JP2000122513A - Optical recording medium, optical recording method, optical recorder, optical reproducing method and optical reproducing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to provide the optical recording medium itself with a function to control the wave fronts of reference light, to always stably obtain the wave fronts necessary for shift multiplex recording and reproducing in spite of a change in recording and reproducing devices and to easily obtain the interchangability between the respective recording and reproducing devices. SOLUTION: The optical recording medium 10 is constituted by forming an optical recording layer 12 capable of recording holograms on a transparent substrate 11 and forming a wave front curving layer 13, which is positive in the nonlinear refractive index induced by light and curves the wave fronts of the incident light by causing the self-convergence of this light, on this optical recording layer 12. A self-divergence layer 14 which acts reverse from the action of the wave front curving layer 13 may be formed between the transparent substrate 11 and the optical recording layer 12. The signal light having data information is subjected to Fourier transform and the optical recording medium 10 is irradiated with this light from the wave front curving layer 13 side and simultaneously, the region irradiated with the signal light the optical recording medium 10 is irradiated with the reference light of the plane waves from the wave front curving layer 13 side. The reference light passes the wave front curving layer 13, by which the wave fronts thereof are curved and the optical recording layer 12 is irradiated with the reference of the curved wave fronts. Then, the shift multiplex recording is made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical recording medium for recording a hologram, and a method and an apparatus for recording data information as a hologram on the optical recording medium and reading the data from the optical recording medium.

[0002]

2. Description of the Related Art As a next-generation computer file memory, a hologram memory having both a large capacity derived from a three-dimensional recording area and a high speed derived from a two-dimensional batch recording / reproducing method has been receiving attention. In hologram memory,
A plurality of data pages can be recorded by being multiplexed in the same volume, and data can be collectively read for each page. Instead of an analog image, the binary digital data “0, 1” is converted into a digital image as “bright, dark” and recorded and reproduced as a hologram, so that digital data can be recorded and reproduced. Recently, specific optical systems of this digital hologram memory system, evaluation of the SN ratio and bit error rate based on the volume multiplexing recording method, or proposal of two-dimensional encoding have been proposed. Research from a more optical point of view is also progressing.

[0003] As a hologram multiplex recording method, there are methods such as wavelength multiplexing and angle multiplexing, but shift multiplex recording has attracted attention because of its applicability to disk type recording media. FIG. 8 shows a document “D. Psaltis, M. Leave”.
ne, A. Pu, G .; Barbasstatis an
dK. Curtis; Opt. Lett. 20 (19
95) 782 ”, which is an example of the volume multiplex recording method.

In the shift multiplex recording method disclosed in this document, a spherical wave obtained by an objective lens is used as a reference beam 32 for irradiating a hologram recording medium 35 simultaneously with a signal beam 31. A plurality of holograms are overwritten in the same area by rotating the disk 35. For example, assuming that the beam diameter is 1.5 mmφ, another hologram can be recorded in almost the same area without crosstalk by moving the disk 35 by several tens of μm. This is based on the fact that the reference light 32 is a spherical wave, which is equivalent to a change in the angle of the reference light 32 due to the movement of the disk 35.

[0005] In addition to this method, the reference "Yong
Hoon Kang, Ki Hyun Kim a
nd Byungho Lee; Opt. Lett.
22 (1997) 739 "proposes a method of performing shift multiplex recording by passing a reference beam through an optical fiber to form a speckle pattern.

As described above, in the conventional method, shift multiplex recording is realized by passing the reference light through an optical element such as a lens or a fiber.

[0007]

One of the advantages of using a disk-type optical recording medium is that the optical recording medium can be carried. Therefore, each optical recording medium needs to be readable by a device other than the device that records the optical recording medium.

However, in the conventional method, since the optical element for controlling the wavefront of the reference light is provided in the recording / reproducing apparatus as described above, in order to be readable by each apparatus, the reference light generated by each apparatus is read. It is necessary to precisely control the wavefront. Therefore, it is required to improve the accuracy of the optical element that controls the wavefront of the reference light, which causes an increase in the cost of the apparatus.

Therefore, the present invention provides an optical recording medium having a function of controlling the wavefront of the reference light, thereby eliminating the need to provide an optical element for controlling the wavefront of the reference light in the recording / reproducing apparatus. The cost of the reproducing apparatus can be reduced, and compatibility between the recording and reproducing apparatuses can be easily obtained.

[0010]

According to the optical recording medium of the present invention, a wavefront bending layer whose wavefront is curved by self-convergence of incident light is formed on one surface of an optical recording layer on which a hologram can be recorded. I do.

According to the optical recording method of the present invention, the signal light for holding data information according to at least one of the light intensity and the polarization direction and the reference light are optically recorded from the wavefront curved layer side of the optical recording medium of the present invention. The data information is recorded as a hologram in the optical recording layer by simultaneously irradiating the layer.

According to the optical reproducing method of the present invention, the signal light for holding data information according to at least one of the intensity and the polarization direction of the light and the reference light are optically recorded from the wavefront curved layer side of the optical recording medium of the present invention. By irradiating the optical recording layer simultaneously with the layer, the data information is recorded as a hologram in the optical recording layer. The data information is read.

[0013]

FIG. 1 shows a case where a plane wave incident light 7 having a Gaussian distribution as a spatial intensity distribution 7a is incident on a nonlinear medium 8 having a positive nonlinear refractive index.

If the refractive index induced by light differs depending on the light intensity, the refractive index differs between a portion where the light intensity is high and a portion where the light intensity is low. When the non-linear refractive index is positive, the higher the light intensity, the higher the refractive index. Therefore, the phase velocity decreases as the light intensity increases.

Therefore, as shown in FIG. 1, when a plane wave incident light 7 having a Gaussian distribution is made incident on a nonlinear medium 8 having a positive nonlinear refractive index, the light passing through the nonlinear medium 8 has a phase velocity near the center. Slows down. As a result, the wavefront 9 of the light passing through the nonlinear medium 8 changes from a flat surface to a curved surface.

Therefore, if the reference light of a plane wave having a Gaussian distribution is transmitted through such a layer made of a nonlinear medium having a positive nonlinear refractive index and then made incident on an optical recording layer capable of recording a hologram. In addition, shift multiplex recording can be performed by using a reference beam having a curved wavefront instead of a plane.

In the optical recording medium of the present invention, such a wavefront curved layer is formed on one surface of the optical recording layer, and the optical recording medium itself has a function of controlling the wavefront of the reference light.
Even if the recording / reproducing device changes, the wavefront required for shift multiplexing recording / reproducing can always be obtained stably, and compatibility between the recording / reproducing devices can be easily obtained. Further, since it is not necessary to provide an optical element for controlling the wavefront of the reference light in the recording / reproducing apparatus, the cost of the recording / reproducing apparatus can be reduced.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [Embodiment of Optical Recording Medium] FIG.
(A) shows an example of the optical recording medium of the present invention.

In this example, a transparent substrate 1 such as a glass substrate is used.
An optical recording medium 12 is formed by forming an optical recording layer 12 on the optical recording layer 1 and forming a wavefront bending layer 13 on the optical recording layer 12.

The optical recording layer 12 may be made of any material as long as it can record a hologram. For example, a polyester having cyanoazobenzene in a side chain represented by a chemical formula shown in FIG. 3 is used. This material is in Japanese Patent Application
As described in detail in Japanese Patent Application No. 0-32834, hologram recording, reproduction and erasing can be performed by light-induced anisotropy due to photoisomerization of side chain cyanoazobenzene.

In order to shift-multiplex record a hologram, the thickness of the optical recording layer 12 must be at least about 10 μm. As the thickness increases, the storage capacity can be increased. The conditions for matching with the readout light used in the method become strict. Therefore, the higher the density is, the more important the wavefront reproduction accuracy is.

The wavefront bending layer 13 may be made of any material as long as the nonlinear refractive index induced by light is positive and the induced nonlinear refractive index is not recorded. A polymer having a photoisomerizable group or a polymer liquid crystal, or a polymer having a photoisomerizable molecule dispersed therein can be used.

These materials are macroscopically isotropic, but when irradiated with linearly polarized light, photoisomerization is induced, thereby causing anisotropy in the refractive index. As the group or molecule that undergoes photoisomerization, those that exhibit large birefringence due to photoisomerization are desirable. For example, those that include an azobenzene skeleton are preferable.

As a polymer or a polymer liquid crystal holding a group or a molecule that undergoes photoisomerization, a large birefringence occurs in the entire polymer or the polymer liquid crystal, and the relaxation time of the birefringence is sufficiently short. Things are desirable. For example, at least one monomer polymer selected from the group of polyesters,
Alternatively, polymethyl methacrylate or polyvinyl alcohol is preferred.

In this example, a material having such characteristics with a molecular structure close to that of the material of the optical recording layer 12 is shown in FIG.
A polyester having a cyanoazobenzene in a side chain represented by the chemical formula shown below is used. Also in this material, birefringence is induced by light-induced anisotropy due to photoisomerization of cyanoazobenzene in a side chain.

FIG. 2B shows another example of the optical recording medium of the present invention. In the example of FIG. 2A, the optical recording layer 12 is on the side opposite to the wavefront bending layer 13, that is, on the transparent substrate. Contrary to the wavefront curved layer 13, a self-diverging layer 14 that has a negative non-linear refractive index induced by light and self-diverges incident light is formed between the optical recording layer 11 and the optical recording layer 12. Self divergent layer 1
4 will be described later.

[Embodiment of Optical Recording Method and Optical Recording Apparatus] FIG. 5 shows an embodiment of the optical recording method and optical recording apparatus of the present invention. The optical recording medium 10 is the optical recording medium of the present invention, for example, the optical recording medium shown in FIG.

As the light source 21, a light source that emits coherent light sensitive to the optical recording layer 12 and the wavefront bending layer 13 of the optical recording medium 10 is used. For example, when the polyester having cyanoazobenzene in the side chain shown in FIGS. 3 and 4 is used as the optical recording layer 12 and the wavefront bending layer 13, respectively, an argon ion laser having a wavelength of 515 nm at which cyanoazobenzene is photoisomerized is used. Used. Light source 2
Light 5 from 1 is a plane wave having a Gaussian distribution.

The light 5 from the light source 21 is split into two light waves by a beam splitter 25, and a shutter 28
Is opened, and the light transmitted through the beam splitter 25 is incident on the spatial light modulator 30 as parallel light by the lenses 22 and 23.

The spatial light modulator 30 displays a two-dimensional digital data image by a computer or the like, not shown. Thus, the light that has passed through the spatial light modulator 30 is intensity-modulated according to the value of each pixel of the two-dimensional digital data image, and becomes signal light 1 having information of the two-dimensional digital data image.

The signal light 1 is converted to a Fourier transform lens 24
Fourier transform is performed on the Fourier transform plane P1 to irradiate the optical recording medium 10. At the same time, the beam splitter 25
Is reflected by mirrors 26 and 27,
The reference light 2 is applied to an area of the optical recording medium 10 to which the signal light 1 is applied.

Thus, the data information of the signal light 1 is recorded on the optical recording medium 10 as a hologram.
In this case, the light 5 from the light source 21 is a plane wave having a Gaussian distribution, and the reference light 2 is also a plane wave having a Gaussian distribution until reaching the optical recording medium 10. As described above, the plane light of the reference light 2 passes through the wavefront curved layer 13 of the optical recording medium 10, so that the wavefront is curved, and the optical recording layer 12 is irradiated with the curved wavefront reference light. . Therefore, shift multiplex recording can be performed.

In this example, a disk-shaped optical recording medium 1
By rotating 0, a plurality of holograms are shifted and multiplex-recorded at different locations in the circumferential direction of the optical recording medium 10. Further, by moving the recording head except the optical recording medium 10 in the radial direction of the optical recording medium 10, shift multiplex recording of the hologram can be performed so as to form concentric recording tracks in the optical recording medium 10.

As the wavefront bending layer 13, the relaxation time of the birefringence induced by the wavefront bending layer 13 is on the order of several seconds to several hundreds of micrometers.
By using a sufficiently fast one on the order of seconds, it is possible to prevent recording or reproduction after hologram recording from being affected.

[Embodiment of Optical Reproducing Method and Optical Reproducing Apparatus] FIG. 6 shows an embodiment of an optical reproducing method and an optical reproducing apparatus according to the present invention. The optical recording medium 10 is the optical recording medium of the present invention, for example, the optical recording medium shown in FIG. 2 (A), and the hologram is subjected to shift multiplex recording by the method described above.

At the time of reproduction, the shutter 28 is closed to block the signal light 1 and the same light as the reference light 2 at the time of recording is read out.
Is applied to the region of the optical recording medium 10 where the hologram is recorded. The irradiated read light 3 is transmitted to the optical recording medium 1.
The light is diffracted by the hologram recorded at 0, and the diffracted light 4 retains the data information of the signal light 1.
The diffracted light 4 is subjected to inverse Fourier transform by a lens 42 to form an image on a photodetector array 44, and the data information of the signal light 1 is read.

However, the hologram to be read includes recorded information, but does not completely reproduce a two-dimensional image of the signal light 1 applied to the optical recording medium 10 during recording. This is because at the time of recording, the signal light which has undergone the self-focusing effect in the wavefront bending layer 13 is recorded, but at the time of reproduction, the signal light does not undergo the self-diverging effect which is the reverse effect. However, since the signal light 1 is a discrete Fourier transform image unlike the reference light 2, the self-converging effect of the wavefront bending layer 13 on the signal light 1 is slight, and it is difficult to reproduce the recorded information. Is no problem.

In order to accurately reproduce a two-dimensional image of the signal light 1, an optical element that gives the opposite effect of the wavefront bending layer 13, that is, an optical element that self-diverges incident light, is provided in the optical path of the diffracted light 4, What is necessary is just to insert it between the optical recording medium 10 and the lens 42 as shown by 46.

Alternatively, without providing the optical element in the optical reproducing apparatus, as shown in FIG. 2B, the optical recording medium 10 is provided on the opposite side of the optical recording layer 12 from the wavefront curved layer 13. What is necessary is just to use what formed the self-diverging layer 14 with a negative nonlinear refractive index.

[Embodiment] The shift multiplex recording / reproduction was actually attempted by the recording / reproduction method described above. As the optical recording layer 12 and the wavefront bending layer 13 of the optical recording medium 10, polyester having cyanoazobenzene in the side chain shown in FIGS. 3 and 4, respectively, was used. However, for comparison, the one without the wavefront curved layer 13 was also used. As the light source 21, the above-described argon ion laser having a wavelength of 515 nm was used.

FIG. 7 shows the result. The horizontal axis represents the shift amount of the optical recording medium 10, and the vertical axis represents the diffraction efficiency (arbitrary unit system) of the hologram diffracted light 4 during reproduction.

The points indicated by black triangles are the results when the optical recording medium 10 without the wavefront curved layer 13 was used. As is clear from this, in the optical recording medium in which the wavefront curved layer 13 is not formed, the reference light is applied to the optical recording layer 12 with a flat wavefront, so that even if the shift amount is changed, a large change in the diffraction efficiency is observed. I can't.

On the other hand, the points indicated by black squares are the results when the optical recording medium 10 having the wavefront curved layer 13 is used. In this case, the diffraction efficiency decreases as the shift amount increases. This is because the wavefront of the reference light incident on the optical recording layer 12 is curved, so that the readout matching condition does not match as the shift amount increases.

From these results, according to the optical recording medium and the optical recording method of the present invention, if the optical recording medium is slightly shifted and the next hologram is recorded at a position where the diffraction efficiency is greatly reduced, the crosstalk is reduced. It can be seen that shift multiplex recording that does not occur can be performed.

[Other Embodiments] The embodiment shown in FIGS. 5 and 6 is a case in which recording and reproduction can be performed by one device, but it is also possible to use a recording-only or reproduction-only device. In an apparatus dedicated to recording, the lens 42 and the photodetector array 44 are unnecessary, and by excluding them, it is possible to reduce the size and weight of the recording head and reduce the cost of the recording apparatus. In a reproduction-only device, a beam splitter 25, a shutter 28, a spatial light modulator 30
The lenses 22, 23, and 24 are unnecessary, and by excluding them, the size and weight of the reproducing head and the cost of the reproducing apparatus can be reduced.

In the above-described embodiment, the signal light holds the data information by the intensity of the light. However, the signal light may hold the data information by the polarization direction of the light. Further, the signal light may hold data information depending on the light intensity and the polarization direction. As described in detail in the above-mentioned Japanese Patent Application No. 10-32834, a polymer or a polymer liquid crystal having a photoisomerizable group in the above-mentioned side chain, or a polymer in which a photoisomerizable molecule is dispersed Such a material exhibiting light-induced birefringence is sensitive to the polarization state of light incident thereon, and can record and store the polarization angle (polarization direction) of the incident light.

[0047]

As described above, according to the present invention, since the optical recording medium itself has the function of controlling the wavefront of the reference light, even if the recording / reproducing apparatus changes, the wavefront necessary for shift multiplexing recording / reproducing is changed. Can always be obtained stably, and compatibility between the recording / reproducing devices can be easily obtained. Further, since it is not necessary to provide an optical element for controlling the wavefront of the reference light in the recording / reproducing apparatus, the cost of the recording / reproducing apparatus can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram provided for explanation of a wavefront bending layer.

FIG. 2 is a diagram showing an example of the optical recording medium of the present invention.

FIG. 3 is a view showing a chemical formula of an example of a material of an optical recording layer of the optical recording medium of the present invention.

FIG. 4 is a diagram showing a chemical formula of an example of a material of a wavefront bending layer of the optical recording medium of the present invention.

FIG. 5 is a diagram showing an embodiment of an optical recording method and an optical recording apparatus of the present invention.

FIG. 6 is a diagram showing an embodiment of an optical reproducing method and an optical reproducing apparatus of the present invention.

FIG. 7 is a diagram showing experimental results.

FIG. 8 is a diagram illustrating an example of a shift multiplex recording method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Signal light 2 ... Reference light 3 ... Reading light 4 ... Diffraction light 10 ... Optical recording medium 11 ... Transparent substrate 12 ... Optical recording layer 13 ... Wave front curved layer 14 ... Self-diverging layer 21 ... Light source 25 ... Beam splitter 30 ... Space Optical modulator 44 ... Photodetector array

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Jiro Manabe 430 Nakai-cho Sakai, Kamigami-gun, Kanagawa Prefecture Inside Green Tech Nakai Fuji Xerox Co., Ltd. In-house F term (reference) 2H049 CA28 CA30 2K008 AA04 BB05 BB06 CC03 DD02 DD12 EE01 FF07 HH18 HH26

Claims (17)

[Claims] 1. An optical recording medium having a wavefront curved layer on one surface side of an optical recording layer capable of recording a hologram, the wavefront of which is curved by self-converging of incident light. 2. The optical recording medium according to claim 1, wherein the material of the wavefront bending layer is a polymer or a polymer liquid crystal having a photoisomerizable group in a side chain. 3. The optical recording medium according to claim 1, wherein the material of the wavefront bending layer is a polymer in which molecules that undergo photoisomerization are dispersed. 4. The optical recording medium according to claim 2, wherein the photoisomerizable group or molecule contains an azobenzene skeleton. 5. The optical recording medium according to claim 2, wherein said polymer or liquid crystal polymer is at least one monomer polymer selected from a polyester group. Medium. 6. The optical recording medium according to claim 1, wherein the material of the optical recording layer and the material of the wavefront bending layer induce birefringence for light having the same wavelength. An optical recording medium characterized by the above-mentioned. 7. The optical recording medium according to claim 1, wherein said optical recording medium has a disk shape. 8. The optical recording layer according to claim 1, wherein a signal light for retaining data information according to at least one of the intensity and the polarization direction of the light and a reference light are simultaneously irradiated on the optical recording layer from the wavefront curved layer side of the optical recording medium. And an optical recording method for recording the data information as a hologram in the optical recording layer. 9. The optical recording method according to claim 8, wherein a hologram is multiplex-recorded in the optical recording layer by moving the optical recording medium. 10. The optical recording method according to claim 8, wherein the optical recording medium is formed in a disk shape, and the optical recording medium is rotated, and the optical recording medium is irradiated with the signal light and the reference light. An optical recording method comprising: moving a head in a radial direction of an optical recording medium. 11. A light source which emits coherent light, a spatial light modulator which modulates light from said light source according to data information to obtain signal light, and said signal light is a wavefront of an optical recording medium according to claim 1. An imaging optical system that irradiates the optical recording layer from the curved layer side; and a reference light that obtains reference light from light from the light source and irradiates the reference light to the optical recording layer from the wavefront curved layer side of the optical recording medium. An optical recording device comprising: an optical system. 12. The optical recording apparatus according to claim 11, wherein said optical recording medium is disk-shaped, said optical recording apparatus comprises a medium drive mechanism for rotating said optical recording medium, said light source, a spatial light modulator, An optical recording apparatus, comprising: a head moving mechanism that moves a recording head including an imaging optical system and a reference light optical system in a radial direction of the optical recording medium. 13. The optical recording layer according to claim 1, wherein a signal light for retaining data information according to at least one of an intensity and a polarization direction of the light and a reference light are simultaneously irradiated on the optical recording layer from the wavefront curved layer side of the optical recording medium. By irradiating the optical recording layer with readout light from the wavefront curved layer side of the optical recording medium in which the data information is recorded as a hologram in the optical recording layer, the data information is read from the hologram. Light regeneration method. 14. The optical reproduction method according to claim 13, wherein a hologram multiplex-recorded in the optical recording layer is reproduced by moving the optical recording medium. 15. The optical reproducing method according to claim 13, wherein the optical recording medium has a disk shape, and the reproducing head for rotating the optical recording medium and irradiating the optical recording medium with the readout light includes: An optical reproducing method characterized by moving the optical recording medium in the radial direction. 16. The optical recording layer according to claim 1, wherein a signal light for retaining data information according to at least one of a light intensity and a polarization direction and a reference light are simultaneously irradiated on the optical recording layer from the wavefront curved layer side of the optical recording medium. By doing, the optical information recording medium in which the data information is recorded as a hologram in the optical recording layer, from the wavefront curved layer side to the optical recording layer, a readout optical system for irradiating readout light, An optical detector that detects diffracted light and reads the data information. 17. An optical reproducing apparatus according to claim 16, wherein said optical recording medium has a disk shape, said optical recording apparatus comprises: a medium drive mechanism for rotating said optical recording medium; The playback head including the
A head moving mechanism for moving the optical recording medium in a radial direction of the optical recording medium.