JPH07114747A - Optical recording medium and its recording method - Google Patents

Abstract

PURPOSE:To enhance multiplicity and to make it possible to perform high- density recording by laminating optical recording layers including the same kinds of materials in such a manner that the interlayer distances thereof are larger than the depth of focus of a light beam spot. CONSTITUTION:The recording layers 10 of the optical recording medium are constituted by laminating the optical recording layers into which three kinds of the optical recording materials A, B, C varying in light absorption regions are respectively incorporated. The optical recording layers are constituted of 13 layers of the optical recording layers as a whole by repetitively laminating the optical recording layers 11 contg. the material A, the optical recording layers 12 contg. the material B and the optical recording layers 13 contg. the material C in this order. Recording is executed by irradiating the optical recording layers 11 with light of a wavelength lambdaA, the optical recording layers 12 with light of a wavelength lambdaB and the optical recording layers 13 with light of a wavelength lambdaC. The optical recording layers having the different light absorption regions are arranged in the upper layer and the lower layer of the one optical recording layer and, therefore, crosstalk are not generated in the optical recording layers of the upper layers and the lower layers.

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 and a recording method thereof, and more particularly to an optical recording medium capable of high density recording and a recording method thereof.

[0002]

2. Description of the Related Art In recent years, there has been a strong demand for the development of a recording technology having a larger capacity due to high-quality image recording and an increase in the amount of data used in a computer, and various new recording technologies have been studied. Among them, in photon mode optical recording, information is recorded by a photochemical reaction, so the degree of freedom of the wavelength and polarization of light is used to take advantage of heat mode optical recording such as magneto-optical recording and phase change recording. It is considered that high-density recording can be performed rather than recording.

As a method for high density recording in photon mode type optical recording, a plurality of photochromic materials having different light absorption regions as disclosed in, for example, Japanese Patent Laid-Open No. 61-203450 are laminated, and these photochromic materials are laminated. A wavelength multiplex recording technique is known in which recording / reproduction is performed by irradiating light with a wavelength corresponding to a material.

FIG. 2 is a diagram showing different light absorption regions of each photochromic material in such wavelength multiplexing recording. As shown in FIG. 2, a photochromic material having different absorption peaks of A, B, C, D, and E is contained in the optical recording layer, and wavelengths λ _A, λ _B, λ _C, corresponding to the respective absorption peaks are contained _.
By irradiating with light of λ _{D and} λ _E , each photochromic material reacts independently, and the absorbance (reflectance) of each photochromic material is changed to perform recording. Further, at the time of reproduction, the change in reflectance is detected using light of each wavelength.

Therefore, in such conventional wavelength multiplex recording, by using n kinds of photochromic materials, n multiplex recording, that is, n times higher density can be achieved. However, the wavelength range that can be used for optical recording is 400
Considering that it is about 900 nm, n kinds of photochromic materials having different absorption bands are required for increasing the density by n times, and further, laser light sources of n wavelengths corresponding to them are required. There was a limit to high density. In such conventional wavelength multiplex recording, practically, the maximum limit was the multiplex recording up to about n = 5.

As another high-density optical recording method, a three-dimensional optical memory technique is known. This three-dimensional optical memory technology is disclosed in, for example, the proceedings of the Kyoto 1992 Symposium on Optical Coalition, page 39. In a three-dimensional memory, a material is used that reacts with the intensity of light in a non-linear manner in some way to cause a change in optical characteristics.

FIG. 4 is a sectional view showing the structure of the recording layer of the optical recording medium of such a three-dimensional memory. Referring to FIG. 4, in the optical recording layer 1, the focus portion 3 of the recording laser 3 is provided.
The recording mark portion 2 is formed in the portion a. In such an optical recording medium, an optical recording material that reacts non-linearly with the light intensity is used, so that the focal portion 3
In non-focus areas other than a, the light intensity is low due to defocusing, the reaction does not proceed and the recording mark area is not formed.

However, as shown in FIG. 3, since the light intensity is relatively high even in the non-focus portion II near the area I of the focus portion 3a of the recording laser 3, the light intensity is not as great as in the area I of the focus portion. With that, the reaction proceeds. Therefore, in order to avoid crosstalk in the depth direction of the three-dimensional memory, it is necessary to make the interlayer pitch L _P of the layer forming the recording mark portion sufficiently larger than the focal depth of the recording light beam spot. Since the value of the depth of focus is usually several μm, the interlayer pitch L _P is preferably 10 μm or more. In the technique disclosed in the above-mentioned lecture proceedings, the interlayer pitch L _P
Is 30 μm. Therefore, such a sufficiently large it becomes necessary to take inter-pitch L _P of the recording layer in the three-dimensional memory, it is impossible to increase the recording density in the depth direction. For example, even if the total thickness of the optical recording layer is about 100 μm, if the interlayer pitch L _P is 30 μm, there are four recording mark layer layers that can be formed in the depth direction, and the recording density is higher than that of a two-dimensional optical memory. It can only be quadrupled.

An object of the present invention is to provide an optical recording medium and a recording method therefor, which solves the above-mentioned conventional problems and enables higher density recording than ever.

[0010]

The optical recording medium of the present invention comprises:
An optical recording layer that is composed of a plurality of optical recording layers that have different light absorption regions and that responds to recording light irradiation in a non-linear manner, and that includes more than one type of material. It is characterized in that the recording layers are laminated such that the interlayer distance between them is larger than the focal depth of the recording light beam spot.

The recording method of the present invention is a method of recording information on the above-mentioned optical recording medium, and recording beam light having a wavelength corresponding to the light absorption region of each material is collected on the optical recording layer of the above-mentioned optical recording medium. It is characterized by irradiating with light and recording information with a multiplicity according to the number of laminated optical recording layers.

Therefore, in the present invention, when n kinds of materials are used as the optical recording material, the number of laminated optical recording layers is n + 1 layers or more, and in the recording method of the present invention, information is recorded at a multiplicity of n + 1 or more. Will be recorded.

[0013]

In the optical recording medium of the present invention, the optical recording layer is formed by laminating more than the number of kinds of the materials, the optical recording layers having different light absorption regions and containing a plurality of materials that cause a photo-nonlinear reaction by irradiation of the recording light. In addition, since the optical recording layers containing the same kind of material are laminated so that the interlayer distance between them is larger than the focal depth of the recording light beam spot, the crosstalk between the same recording materials is sufficiently low. As a result, it is possible to perform multiplex recording that exceeds the number of types of materials, and it is possible to achieve higher density recording than in the past.

[0014]

1 is a sectional view showing the structure of an optical recording medium according to an embodiment of the present invention. Referring to FIG. 1, a recording layer 10 of this optical recording medium is formed by laminating optical recording layers containing three kinds of optical recording materials A, B and C having different light absorption regions. . Optical recording layer 11 containing material A, optical recording layer 12 containing material B, and material C
The optical recording layer 13 containing is repeatedly laminated in this order,
It is composed of 13 optical recording layers as a whole. The optical recording layer 11 containing the material A is irradiated with light of wavelength λ _A , the optical recording layer 12 containing the material B is irradiated with light of wavelength λ _B , and the optical recording layer 13 containing the material C is irradiated. Recording is performed by irradiating light of wavelength λ _C. In this case, as shown in FIG. 1, since the optical recording layers having different light absorption regions are arranged in the upper layer and the lower layer of one optical recording layer, the recording of one optical recording layer results in the upper layer. Also, crosstalk does not occur in the lower optical recording layer.

Further, the interlayer pitch L _P of the same kind of optical recording layers
Is configured to be larger than the depth of focus of the recording light beam spot. Recording material of the optical recording layer used in the present invention since it is a material that reacts optical nonlinear manner with respect to the recording beam exposure, larger than the focal depth of the recording light beam spot of the interlayer distance L _P of the same kind of the optical recording layer As a result, the recording light for one optical recording layer does not cause a reaction with another optical recording layer of the same kind. The interlayer distance L _P of such optical recording layers of the same kind is preferably set to be a distance three times or more the depth of focus of the recording light beam spot.

In the embodiment shown in FIG. 1, three kinds of optical recording materials having different light absorption regions are used and an optical recording medium having a multiplicity of 13 is used. The recording density is remarkably increased as compared with the conventional wavelength multiplexing recording. be able to. Further, according to the present invention, if the interlayer distance of the same kind of optical recording layer is made larger than the focal depth of the recording beam spot, the interlayer distance of different kinds of optical recording layers does not need to be larger than the focal depth. Three
It is possible to realize higher density recording than the dimensional memory.

The optical recording medium of the present invention comprises A / B / C / A / in which three kinds of optical recording layers are laminated as in the embodiment shown in FIG.
The structure is not limited to B / C / ..., but various modifications are possible. For example, if the interlayer distance L _P of the same kind of the optical recording layer may be relatively short, different materials A, a configuration such as A / B / A / B / ...... / A / B with B Good. In addition, the interlayer distance L of the same kind of optical recording layer
If it is necessary to lengthen _P , four or more kinds of materials may be used to form a structure of A / B / C / D / A / B / C / D .... Further, the adjustment of the interlayer distance L _P of the same type of optical recording layer may be performed by changing the thickness of each optical recording layer.

In the present invention, it is not always necessary to make the order of laminating the optical recording layers the same, and the order of laminating the optical recording layers may be changed depending on the ease of occurrence of crosstalk. It is possible, for example, if the materials A and C easily cause crosstalk between recording layers of the same material and the material B hardly causes crosstalk, A / B / C / B / A /
B / C / B / A ... By increasing the interlayer distance between the optical recording layer of the material A and the optical recording layer of the material C and shortening the interlayer distance of the optical recording layer of the material B. Good.

The recording light used in the present invention will be described below.
On the optical recording material that responds to the irradiation in an optical nonlinear manner
explain. The optical nonlinearity that reacts in the photon mode is
There are two types. First type of light
The linearity is a non-linearity due to the two-photon process as shown in FIG.
It is sex. In such a two-photon process, the ground state
The molecule A in G is the photon hν₁Absorbs, inside
State E₁Is excited by (process). Next, this intermediate encouragement
Upright E₁Another photon hν during the lifetime of₂
Is excited, the excited state E is further excited.₂Transition to
Do (process). This excited state E₂Changes the molecular structure
Is converted into a molecular structure of A ′. E ₁To the excited state of
If the molecule does not absorb photons within its lifetime,
Return to the ground state G again (process). Therefore, photon density
At low degrees, ie low light intensity, most molecules
The reaction does not proceed through the process of →. Photo again
If the light density is high, that is, if the light intensity is high, the →
The number increases and the reaction proceeds. This intermediate state E₁Is the actual situation
And therefore the photon hν₁Normal weak corresponding to
Molecule A exhibits absorption as measured by bright light. Intermediate excited state
E₁May be in an electronically excited state or have a molecular structure
It may be in a partially changed state. The photon hν₁And hν
₂May have different wavelengths or may have the same wavelength.

An example of the optical recording material exhibiting the above-mentioned optical non-linearity is a chromene derivative represented by the following formula.

[0021]

[Chemical 1]

In the molecule of the above chromene derivative, the state in which the three-membered ring of (2) is opened exists between (1) and (2) as a thermally unstable intermediate state, and is exposed to visible light. Photoisomerization proceeds from (1) to (2) in a two-photon manner. By attaching an appropriate modifying group to this molecule, it is possible to change the absorption spectrum and thus obtain a material that reacts in an optically non-linear manner with different light absorption regions.

Next, the second type of optical nonlinearity will be described. FIG. 6 shows the energy level diagram for this second type of non-linearity. As shown in FIG. 6, the intermediate state E ₁ does not exist as a real state but exists only as a virtual state. Therefore, unlike the first type, this molecule A does not show absorption for one photon and absorbs only when two photons come at the same time, and transitions from the ground state G to the excited state E ₂ . Do (process,). In this excited state E ₂ , the molecular structure is changed to form a molecule A ′. The mechanism of manifestation of this type of optical nonlinearity is similar to the mechanism of optical harmonic generation. Such a second
As for the first type of non-linearity, as in the first type of non-linearity, the reaction does not proceed with a weak light intensity, but only with a strong light intensity. In the case of this second type, the light absorption region of the material is not in the wavelength band of light used apparently, but in a wavelength band shorter than that.

As a material exhibiting the above-mentioned second type of optical nonlinearity, there is a spiropyran-based molecule represented by the following formula.

[0025]

[Chemical 2]

In the state of (1), the molecule has a visible region (4
Since it has no absorption in (00 to 800 nm), for example, YA
The reaction does not occur even if the laser having the wavelength of 532 nm, which is the SHG wave of the G laser, is irradiated with weak power. However, when the same laser is irradiated with a strong power, a photochromic reaction occurs in an optical nonlinear manner, and the state changes to (2).

Even in this molecule, the absorption spectrum can be changed by appropriately modifying the molecule, and therefore, a material which reacts in an optical non-linear manner with different light absorption regions can be obtained. Incidentally, it is known that a certain type of spiropyran-based compound causes a reaction to progress non-linearly by the action of heat generated during light irradiation, but this is not an optical non-linearity and is outside the scope of the present invention. .

[0028]

In the optical recording medium of the present invention, a plurality of optical recording layers, which have different light absorption regions and respond to the irradiation of recording light in an optical non-linear manner, are laminated. Therefore, the multiplicity can be remarkably increased as compared with the conventional optical recording medium, and high density recording can be performed. In addition, since the optical recording layers of the same kind are laminated so that the interlayer distance is larger than the depth of focus of the recording light beam spot, high density recording is possible without causing crosstalk between the optical recording layers of the same type. It can be used as a high capacity optical recording medium.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an optical recording medium of an embodiment according to the present invention.

FIG. 2 is an absorption spectrum diagram for explaining conventional wavelength multiplexing recording.

FIG. 3 is a schematic diagram for explaining an influence of a recording light beam spot in the vicinity of a focal portion.

FIG. 4 is a sectional view showing an optical recording medium of a conventional three-dimensional memory.

FIG. 5 is an energy level diagram for explaining the first type of optical nonlinearity.

FIG. 6 is an energy level diagram for explaining a second type of optical nonlinearity.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... Optical recording medium 11 ... Optical recording layer of material A 12 ... Optical recording layer of material B 13 ... Optical recording layer of material C _LP ... Interlayer distance between optical recording layers of the same kind

Claims (2)

[Claims] 1. An optical recording layer comprising a plurality of materials having different light absorption regions and reacting to the irradiation of recording light in an optical non-linear manner is formed by laminating in excess of the number of kinds of the materials. An optical recording medium comprising: the optical recording layer containing the material described above, wherein the interlayer distance between them is larger than the depth of focus of the recording light beam spot. 2. The optical recording layer of the optical recording medium according to claim 1, wherein recording beam light having a wavelength corresponding to a light absorption region of each material is condensed and irradiated, and the number of stacked optical recording layers is increased. A recording method characterized in that information is recorded at a multiplicity according to.