JP3206226B2 - Optical information recording medium and reproducing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information recording medium for recording / reproducing information using a laser beam and a method for reproducing information recorded on the medium.

[0002]

2. Description of the Related Art It is the conditions of reproduction that determine the current limit of the recording / reproduction density of optical disks (including magneto-optical disks). This is because the recording area becomes the recording area above a certain temperature of the recording layer, so the recording area can be reduced in principle by controlling the incident light power, but in the case of reproduction, the optical diffraction limit determines the reproduction performance. Because it will. According to the diffraction theory, the cutoff spatial frequency is given by 2 (NA) / λ, where λ is the wavelength and NA is the aperture of the lens, and a recording mark with a spatial frequency higher than this cannot be reproduced. In order to improve the recording and reproducing densities, it is only necessary to use a short-wavelength laser beam or to increase the numerical aperture of the lens in the pickup optical system. However, as a method using a short-wavelength laser beam, SH using a nonlinear optical material is used.
Although there is a device using G, it is difficult for SHG light to be subjected to high-speed external modulation, and it takes time to reach a practical level. On the other hand, when the numerical aperture of the lens is increased, the depth of focus becomes shallower, so that it becomes difficult to perform focus servo.
Problems such as an increase in spherical aberration occur.

As a method of overcoming such a reproduction limit, a magnetically induced super-resolution (MS) using a devised recording medium has been proposed.
R) method has been proposed (Journal of the Japan Society of Applied Magnetics; Vo)
l. 15, No. 5, 1991, p. 838-843,
etc). This method is a method in which a readout layer or a readout layer and a switching layer are provided on a conventional recording layer, and a mark recorded at high density can be read out with high density using a magnet. This method includes two methods, each of which is a FAD (Front Aperture Detectio).
n), RAD (Rear Aperture Detection).

In the FAD, as shown in FIG. 11, basically, a recording layer 11, a switching layer 12, and a readout layer 13 are provided.
Using a magneto-optical recording medium 10 having an exchange-coupling three-layer film made of, recording is performed on the recording layer 11 by a normal method. Done. Reproduction is performed by irradiating a laser beam with a reproduction power of 1.5 mW or more higher than usual and applying a read magnetic field Hr. By the irradiation of the laser beam with the reproducing power, a high-temperature region R is formed behind the spot S of the laser beam, and when the temperature becomes equal to or higher than the masking temperature near the Curie temperature of the switching layer 12, the reading layer 13 and the recording layer 11 The exchange coupling force between them becomes weaker, the magnetization of the readout layer 13 having a small coercive force is aligned with the direction of the readout magnetic field Hr, and the readout layer 13 in the high temperature region R is adjusted.
Are all "0", and the same state as the magnetization of the recording layer 11 is preserved in the readout layer 13 in a lower temperature region. In other words, the high temperature area R is a mask area from which information cannot be read, and information can be read only from the crescent-shaped area in the spot S. Accordingly, the same state as when the spot diameter of the laser light is effectively reduced is obtained, and it becomes possible to read a recording mark smaller than the spot diameter of the laser light. In the drawing, M indicates a recording mark.

In the RAD, as shown in FIG. 12, a magneto-optical recording medium 20 basically having an exchange-coupled two-layer film composed of a recording layer 21 and a readout layer 22 is used, and recording is performed on the recording layer 21 by an ordinary method. The same recording is performed on the readout layer 22 by the exchange coupling force. Playback is
First, the magnetization of the readout layer 22 is reversed by applying an initialization magnetic field Hi in advance, and then a laser beam is irradiated with a reproduction power of about 2.5 mW and the readout magnetic field Hr is applied. In this case, since the readout layer 22 in the spot S has been initialized, the readout layer 22 becomes a mask area from which information cannot be read out, and only in the high-temperature area R in the spot S , the recording layer under the readout magnetic field Hr. The magnetization information 21 is copied to the readout layer 22, and the information can be read out. Therefore, in the same manner as described above, the state becomes the same as when the spot diameter of the laser light is effectively reduced, and it becomes possible to read a recording mark smaller than the spot diameter of the laser light. In the conventional method, when the track pitch is smaller than the spot diameter of the laser beam, the spot protrudes into an adjacent track, and crosstalk increases.
As described above, in the RAD, as long as the signal detection area, which is a high-temperature area, does not protrude into an adjacent track, no crosstalk occurs. Therefore, the density in the track width direction can be improved.

Further, FAD, RAD and the like are applied to a magneto-optical recording medium and are not suitable for an optical information recording medium provided with a recording film of another dye or the like.

Therefore, the present invention overcomes the above-described problem that it is impossible to read out a minute mark from a recording mark of an optical information recording medium recorded at high density due to an optical diffraction limit of a pickup. It is an object of the present invention to provide a super-resolution reproduction method applicable to all types of optical information recording media.

[0008]

According to the present invention, in order to solve the above-mentioned problems, in an optical information recording medium for recording / reproducing information with a laser beam, at least the transmittance of a recording layer on the reading surface side is changed by temperature change. Are provided in a plurality of layers , and each of the variable transmittance films is such that the threshold temperature at which the transmittance becomes a predetermined value decreases as the distance from the recording layer increases or moves away from the recording layer. The optical information recording medium is characterized in that it is set to increase according to the following. Further, according to the present invention, there is provided an optical information recording medium according to the above configuration, wherein the plurality of transmittance variable films have a large transmittance at room temperature and a reduced transmittance at a high temperature side. Is done. Further, according to the present invention, in the above configuration,
The optical information recording medium is characterized in that the plurality of transmittance variable films have a low transmittance at room temperature and an increase in transmittance at a high temperature side. Further, according to the present invention, the optical information recording medium is irradiated from the substrate side with a laser beam having a power for raising the transmittance variable film to a temperature equal to or higher than a threshold temperature. home,
There is provided a reproducing method characterized by reading recorded information from an area other than an area where the transmittance of the transmittance variable film decreases below a predetermined value. Further, according to the present invention, the optical information recording medium is irradiated from the substrate side with a laser beam having a power to raise the transmittance variable film to a temperature equal to or higher than a threshold temperature, and the laser beam is irradiated in Among them, there is provided a reproducing method characterized by reading recorded information from an area where the transmittance of the transmittance variable film increases to a predetermined value or more.

[0009]

[Function] By providing a plurality of transmittance variable films having a small transmittance near room temperature and an increase in transmittance at a high temperature side, a region having a large transmittance and a transmittance at room temperature within a spot diameter of the laser beam. A region with a small value close to it is formed,
A region having a large transmittance has a diameter smaller than the spot diameter of the laser beam, and a small recording mark can be reproduced. Further, by providing a plurality of transmittance variable films having a large transmittance near room temperature and a decrease in transmittance on the high temperature side, the transmittance is close to that at room temperature within a laser beam spot diameter within a small transmittance area. A region with a large value is formed,
A region having a large transmittance has a diameter smaller than the spot diameter of the laser beam, and a small recording mark can be reproduced.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. FIG. 1 shows a configuration example of an optical information recording medium according to the present invention. This optical information recording medium is composed of a plurality of transmittance variable films 2 on a substrate 1,
Protective film 3, magnetic film 4, second protective film 3 'and reflective film 5
Are sequentially stacked, and is characterized in that a plurality of transmittance variable films 2 are provided as compared with the conventional layer configuration. In addition,
The first protective film 3 has a function of enhancing the Kerr rotation angle in addition to the original protective function, and the second protective film 3 'also has a function of accumulating heat of the laser beam.

As the transmittance variable film used in this configuration example, the following two types can be used. (1) A film whose transmittance changes with temperature.The transmittance is large at room temperature, and the transmittance decreases at high temperatures. (2) A film whose transmittance changes with temperature and has a small transmittance at room temperature. Specifically, the former film has a transmittance of about 90% at room temperature with respect to, for example, a laser beam having a wavelength of 780 nm (a reflectance of about 10%).
%), And when the temperature is set to 100 ° C. or higher, the transmittance is conversely reduced to nearly 10%, preferably to 0%. FIG. 2 shows an example of the relationship between the transmittance and the wavelength of the variable transmittance film at room temperature and temperature T ′. Here, T 'is a temperature at which the transmittance becomes 10% or less when a laser beam having a wavelength of 780 nm is irradiated. Further, the latter film is specifically a film that exhibits the opposite behavior to the former film.

The number of layers constituting the variable transmittance film 2 is at least two or more. If the temperature at which the transmittance of the variable transmittance film reaches a predetermined value is the threshold temperature Tt, the thickness of each film is determined. By appropriately selecting the threshold temperature Tt in relation to the thermal conductivity, as a whole, as shown in FIG. 3, a region having a high transmittance (hereinafter, referred to as an opening) is provided for each variable transmittance film. It becomes narrower as approaching the membrane 4. FIG. 3 shows four layers of variable transmittance films (a, a in order from the substrate 1 side).
b, c, d) are diagrams showing the temperature distribution in the case of providing
(A) shows a high temperature region where the temperature is equal to or higher than the threshold temperature Tt,
FIG. 3B shows the relationship between temperature and distance in each transmittance variable film. For example, when a material having a low thermal conductivity is used as a material constituting each transmittance variable film, as shown in FIG. 4, as the threshold temperature Tt of each transmittance variable film approaches the magnetic layer 4, The material is selected so that it becomes smaller gradually (as it moves away from the substrate 1). When a transmittance variable film having a small thermal conductivity is used, the temperature distribution becomes flat and uniform as it approaches the magnetic film 4, so that the opening may become too narrow and may not be opened in some cases. When the relationship between the threshold temperatures between the variable films is set as described above, an opening having an appropriate size can be formed. On the other hand, when a material having a high thermal conductivity is used as a material constituting each transmittance variable film, as shown in FIG. 5, as the threshold temperature Tt of each transmittance variable film approaches the magnetic layer 4, The material is selected so as to increase gradually (as the distance from the substrate 1 increases). When a transmittance variable film having a large thermal conductivity is used, there is no large difference in temperature distribution between the respective transmittance variable films, so that the opening of the transmittance variable film on the magnetic film 4 side may be too large. When the relationship between the threshold temperatures between the transmittance variable films is set as described above, an opening having an appropriate size can be formed.

In the above description, the formation of an appropriate opening has been described in relation to the threshold temperature and the thermal conductivity of each transmittance variable film. However, any layer structure can be adopted as long as the layered structure is such that the size of the opening on the magnetic film is smaller than the spot diameter of the laser beam.

In the variable transmittance film 2, the type of the above (1),
That is, as a material of a film whose transmittance decreases on the high temperature side, for example, 80 to 90 nm in a wavelength range of 600 to 800 nm.
%, A TiO ₂ / Ag / TiO ₂ laminated film having a particularly high transmittance can be used. The metal oxide film is a transparent high-refractive-index dielectric film, and the metal interposed therebetween may be Au, Cu, or the like other than Ag, or an alloy thereof instead of the metal. (Cl or the like) may be added. Here, the optical film thickness is important.
Further, in order for the transmittance to decrease as the temperature rises, the metal in between bonds with the oxygen of the metal oxide film at 100 to 200 ° C., increasing the light absorption, and dissociating again at room temperature to return to the original state. Elements such as those described above are preferred. Therefore, not only one element but also a plurality of elements (alloys) may be used, and the above metals may be added with a halogen element. The film thickness is, for example, 300 ° for the metal oxide film and 120 ° to 130 ° for the metal or alloy film, and the transmittance is about 90% near a wavelength of 780 nm. As the metal oxide, Ta ₂ O _{5 and the} like can be used in addition to TiO ₂ . In the present invention,
Since a plurality of variable transmittance films are provided, for example, TiO ₂ / A
g / TiO ₂ / Ag / TiO ₂ or the like can be used.

As the film material of the variable transmittance film 2 (including the above (1) and (2) types), a conductive polymer exhibiting reversible thermochromism can be used. As such a conductive polymer, those having a basic skeleton shown in Chemical formula 1 and having an appropriate alkyl chain or the like added to a side chain are preferably used.

Embedded image

In the above-described conductive polymer, a change in side chain conformation causes steric hindrance, resulting in a change in effective conjugation length. Therefore, a phenomenon of absorption spectrum change due to thermochromism occurs. Therefore, the intended purpose can be achieved by utilizing this phenomenon. Here, as an example, FIG. 6 shows a change in absorption spectrum (thermochromism) of poly (3-docosylthiophene) represented by the following formula 2 with temperature.

Embedded image

Further, the film material of the transmittance variable film 2 (the
(Including those of the types (1) and (2)) include metal complex salts exhibiting reversible thermochromism, condensed aromatic ring-substituted ethylene derivatives, liquid crystals and the like.

The type of medium to which the present invention is applied is not limited as long as it can be reproduced by light, and it can be applied to a magneto-optical disk or a dye-based optical disk.

In the present invention, the method of lamination, the thickness, etc. of the plurality of variable transmittance films can be selected in accordance with a desired reproduction shortest mark. Alternatively, whether to use the light blocking portion is determined by the property of the material of the transmittance variable film to be used.

Usually, Tb is used as a magnetic film of a magneto-optical recording medium.
When an FeCo-based amorphous alloy is used, the Curie temperature of the magnetic film is 200 ° C. or higher, and the recording condition is a recording magnetic field of 200 to 200 ° C.
300 Oe, optimal recording laser power (laser power when C / N is maximized) 7 to 8 mW. Wavelength 780n
The spot diameter of the laser beam of m is about 1.8 μm, but the temperature region above the Curie point is near the center of the spot of the laser beam. Recording is possible. Therefore, in the present invention, the recording method itself is the same as before, but by appropriately setting the recording power and the pulse width of the laser beam,
Enable higher density recording.

At the time of reproduction, a laser beam is radiated to read out information recorded on the magnetic film (recording layer) 4. At this time, the value of the reproduction power is adjusted by the transmittance variable film described above. 2 is selected to a value that raises the temperature to a temperature equal to or higher than Tt. In this case, the temperature distribution within the spot diameter of the laser beam becomes a curve as shown in FIG. When the disk is rotating, the high temperature region (T ≧ Tt) is distributed asymmetrically within the spot diameter. By utilizing the fact that the area of the high-temperature region becomes smaller than the spot area, it becomes possible to reproduce a minute recording mark smaller than the spot diameter of the laser beam. FIG. 8 and FIG. 9 schematically illustrate the reproduction (reading) method of the present invention. In the present invention, the readout method may be either normal transmission OFF (meaning small transmittance) and ON in a high temperature part (meaning large transmittance) or normal transmission ON and OFF in a high temperature part. .

Here, a case where a magneto-optical recording medium is manufactured will be described as an example of manufacturing an optical information recording medium according to the present invention.
Al / SiN / TbFeCo /
TiO ₂ / Ag / Ti between the SiN four-layer film and the substrate
A transmittance variable film composed of O ₂ / Cu / TiO ₂ was newly provided to obtain a magneto-optical recording medium according to the present invention. For this magneto-optical recording medium and a conventional magneto-optical recording medium, a recording / reproducing wavelength λ = 780 nm, a disk rotation speed of 1800 rpm,
Reproduction power about 3 mw (in the case of the magneto-optical recording medium of the present invention:
The C / N for each recording mark length was examined at 1.3 mW for a conventional magneto-optical recording medium. As shown in FIG. 10, in the present invention, C / N ≧ 45 dB was obtained even with a recording mark length of 0.4 μm as compared with the conventional example. It became clear that image reproduction was achieved.

[0023]

According to the present invention, a recording mark recorded at a high density exceeding the diffraction limit by a transmittance variable film having a plurality of layers is kept in a high density state by using a conventional pickup optical system. A super-resolution reproduction method capable of reproduction is provided.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing one configuration example of a magneto-optical recording medium according to the present invention.

FIG. 2 is a diagram showing the relationship between transmittance and wavelength at room temperature and T ′ of a transmittance variable film.

FIG. 3A is a diagram showing a high-temperature region of each transmittance variable film.
(B) is a diagram showing the relationship between temperature and distance in each transmittance variable film.

FIG. 4 is an explanatory diagram in the case where the threshold temperature of each transmittance variable film decreases as approaching the magnetic film.

FIG. 5 is an explanatory diagram in the case where the threshold temperature of each transmittance variable film increases as approaching the magnetic film.

FIG. 6 is a diagram showing a change in absorption spectrum of poly (3-docosylthiophene) with temperature.

FIG. 7 is a diagram showing a temperature distribution within a spot diameter of a laser beam when a reproduction laser beam is irradiated.

FIG. 8 is a diagram schematically showing a reproduction method of the present invention.

FIG. 9 is a diagram schematically showing another reproduction method according to the present invention.

FIG. 10 is a diagram showing characteristics of a production example of a magneto-optical recording medium of the present invention in comparison with a conventional medium.

FIG. 11 is an explanatory diagram of FAD in the magnetic induction super-resolution method.

FIG. 12 is an explanatory diagram of RAD in the magnetic induction super-resolution method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Variable transmittance film 3 Protective film 4 Magnetic film (recording layer) 5 Reflective film S Laser spot A Light transmission area B Light absorption area

Claims (5)

(57) [Claims] 1. A optical information recording medium for recording / reproducing information with a laser beam, and variable transmittance film transmittance varies with temperature changes the reading surface side of at least a recording layer is provided a plurality of layers, yet Each transmittance variable membrane transmits
When the threshold temperature at which the rate reaches a predetermined value moves away from the recording layer,
Therefore, as it becomes smaller or moves away from the recording layer,
An optical information recording medium, which is set so as to be larger . 2. The optical information recording medium according to claim 1, wherein the plurality of variable transmittance films have a large transmittance at room temperature and a reduced transmittance at a high temperature. 3. The optical information recording medium according to claim 1, wherein the plurality of transmittance variable films have low transmittance at room temperature and increase transmittance at a high temperature. From the substrate side to 4. Any of the optical information recording medium according to claim 1 to 3, and a laser beam of a power to raise the temperature of the transmittance variable film above a threshold temperature, the laser beam spot A reproduction method comprising reading recorded information from an area within a diameter other than an area where the transmittance of the transmittance variable film decreases below a predetermined value. From the substrate side to 5. Any of the optical information recording medium according to claim 1 to 3, and a laser beam of a power to raise the temperature of the transmittance variable film above a threshold temperature, the laser beam spot A reproduction method comprising: reading out recorded information from an area in which the transmittance of the transmittance variable film is increased to a predetermined value or more in an area within a diameter.