JP2007199092A - Optical recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical recording medium capable of uniformly recording optical information in the thickness direction even when a recording layer is thick when performing the recording by means of a transmission type hologram. <P>SOLUTION: The optical recording medium comprises a recording layer 1 which makes recording light 4 incident from an incident surface 25 and records the optical information, wherein light absorptance (r) for the recording light 4 of the recording layer 1 increases toward the propagating direction of the recording light 4 from the incident surface 25. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical recording medium having a recording layer for recording data three-dimensionally, such as a holographic memory or a multilayer optical memory.

  Examples of high-capacity optical recording media currently in use or under consideration include digital versatile discs (DVD), Blu-ray discs, and high definition DVDs (HD DVD). It is done. However, the capacity of these media is considered to be about 100 GB at the maximum, and data represented by holographic memory and multilayer optical memory is recorded three-dimensionally as a high capacity optical recording medium having a capacity larger than this. Optical recording media that perform (volume recording) have been studied.

  Optical recording media that perform volume recording, such as holographic memory and multilayer optical memory, record data not only in the in-plane direction of the recording layer but also in the thickness direction of the recording layer, and recording is performed over three dimensions of the recording layer. Therefore, the capacity can be increased. In the holographic memory, multiplex recording is performed many times at the same place in the surface by various multiplexing methods. In a multilayer optical memory, a recording bit is formed in each of the multilayer recording layers at the same place in the plane.

In an optical recording medium for volume recording, in order to increase the capacity, as the recording layer becomes thicker, the travel of incident light through the recording layer tends to become longer. In the holographic memory, it is possible to increase the capacity by uniformly recording the interference fringes over the entire thickness of the recording layer. Even in a multi-layer optical memory, the capacity can be increased by forming recording bits uniformly over the entire thickness of the recording layer.
And in order to record an interference fringe uniformly, the proposal which reduces the optical density (light absorption rate) of a recording layer along the advancing direction of recording light is made | formed (for example, refer patent document 1).
JP 2004-280899 A (paragraphs 0004 to 0009)

  If the recording layer is made thicker to increase the capacity, the proposed optical density of the recording layer is reduced along the traveling direction of the recording light. With regard to the type hologram, it has been considered that optical information of interference fringes and bits cannot be recorded uniformly.

  Therefore, an object of the present invention is to provide an optical recording medium capable of recording optical information uniformly in the thickness direction even when the recording layer is thick when recording with a transmission hologram.

  In order to solve the above-mentioned problems, the optical recording medium of the present invention has a recording layer that records the optical information of the hologram and the bit by entering the recording light from the incident surface, and the light absorption rate of the recording layer with respect to the recording light is , And increasing from the incident surface in the traveling direction of the recording light.

  According to the present invention, an optical recording medium capable of recording optical information uniformly in the thickness direction even when the recording layer is thick can be provided.

  Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. In the embodiment, an optical recording medium that performs volume recording so as to record a plurality of data in the thickness direction of the recording layer will be described. Specifically, holographic recording optical information using holography (interference) The memory will be described.

  As shown in FIG. 1A, the holographic memory according to the embodiment of the present invention has a recording layer 1. The recording layer 1 records a hologram when the recording light 4 and the reference light are incident from the incident surface 25. Further, if necessary, the light transmissive substrates 2 and 3 may be provided on the incident surface 25 and the emission surface 26 of the recording layer 1, respectively.

  As is well known, optical information is multiplexed and recorded as interference fringes 8 on the recording layer 1 by irradiating the recording layer 1 with reference light and recording light 4. As the multiplexing method, a shift multiplexing method, an angle multiplexing method, a wavelength multiplexing method, a phase code multiplexing method, a polytopic multiplexing method, or a multiplexing method based on a combination thereof can be used. In the case of the angle multiplexing method, the incident angle of the reference light is changed for each recording light 4 of each optical information to be multiplexed recorded. Specifically, first, the reference light set at a predetermined incident angle is adjusted to the recording light 4 of the optical information, and the recording light 4 and the reference light are irradiated to the recording portion of the recording layer 1 to thereby record the recording layer. 1, interference fringes 8 are formed as shown in FIG.

  When the interference fringes 8 are recorded on the recording layer 1, the recording layer 1 changes in the recording layer 1 along the brightness and darkness of the interference fringes 8. The Examples of the material of the recording layer 1 currently under consideration include a photopolymer material, a dye-type material for refractive index modulation, silver halide, dichromated gelatin, a photorefractive material, and a photochromic material.

  Photopolymer materials are attracting attention and are being developed because of their high diffraction efficiency, low noise, and good storage stability if they are completely fixed after recording. A typical photopolymer material contains a binder, a polymerizable monomer, a sensitizing dye, a polymerization initiator, and the like. Moreover, in order to suppress shrinkage | contraction at the time of superposition | polymerization, you may use the thermosetting resin produced by superposing | polymerizing a polyol and isocyanate instead of a binder.

  As the binder and the polymerizable monomer, those having different refractive indexes are used. During recording, bright portions and dark portions of the interference fringes 8 are formed in the recording layer 1. In the bright part of the interference fringes 8, for example, when a radical polymerization monomer is used, the sensitizing dye excites and emits electrons, the emitted electrons move to the polymerization initiator, and the polymerization initiator generates radicals. Then, the radical moves to the monomer and the monomer polymerization reaction is performed. As a result, in the bright part of the interference fringe 8, monomers that are superposed from the adjacent dark part are collected and the monomer becomes rich, and in the dark part of the interference fringe 8, the binder becomes rich corresponding to the loss of the monomer. . The interference fringes 8 are recorded by generating different refractive indexes in the bright and dark portions of the interference fringes 8 due to these mono-rich and binder-rich. As a result, the optical information is recorded in the recording layer 8 as stripes expressed as different refractive indexes and light transmittances. In addition, when a cationic polymerization monomer is used, a reaction occurs with an acid instead of a radical.

  Next, the incident angle of the reference light is changed, and the reference light is adjusted to the recording light 4 of other optical information. Then, the interference fringes 8 are formed at the same location as the recording location where one optical information is recorded, so that other optical information is recorded in an overlapping manner. Hereinafter, a plurality of pieces of optical information are recorded in one recording location of the recording layer 1 by combining reference light having different incident angles for each recording light 4 of other optical information to be multiplexed recorded. The recording layer 1 can perform multiple recording using angle selectivity.

  Then, when the multiplex recording of the predetermined optical information in one recording location is completed, the recording layer 1 moves to another recording location in the stop and go manner. Other recording locations and one recording location may or may not partially overlap. The positioned recording light 4 and reference light multiplex-record a plurality of optical information in other recording locations in the same manner as multiplex recording at one recording location.

  When the recording of the predetermined optical information on the recording layer 1 is completed, the polymerizable monomer that has not been used for the recording of the optical information is fixed by being exposed to the whole surface with a laser or a white light source, or by heat treatment.

  As is well known, the hologram as optical information recorded in the recording layer 1 is read out as reproduction light generated by irradiating each recording location with reproduction illumination light. Then, each of the plurality of pieces of optical information multiplexed and recorded at each recording location is read by irradiating the reproduction illumination light set at the incident angle of the reference light when each piece of optical information is recorded.

  The optical recording medium emits reproduction light from a transmission type optical recording medium in which a surface on which reproduction illumination light is incident and a surface on which reproduction light is emitted are opposed to each other, and a surface on which reproduction illumination light is incident. There is a reflection type optical recording medium. A reflective layer is provided on a surface of the reflective optical recording medium different from the surface on which the reproduction illumination light is incident. The reproduction illumination light and the reproduction light are reflected by the reflection layer and emitted from the same surface as the incident surface. On the other hand, a transmissive optical recording medium does not have a reflective layer. A transmissive optical recording medium is suitable for the embodiment.

  The recording layer 1 is a layer on which optical information is recorded by being irradiated with light, and has a thickness of 200 μm or more, preferably 0.2 to 2.5 mm, more preferably 0.5 to 2.5 mm. . By setting the thickness to 200 μm or more, it is possible to satisfy the number of multiplex recordings required when a holographic memory is commercialized. Moreover, by making it 2.5 mm or less, the material which does not contribute to the recording of optical information can be reduced, and the cost can be reduced at the time of commercialization.

  The polymerizable monomer is not particularly limited as long as it has a polymerizable group, and examples thereof include radically polymerizable monomers, cationically polymerizable monomers, and combinations of these monomers, specifically, epoxy groups, ethylenic monomers. Examples thereof include compounds containing a polymerizable group such as an unsaturated group. The polymerizable monomer only needs to contain one or more of these polymerizable groups in the molecule, and the polymerizable monomer containing two or more polymerizable groups in the molecule may have different polymerizable groups. And the same thing may be used.

  The sensitizing dye is not particularly limited as long as it has absorption at the recording wavelength of the recording light 4, but preferably has a low light absorption coefficient ε of the sensitizing dye itself at the recording wavelength. As shown in FIG. 1B, the light absorption rate r is increased because the sensitizing dye absorbs the recording light 4 in the recording layer 1. The light absorption rate r depends on the concentration of the sensitizing dye in the recording layer 1. In order to increase the light absorption rate r, the concentration of the sensitizing dye may be increased, and in order to decrease the light absorption rate r, the concentration of the sensitizing dye may be decreased. As shown by the solid line 6 in FIG. 1B, the light absorption rate r with respect to the recording light 4 of the recording layer 1 increases continuously or intermittently as the distance from the incident surface 25 increases. That is, it increases from the incident surface 25 in the traveling direction of the recording light 4. In order to increase in this way, the concentration of the sensitizing dye is increased from the incident surface 25 toward the traveling direction of the recording light 4. Further, as indicated by the solid lines 6a to 6c, the light absorption rate r may increase in a stepwise manner in multiple stages. This stepwise increase can also be realized by increasing the concentration of the sensitizing dye stepwise. Accordingly, the recording layer can be easily manufactured by a manufacturing method described later.

As shown in FIG. 1C, the photon density n of the recording layer 1 due to the recording light 4 decreases from the incident surface 25 toward the traveling direction of the recording light 4. The reason for this decrease is that the recording light 4 travels while being absorbed by the sensitizing dye. The total decrease amount of the recording light 4 in the recording layer 1 is an amount obtained by subtracting the intensity of the transmitted light 5 emitted from the recording layer 1 from the intensity of the recording light 4 incident on the recording layer 1. The transmittance, which is the ratio of the intensity of the transmitted light 5 to the intensity of the recording light 4, is desirably 20% or more and 70% or less. When the transmittance is less than 20%, interference fringes may be generated only on the incident surface 25 side of the recording layer 1, and the interference fringes may be incomplete or not generated on the emission surface 26 side. If the transmittance exceeds 70%, the recording light 4 is difficult to be absorbed, and the reaction does not proceed, so-called poor sensitivity.
As shown in FIG. 1D, the product (r × n) of the light absorption rate r and the photon density n exceeded the lower limit value 7 of the product that can generate interference fringes in the entire region of the recording layer 1. This product (r × n) has a correlation with the density of photons and electrons that contribute to the reaction. The lower limit value 7 may be considered as a constant, and when the product exceeds the lower limit value 7, the density of electrons necessary for generating interference fringes can be ensured. The reason why the lower limit 7 is exceeded in the entire area of the recording layer 1 is that the light absorption rate r increases so as to compensate for the decrease in the photon density n. Then, as shown in FIG. 1 (e), the interference fringes 8 could be generated over the entire area of the recording layer 1.

On the contrary, as shown in FIG. 1B, the optical absorption rate r with respect to the recording light 4 of the recording layer 1 continuously or intermittently increases as the distance from the incident surface 25 increases. In the case of recording, as shown in FIG. 1C, the recording light 4 is incident so that the photon density n of the recording light 4 of the recording layer 1 continuously or intermittently decreases as the distance from the incident surface 25 increases. What is necessary is just to enter from the surface 25. According to the recording method to be incident in this way, as shown in FIG. 1D, the product (r × n) can be kept almost constant over the entire area of the recording layer 1, so that the entire area of the recording layer 1 can be maintained. The product (r × n) can exceed the lower limit of 7. Further, in this recording method, the product (r × n) at an arbitrary distance z in the recording layer 1 is not less than 0.8 times the product (r × n) at the incident surface 25 (when the distance z is zero). It is desirable to enter recording light that can realize a photon density distribution that is 1.2 times or less. As a result, the product (r × n) at an arbitrary distance z in the recording layer 1 can be kept substantially constant, so that the product (r × n) exceeds the lower limit 7 in the entire area of the recording layer 1. Is possible.
Specific examples of the sensitizing dye include cyanine, merocyanine, phthalocyanine, azo, azomethine, indoaniline, xanthene, coumarin, polymethine, diallylethene, fulgidofluorane, anthraquinone, styryl. Known organic dyes such as those of the system are mentioned. In addition to these, a complex dye may be used as the sensitizing dye.

  Examples of the polymerization initiator include a radical generator, a cation generator, and an acid generator.

  Examples of the binder include chlorinated polyethylene, polymethyl methacrylate, copolymers of methyl methacrylate and other (meth) acrylic acid alkyl esters, copolymers of vinyl chloride and acrylonitrile, polyvinyl acetate, polyvinyl alcohol, polyvinyl formal, Examples thereof include polyvinyl butyral, polyvinyl pyrrolidone, ethyl cellulose, acetyl cellulose, and polycarbonate. The refractive index of the binder preferably has a large difference from the refractive index of the polymerized polymer (polymer). However, in a combination in which the difference in refractive index is too large, the compatibility between the binder and the polymerizable monomer is lowered, and as a result, light scattering may increase, so a binder with an appropriate refractive index is required. .

  The recording layer 1 is a recording layer of this type of optical recording medium such as a sensitizer, an optical brightener, an ultraviolet absorber, a heat stabilizer, a chain transfer agent, a plasticizer, and a colorant, if necessary. Those commonly used in the formation of s may be included.

  As a material other than the photopolymer material of the recording layer 1 that is currently being studied, a refractive index modulation dye-type material can be cited. The refractive index modulating dye-type material causes a coloring reaction or a decoloring reaction of the refractive index modulating dye in the bright part of the interference fringe 8, and the refractive index difference or the transmittance difference is compared with the dark part of the interference fringe 8. It causes a difference in physical property values. The refractive index modulating dye-type material has a binder, a sensitizing dye, a refractive index modulating dye, and an acid generator. Similar to the photopolymer, when the interference fringe 8 is generated, the sensitizing dye is excited and electrons are emitted in the bright part of the interference fringe 8. The emitted electrons move to the acid generator, and acid is generated from the acid generator. With this acid, the refractive index modulating dye is decolored by a decoloring reaction, and the refractive index changes. Alternatively, the refractive index modulating dye is colored by a color development reaction, and the refractive index changes. In the case of a color development reaction, the refractive index modulation dye may be particularly referred to as a dye precursor.

  In addition, examples of the material of the recording layer currently under consideration include silver halide, dichromated gelatin, photorefractive material, and photochromic material.

  Although silver halide has high sensitivity and relatively high resolution, it has a problem that it requires wet processing and is complicated, has a large scattering, and is inferior in light resistance. Therefore, there is much room for improvement as a memory application.

  Bichromated gelatin has the characteristics of high diffraction efficiency and low noise, but has low sensitivity and poor recording storage stability, so there is room for improvement in memory applications.

  Photorefractive materials have the feature of being rewritable, but they require a high voltage application during recording and have poor record retention, so there is room for improvement in memory applications.

  Photochromic materials also have the feature of being rewritable, but they have room for improvement as memory applications because of their extremely low sensitivity and poor record retention.

  Therefore, the recording layer 1 material such as silver halide, dichromated gelatin, photorefractive material, photochromic material, photopolymer material, and refractive index modulation dye-type material may be combined. For example, by combining a refractive index-modulating dye-type material that causes a coloring reaction or a decoloring reaction and a photopolymer material that causes a polymerization reaction, it becomes possible to fix the refractive index-modulating dye-type material, Since the refractive index difference can be easily obtained, the suitability of the recording layer 1 as a material can be obtained synergistically. In addition, even when a photorefractive material and a photopolymer material are combined, it is possible to fix the photorefractive material as well, and a refractive index difference can be easily obtained. Aptitude can be acquired synergistically. It has suitability as a material for the recording layer 1.

  Further, the optical recording medium may include light transmissive substrates 2 and 3 on the left and right sides of the recording layer 1 in order to hold and protect the recording layer 1. The thickness of the light transmissive substrates 2 and 3 may be about 0.05 to 1.2 mm. Examples of the material of the light-transmitting substrates 2 and 3 include inorganic substances such as glass, polycarbonate, triacetyl cellulose, cycloolefin polymer, polyethylene terephthalate, polyphenylene sulfide, acrylic resin, methacrylic resin, polystyrene resin, vinyl chloride resin, and epoxy. Resins such as resin, polyester resin, and amorphous polyolefin may be used. Of these, glass, polycarbonate, and triacetyl cellulose are preferable because of their low birefringence. The light transmissive substrates 2 and 3 may be made of the same material or different materials. The surfaces of the light transmissive substrates 2 and 3 may be provided with an antireflection coating, an oxygen permeation prevention coating, a moisture permeation prevention coating, a UV cut coating, or the like as necessary.

  A holographic memory according to an embodiment of the present invention includes a step of first producing two or more types of recording materials having different sensitizing dye concentrations, and a step of applying these recording materials in multiple layers in the order of the concentrations. It can be obtained by the method. Further, instead of the coating process, the recording materials may be poured into the mold of the recording layer 1 in the order of concentration. Since the recording material is only the difference in the concentration of the sensitizing dye, the influence of the interface formed between the recording materials is almost negligible. In both the coating process and the pouring process, the recording material in the vicinity of the interface melts and a gentle concentration distribution is formed, so that the influence of the interface can be ignored.

  According to the holographic memory according to the embodiment, the interference fringes 8 can be generated over the entire thickness direction even when the thickness of the recording layer 1 is increased. The advantage of this effect will be described by comparing this effect with Example 1 and Example 2 of other holographic memory examples.

  As shown in FIG. 2A, the optical recording medium serving as the holographic memory of Example 1 is similar to the holographic memory of the embodiment of FIG. 3. As shown in FIG. 2 (b), the holographic memory of Example 1 and the holographic memory of the embodiment include the curve of the light absorption rate r of the recording layer 11 and the light of the recording layer 1 of FIG. 1 (b). Only the tendency of the curve 6 of the absorption rate r is different. The light absorption rate r of the recording layer 11 of Example 1 is constant regardless of the distance z from the incident surface 25 of the recording layer 11. This constant tendency can be realized by making the concentration of the sensitizing dye constant regardless of the distance z. In most of the holographic memories used conventionally, it is considered that a recording layer in which the concentration of the sensitizing dye is constant regardless of the distance z as in Example 1 is employed.

  As shown in FIG. 2C, the photon density n by the recording light 4 incident on the recording layer 11 decreases as the distance z increases. The amount of decrease is larger than the photon density n in FIG. This is presumably because the light absorption rate r on the incident side is higher in Example 1 than in the embodiment, so that high-density photons present on the incident side are absorbed with a high probability.

  As shown in FIG. 2D, the product (r × n) of the light absorption rate r and the photon density n decreases as the distance z from the incident surface 25 of the recording layer 11 increases. If the thickness of the recording layer 11 is increased so that the distance z is 200 μm or more, there may be a case where the lower limit of the product (r × n) capable of generating interference fringes in the region on the exit surface 26 side is considered. It is done. As a result, as shown in FIG. 2E, in the region where the product (r × n) is below the lower limit value, incomplete interference fringes 9 are generated or no interference fringes are generated. On the other hand, in the vicinity of the incident surface 25 on the incident surface 25 side, the product (r × n) is larger than necessary with respect to the lower limit value 7. Corresponding to these, in the embodiment, as shown in FIG. 1 (d), the product (r × n) in the vicinity of the incident surface 25 on the incident surface 25 side is lowered, and the product on the exit surface 26 side is lowered. It has succeeded in raising (r × n). This success is due to the fact that the light absorptance r is lowered on the incident surface 25 side and increased on the exit surface 26 side, as shown in FIG.

  The holographic memory records information in the thickness direction as interference fringes. Therefore, if the interference fringes are not sufficiently recorded in the thickness direction, the recording capacity is lowered. If the light absorption rate r of Example 1 is constant as shown in FIG. 2B, the photons that reach the position z decrease as the distance z increases, resulting in non-uniform recording. A decrease in capacity occurs. In order to improve this, in the embodiment as shown in FIG. 1B, a gradient is given to the light absorption rate r, the decrease in the photon density n is compensated by the increase in the light absorption rate r, and the product (r × n) is It is set to be constant. As a result, the interference fringes are recorded uniformly and the recording capacity can be ensured.

  Next, the optical recording medium of the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

<Manufacture of optical recording media>
(Example 1, Comparative Example 1)
In Example 1, Sample A of an optical recording medium was manufactured using a photopolymer material. Also in Comparative Example 1, Sample B of an optical recording medium was manufactured using a photopolymer material. First, three types of coating solutions # 1, # 2, and # 3 were prepared in order to produce a recording layer. Each material of the binder, monomer, polymerization inhibitor, sensitizing dye, polymerization initiator, and dichloromethane as a solvent shown in Table 1 was weighed so that the mass ratio shown in Table 1 was obtained. The coating liquids # 1, # 2, and # 3 differ only in the mass ratio of the sensitizing dye. Weighing was performed under a red light.

In Table 1, CAB531-1 represents cellulose acetate butyrate (Eastman Chemical Co., Ltd.), POEA represents 2-phenoxyethyl acrylate (Cas No. 48145-04-6), and MEHQ represents 4 Represents methoxyphenol (Cas No. 150-76-5), DEAW represents cyclopentanone-2,5-bis {[4- (diethylamino) phenyl] methylene} (Cas No. 38394-53-5); MBO represents 2-mercaptobenzoxazole (Cas No. 2382-96-9), and o-Cl-HABI represents 2,2-bis [o-chlorophenyl] -4,4,5,5-tetraphenyl-1 , 1-biimidazole (Cas No. 1707-68-2).
Each material was put into a brown eggplant-shaped flask under a red lamp, and stirred for 3 hours with a stirrer to obtain three types of coating liquids # 1, # 2, and # 3.

  Next, for sample A, coating solution # 1 was applied onto light transmissive substrate 2 using a coater. The applied coating solution # 1 was dried at 40 ° C. for 3 minutes. Further, overcoating was performed by applying the coating solution # 2 on the dried coating solution # 1 using the same coater. The applied coating solution # 2 was dried at 40 ° C. for 3 minutes. Finally, overcoating was performed by applying the coating solution # 3 onto the dried coating solution # 2 using the same coater. The coated sample was dried at 40 ° C. for 3 minutes to complete a recording layer corresponding to the recording layer 1 in FIG. A glass plate serving as the light transmissive substrate 3 was bonded to the recording layer 1 to complete the sample A optical recording medium. In sample A, the concentration of the sensitizing dye increases stepwise from the incident surface 25 side, and the light absorptance r also increases stepwise from the incident surface 25 side as indicated by the solid lines 6a to 6c in FIG. It is thought that it has risen. That is, Sample A of Example 1 corresponds to the optical recording medium of the embodiment.

  For sample B, coating solution # 2 was applied three times on another light-transmitting substrate 2 using a coater. The coated sample was dried at 40 ° C. for 3 minutes to complete a recording layer corresponding to the recording layer 11 in FIG. A glass plate serving as the light transmissive substrate 3 was bonded to the recording layer 11 to complete the optical recording medium of Sample B. In sample B, it is considered that the concentration of the sensitizing dye is constant and the light absorption rate r is also constant as shown in FIG. That is, Sample B of Comparative Example 1 corresponds to the optical recording medium of Example 1.

(Example 2, comparative example 2)
In Example 2, Sample C of an optical recording medium was manufactured using a dye-type material for refractive index modulation. Also in Comparative Example 2, a sample D of an optical recording medium was manufactured using a dye-type material for refractive index modulation. First, three types of coating solutions # 4, # 5, and # 6 were prepared in order to produce a recording layer. Binders shown in Table 2, acid-decolorable dyes, acid generators, sensitizing dyes, and each material of methylene chloride and acetonitrile as solvents were weighed so as to have the mass ratio shown in Table 2. The coating liquids # 4, # 5, and # 6 differ only in the mass ratio of the sensitizing dye. Weighing was performed under a red light.

  In Table 2, PMMA-EA represents methyl methacrylate and ethyl acrylate (Aldrich, average Mw: 39500), and Dye A represents a quaternary ammonium salt represented by the chemical formula (a).

  The acid generator A represents diphenyliodonium hexafluorophosphoric acid (Cas No. 58109-40-3), and the dye B represents a cyanine dye represented by the chemical formula (b).

  Then, each material was put into a brown eggplant-shaped flask under a red lamp, and stirred for 3 hours with a stirrer to obtain three types of coating liquids # 4, # 5, and # 6.

  Next, for sample C, coating solution # 4 was applied onto light transmissive substrate 2 using a coater. The applied coating solution # 4 was dried at 40 ° C. for 3 minutes. Further, overcoating was performed by applying the coating solution # 5 onto the dried coating solution # 4 using the same coater. The applied coating solution # 5 was dried at 40 ° C. for 3 minutes. Finally, overcoating was performed by applying the coating solution # 6 onto the dried coating solution # 5 using the same coater. The coated sample was dried at 40 ° C. for 3 minutes to complete a recording layer corresponding to the recording layer 1 in FIG. Thereafter, drying was performed at 40 ° C. for 48 hours. In sample C, the concentration of the sensitizing dye increases stepwise from the incident surface 25 side, and the light absorption rate r also increases stepwise from the incident surface 25 side as indicated by solid lines 6a to 6c in FIG. It is thought that it has risen. That is, Sample C of Example 2 corresponds to the optical recording medium of the embodiment.

  For sample D, coating solution # 5 was applied three times on another light-transmitting substrate 2 using a coater. The coated sample was dried at 40 ° C. for 48 hours to complete a recording layer corresponding to the recording layer 11 in FIG. 2A, and an optical recording medium of Sample D was completed. In sample D, it is considered that the concentration of the sensitizing dye is constant and the light absorption rate r is also constant as shown in FIG. That is, Sample D of Comparative Example 2 corresponds to the optical recording medium of Example 1.

<Measurement of recording layer thickness>
Then, the thicknesses of the recording layers of Samples A to D were measured. For measuring the thickness of the optical information recording layers 1 and 11, DIGITAL MICROMETER made by Sony was used. The thicknesses of the optical information recording layers 1 and 11 were calculated by subtracting the thicknesses of the light transmissive substrates 2 and 3 from the total thickness of the measured optical recording medium. As shown in Table 3, the thickness of the recording layer of Sample A of Example 1 was 250 μm. The thickness of the recording layer of Sample C of Example 2 was 500 μm. The thickness of the recording layer of Sample B of Comparative Example 1 was 250 μm as with Sample A. The thickness of the recording layer of Sample D in Comparative Example 2 was 500 μm as in Sample C.

<Measurement of diffraction efficiency of optical recording medium and calculation of M #>
Optical information was recorded and reproduced on each of the manufactured optical recording media of Samples A to D. Then, the diffraction efficiency η during recording / reproduction was measured by a diffraction efficiency measuring apparatus M shown in FIG.

  The YAG laser light (wavelength: 532 nm) irradiated from the YAG laser source 31 passes through the objective lens 32, the lens 33, the beam splitter 34, and the mirror 35, thereby being recorded as the recording light L1 on the optical recording medium OM (samples A to D). Equivalent) surface S1. At this time, the incident angle of the recording light L1 with respect to the surface S1 of the optical recording medium OM was set to 15 degrees, and the spot diameter was set to 8 mmφ. Then, the light separated by the beam splitter 34 was combined with the recording light L1 through the mirror 35 as the reference light L3. As a result, optical information was hologram-recorded by forming interference fringes in the recording layer of the optical recording medium OM. In this diffraction efficiency measuring apparatus M, optical information was multiplexed and recorded on the recording layer of the optical recording medium OM by schedule recording. With this schedule recording, the same diffraction efficiency η can be obtained during multiplexing.

For each of such multiplexed optical information, the He-Ne laser beam L2 having a wavelength of 633 nm is optically recorded from the He-Ne laser source 38 via the mirror 39 and the mirror 40 at the time of recording and reproduction. The incident light was incident on the back surface S2 of the medium OM at an incident angle of about 18 degrees Bragg. And the change of the diffraction efficiency (eta) with respect to the exposure amount at this time was observed. In order to calculate the diffraction efficiency η, the light quantity of the diffracted light (reproduced light) of the He—Ne laser was measured by the power meter 41 provided on the surface S1 side of the optical recording medium OM. The diffraction efficiency η is calculated by the formula (1) based on the incident light amount of the He—Ne laser incident on the back surface S2 of the optical recording medium OM (the light amount emitted from the He—Ne laser source 38) and the measured light amount of the diffracted light. Calculated.
η (%) = diffracted light quantity / incident light quantity × 100 (1)

Next, an M number M # serving as an index of the size of the storage capacity was calculated by Equation (2) based on the diffraction efficiency η for recording and reproduction for each multiplexed optical information. In this calculation, an M number M # with the thickness of the recording layer normalized to 100 μm was calculated. For normalization, the measured thickness of the recording layer was used. The M number M # increases as the optical information is multiplexed, and increases as the diffraction efficiency of the individual optical information increases. Therefore, the M number M # is considered to represent the storage capacity relatively. That is, the larger the M number M #, the larger the storage capacity. The reason why the M number M # is standardized by the thickness of the recording layer is to eliminate the influence of the thickness of the recording layer and compare the samples A to D.
M # = Σ (η ^1/2 ) (2)

  Table 3 shows the M numbers M # of samples A to D. The M number M # of the sample A of Example 1 was 4.0. The M number M # of the sample C of Example 2 was 1.6. The M number M # of Sample B of Comparative Example 1 was 3.5. The M number M # of the sample D of Comparative Example 2 was 1.2.

  The M number M # of Example 1 is larger than the M number M # of Comparative Example 1. From this, it can be seen that the recording layer of Example 1 has a larger recording capacity. Since the number of times of multiplexing is the same in Example 1 and Comparative Example 1, it can be seen that the recording layer of Example 1 has a higher diffraction efficiency η than Comparative Example 1. This is considered to be because the uniform interference fringes are generated in the first embodiment deeper in the thickness direction. Since Example 1 corresponds to the optical recording medium of the embodiment, and Comparative Example 1 corresponds to the optical recording medium of Example 1, according to the optical recording medium of the embodiment, the thickness of the recording layer is Even if it is as thick as 250 μm, it is considered that uniform interference fringes can be generated deeper in the thickness direction.

  In addition, the M number M # of Example 2 is larger than the M number M # of Comparative Example 2. This shows that the recording layer of Example 2 has a larger recording capacity. Since the number of multiplexing is the same in Example 2 and Comparative Example 2, it can be seen that the recording layer of Example 2 has a higher diffraction efficiency η than Comparative Example 2. This is presumably because the uniform interference fringes are generated deeper in the thickness direction in Example 2. Example 2 corresponds to the optical recording medium of the embodiment, and Comparative Example 2 corresponds to the optical recording medium of Example 1. Therefore, according to the optical recording medium of the embodiment, the thickness of the recording layer is Even if it is as thick as 500 μm, it is considered that uniform interference fringes can be generated deeper in the thickness direction.

  As described above, according to the embodiment, Example 1, and Example 2, it is possible to provide an optical recording medium capable of recording optical information uniformly in the thickness direction even when the recording layer is thick.

(A) is a schematic diagram of the optical recording medium according to the embodiment, (b) is a graph of the light absorption rate r with respect to the distance z from the incident surface of the recording layer, and (c) is a photon density with respect to the distance z. (d) is a graph of the product (r × n) of the optical absorptance r and the photon density n with respect to the distance z, and (e) is a diagram schematically showing how interference fringes are generated. is there. (A) is a schematic diagram of the optical recording medium of Example 1, (b) is a graph of the light absorptance r with respect to the distance z from the incident surface of the recording layer, and (c) is the photon density n of the distance z. It is a graph, (d) is a graph of the product (r × n) of the light absorptance r and the photon density n with respect to the distance z, and (e) is a diagram schematically showing how interference fringes are generated. It is a schematic diagram of a diffraction efficiency measuring apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Recording layer 2, 3 Light transmissive substrate 4 Recording light 5 Transmitted light 6 Optical absorptance curve 6a Lower optical absorptance curve 6b Middle optical absorptivity curve 6c Upper optical absorptivity curve 7 Product which can produce interference fringes Lower limit of (r × n) 8 Interference fringes 9 Incomplete or non-generated interference fringes 11, 12, 13 Recording layer 14, 15 Reflective layer 16, 17 Condensed recording light 18, 19 Assuming that light absorption is zero Photon density curve 20, 21 Photon density curve 22 Product curve when the recording layer is thin 23 Product curve when the recording layer is thick 25 Entrance surface 26 Exit surface or reflection surface

Claims (4)

An optical recording medium having a recording layer for recording optical information by recording light entering from an incident surface,
The optical recording medium according to claim 1, wherein the light absorption rate of the recording layer with respect to the recording light continuously or intermittently increases as the recording layer moves away from the incident surface.   The optical recording medium according to claim 1, wherein the recording layer has a thickness of 200 μm or more.   The optical recording medium according to claim 1, wherein a transmission hologram is recorded when the recorded optical information is read out.   4. The optical recording medium according to claim 1, wherein a transmittance of the recording layer with respect to the recording light is 20% or more and 70% or less.

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