JPH07304258A - Optical recording medium and signal recording method thereof - Google Patents

Abstract

(57) [Summary] [Structure] An optical recording medium is formed by forming a saturable absorption dye-containing layer 3 and a reflective layer 4 on a transparent substrate 1, and the saturable absorption dye-containing layer has an extinction coefficient. The information signal is recorded by the change of. [Effects] The optical recording medium having such a structure exhibits excellent super-resolution and reproduces information signals from the diffraction limit λ / 2 of the reproduction optical system.
Even when formed as a pit pattern with a cycle shorter than NA, a reproduced signal with a high C / N ratio of 45 dB or more can be obtained. Therefore, according to the present invention, for example, the wavelength of the reproduction light can be shortened, the numerical aperture NA of the focus lens can be increased, the signal demodulation method can be changed without making any significant changes on the apparatus side, and for example, about four times the current value can be obtained. It is possible to store the amount of recorded information in an optical recording medium of the same size.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical recording medium for reproducing a signal by irradiating a reproducing light from a substrate side and detecting a change in the amount of reflected light, and a signal recording method thereof.

[0002]

2. Description of the Related Art In recent years, in the field of information recording, researches on optical information recording methods have been advanced in various places. This optical information recording system can perform non-contact recording / reproducing, can achieve a recording density higher than the magnetic recording system by an order of magnitude, and supports read-only, write-once, and rewritable memory types. A wide range of applications from industrial to consumer use are considered as those that have a number of advantages such as being able to do and can realize an inexpensive large-capacity file.

Among the above-mentioned memory types, a read-only type optical recording medium is a digital audio disc (so-called compact disc, CD), an optical video disc (so-called laser disc, LD), or a CD.
-ROM etc. are already widely used.

In these read-only type optical recording media, pits are usually recorded on a transparent substrate in a pattern corresponding to an information signal as, for example, a concave-convex shape or a layer for changing an optical constant, and this recording pattern is formed. , A reflective layer made of a metal material such as Al is adhered and formed. In such an optical recording medium, reproducing light such as laser light is irradiated from the transparent substrate side to check the presence / absence of pits in the reproducing spot.
The information is reproduced by identifying by detecting the intensity of the reflected light.

By the way, in the read-only optical recording medium, digitization of VTR and high-definition TV (H
Further improvement in recording density is required in order to secure a capacity capable of supporting DTV) and the like. On the other hand, the size of the optical recording medium is required to be small for the convenience of operation, and the improvement of the recording density is also demanded from such requirements.

Here, as a means for improving the recording density of the optical recording medium, it is first considered to make the recording pattern finer, for example, to shorten the pit period. However, since the reproduction optical system has a diffraction limit λ / 2NA (λ: reproduction light wavelength NA: objective lens numerical aperture of the optical system) that cannot reduce the spot diameter further, if the pit cycle becomes too short, reproduction will occur. A situation occurs in which a plurality of pits are duplicated in the spot, which causes a problem that the information signal cannot be reproduced. That is, the reproducing apparatus has an MTF (Modu) that is an index of the resolution determined by the reproducing optical system.
There is a cut-off spatial period of the transfer transfer function.

For this reason, the signal code is compressed with the pit period unchanged, or the numerical aperture NA of the objective lens of the optical system is increased so as to cope with a recording pattern with a short pit period. Attempts have been made to improve the diffraction limit of reproduction light by shortening the reproduction light wavelength. In addition, recently, super resolution (super
A method called "resolution" is drawing attention as a method that can be applied to a recording pattern with a short pit period.

[0008] Super-resolution means setting an aperture (aperture) smaller than the diffraction limit of the irradiation light at the object point position so that the apparent spot diameter of the irradiation light becomes smaller than the diffraction limit, thereby increasing the resolution. The principle is to improve. Regarding this super-resolution, for example, “Charles W.
McCutchen, “Super-resolution in Microscopy and the
Abbe Resolution Limit. ”Journal of Optical Societ
y of America, 57 (10), 1190 (1967) ”, Tony Wilson
and Colin Sheppard, “Theory and Practiceof Scannn
ing Optical Microscopy. ”, Academic Press (London),
1984 ”and the like.

In order to actually apply such super-resolution to the signal reproduction of the optical recording medium, it is necessary that the aperture also moves following the movement of the reproducing light on the optical recording medium.

As an example in which super-resolution is applied to a magneto-optical recording medium, the applicant of the present application discloses the magnetic Kerr effect of the magneto-optical recording / reproducing system in Japanese Patent Application Laid-Open Nos. 1-143041 and 1-143042. We propose a method to achieve high-density recording by making the area narrower than the spot diameter of the reproducing light to develop the super-resolution effect. However, this method is used only in a magneto-optical system, and cannot be applied to an ordinary optical recording system that does not use a magnetic head.

As a super-resolution method applicable to a general optical recording system, Japanese Patent Laid-Open No. 2-96926 proposes to use a photoresponsive material for the reflective layer. The principle of this super-resolution reproduction is as follows.

That is, in an optical recording medium using a photoresponsive material for the reflective layer, when reproducing light is irradiated, there is a light amount distribution in which the light amount increases in the center of the reproducing light spot. It is possible to change the optical characteristics of only the central portion. The portion where the optical characteristics have changed functions as an aperture.

When the aperture is formed in this manner, even if a plurality of pits are present in the spot of the reproduction light, only the pits present in the aperture are detected, and the other pits are masked. It will not be detected as a state. Therefore, it is possible to perform signal reproduction from a fine recording pattern in which pits are formed with a cycle shorter than the diffraction limit of the reproduction optical system.

However, in this publication, as a photoresponsive material, a nonlinear optical material whose optical characteristics are directly changed by reproduction light, or a phase in which the optical characteristics are indirectly changed by heat generation due to absorption of reproduction light. Only the change material is described, and no specific material is mentioned.
Therefore, it can be said that its realization is difficult.

Therefore, as specific photoresponsive materials, chalcogenite materials and saturable absorption dyes have been proposed. A chalcogenite-based material undergoes a phase change from a solid state to a melt state by laser heating, and at that time, the complex refractive index changes greatly. On the other hand, the saturable absorbing dye is a photoresponsive material proposed by the applicant in Japanese Patent Application No. 5-26805, and is irradiated with a certain amount of light or more.
It is a pigment material that exhibits a phenomenon such that the absorptance becomes 0 when it is in an excited state, that is, a saturable absorption phenomenon.

For example, when reproducing light is irradiated onto an optical recording medium in which a saturable absorbing dye-containing layer containing the saturable absorbing dye and a reflective layer are formed on a substrate having pits and projections formed in a concavo-convex shape, As described above, since the reproduction light spot has a light amount distribution in which the light amount increases toward the center, the saturable absorption occurs more strongly in the central portion where the light amount exceeds a certain amount. Since the absorptivity is lowered in this saturable absorption region, the amount of light reaching the reflective layer is large and a high reflectance is obtained as compared with other regions. Therefore, this saturable absorption region functions as an aperture, and even if a plurality of pits are duplicated in the reproduction light spot, only the pits present in this saturable absorption region are detected.

[0017]

As described above, various photo-responsive materials for forming an aperture have been proposed so far, and their application to various medium configurations has been attempted, but they have not reached the level of practical use. Is the reality.

Therefore, the present invention has been proposed in view of such a conventional situation, is excellent in super-resolution and is formed in a pit formed in a cycle shorter than the diffraction limit λ / 2NA of the reproducing optical system. It is an object of the present invention to provide an optical recording medium from which a reproduction signal having a high C / N ratio can be obtained from the recording pattern.

[0019]

In order to achieve the above object, the optical recording medium of the present invention comprises a saturable absorbing dye-containing layer and a reflective layer formed on a transmissive substrate. An information signal is recorded in the dye-containing layer by changing the extinction coefficient. The saturable absorption dye contained in the saturable absorption dye-containing layer is a naphthalocyanine derivative.

Further, the signal recording method of the optical recording medium of the present invention is the above-mentioned saturable absorption dye-containing layer for an optical recording medium having a saturable absorption dye-containing layer and a reflective layer formed on a transparent substrate. The signal is recorded by changing the extinction coefficient of P by the photobleaching method.

[0021]

In the optical recording medium in which the saturable absorbing dye-containing layer and the reflecting layer are formed on the transparent substrate and the information signal is recorded in the saturable absorbing dye-containing layer by the change of the extinction coefficient, When recording is performed with a pit pattern having a period shorter than the diffraction limit λ / 2NA of the reproducing optical system, for example, by forming an uneven shape on the substrate, the C / N ratio is higher than that of an optical recording medium on which an information signal is recorded. And a reproduction signal with high error is obtained.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, the optical recording medium of this embodiment is constructed by forming a high refractive index layer 2, a saturable absorbing dye layer 3 and a reflective layer 4 on a transparent substrate 1. And
The pits 5 have a pattern corresponding to an information signal and are formed by changing the extinction coefficient of the saturable absorbing dye-containing layer. The extinction coefficient here is the coefficient of the imaginary part of the complex refractive index, and indicates the degree of light absorption.

The principle of signal recording in the saturable absorbing dye-containing layer and its super-resolution reproduction by changing the extinction coefficient in this manner will be described below. FIG. 2 shows the relationship between the extinction coefficient and the reflectance of the saturable absorbing dye-containing layer 3 when the optical recording medium is irradiated with laser light from the substrate side. The detailed medium structure is as follows.

Transparent substrate: glass substrate High refractive index layer: ZnS vapor deposition film having a film thickness of 85 nm Saturable absorption dye-containing layer: (tri-n-hexylsiloxy) silicon naphthalocyanine (SINC) and polymethylmethacrylate (PMMA) Coating film having a film thickness of 220 nm, which is a reflection layer: an Al vapor deposition film having a film thickness of 400 nm

As can be seen from FIG. 2, the reflectance of the medium is
It changes depending on the extinction coefficient of the saturable absorbing dye-containing layer.
The change is not constant, and when the extinction coefficient is in the range of 0 to 0.2, the extinction coefficient decreases from about 60% to about 1% while decreasing the rate of change with the increase of the extinction coefficient. 0.2 ~
In the range of 0.3, there is almost no change and the value of about 1% is maintained. Then, when the extinction coefficient becomes 0.3 or more, the extinction coefficient increases with the increase of the extinction coefficient.

In the above optical recording medium, super-resolution reproduction is possible by utilizing the change in reflectance that occurs when the extinction coefficient of the saturable absorbing dye-containing layer is changed within the range of 0.1 to 0.3, for example. Signal recording.

First, in this case, the extinction coefficient of the saturable absorbing dye-containing layer 3 in the state where no signal is recorded is 0.3.
Is. In the saturable absorption dye-containing layer 3, a low extinction coefficient portion having an extinction coefficient of 0.2, which is lower than the initial value by 0.1, is formed in a pattern corresponding to the information signal, so that signal recording is performed. Done. The portion where the extinction coefficient is reduced corresponds to the pit 5. In this case, since the purpose is super-resolution reproduction, the pits 5 are formed with a period shorter than the diffraction limit of the reproduction optical system.

As a method of reducing the extinction coefficient, for example, there is a photobleaching method in which the coloration state of the saturable absorbing dye-containing layer is selectively changed by irradiating a laser beam. The wavelength of this laser beam is appropriately selected depending on the optical characteristics of the saturable absorbing dye used.

By changing the extinction coefficient of the saturable absorbing dye-containing layer 3 in this manner, the super-resolution reproducing mechanism from the optical recording medium on which signal recording is performed is shown in FIGS. 3 (a) and 3 (b).
Will be described with reference to.

First, FIG. 3A shows a state in which reproduction light is applied to the optical recording medium, and three pits 5 are overlapped in the reproduction light spot S.

When reproducing light is irradiated onto the optical recording medium, a light quantity distribution in which the quantity of light becomes larger in the center as shown in FIG. 3B is generated in the reproducing light spot S, and the saturable absorption dye-containing layer becomes closer to the center. Is strongly saturable, and the extinction coefficient decreases by 0.1 or more in a region Sa that exceeds a certain light amount A. Here, the region where the extinction coefficient decreases by 0.1 or more is referred to as a saturable absorption region.

That is, in the saturable absorption region S _a , the extinction coefficient of the pit 5 is reduced from 0.2 to 0.1 (or less), and the other part is reduced from 0.3 to 0.
To 2. Here, referring to FIG. 2 shown above, when the extinction coefficient of the saturable absorbing dye-containing layer 3 is 0.1, the reflectance of the medium is about 10%.
The reflectance of the medium is about 1% when the extinction coefficient of is 0.2.
Is. That is, in the saturable absorption region S _a , the pit portion has a reflectance about 10 times that of the other portions.

On the other hand, in the area S _b other than the saturable absorption area, the extinction coefficient of the portion of the pit 5 is 0.2 and the extinction coefficient of the other portions is 0.3, which are both before irradiation with the reproducing light. Is the same. Here, referring again to FIG. 2, when the extinction coefficient of the saturable absorbing dye-containing layer 3 is 0.2, the reflectance of the medium and the extinction coefficient of the saturable absorbing dye-containing layer 3 are 0.3. The reflectivity of the medium is about 1%. That is,
In the saturable absorption area S _a , the pit 5 has a reflectance about 10 times that in the other areas, whereas in the area S _b other than the saturable absorption area, the pit 5 and the non-pit area The part will show almost the same reflectance.

Therefore, in such an optical recording medium,
Even if the pits 5 are duplicated in the reproduction light spot S, the pits 5 existing in the region S _b other than the saturable absorption region are in a state equivalent to being masked,
Only the pits in which the saturable absorption area S _a exists are detected. Therefore, the signal is reproduced from the recording pattern of the pits formed in a cycle shorter than the diffraction limit of the reproducing optical system.

The C / N ratio of the reproduced signal thus obtained is higher than that of a reproduced signal obtained from a pit pattern recorded by forming a concavo-convex shape on the substrate, for example, being 45 dB or higher, and the number of errors is extremely small. It can be said to be highly reliable.

As the saturable absorbing dye contained in the saturable absorbing dye-containing layer, one having a large absorption peak at the wavelength of the reproduction light used is selected.

For example, when the reproduction light has a wavelength of about 780 nm, a naphthalocyanine dye, a cyanine dye,
Examples thereof include phthalocyanine dyes, and among them, naphthalocyanine dyes are suitable. In addition, the wavelength of the reproduction light is 630
In the case of ˜690 nm, porphyrin-based dyes, cyanine-based dyes, etc. can be used, and in the case of 400-600 nm, azo-based dyes, cyanine-based dyes, etc. can be used.

The saturable absorbing dye-containing layer is formed, for example, by dissolving the saturable absorbing dye and a polymer such as polymethylmethacrylate in a solvent, applying the dye solution by spin coating, and drying.

The reflective layer is selected to have a reflectance that meets the standard of the reproducing device. For example a compact disc,
When applied to a reproducing apparatus such as a laser disk or a write-once compact disk, a metal thin film or a dielectric film formed by a vacuum thin film manufacturing method typified by an aluminum thin film and a gold thin film used in these disks. Body thin film etc. are mentioned.

The high refractive index layer is additionally provided to enhance the light absorption efficiency of the saturable absorbing dye layer,
It is composed of a material having a relatively higher refractive index than the material of the substrate and the saturable absorbing dye-containing layer, for example, an inorganic material such as ZnS.

Here, the film thickness of the high refractive index layer is λ, where λ is the wavelength of the reproduction light and n is the real part refractive index of the high refractive index layer.
The film thickness is preferably / 4n. When the high refractive index layer is formed with this thickness, the phase difference between the reflected light at the interface between the high refractive index layer and the substrate and the reflected light at the interface between the high refractive index layer and the saturable absorbing dye-containing layer is obtained. The angle is 180 °, which satisfies the so-called non-reflection condition. As a result, the Fresnel reflection at the saturable absorbing dye-containing layer is maximized and the intensity of the reproduced signal can be increased.

The substrate on which these functional films are formed is a polycarbonate substrate, a polyolefin substrate, or a glass 2P.
(Photopolymer method) Examples include substrates.

Next, an optical recording medium was actually manufactured and its reproducing characteristics were examined. The optical recording medium prepared in this example is composed of a glass disk substrate 1, a ZnS high refractive index layer 2,
A saturable absorption dye-containing layer 3 made of (tri-n-hexylsiloxy) silicon naphthalocyanine (SINC) and polymethylmethacrylate, and an Al reflective layer 4 are formed in this order. Such an optical recording medium was prepared as follows.

First, a ZnS high-refractive index layer 2 was formed on a glass disk substrate 1 having an outer diameter of 12 cm, an inner diameter of 15 mm and a thickness of 1.2 mm by a vacuum deposition apparatus (manufactured by Nichiden Anerva Co., trade name EVD).
A film was formed using 500A. The film forming conditions are shown below.

Degree of ultimate vacuum: 3 × 10 ^-4 P Vacuum degree during vapor deposition: 1 × 10 ^-3 P Deposition source: ZnS having a purity of 99.99% Heating of the deposition source: Resistance heating method Deposition rate: 0.4 to 0 0.5 nm / s

During the vapor deposition, the substrate was not heated and was rotated around the vapor deposition source in order to make the film thickness uniform.
Further, the film thickness of the vapor deposition film was controlled while monitoring the transmission spectrum with an optical film thickness monitor to which a fiber spectroscope was applied, and finally the film thickness was adjusted to 85 nm. This film thickness is λ / 4 at which the high refractive index layer exhibits its effect most.
It is a film thickness corresponding to n (λ: reproduction light wavelength: λ, n: real part refractive index of the high refractive index layer).

Next, the saturable absorption dye-containing layer 3 was spin-coated on the high refractive index layer 2 (manufactured by Mikasa Co., Ltd., product name 700L).
Was formed using.

The dye solution for spin coating is bis (tri-n-hexylsiloxy) silicon naphthalocyanine (S
INC) and polymethylmethacrylate (PMMA)
Cyclohexanone (product name ANON) with SINC: P
It was dissolved at a composition ratio of MMA: ANON = 1: 10: 300 (weight ratio). Structural formulas of SINC, PMMA, and ANON are shown in Chemical formulas 1 to 3, respectively.

[0050]

[Chemical 1]

[0051]

[Chemical 2]

[0052]

[Chemical 3]

After the spin coating film was coated, the solvent was dried by leaving it in the vacuum environment at a temperature of 80 ° C. for 2 hours. The film thickness after drying was 220 nm. Further, the complex index of refraction was measured by an ellipsometer, and found to be 1.6 near the wavelength of 780 nm of the reproduction light used in this example.
It was -0.3i.

Then, in the saturable absorbing dye-containing layer 3,
Signals were recorded by reducing the extinction coefficient with a pattern corresponding to the information signal. The extinction coefficient was reduced by using the optical system for photobooching shown in FIG.

The optical system used for photobleaching is composed of two systems: an exposure system for irradiating the saturable absorbing dye-containing layer with an exposure laser and a servo system for performing focus servo of this exposure laser. It comprises an exposure laser light source unit 11, a dichroic prism 12, an objective lens 13, a focus servo laser light source unit 14, a polarization beam splitter 15, and a quarter wavelength plate 16.
The disk 6 having the high refractive index layer 2 and the saturable absorption dye-containing layer 3 formed on the substrate 1 is arranged with the saturable absorption dye-containing layer 3 side facing the objective lens 13.

In this optical system, the exposure laser light source unit 11
The emitted laser light L _f passes through the dichroic prism 12 and the objective lens 13 and is condensed on the saturable absorption dye-containing layer 23. In the saturable absorption dye-containing layer 3, the extinction coefficient of the portion where the exposure laser light L _f is condensed is reduced, and the pit 5 is formed.

On the other hand, the focus servo laser light source unit 1
The laser beam L _s emitted from the polarization beam splitter 4, the polarization beam splitter 15, the quarter-wave plate 16, the dichroic prism 12
Then, the light is condensed on the saturable absorption dye layer 3 through the objective lens 13. Then, the reflected light reaches the polarization beam splitter 15 via a route opposite to that of the emitted light, and is further received by a light receiving portion (not shown) in the future. A change in the distance between the saturable absorption dye-containing layer 3 and the optical system is detected from the change in the amount of light received by the light receiving unit, and the focus servo is performed by controlling the position of the optical system based on the change.

In this example, the third harmonic of YAG (wavelength 355 nm) was used as the exposure laser light, and the He-Ne laser was used as the servo laser light. The numerical aperture NA of the objective lens 13 is 0.9.

Further, the irradiation of the exposure laser was performed from the air side of the saturable absorption dye-containing layer 3 to the rotating disk 6 so that the pits 5 were arranged in a spiral shape. The pit period is 0.6 μm.

Wavelength 7 of the photo-bleached part
The complex refractive index at 80 nm was 1.6-0.2i as measured by an ellipsometer. Since the initial complex refractive index was 1.6 to 0.3i, this photobleaching reduced the extinction coefficient of the saturable absorbing dye-containing layer by 0.1. In addition, the measurement of the refractive index was performed on an area other than the area where the signal recording was performed, with the track pitch narrower (0.3 μm pitch) than in the case of the signal recording, and the area where the exposure laser was continuously irradiated. went.

Next, the Al reflection layer 4 was formed on the saturable absorption dye-containing layer 3 on which the signal recording was carried out in this manner by using a vacuum vapor deposition apparatus (product name EVD500A, manufactured by Nichiden Anerva Co., Ltd.). An optical recording medium was created. The vapor deposition conditions are as follows.

Degree of ultimate vacuum: 3 × 10 ^-4 P Degree of vacuum during vapor deposition: 1 × 10 ^-3 P Deposition source: Al having a purity of 99.99% Heating of the deposition source: electron gun Deposition rate: 1-2 nm / s

During the vapor deposition, the substrate was not heated, but was rotated around the vapor deposition source in order to make the film thickness uniform.
Further, the vapor deposition rate of the vapor deposited film was controlled by a quartz type film thickness monitor so that the final film thickness was 400 nm.

The reflectance of the optical recording medium prepared as described above was about 1% at a wavelength of 780 nm.

Then, with respect to this optical recording medium, signal reproduction was performed and the C / N ratio was measured. A reproduction optical system used for signal reproduction is shown in FIG.

This reproducing optical system includes a reproducing laser light source unit 3
1, a polarization beam splitter 32, a quarter-wave plate 33, and an objective lens 34. The optical recording medium reproduced by this reproducing optical system is arranged with the substrate 1 side facing the objective lens 34 of the reproducing optical system. In this reproducing optical system, the laser light LB emitted from the reproducing light source unit passes through the polarization beam splitter 32, the quarter wavelength plate 33 and the objective lens 34 and is condensed on the optical recording medium 6. Then, the reflected light from the optical recording medium 6 is the objective lenses 34, 1
It reaches the polarization beam splitter 32 via the / 4 wavelength plate 33, is reflected here, and is detected by a light receiving element (not shown) such as a photodiode. The apparatus conditions used in this example are shown below.

Reproducing laser light source: semiconductor (AlGaAs) laser having a wavelength of 780 nm Objective lens numerical aperture NA: 0.53 Laser power: 4-5 mW Disk linear velocity: 4 m / s

As a result of reproducing with such a reproducing optical system, the C / N ratio of the reproduced signal was 45 dB or more, and a sufficiently practical reproducing characteristic was obtained.

In the case of the reproducing optical system, the cutoff spatial period λ / 2NA is 0.736 μm, and the pit period of the recording pattern formed on the optical recording medium is 0.6.
Larger than μm. Therefore, the recording pattern of the optical recording medium cannot be read unless the super-resolution phenomenon appears in the optical recording medium. In such a situation, the optical recording medium has a high C / N ratio of 45 dB or more, which confirms that signal reproduction is performed in the super-resolution state.

In the above description, a read-only type in which an information signal is recorded in advance on the saturable absorbing dye-containing layer and only the information signal is reproduced is used. However, the optical recording medium of the present invention is
Not only the read-only type but also the write-once type can be used.

In this case, a guide groove for performing tracking servo is formed on the substrate in a concavo-convex shape, and an information signal is not recorded in the saturable absorbing dye-containing layer so that it can be written by a user. Keep it. Writing by the user is also performed by reducing the extinction coefficient of the saturable absorbing dye-containing layer in a pattern corresponding to the information signal. The information signal additionally recorded in this way can be super-resolution reproduced by the same mechanism as the information signal recorded exclusively for reproduction, and a reproduction signal having a high C / N ratio can be obtained in the same manner.

[0072]

As is apparent from the above description, the optical recording medium of the present invention comprises a saturable absorbing dye-containing layer and a reflective layer formed on a transparent substrate. In addition, since the information signal is recorded by the change of the extinction coefficient, it exhibits excellent super-resolution, and when the information signal is formed as a pit pattern with a period shorter than the diffraction limit λ / 2NA of the reproducing optical system. However, a reproduced signal with a high C / N ratio of 45 dB or more can be obtained.

Therefore, according to the present invention, the wavelength of the reproduction light can be shortened, the numerical aperture NA of the focus lens can be increased, and the signal demodulation method can be changed without making any significant changes on the apparatus side. It is possible to store about twice the amount of recorded information in an optical recording medium of the same size.

If these high-density recording techniques are applied to the device side, the recording density on the optical recording medium will be several tens of the current level.
Can be doubled. As a result, CDs such as digital video discs and high definition video discs
It can be configured in a size, which can be said to be extremely useful industrially.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of an essential part showing a configuration example of an optical recording medium to which the present invention is applied.

FIG. 2 is a characteristic diagram showing the relationship between the extinction coefficient of the saturable absorbing dye-containing layer and the reflectance of the medium.

FIG. 3 (a) is a plan view showing a state in which pits are overlapped in a reproduction light spot, and FIG. 3 (b) is a light amount distribution in the reproduction light spot and the erasing of the saturable absorbing dye-containing layer. It is a characteristic view which also shows an extinction coefficient distribution.

FIG. 4 is a schematic diagram showing the configuration of an optical system for photobleaching for reducing the extinction coefficient of the saturable absorbing dye-containing layer.

FIG. 5 is a schematic diagram showing a configuration of a reproduction optical system for reproducing a signal from an optical recording medium.

[Explanation of symbols]

 1 Transparent Substrate 2 High Refractive Index Layer 3 Saturable Absorption Dye Containing Layer 4 Reflective Layer 5 Pit

Claims (3)

[Claims] 1. A saturable absorption dye-containing layer and a reflective layer are formed on a transparent substrate, and the saturable absorption dye-containing layer has an information signal recorded by a change in extinction coefficient. Characteristic optical recording medium. 2. The optical recording medium according to claim 1, wherein the saturable absorbing dye contained in the saturable absorbing dye containing layer is a naphthalocyanine derivative. 3. An extinction coefficient of the saturable absorbing dye-containing layer is changed by a photobleaching method for an optical recording medium having a saturable absorbing dye containing layer and a reflective layer formed on a transparent substrate. 2. The signal recording method for an optical recording medium according to claim 1, wherein the information signal is recorded.