Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
2001-05-11
2004-09-28
Hindi, Nabil (Department: 2651)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C369S112230
Reexamination Certificate
active
06798733
ABSTRACT:
This invention relates to an optical recording method capable of high density recording on optical recording media such as phase change optical recording media, and phase change optical recording media to which the optical recording method is applicable.
BACKGROUND OF THE INVENTION
Great attention is now paid to optical recording media capable of high density recording and erasing the once recorded information for rewriting. Among such rewritable optical recording media, phase change recording media are designed such that recording is performed by irradiating a laser beam to a recording layer to change its crystalline state and reading is performed by detecting the change of reflectivity of the recording layer associated with that state change. The phase change recording media are of greater interest because the drive unit used for their operation may have a simple optical system as compared with that used for magneto-optical recording media.
For the phase change recording layer, calcogenide materials such as Ge—Sb—Te are often used because of a greater difference in reflectivity between the crystalline and amorphous states and a relatively high stability in the amorphous state.
When information is recorded in a phase change optical recording medium, the recording layer is irradiated with a laser beam having a high power (recording power) sufficient to heat the recording layer at or above its melting point. In the region where the recording power is applied, the recording layer is melted and then rapidly cooled, forming a recorded mark in the amorphous state. To erase the recorded mark, the recording layer is irradiated with a laser beam having a relatively low power (erasing power) sufficient to heat the recording layer above its crystallization temperature, but below its melting point. The recorded mark to which the erasing power is applied is heated above the crystallization temperature and then slowly cooled, resuming the crystalline state. Therefore, the phase change optical recording medium allows for overwriting simply by modulating the intensity of a single laser beam.
In order to increase the recording density and transfer rate of a recording medium, attempts have been made to reduce the wavelength of recording/reading beam, to increase the numerical aperture of an objective lens in a recording/reading optical system, and to increase the linear velocity of the medium. When a recording laser beam is irradiated to a medium rotating at a linear velocity V, the recording laser beam defines on the surface of the recording layer a spot having a diameter represented by &lgr;/NA wherein &lgr; is the wavelength of the laser beam and NA is the numerical aperture of the objective lens. The spot diameter &lgr;/NA divided by the linear velocity V, i.e., (&lgr;/NA)/V gives the time of irradiation of laser beam to the recording layer, that is, the time taken for passage across a beam spot. As the recording density and transfer rate increase, the irradiation time of laser beam to the recording layer becomes shorter and shorter. This makes it difficult to optimize overwriting conditions.
Problems arising from overwriting at an increased linear velocity are discussed below.
An increased linear velocity leads to a shortened irradiation time of recording beam. It is then a common practice to increase the recording power in proportion to the increased linear velocity for preventing the heated temperature of the recording layer from lowering. However, as the linear velocity increases, the rate of cooling following recording beam irradiation increases. To form an amorphous recorded mark, the recording layer, once melted by recording beam irradiation, must be cooled at or above a rate corresponding to the crystallization speed. For the given construction of recording layer and the given thermal design of medium, the cooling rate of the recording layer depends on the linear velocity. The cooling rate becomes higher at a higher linear velocity and lower at a lower linear velocity.
On the other hand, to erase the amorphous recorded mark (to recrystallize), erase beam must be irradiated such that the recording layer may be held for at least a predetermined time at a temperature between the crystallization temperature and the melting point. An attempt to increase the erasing power in proportion to the increased linear velocity for preventing the heated temperature of the recording layer from lowering has a less likelihood to erase the recorded mark because the irradiation time is reduced as a result of the increased linear velocity.
Therefore, to increase the linear velocity for improving the transfer rate, the recording layer must be formed of a composition having a relatively high crystallization speed such that recrystallization is completed within a relatively short time (as disclosed in JP-A 1-78444 and JP-A 10-326436), or the medium must be structured so as to retard heat release from the recording layer (slow cooling structure). It is also believed that the medium of slow cooling structure is also advantageous for preventing any drop of recording sensitivity which can otherwise occur as a result of an increased linear velocity, as described in JP-A 7-262613 and 8-63784.
The inventors made an experiment of overwriting on a phase change optical recording medium at a high transfer rate. The phase change optical recording medium used had a recording layer of a composition having a high crystallization speed so as to enable erasion at a high linear velocity and was structured for slow cooling. An attempt was made to gradually reduce the window margin Tw in order to increase the transfer rate. It became difficult to reduce the jitter when the signal length corresponding to the shortest recorded mark which was reduced in accordance with the reduction of window margin Tw was reduced below a certain value.
SUMMARY OF THE INVENTION
An object of the invention is to provide an optical recording method capable of increasing a transfer rate while minimizing the jitter. Another object of the invention is to provide an optical recording medium to which the method is applicable.
These and other objects are achieved by the invention which is defined below.
(1) An optical recording method for recording information in an optical recording medium having a recording layer by irradiating a laser beam thereto through an optical system, wherein provided that the laser beam has a wavelength &lgr;, the optical system includes an objective lens having a numerical aperture NA, the window margin is Tw, and a signal length corresponding to the shortest recorded mark is n·Tw, the recording is carried out under the conditions: &lgr;/NA≦680 nm, and n·Tw≦22 ns.
(2) The optical recording method of (1) wherein provided that the laser beam used to form the shortest recorded mark has an emission time Tmin, the recording is carried out under the condition: 0.113≦Tmin/(n·Tw)≦1.0.
(3) The optical recording method of (1) or (2) wherein said recording layer is a phase change recording layer.
(4) The optical recording method of any one of (1) to (3) wherein the recording is carried out at a linear velocity V which satisfies the condition:(&lgr;/NA)/V≦60 ns.
(5) The optical recording method of any one of (1) to (4) wherein said optical recording medium includes the recording layer, a dielectric layer, and a reflective layer stacked in the order from closer to remoter one as viewed from the incident side of the recording laser beam,
the reflective layer has a thermal conductivity K
R
of at least 100 W/mK, and the dielectric layer disposed between the recording layer and the reflective layer has a thermal conductivity K
2D
of at least 1 W/mK.
(6) The optical recording method of any one of (1) to (5) wherein said optical recording medium includes a light-transmitting substrate, a dielectric layer, and the recording layer stacked in the order from closer to remoter one as viewed from the incident side of the recording laser beam, wherein
the dielectric layer disposed between the light-transmitting substrate and the recor
Inoue Hiroyasu
Kato Tatsuya
Shingai Hiroshi
Utsunomiya Hajime
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
TDK Corporation
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