and curve 19 for an information-coding pit 16 recorded after the domain center.
The signals are sampled from the respective signal curves, at the three positions of the light spot, i.e. A: 2(W/16) before the domain center, C: the domain center, and D: 2(W/16) after the domain center. The values of the signals are represented by S(front), S(center), and S(back).
With information-coding pit 15 recorded at the domain center C, the values of S(front) and S(back) are equal to each other and are close to S(center). In other words, the information-coding pit is judged to be recorded at the domain center when S(front) and S(back) are equal to each other and close to S(center).
With information-coding pit 14 recorded at A, 2(W/16) before the domain center, the S(front) is weak, S(center) is a little stronger, and S(back) is further stronger. In other words, the information-coding pit is judged to be recorded at the position before the domain center when the difference of S(front) and S(back) is large and S(front) is weaker than S(back).
With information-coding pit 16 recorded at D, 2(W/16) after the domain center, the S(back) is weak, S(center) is a little stronger, and S(front) is further stronger. In other words, the information-coding pit is judged to be recorded at the position before the domain center when the difference of S(front) and S(back) is large and S(front) is stronger than S(back).
In
In the system of
In
In this embodiment also, the domain for the recording is divided into 16 divisions (16 channel bits), and three widths in the track direction of the information-coding pits are utilized: 6(W/16), 10(W/16), and 14(W/16) where W denotes the width of the recording domain. With the width W of 200 nm, the width of the smallest information-coding pit, 6(W/16), is 75 nm, which makes easy the recording in comparison with the aforementioned embodiment.
Eight levels can be designed, for example, as below.
Signals of 6(W/16), 10(W/16), and 14(W/16) taken out selectively from the distribution of reproduced signals in
The above description explains the recording and reproduction with three kinds of pit width and frontward or backward shift of the information-coding pit, but it is not limited thereto. For example, the information-coding pit may be shifted forward or backward by two or more steps from the center in the domain. When the information-coding pits are recorded with multiple degrees of shift, the degree of the shift needs to be detected. The degree of the shift can be detected by the absolute value of the difference of the reproduced signals sampled.
As described above, when the information-coding pits are recorded in multiple levels of the information-coding pits, the multi-level information can be reproduced by detecting the direction and degree of the shift.
The points of the sampling of the reproduced signal are not limited to three points. The number of the sampling points may be two, or more than three provided that the width and position in the track direction of the information-coding pit can be detected.
This application claims priority from Japanese Patent Application No. 2004-165875 filed on Jun. 3, 2004, which is hereby incorporated by reference herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-165875 | Jun 2004 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP05/10445 | 6/1/2005 | WO | 00 | 4/28/2006 |
