Patent Application
20070243472
Various beams, namely, recording signal beam (incident recording beam) L1, recording reference beam L2, reading reference beam L3, etc. (all described later, in detail) are irradiate from the incident side (i.e. upper side in the figure) toward the transparent substrate 8 and successively enter the interior of the holographic recording medium 1 through the transparent substrate 8.
The substrate 2 is made of glass, plastic material, such as polycarbonate, or metallic material, such as Ni-alloy, Ag-alloy and Al-alloy. The substrate 2 may be provided in the form of a card or a disc. In case of a disc-shaped substrate 2, it is desirable that format signals for controlling a recording of the information into the holographic recording medium 1 or a readout of the information from the holographic recording medium 1 are recorded in the form of concavities and convexities (irregularities). In arrangement, the substrate 2 is positioned on one side of the two-state variable layer 5 in the thickness direction. This side of the two-state variable layer 5 is opposite to the incident side of the holographic recording layer 7.
The reflecting layer 3 is formed by appropriate material, for example, Al, Ag, Al or Ag based alloy film, dielectric mirror having dielectrics laminated in multilayer, etc.
As materials for the first dielectric layer 4 and the second dielectric layer 6, there are preferably recommended inorganic transparent materials, for instance, Zn—S, Si—N, Si—O, Al—O, In—O, Zn—O, Sn—O, Ga—N, B—N, Al—N and their mixtures (e.g. ZnS−SiO2, In2O3—SnO2, In2O3—ZnO). Alternatively, organic transparent resin (e.g. polyolefin plastic, petroleum resin) is available for the first dielectric layer 4 and the second dielectric layer 6.
The number of dielectric layers is appropriately established from the viewpoint of improving optical characteristics, read-write characteristics, environment-resistance capability and so on. Corresponding to desired characteristics, different materials may be selected from the above-mentioned group of materials, forming respective layers in combination.
The two-state variable layer 5 is made from a special material. This material can play either a first state with a lower reflectively (e.g. a first reflectivity) or a second state with a higher reflectivity (e.g. second reflectivity) state in response to a light having a predetermined wavelength. By accepting a light, the same material can change its condition from the first state to the second state, reversibly or irreversibly. In other words, the two-state variable layer 5 may be either a reflectivity-variable layer changeable between the first state and the second state reversibly or another reflectivity-variable layer changeable from the first state with the first reflectivity to the second state with the second reflectivity irreversibly.
For the two-state variable layer 5, the above material is selected from various materials: phase change material; inorganic material; and organic dye material. In common with these materials, each reflectivity can be altered dependently of changes in terms of amount of irradiated beam and temperature. From these materials, it is desirable to select an appropriate material whose reflective property is characterized by a lower reflectivity of 4˜5% and a higher reflectivity exceeding at least 10% in response to incident beams. It should be noted that the reflectivity of the phase change material is changeable reversibly by heat, while the reflectivity of the inorganic material and that of the organic dye are together changeable irreversibly by heat.
When adopting the phase change material for the two-state variable layer 5, it is preferable that the same material has two conditions of an amorphous state and a crystallized state and that its reflectivity in the crystallized state is higher than that in the amorphous state.
Recommended for the phase change material are chalcogen-compounds (e.g. Sb—Te, Ge—Sb—Te, Ag—In—Sb—Te, etc.), which contains Se, Te, or the like, materials each having any of the above chalcogen-containing compounds as a base material and other additive elements, Te-containing oxide (e.g. Te—O—Pd) and so on.
Further, depending on its composition, the phase change material can be classified to an eutectic type material or a compound type material in terms of its structure in the crystallized state. Either eutectic type material or compound type material is available for the two-state variable layer 5.
When adopting an inorganic material having its reflectivity changeable irreversibly for the two-state variable layer 5, there are available oxide (e.g. Ag—O), Si—Cu alloy, Ge—Bi—N alloy and so on.
In adopting the oxide, such as Ag—O, the two-state variable layer 5 can be provided with a high reflectivity since it is physically deformed by irradiation of light. In adopting Si—Cu alloy, the layer 5 can be brought into a state of low reflectivity with a layer's structure where a Si-layer is laminated on a Cu-alloy layer. While, the layer 5 can be provided with a high reflectivity since the Si-layer and the Cu-alloy layer are alloyed to Si—Cu alloy by irradiation of light. In adopting Ge—Bi—N alloy, the layer 5 can be brought into a state of low reflectivity with a layer's structure having Bi—N and Ge—N in their mixed state. While, the layer 5 can be provided with a high reflectivity when N (nitrogen) is dissolved by irradiation of light, so that fine pores of nitrogen gas are dispersed.
When adopting an organic dye material having its reflectivity changeable irreversibly for the two-state variable layer 5, there are available cyanine dye, phthalocyanine dye, azo dye, etc. each of which has a reflectivity changed by irradiation of light.
In case of the two-state variable layer 5 made from the phase change material, preferably, respective materials and film thicknesses of the first dielectric layer 4, the two-state variable layer 5 and the second dielectric layer 6 are determined so that a difference in reflectivity between the amorphous state of the phase change material and the crystallized state of the same material gets larger. In other words, their materials and film thickness are selected in a manner that a difference between a layer's reflectivity in the amorphous state and another layer's reflectivity in the crystallized state increases due to an interaction of different refractive indexes “n” and different extinction coefficients “k” of the two-state variable layer 5 (in the amorphous state and the crystallized state) with respective refractive indexes “n” and respective extinction coefficients “k” of materials for the dielectric layers 4, 6.
If the two-state variable layer 5 is made from the organic dye material, the structure of the medium 1 may be modified so as to eliminate the first dielectric layer 4. In such a case, respective materials and film thicknesses of the two-state variable layer 5 and the second dielectric layer 6 have only to be determined so that a difference in reflectivity between a high-reflective state of the two-state variable layer 5 and a low-reflective state of the layer 5 grows larger.
The holographic recording layer 7 is produced by use of a material whose optical constants (e.g. refractive index, reflectivity, absorptivity) are modulated in between a layer's part where a recording signal beam and a recording reference beam (both described later) weaken each other and another layer's part where the recording signal beam and the recording reference beam intensify each other. For the holographic recording layer 7, there are available a variety of materials, for instance, photopolymer, chalcogenide compound, dye, dye-additive thermoplastic, photorefractive crystal and so on.
Note that the holographic recording layer 7 is classified broadly into either “rewritable” type layer or so-called “write-once” type layer allowing of writing once.
In adopting the rewritable holographic recording layer 7, it is desirable that a reflectivity variable layer whose reflectivity is changeable between a high value and a low value reversibly is employed for the two-state variable layer 5 in view of realizing a rewritable holographic recording medium.
On the other hand, in adopting the write-one holographic recording layer 7, either a reflectivity variable layer whose reflectivity is changeable between a high value and a low value reversibly or another reflectivity variable layer whose reflectivity is changeable from a low value to a high value irreversibly is usable for two-state variable layer 5.
Provided that the holographic recording layer 7 is formed with a thickness and a strength enough to double as the function of a substrate, for example, any of photopolymer, dye-additive thermoplastic and photorefractive crystal, the holographic recording medium 1 may be modified so as to eliminate the above substrate 2. Then, the above-mentioned layers (i.e. the dielectric layers, the two-state variable layer 5, the reflecting layer, etc.) have only to be appropriately laminated on the holographic recording layer 7 depending on required specifications.
We now describe a method of recording information into the holographic recording medium 1 constructed above.
The interference fringes 71 are produced in the holographic recording layer 7 since the recording signal beam L1 and the recording reference beam L2 irradiated above interfere with each other in the same layer 7, so that the information is recorded in the holographic recording medium 1. In this embodiment, a laser beam having a wavelength of 405 nm, 514.5 nm or 532 nm is used for the recording signal beam L1 and the recording reference beam L2.
When recording the information in the holographic recording layer 7, the two-state variable layer 5 is established to have a low reflectivity for the recording signal beam L1 and the recording reference beam L2. This establishment of the low reflectivity may be accomplished at that stage of shipment of the holographic recording medium 1 or accomplished before recording the information.
The establishment of the low reflectivity comes from a purpose of preventing the recording signal beam L1 and the recording reference beam L2 reflected on the two-state variable layer 5 from entering into the holographic recording layer 7 again and subsequently forming unnecessary interference fringes 71 in the layer 7 and another purpose of lowering unnecessary exposure of the holographic recording layer 7 due to reflected beams. With the establishment, it is possible to prevent a reduction of the number of possibilities of repeatedly recording (multiplexing) various information in an identical place in the holographic recording layer 7 (recording multiplicity).
Now, we describe a reflectivity setting process (i.e. reflectivity setting step) of changing (transition of) the two-state variable layer 5 from the first state of a low reflectivity (i.e. the first reflectivity) for the recording signal beam L1 and the recording reference beam L2 to the second state of a high reflectivity (i.e. the second reflectivity) for a later-mentioned reading reference beam (or reading beam) L4. In reading the information from the holographic recording layer 7, the readout operation could be realized effectively if the reflectivity of the two-state variable layer 5 has been established highly (i.e. the second reflectivity) for the reading reference beam LA. If adopting a phase change material for the two-state variable layer 5, its amorphous state corresponds to the first state with the first reflectivity (low reflectivity), while a crystallized state of the phase change material corresponds to the second state with the second reflectivity (high reflectivity).
In order to change the reflectivity of the two-state variable layer 5 from a lower value (e.g. the first reflectivity) to a higher value (the second reflectivity), a laser beam (i.e. setting beam or third beam) L3 suitable to the material of the two-state variable layer 5 is irradiated from the side of the transparent substrate 8 to a desired area in the layer 5 after completing to record the information in the holographic recording layer 7. The setting beam 13 is generated by the beam generator 33 (forming a first beam generator at reading). In common with the above-mentioned materials for the two-state variable layer 5, each reflectivity changes since the layer 5 is heated to produce an increase in temperature. In case of adopting the phase change material for the two-state variable layer 5, the reflectivity is changed by irradiating the setting beam L3 to the layer 5 so that its temperature becomes more than a crystallization temperature and subsequently cooling down the layer 5. Even when changing the reflectivity of the two-state variable layer 5 from a high one to a low one, the reflectivity may be altered by use of the setting beam (i.e. the third beam) 13 generated the beam generator 33 (forming a first beam generator at reading).
The irradiation of the setting beam L3 may be carried out by combining the following conditions in accordance with an adopted material appropriately:
(1) Wavelength: 300 nm ˜1500 nm, Beam power: 200 mW˜2000 mW
(2) Spot-diameter of laser beam L3: 0.3 μm˜100 μm
(3) Relative speed (scanning rate) against the holographic recording medium 1: 3 m/sec˜50 m/sec
(4) Track feeding pitch: 3 μm˜50 μm
If provided a holographic recording medium 1 in the form of a disc, it is mounted on a mount T and successively, a setting beam 13 is irradiated while rotating the medium 1 so as to satisfy the above condition (3), as shown in
Next, we describe a reading method of reading the information from the holographic recording medium 1.
As shown in
In this embodiment, the reading reference beam LA has the same wavelength as the recording signal beam L1 and the recording reference beam L2. As for beam intensity (laser power), each power of the recording signal beam L1 and the recording reference beam L2 may be larger than or equal to the power of the reading reference beam LA.
As shown in the figure, the reading reference beam L4 is irradiated toward the holographic recording layer 7 in a direction to make an incident angle −θR with the normal line, which is symmetrical to the above incident angle θR of the recording reference beam L2. In this case, the reading reference beam LA reflected by the two-state variable layer 5 becomes a phase conjugate beam L6. The reading reference beam L4 (the phase conjugate beam L6) is diffracted by the holographic recording layer 7, so that the reading beam L5 is emitted in a direction to make an angle θS with the normal line Ln.
If allowing the reading reference beam LA to enter the holographic recording layer 7 at an angle to produce the phase conjugate beam L6, then the reading beam L5 is emitted at the same angle θS as the incident angle of the recording signal beam L1. Therefore, it is possible to share an optical system at both recording and reading, allowing a miniaturization of a recording/reading apparatus. Additionally, even if a record-optical system, e.g. a condenser lens has an aberration, it is almost cancelled at a reading, allowing a provision of reading images with reduced distortion and noise in the reading having an improved SNR.
In the recording method of
In
For appropriate recording/reading operations, it is required that the two-state variable layer 5 has a reflectivity less than 5% and a modulation rate more than 50% at recording.
Assume, Ra and Rc represent a reflectivity (at recording) of the two-state variable layer 5 in the amorphous state and a reflectivity (at reading) of the two-state variable layer 5 in the crystallized state, respectively. Then, a modulation rate R can be obtained by a calculation of (Rc−Ra)/Rc.
Regarding the holographic recording medium 1 of this embodiment, we examined its reflectivity Ra before the two-state variable layer 5 is crystallized (i.e. amorphous state), reflectivity Rc and modulation rate R after crystallization with parameters of respective thicknesses of the first dielectric layer 4, the two-state variable layer 5 and the second dielectric layer 6.
The holographic recording medium 1 was made as follows:
Note, the first dielectric layer 4 and the second dielectric layer 6 are characterized by 2.1 in refractive index n and 0 in extinction coefficient k, commonly. The two-state variable layer 5 in a state of low reflectivity (amorphous state) has 3.91 in the refractive index n and 2.53 in the extinction coefficient k, while the same layer 5 in a state of high reflectivity (crystallized state) has 2.68 in the refractive index n and 5.00 in the extinction coefficient k. The reflecting layer 3 has 0.07 in the refractive index n and 4.1 in the extinction coefficient k.
From the figure, it will be understood that if the laser beam has a wavelength larger than 360 nm, the modulation rate R (after crystallization) of the two-state variable layer 5 exceeds 50% and a difference in reflectivity between the amorphous state and the crystallized grows larger advantageously. Additionally, it is preferable that the two-state variable layer 5 has a modulation rate R (after crystallization) more than 80% for the laser beam having a wavelength within the range of 480 nm ˜800 nm and a difference in reflectivity between the amorphous state and the crystallized grows larger.
Further, we examined both reflectivity and modulation rate of the two-state variable layer 5 while changing a thickness of the first dielectric layer 4 within the range of 1 nm ˜30 nm, a thickness of the two-state variable layer 5 within the range of 1 nm ˜50 nm and a thickness of the first dielectric layer 6 within the range of 10 nm ˜200 nm. If the first dielectric layer 4 is formed with a thickness of 3 nm ˜30 nm, the two-state variable layer 5 with a thickness of 5 nm ˜40 nm and the second dielectric layer 6 is formed with a thickness of 20 nm ˜200 nm, then the reflectivity at a wavelength of 480 nm ˜800 nm exhibits a tendency similar to
For instance, if adopting a laser beam having a wavelength of 532 nm, the modulation rate exceeding 50% is accomplished on condition of the first dielectric layer 4 with a film thickness of 3 nm ˜30 nm, the two-state variable layer 5 with a film thickness of 5 nm ˜40 nm and the second dielectric layer 6 with a film thickness of 20 nm ˜200 nm.
Next, a multiplexing was carried out to the holographic recording layer 7 in the holographic recording medium 1 produced in the above way. In the multiplexing, the incident angle θS of the recording signal beam L1 with the normal line Ln is set to 0 degree (vertical incident beam) while varying the incident angle θR of the recording reference beam L2 (parallel beam) with the normal line Ln. Subsequently, we examined respective powers of the reading beam L5 emitted from the holographic recording layer 7. The reading beam L5 is emitted in a vertical direction. The change of the two-state variable layer 5 from the amorphous state to the crystallized state was accomplished by irradiating the laser beam L3 of 500 mW˜900 mW and 810 nm in wavelength under conditions of the beam spot diameter of 2 μm˜5 μm, the laser scanning rate of 5 m/sec and the track feeding pitch of 6 μm˜20 μm.
For comparison, we made a comparative holographic recording medium under the same conditions except for the use of Al-layer in place of the two-state variable layer 5 and further examined the power of a reading beam after performing the similar recording. The comparative holographic recording medium is provided with a reflecting layer 3 of 200 nm in thickness, an Al-layer of 200 nm in thickness, no first dielectric layer 4 and no second dielectric layer 6. The recording into the holographic recording layer 7 was performed seventeen times (recording multiplicity: 17). In the embodiment, the laser beam having a wavelength of 532 nm was employed. The recording of information was carried out by irradiating a recording signal beam L1 powered with 6 mW/cm2 and a recording reference beam L2 powered with 6 mW/cm2 for 2 seconds.
The examination results are shown in
CNR=10 log10(C/N) (1)
where “C” represents a maximum power of a readout beam (diffracted beam) in each recording multiplicity shown in
As shown in
It is found that the layered structure adopting the two-state variable layer 5 can produce appropriate readout beams at a large multiplexing number, while the comparative structure adopting Al-layer in place of the two-state variable layer 5 produces readout beams with noises at a large multiplexing number.
The potential reasons for the difference between the holographic recording medium 1 and the comparative medium are as follows. That is, as the reflectivity of two-state variable layer 5 is lowered for a predetermined wavelength at the stage of recording the information into the holographic recording layer 7, the formation of unnecessary interference fringes 71 due to the re-entry of the reflected recording beams L1 and L2 into the holographic recording layer 7 is suppressed in the holographic recording medium 1, so that the beam reflected by the interference fringes 71 has little effect on the reading operation. On the contrary, the comparative medium is provided with Al-layer of a high reflectivity. Thus, the recording beams L1 and L2 are easily reflected by the Al-layer and subsequently enter the holographic recording layer 7 again, producing a large number of interference fringes 71 unnecessary at reading. In this way, the beam reflected by the interference fringes 71 has a great effect on the reading operation.
From above, it will be understood that even if the integrated irradiation value of the recording beams L1 and L2 gets more than double of an integrated irradiation value in the comparative medium adopting Al-layer, the holographic recording medium 1 adopting the two-state variable layer 5 allows the information to be reproduced favorably. Thus, it should be said that the recording multiplicity in adopting the two-state variable layer 5 is substantially doubled of the recording multiplicity in adopting Al-layer. In the holographic recording medium adopting the two-state variable layer 5, the readout beam can be produced with sufficient power. The reason is that the exposure of the holographic recording layer 7 (by reflected beams) can be held down since the reflectivity of the two-state variable layer 5 is lowered to light at recording.
By irradiating the recording signal beam L1 to the holographic recording layer 7 at the incident angle θS of 0 degree (vertical incidence) with the normal line Ln and also irradiating the recording reference beam L2 to the holographic recording layer 7 at a variable incident angle θR with the normal line Ln, we recorded the recording signals of 22.5 kbit/page on 2-4 modulation in 7-times multiplexing. Thereafter, we reproduced the information by irradiating the reading reference beam L4 at an incident angle −θR with the normal line Ln (corres. a phase conjugate reference beam of
In succession, we sorted the power of signals at respective positions corresponding to respective recording bits in an image reproduced by the reading beam L5 into signal levels in 256-gradation. Further, we drafted respective histograms by further sorting each recording bit into either “0” or “1”. Assuming that μ1 and σ02 represent an average of the histogram “0” and its deviation, respectively, and μ1 and σ12 represent an average of the histogram “1” and its deviation, respectively, then the SNR of the readout beam is represented by
SNR=(μ1−μ0)/(σ12+σ02)1/2 (2)
The expression (2) implies that the further the distributions of “0” and “1” are separated from each other or the smaller their dispersions of the distributions get, the more a noise of the readout beam is lowered.
According to this embodiment, it is established that the incident angle θS of the recording reference beam L1 with the normal line Ln becomes equal to 0 degree (vertical incidence). Thus, since the incident angle of the recording signal beam L1 coincides with the exit angle of the readout beam L5, the recording/readout of information can be accomplished with a simplified optical system. Furthermore, owing to the adoption of a phase conjugate beam at reading, it is possible to provide a readout image with a reduced image distortion.
Even if adopting inorganic materials or organic dye materials for the two-state variable layer 5, preferable recording/readout operations could be accomplished similarly to the case of the holographic recording medium 1 having the two-state variable layer 5.
In the holographic recording medium 1 of this embodiment, since the reflectivity of the two-state variable layer 5 against the light having a predetermined wavelength is lowered at recording the information in the holographic recording layer 7, it is possible to prevent a formation of unnecessary diffraction grating due to the formation of unnecessary interference fringes 71 by lights reflected by the two-state variable layer 5 and entering the holographic recording layer 7 again, whereby the recording multiplicity can be increased. Further, since the holographic recording medium 1 of this embodiment can produce the interference fringes 71 as the information derived from only interference between the recording signal beam L1 and the recording reference beam L2 both entering the holographic recording layer 7, allowing a performance of favorable recording.
Again, as the above-mentioned interference fringes 71 are recorded in the holographic recording layer 7, it is possible to attain a favorable SNR at reading the holographic recording layer 7 with the reading reference beam L4.
A holographic recording medium 9 in accordance with the second embodiment of the present invention will be described with reference to
In this embodiment, the holographic recording medium 9 is provided with a disc-shaped substrate 21. The holographic recording medium 9 contains format signals for controlling recording or reading of information, which are recorded in the form of irregularities. The format signals comprise tracking signals, address signals, recording/reading control signals and so on. The holographic recording medium 9 differs from the above-mentioned holographic recording medium 1 of the first embodiment in that a smoothing layer 10 and a reflecting layer 11 are interposed between the reflecting layer 3 and the first dielectric layer 4, in sequence from one side of the layer 3. The other constitution is identical to that of the first embodiment.
A servo beam L7 for reading the format signals is generated by a servo-beam generator 37. The servo beam L7 is irradiated to the holographic recording medium 9 from one side facing the transparent substrate 8. Similarly to the holographic recording medium 1 mentioned before, other beams for recording and reading the information are also irradiated from the medium's side facing the transparent substrate 8.
For the smoothing layer 10, there is available a material having fluidity in its early stage, curing by heat, light or oxidization, and optical characteristics allowing a transmission of the servo beam L7, for example, epoxy resin.
The reflecting layer 11 is formed by material that is characterized by allowing a transmission of the servo beam L7 and reflecting the recording signal beam L1, the recording reference beam L2 and the reading reference beam LA all transmitted through the holographic recording layer 7. For the reflecting layer 11, there are available, for instance, amorphous carbon, DLC (diamond-like carbon), these carbon materials containing either H or N, Si, SiHx, SiNx and so on.
In the modification, the reflecting layer 11 may be formed by a dichroic mirror layer.
Then, the dichroic mirror layer comprises a lamination of one layer of high refractive-index material (e.g. Nb2O5, Ta2O5, ZnS—SiO2, Si3N4, TiO2) and another layer of low refractive-index material (e.g. MgF2, SiO2).
Note that the reflecting layer 11 having a “dichroic mirror” function would offer an advantage in the possibility of separating the servo beam L7 and the recording/reading beams for the holographic recording layer 7 from each other.
Owing to the interposition of the smoothing layer 10 and the reflecting layer 11 between the reflecting layer 3 and the first dielectric layer 4, it becomes easy to read out the format signals. The lamination of the holographic recording medium 9 is convenient for a medium in the form of a disc.
The recording/reading methods of the holographic recording medium 9 are similar to those of the holographic recording medium 1 in the first embodiment and therefore, their explanations are eliminated.
According to the second embodiment, the holographic recording medium 9 is capable of accomplishing appropriate recording and reading as well as the holographic recording medium 1 of the first embodiment.
In common with the first and second embodiments mentioned above, there is provided, between the information recording step (i.e. formation of the interference fringes 71 in the holographic recording layer 7) and the information reading step (i.e. reproducing information from the layer 7), a reflectivity setting process for changing the two-state variable layer 5 from its low-reflectivity state to the high-reflectivity state. In the reflectivity setting process, the setting beam L3 for bringing the two-state variable layer 5 into the high-reflectivity state is irradiated to establish a reflectivity of the layer 5. In the previous embodiments, the phase change material is employed as the two-state variable layer 5. In the reflectivity setting process, it is executed to change the layer 5 from the amorphous state to the crystallized state.
On the contrary, according to the third embodiment, the recording signal beam L1 or the recording reference beam L2 is irradiated so as to reach the two-state variable layer 5 as shown in
In various materials suitable for the two-state variable layer 5, a phase change material containing chalcogen element is crystallized in a cooling process after heating up the layer 5. Thus, if adopting the phase change material containing chalcogen element, the recording into the holographic recording layer 7 is completed in advance of the crystallization of the two-state variable layer 5. Therefore, there is no possibility that the reflectivity has changed in the middle of recording the information, improving the quality of recording operation.
In connection, if adopting a light-sensitive (photon-mode) material for the holographic recording layer 7, then the holographic recording can be completed before the two-state variable layer 5 changes since the photon-mode material has a reaction rate higher than that of a temperature-sensitive (heat-mode) material provided for the two-state variable layer 5. Thus, it is possible to accomplish the holographic recording under condition of less effect by the reflecting beam advantageously.
Consequently, it is possible to eliminate the above setting process of bringing the two-state variable layer 5 into the high-reflectively state for the reading reference beam L4 in advance of the reading operation. Thus, as there is no need of providing the beam generator 33 for the setting beam L3, the recording/reading apparatuses can be simplified.
Alternatively, if forming the holographic recording layer 7 by photo polymer material, it is necessary to make the holographic recording layer 7 fixed and exposed all over the layer's area in order to complete the polymerization of monomer after recording. Under such a situation, if forming the two-state variable layer 5 by the phase change material, there arises the need of providing the above-mentioned reflectivity setting process. Therefore, if allowing a wavelength of a light used in the reflectivity setting process to accord with a wavelength of the light for exposing the monomer in the holographic recording layer 7, then it is possible to perform the process of changing the two-state variable layer 5 to the crystallized state at the same time of photo polymerization, whereby the recording/reading apparatuses can be simplified.
In conclusion, according to the present invention, it is possible to increase the number of multiplexing and also possible to accomplish the reading operation for the holographic recording medium with an improved SNR.
Finally, it will be understood by those skilled in the art that the foregoing descriptions are nothing but embodiments of the disclosed holographic recording medium, the recording/reading methods and the recording/reading apparatuses for the holographic recording medium and therefore, various changes and modifications may be made within the scope of claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| P2006-114460 | Apr 2006 | JP | national |
| P2006-279812 | Oct 2006 | JP | national |
| P2007-039046 | Feb 2007 | JP | national |
