Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
1998-08-25
2002-05-14
Tran, Thang V. (Department: 2651)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C369S275100
Reexamination Certificate
active
06388984
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical information recording medium capable of recording and reproducing information at high speed and high density by using light beams, etc., and to a method for recording and reproducing information by using the optical information recording medium.
BACKGROUND OF THE INVENTION
Studies and developments have been conducted in the field of optical information recording media capable of recording and reproducing at high speed and high density. In the optical information recording medium, a thin film comprising a chalcogen material etc. provided on a substrate is locally heated by irradiation with laser beams. The thin film in the medium is able to change between an amorphous state and a crystalline state. The two states are different in the optical constants (refractive index n, extinction coefficient k) in accordance with irradiation conditions. And in the optical information recording medium, a signal is detected as a difference of an amount of the reflected light or an amount of the transmitted light with respect to the laser beams having a predetermined wavelength.
In such a phase change type recording, the laser beams are fluctuated between two levels, namely, the recording level and the erasing level, in accordance with the information signal and irradiated on the information track on the medium. Thus, a new signal can be recorded while erasing a previously recorded signal (Publication of Japanese Patent Application (Tokkai Sho) No. 56-145530). Since this method does not require parts of a magnetic circuit that are required in a magneto-optical recording, the head can be simplified. Moreover, since signals can be erased and recorded simultaneously, the rewriting time can be shortened. Consequently, this method is advantageous in recording information.
In order to realize a phase change recording at high density, in general, a smaller recording mark is formed by shortening the wavelength of the light source used for recording and by enhancing the NA (numerical aperture) of an objective lens. Thus, the linear density of the recording mark on the substrate in the circumference direction and the track density in the direction of the diameter are improved. Recently, a mark edge recording method has been dominant. In this method, the length of the recording mark contains information.
Furthermore, as one of the effective means for realizing a high density recording, a land & groove recording has been suggested. In this method, both a groove for guiding laser beams, which is provided on the substrate, and a land provided between the guiding grooves are used as information tracks (Publication of Japanese Patent Examined Application (Tokko Sho) No. 63-57859). However, in the land & groove recording, the track direction of the recording mark, namely, a pitch in the direction of the disc diameter, is almost half as that of the case where recording is performed on either a groove or a land. As a result, during reproducing, crosstalk often occurs. In other words, the signal of the recording mark of the track adjacent to the recording mark to be reproduced appears in a spot on which laser beams are converged. Thus, the quality of the reproduced signals is deteriorated.
In order to reduce the crosstalk, some methods have been disclosed. One of the methods so as to reduce the crosstalk without providing a particular signal proceeding circuit includes the steps of: approximately equalizing the width of a land and that of a groove; and setting the depth of the groove to be in the range of &lgr;/7n to &lgr;/5n (&lgr;: wavelength of a laser beam, n: refractive index of the substrate material) (Publication of Japanese Patent Application (Tokkai Hei) No.5-282705, Tokkai Hei No. 6-338064)
Moreover, the reflectance and the phase of the reflected light change in accordance with the change of the state due to recording. According to some disclosures, it is also effective to specify the reflectance ratio or the phase difference so as to reduce the crosstalk. For example, R
1
and R
2
satisfy the relation: 0≦R
2
/R
1
≦0.2, when a higher reflectance in the reflectance before recording and the reflectance after recording is R
1
, and a lower reflectance is R
2
(Publication of Japanese Patent Application (Tokkai Hei) No.7-287872); R
1
and R
2
satisfy the relation: R
2
/R
1
≦0.15 and 10% ≦R
1
≦40% (Publication of Japanese Patent Application (Tokkai Hei) No.8-329529); and the phase difference &Dgr;&phgr; satisfies the relation: (m−0.1)&pgr;≦&Dgr;&phgr;≦(m+0.1&pgr;) [rad] (Publication of Japanese Patent Application (Tokkai Hei( No.8-329528).
All of the information recording media disclosed in the above mentioned Patent Applications are the reflectance reducing type recording media where the reflectance is reduced by recording. As a recording medium using the change of the reflectance, besides the reflectance-reducing type recording, there is a reflectance increasing type recording medium. In the land & groove recording, in general, the reflectance reducing type medium can easily reduce the crosstalk. The reflectance increasing type medium is disadvantageous in this respect. The reasons of this will be described as follows.
As described in Publication of Japanese Patent Application (Tokkai Hei) No.7-287872, when the reflectance is increased, the interference by the reflected lights between the adjacent tracks is increased. When the lower reflectance is brought closer to 0, the interference is reduced. Thus, even if the interference is great in the region where the reflectance is high, the optimization of the depth of the groove allows the change in the reflectance not to affect the adjacent tracks. In other words, as the lower reflectance is closer to 0, the crosstalk can advantageously reduced.
However, the reflectance before recording, namely, the reflectance of the base region other than the recording mark is desired to be high to some extent. In general, the necessary reflectance is said to be 10-15 % or more. The reason for this is as follows. (1) A sufficient amount of the reflected light from the recording medium is needed when the servo control such as focusing, tracking, etc. is performed on the recording medium. (2) A recording medium generally has a sector structure in which one circle of track is divided into some sectors and at the head of the sector on the substrate, concavities and convexities showing address numbers are provided. In order to detect its signal, a sufficient amount of the reflective light is needed.
From the above, in the case of the reflectance reducing type medium, if the reflectance after recording is brought closer to 0 while making the reflectance before recording great, the change in the reflectance can be increased, and in turn a sufficient signal strength can be obtained. In addition, the crosstalk can be reduced. On the other hand, in the case of the reflectance increasing type medium, if the reflectance before recording is sufficiently increased, the reflectance after recording is also required to be further increased so as to increase the change in the reflectance. Therefore, neither reflectance before recording nor reflectance after recording can be brought closer to 0. As a result, the reflectance increasing type medium is disadvantageous in terms of reducing crosstalk.
From the above mentioned reasons, all of the recording media disclosed in the above mentioned Patent Applications are reflectance reducing type media. Publication of Japanese Patent Application (Tokkai Hei) No.8-329529 also discloses the reflectance increasing type medium. However, the medium is described as a comparative embodiment where crosstalk is great. The reflectance increasing type medium having a reduced crosstalk is not disclosed.
As mentioned above, from the respect of the crosstalk, the reflectance increasing type recording medium is disadvantageous in realizing a high density recording. However, the reflectance increasing type medium has advantageous characteristics as a practical
Furukawa Shigeaki
Kitaura Hideki
Nagata Ken'ichi
Nishiuchi Ken'ichi
Yamada Noboru
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
Tran Thang V.
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