Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2001-04-26
2003-05-20
Hindi, Nabil (Department: 2655)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S047510, C369S275300
Reexamination Certificate
active
06567367
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a multilevel recording and reproduction method and a phase change multilevel recording medium.
BACKGROUND ART
As the volume of information increases in recent years, there are growing demands for recording media capable of reading (reproducing) and writing a large amount of data with high density and at high speed. Optical recording media, particularly optical discs, are expected to meet such demands. The optical discs are available in two different types: a write-once type that allows the user to record data only once, and a rewritable type that allows the user to record and erase data as many times as they wish. Examples of the rewritable optical disc include a magnetooptical medium that utilizes a magneto-optical effect and a phase-change medium that utilizes a change in reflected light intensity accompanying a reversible crystalline state change.
The phase change medium can be written and erased by simply modulating the power of a laser beam without requiring an external magnetic field and thus has the advantage of being able to reduce the size of a recording and reproducing apparatus. It is also possible to enhance the recording density by using a light source with a shorter wavelength without specially changing the material of a recording layer of media which are currently recorded and erased with a light source with a commonly used wavelength of about 800 nm.
In the currently available rewritable phase change recording media, the crystalline state is taken as an unrecorded/erased state, and amorphous mark is formed. The amorphous mark is formed typically by heating the recording layer to a temperature higher than the melting point and quickly cooling it. Erasure (crystallization) is done by heating the recording layer to a temperature higher than the crystallization temperature of the recording layer, but lower than a temperature just above the melting point or the melting point itself. In a so-called one-beam overwritable phase change medium, the erasure and re-recording processes can be performed only by modulating the intensity of one focused light beam. In the 1-beam overwritable phase change medium, the layer configuration of the recording medium and the circuit configuration of the drive become simple Hence, this medium draws attention as a possible medium for use in an inexpensive, high-density, large-capacity recording system.
As described above, the phase change medium can increase the recording density by shortening the wavelength of a focused light beam to reduce its diameter and therefore the size of recorded marks. At present, laser diodes with a wavelength of 780 nm and an output of about 50 mW are widely available at low prices and applied to a phase change recording technology for rewritable compact discs, for example. Laser diodes with 630-660 nm are also available recently and a rewritable DVD is nearing the practical use along with the development of a high-output red laser diodes with an output of about 30 mW. With the demands for a higher density continuing, attempts to realize the recording density about two to three times that of the DVD by using a blue laser (about 400 nm) diode are actively under way though at a very preliminary stage of development.
There is naturally a limit to the recording density, however, if an increase in density of the phase change medium depends simply on the shortening of the wavelength of the light source. There are many problems to be solved as to the longevities of the laser diodes with short wavelengths and high outputs, and it will take time before such high-output laser diodes, though experimentally successful, can be put to practical use. Further, as the spot becomes smaller in size, problems arise, for example, an increased influence of tilt of the surface of a focused point and a reduced focus offset margin due to a shallower focal depth. Another question whether the amorphous marks, when they become smaller than 0.01 &mgr;m, can remain stable, has not been solved yet.
An effort to increase the recording density of a magnetooptical recording medium dependent solely on the miniaturization of the read/write beams will naturally encounter a limitation due to the optical resolution capability (limit). In a phase change medium in particular, a so-called magnetic super-resolution phenomenon cannot be expected. Although there are some proposals on a super-resolution phenomenon utilizing a change in refractive index due to temperature changes, this method has an intrinsic problem that the recorded marks will deteriorate over repeated reading operations.
Spotlighted as one of the methods that transcend the limitation of the optical resolution capability (limit) and allow for an increased density beyond the optical resolution limit is a multilevel recording. This is a technology for a read-only compact disc which, rather than modulating the mark length, controls the depths of pits in a substrate in multiple levels to express the modulation in multiple values (“15GB and No Blue Laser”, Data Storage, April 1994 issue, cover story and pp27-32).
Such a multilevel recording that expresses the modulation in multiple values is realized in principle by controlling a continuous change in the reflected light intensity (modulation) in a finite number of discrete levels. It is a natural course of events to apply to the multilevel recording the phase change medium that performs information read and write operations by using a chance in the reflected light intensity.
However, no recording medium is currently available that takes advantage of the phase change recording to realize the capability of actually performing such a recording in multilevel levels, or preferably overwriting repetitively. This is because both the phase change medium and the recording method that record data at a plurality of modulation levels with good reproducibility are still in the development stage. Typically, the recording levels are two states-crystal and amorphous states-or three states at most (JP-A 61-3324, 62-259229 and 10-124925).
There is also an effort to control an average optical characteristic in multiple levels by changing a mixture ratio of different crystalline states or of crystalline and amorphous states.
However, an optical characteristic difference among different crystalline states is too small to identify and it is difficult to control the mixture ratio of crystalline and amorphous states in multiple levels with good reproducibility. Obtaining the four or more levels with good reproducibility is not easy. Such a mixed state is unstable and the amorphous portion easily transforms into crystal, gibing rise to a problem of poor stability of recorded information over time.
DISCLOSURE OF THE INVENTION
The problems described above can be solved by causing a recrystallization to occur in the recording layer during the solidifying of she recording layer melted by the recording beam and by using the recrystallization in controlling the size of the amorphous mark in multiple levels.
In summary, this invention includes the following inventions.
(1) A multilevel recording/reproducing method comprising the steps of: radiating a recording energy beam against an information recording medium having a recording layer to locally melt the recording layer, the recording layer being adapted to produce a phase change between a crystalline state and an amorphous state upon being radiated with an energy beam; and forming an amorphous mark by cooling during a solidifying process to record information in the medium; wherein the size of the amorphous mark is controlled mainly by a competition between a recrystallization process and an amorphization process during the solidifying process; wherein an intensity of reflected light from a reproducing light beam radiated region is controlled in three or more multiple recording levels according to an optical characteristic difference between a crystalline region and a amorphous region and their areas.
(2) A method according to item (1), wherein a recording energy b
Horie Michikazu
Ohno Takashi
Hindi Nabil
Mitsubishi Chemical Corporation
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