JP2005250263A - Holographic memory device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a holographic memory device which copes with a higher writing speed/higher capacity. <P>SOLUTION: The volume holographic memory device has: a signal light beam optical system with a recording medium 10 mounted, which makes a coherent signal light beam 4 incident via a Fourier transform lens 9 on the recording medium 10; a reference light beam optical system for making a coherent reference light beam 5 incident on the recording medium 10; and a means 12 for intersecting the reference light beam 5 with the signal light beam 4 within the recording medium 10. An optical path of the signal light beam optical system is provided with a plurality of spatial optical modulation means 110 to 140 which are made successively switchable. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a technique suitable for use in a holographic memory device using the principle of holography.

  As a small-sized and large-capacity next-generation recording medium, a holographic memory device that records hologram information on a photopolymer material or a photorefractive material is expected.

  The holographic memory device converts digital data into spatial light on / off signals, that is, spatial light (signal light) consisting of bright and dark dot patterns on a plane, and adds appropriate reference light to the light. An interference pattern is obtained and recorded in a hologram to form and write a holographic memory, and the holographic memory uses the same light as the reference light to receive the reproduced dot pattern image on the photoelectric detector array. The output signal is processed by an electronic circuit, converted back to digital data, and read.

The following technologies are known as such holographic memory devices.
JP 11-282330 A

In writing (recording) the holographic memory as described above, a liquid crystal panel is used as a spatial light modulator, and necessary digital data is sequentially switched and displayed on the liquid crystal panel to modulate spatial light.
In such a holographic memory device, in order to realize high-speed recording / reproduction, the spatial light modulator is required to have a data switching rate of several hundred to several kHz.

However, when switching the display data of the liquid crystal panel, considering the response speed of the liquid crystal panel, the drive limit is about several tens of Hz to 200 Hz, and it cannot respond to the high speed writing and large capacity of the holographic memory device. There was a problem.
Furthermore, since the display on the liquid crystal panel is line-sequential scanning, if writing is to be performed with a uniform light density, the actual writable time becomes very short, which is practically difficult to implement.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an object of providing a holographic memory device that can cope with an increase in writing speed and capacity.

The holographic memory device of the present invention can be mounted with a recording medium that records therein a three-dimensional optical interference pattern of at least two coherent lights as a spatial change in its refractive index,
A signal light beam optical system for injecting a coherent signal light beam into the recording medium via a Fourier transform lens; and
A reference light beam optical system for injecting a coherent reference light beam into the recording medium;
Means for intersecting the reference light beam with the signal light beam inside the recording medium, and a volume holographic memory device comprising:
A plurality of spatial light modulation means are provided in the optical path of the signal light beam optical system, and these spatial light modulation means can be sequentially switched to solve the above problem.
In the present invention, the spatial light modulation means includes a spatial light modulator and a pair of spatial light modulator switching means for selectively irradiating the spatial light modulator with a signal light beam. Is preferred.
The spatial light modulator switching means of the present invention can be configured such that a diffraction efficiency variable layer whose diffraction efficiency changes by applying an electric field is sandwiched between a pair of transparent electrodes.
Further, in the present invention, the diffraction efficiency variable layer is provided in parallel with the transparent electrode, and incident light incident through the transparent electrode layer can be switched between reflection and transmission according to the electric field strength applied between the transparent electrodes. Thus, it is possible to adopt means.
In the present invention, when the spatial light modulator repeats a display cycle including a display transition state and a display saturation state, the spatial light modulator switching unit corresponding to the spatial light modulator includes the saturation display state. And repeating the transmission cycle for switching the diffraction state and the transmission state in correspondence with the display cycle so that the signal light beam is irradiated to the spatial light modulator in the display space. The light modulation means may be provided with a control means for performing sequential recording and recording on the recording medium.

  The holographic memory device of the present invention can be equipped with a recording medium that records a three-dimensional optical interference pattern of at least two coherent lights as a spatial change in refractive index therein, and a coherent signal light beam. A signal light beam optical system incident on the recording medium via a Fourier transform lens, a reference light beam optical system incident on the recording medium with a coherent reference light beam, and the reference light beam inside the recording medium A volume holographic memory device having a crossing with respect to the signal light beam and a means for changing the crossing angle, wherein a plurality of spatial light modulation means are provided in the optical path of the signal light beam optical system. Spatial light modulation means can be sequentially switched, and two-dimensional digital data is sequentially transmitted to the plurality of spatial light modulation means. Two-dimensional digital data in each spatial light modulation means is displayed by switching so that the signal light beam sequentially enters each spatial light modulation means corresponding to this display and recording (writing) on the recording medium. Even if the display speed (display speed) is about several tens to 200 Hz, the high-speed data rate is supported by switching the signal light beam to the spatial light modulation means at about several hundred to several kHz. It becomes possible to write.

  In the present invention, the spatial light modulation means includes a spatial light modulator and a pair of spatial light modulator switching means for selectively irradiating the spatial light modulator with a signal light beam. Then, by sequentially selecting the spatial light modulator on which the signal light beam is incident by the pair of spatial light modulator switching means, the signal light beam is changed according to the two-dimensional digital data displayed on the plurality of spatial light modulation means. Sequential spatial modulation can be performed. Therefore, even if the spatial light modulator is a liquid crystal panel or the like and the display speed is about several tens Hz to 200 Hz, the spatial light switching means switches the incidence switching of the signal light beam to the spatial light modulator from several hundred to Writing at a high data rate can be performed by modulating the signal light with a spatial light modulator that is in a display state sufficient for modulation of the display signal light beam.

  The spatial light modulator switching means of the present invention corresponds to the display state in the spatial light modulator by sandwiching a diffraction efficiency variable layer whose diffraction efficiency is changed by applying an electric field between a pair of transparent electrodes. By applying an electric field to the electrode of the spatial light modulator switching means, the diffraction efficiency of the variable diffraction efficiency layer is changed, so that the signal light beam is incident on the spatial light modulator, and the signal is transmitted by the spatial light modulator. The light beam can be spatially modulated, whereby the spatial light modulator that performs the spatial modulation of the signal light beam can be switched at about several hundred to several kHz.

The variable diffraction efficiency layer includes a first region having an isotropic refractive index and a second region having a refractive index different from that of the first region when an electric field is applied. be able to.
The light diffracted by the linear region to which the electric field is applied preferably satisfies the Bragg condition represented by the following formula.
2Λ · sinθ _i '= λ'
(Where Λ is the refractive index distribution period of the diffraction efficiency variable layer, θ _i ′ is the angle formed by the incident locus of light in the diffraction efficiency variable layer and the equirefractive index surface in the diffraction efficiency variable layer, and λ ′ is diffraction. Effective wavelength of light in the efficiency variable layer)

  In this variable diffraction efficiency layer, there is provided a variable diffraction efficiency layer having a first region having an isotropic refractive index and a second region having a refractive index different from that of the first region when an electric field is applied. Since the first region and the second region existing between the transparent electrodes are different in refractive index when a voltage is applied between the transparent electrodes, the light is emitted from the low refractive index layer. The scattered light is Bragg diffracted. Here, there is a close correlation between the difference in the refractive index (difference) between the first region and the second region and the diffraction efficiency. However, it is not diffracted by any light, but only light that satisfies certain conditions such as wavelength and incident angle is diffracted.

  Further, in the present invention, the diffraction efficiency variable layer is provided in parallel with the transparent electrode, and incident light incident through the transparent electrode layer can be switched between reflection and transmission according to the electric field strength applied between the transparent electrodes. The incident-side spatial light modulator switching means is arranged so that the signal light beam diffracted in the diffraction efficiency variable layer is incident on the spatial light modulator, and the signal light beam modulated by the spatial light modulator is A spatial light modulator switching means on the emission side is arranged for going to the recording medium, and the signal light beam is sequentially switched to a spatial light modulator in a display state capable of spatial modulation by switching the electric field application to the transparent electrode. be able to.

  In the present invention, when the spatial light modulator repeats a display cycle including a display transition state and a display saturation state, the spatial light modulator switching unit corresponding to the spatial light modulator includes the display saturation state. And repeating the transmission cycle for switching the diffraction state and the transmission state in correspondence with the display cycle so that the signal light beam is irradiated to the spatial light modulator in the display space. Since the light modulation means is provided with control means for sequentially switching and recording on the recording medium, one spatial light modulation means starts to display two-dimensional digital data on the spatial light modulator at time t0. As the display transition state, the display saturation state has reached the display state where sufficient spatial modulation can be performed at time t1. Later, at time t2, the spatial light modulator switching means is switched so that the signal light beam enters the spatial light modulator, writing at the recording medium starts at time t3, and the display saturation state at the spatial light modulator at time t4. Is switched to the display transition state to the next display, and the control means controls so that the writing of the one data is sequentially repeated by the plurality of spatial light modulation means. As a result, the display cycle and the transmission cycle from time t0 to time t4 are repeated to perform data switching of about several hundreds to several kHz, and writing corresponding to a high-speed data rate can be performed.

  According to the present invention, two-dimensional digital data is sequentially displayed on the plurality of spatial light modulators, and the recording medium is switched so that a signal light beam is sequentially incident on each spatial light modulator corresponding to the display. Even if the speed (display speed) for displaying the two-dimensional digital data in each of the spatial light modulators is about several tens of Hz to about 200 Hz, the spatial light modulator of the signal light beam is recorded. It is possible to perform writing corresponding to a high-speed data rate by performing switching to about several hundred to several kHz.

  Hereinafter, an embodiment of a holographic memory device according to the present invention will be described with reference to the drawings.

In an embodiment in which a volume holographic memory is actually applied to an optical information recording / reproducing apparatus, for example, an angle multiplex recording method in which recording is performed by changing the irradiation angle of reference light in the same space of a recording medium will be described. In the angle multiplexing method, recording and reproduction of information for one page of an image is performed at a preset irradiation angle with respect to the reference light, and the reference light irradiation angle is repeatedly changed to approximately the same position on the recording medium, so that information of a plurality of pages is recorded. Record and play back. It is necessary to have an angle width that does not cause crosstalk from a page image recorded at an adjacent angle during reading.
As a recording medium used, a photopolymer material photorefractive material that records a three-dimensional optical interference pattern as a spatial change in the refractive index in the crystal is used.

FIG. 1 is a schematic diagram showing a holographic memory device according to this embodiment.
As shown in FIG. 1, the holographic memory device according to the present embodiment has a laser light source device 1 and polarized light that divides a light beam emitted from the laser light source device 1 into a signal light beam 4 and a reference light beam 5. A beam splitter 3, a light beam expander 7 that expands the signal light beam 4 separated by the polarization beam splitter 3 into parallel light of a predetermined diameter, a plurality of spatial light modulators 110 to 150, a reflecting mirror 6, and a Fourier transform A lens 9, a beam expander 11 for causing the reference light beam 5 to enter the recording medium 10, a Fourier transform lens 13, an inverse Fourier transform lens 20 through which diffracted light from the recording medium 10 by the reference light beam 5 is reproduced, and this Two-dimensional photodetector array 21 irradiated with diffracted light, control means (not shown) for controlling each of these parts, half-wave plates 14 and 15, It is assumed to have.

In the present embodiment, the light beam emitted from the laser light source device 1 is converted into linearly polarized light having an arbitrary polarization axis by the half-wave plate 14, and then the signal light beam 4 and the reference light beam are converted by the polarization beam splitter 3. 5, and each is guided to the optical path of the signal light beam optical system and the reference light beam optical system. The polarized beam 4 that has just exited the polarization beam splitter 3 is S-polarized light (light having an electric field component perpendicular to the plane formed by the light incident on and emitted from the polarization beam splitter 3), and the polarized beam. The reference light beam 5 that has just exited from the splitter 3 is P-polarized light (light having an electric field component parallel to a plane formed by light incident on and output from the polarization beam splitter 3). 5 is converted into S-polarized light by the half-wave plate 15.
The half-wave plates 14 and 15 are made of a material having uniaxial refractive anisotropy, but the phase difference can be varied by arbitrarily selecting the principal axis (abnormal axis) orientation with respect to the incident optical axis. It acts as a phase difference plate and gives a phase difference of π (= λ / 2) at the maximum, that is, a polarization axis rotation of 90 °. Such a phase difference of π is referred to as first order or zero order, and that which is an integral multiple of π is referred to as multi-order. The half-wave plates 14 and 15 of the present embodiment are first order. But it doesn't matter.
The half-wave plate 14 adjusts the intensity of the reference light and the signal light.

The signal light beam 4 emitted from the polarization beam splitter 3 is incident on the recording medium 10 through the beam expander 7, the spatial light modulators 110 to 150, the reflecting mirror 6, and the Fourier transform lens 9 in the signal light beam optical system. . In other words, the beam expander 7 expands the beam into parallel light having a predetermined diameter. Further, after being spatially modulated in accordance with the recording page data by the spatial light modulators 110 to 150, that is, after the signal light beam is spatially modulated as a two-dimensional lattice pattern for transmission / nontransmission for each pixel, the Fourier transform lens 9. Is subjected to Fourier transform, condensed on the recording medium 10, and imaged as a Fourier transform image in the recording medium 10. At this time, the polarization direction of the light may be controlled by the ½ wavelength plate, and may be controlled to a predetermined light intensity by the ND filter.
The beam expanders 7 and 11 are not simply a combination of lenses, but include an objective lens, a pinhole, and a lens. The objective lens is characterized in that a high magnification can be obtained with a short optical path, and the pinhole functions as a “spatial filter” that removes a wavefront error contained in the beam emitted from the objective lens. This is because the wavefront error is not preferable because it disturbs the two-light interference.

  On the other hand, in the reference light beam optical system, the reference light beam 5 is incident on the recording medium 10 through the beam expander 11 and the reflecting mirror 6, and the two light beams intersect at a position inside the medium. The reference light beam 5 that has passed through the polarization beam splitter 3 is expanded into parallel light having a predetermined diameter by a beam expander 11, but the light intensity may be further controlled by an ND filter. Thereafter, the light is incident on the recording medium while being controlled at a predetermined angle by the pager reflector 12. The paging operation of the pager reflecting mirror 12 is controlled so that the reference light having different angles is incident on the same part of the recording medium by the deflection angle and the mirror slide.

  The signal light beam transmitted through the Fourier transform lens 9 and the reference light reflected from the reflecting mirror 6 form a hologram in the recording medium 10. That is, when recording data, the recording medium 10 is simultaneously irradiated with the signal light and the reference light, and the refractive index changes in the medium to record the interference pattern. The hologram formation time is controlled by the automatic shutter of the laser light source device 1.

  On the other hand, at the time of reproduction, only the reference light from the reflecting mirror 6 is irradiated onto the recording medium 10 due to the shutter being not shown, and the diffracted light from the recording medium 10 is two-dimensional light including the CCD via the inverse Fourier transform lens 20. An image is formed on the detector array 21. The CCD pixel and the LCD pixel are adjusted to correspond to 1: 1. The correspondence of these pixels includes not only 1: 1 but also various patterns such as 4: 1 or 9: 1. Thus, when reproducing information, data can be read by irradiating only the reference light 5 to the interference pattern recorded on the recording medium 10.

  The spatial light modulators 110 to 150 have substantially the same configuration, and the signal light beams are transmitted to the spatial light modulators 110 to 150 from the same direction with respect to the optical path of the signal light beam optical system irradiated from the beam expander 7. Are provided in parallel so that they can be incident at the same angle.

The spatial light modulator 110 includes a spatial light modulator 111 that can display two-dimensional digital data such as a liquid crystal panel, and a signal that is expanded into parallel light of a predetermined diameter by the beam expander 7 on the spatial light modulator 111. A pair of spatial light modulator switching means 112 and 113 for selectively irradiating light and making the signal light spatially modulated by the spatial light modulator 111 incident on the reflecting mirror 6 are provided. The liquid crystal panel 111 has a pair of polarizing plates attached to both sides thereof in a parallel Nicol arrangement so that incident polarized light and outgoing polarized light are equal.
The spatial light modulator switching means 112 and 113 have substantially the same configuration, and the incident surfaces are symmetrical with respect to the optical axis of the spatial light modulator 111 and are symmetrical with respect to the spatial light modulator 111. It is provided to be.
In the above configuration, since the coherence of the laser light source device 1 is finite, the optical path difference between the signal light beam and the reference light beam is configured to be as small as possible. The spatial light modulator switching units 112 and 113 are respectively A diffraction efficiency variable layer 116, which will be described later, is arranged in the opposite direction so as to be symmetric with respect to the spatial modulator 11.

FIG. 2 is a schematic cross-sectional view showing the spatial light modulator switching means of the holographic memory device in this embodiment.
As shown in FIG. 2, the spatial light modulator switching means 112 has a diffraction efficiency variable layer 116 whose diffraction efficiency is changed by applying an electric field, sandwiched between a pair of transparent electrodes 114 and 115, and these diffraction efficiency variable layers 116. The transparent electrodes 114 and 115 are provided on the entire surface serving as an optical path in the spatial light modulator switching unit 112. In the spatial light modulator switching means 112, the switching speed of the diffraction efficiency variable layer 116 reaches several tens of kHz, which is no longer than the switching speed of liquid crystal or the like.

  As shown in FIG. 3, the variable diffraction efficiency layer 116 includes a first region 117 having an isotropic refractive index and a second region 118 having an anisotropic refractive index (uniaxial or biaxial) in the thickness direction. The hologram recording material has a structure that is periodically repeated. The second region 118 has a refractive index different from that of the first region 117 when an electric field is applied.

FIG. 3 is an enlarged cross-sectional view schematically showing a part of the diffraction efficiency variable layer of the spatial light modulator switching means in the present embodiment.
In the present embodiment, the first region 117 is made of a polymer resin, and the second region 118 is made of a liquid crystal. In the first region 117 indicated by hatching in FIG. 3, the polymer resin is present in high density, and the second region 118 surrounded by the ellipse is filled with liquid crystal molecules in high density. The liquid crystal used for the second region 118 is a nematic liquid crystal and has a positive or negative dielectric anisotropy. When an electric field is applied to the liquid crystal, the liquid crystal responds to the electric field by its own dielectric anisotropy. To change the orientation state.
The direction in which the alignment state of the liquid crystal changes depends on whether the sign of dielectric anisotropy of the liquid crystal is positive or negative. If the liquid crystal has a positive dielectric anisotropy, the liquid crystal molecules are parallel to the electric field direction. In the case of movement and negative dielectric anisotropy, a liquid crystal molecule that moves so as to be perpendicular to the electric field direction is used. As the liquid crystal molecules move in this way, the refractive index of the second region 118 changes.
In addition, as the liquid crystal used for the second region 118, PDLC described in JP 2000-515996A can be used.

The diffraction efficiency variable layer 116 is a volume hologram, and light satisfying the Bragg diffraction condition represented by the following formula (1) is strongly diffracted (diffraction efficiency is large), and light outside the Bragg diffraction condition is hardly diffracted (diffraction). Efficiency is zero or very small).
2Λ · sin θi ′ = λ ′ (1)
(Where Λ is the refractive index distribution period of the diffraction efficiency variable layer 116, θi ′ is the angle formed by the incident locus of light in the diffraction efficiency variable layer and the equal refractive index surface in the diffraction efficiency variable layer, and λ ′ is diffraction. This is the effective wavelength of light in the efficiency variable layer.) The above-mentioned iso-refractive index surface is along the line III-III in FIG. 3 when a voltage is applied to a part of the diffraction efficiency variable layer 116 shown in FIG. In other words, the surface has the same refractive index value in the refractive index distribution of the cross section.

  The diffraction efficiency variable layer 116 has a refractive index of the liquid crystal constituting the second region 118 when the electric field is not applied under the Bragg condition with respect to the wavelength of the expanded signal light 4 and the polymer resin of the surrounding first region 117. It is fabricated so as to be approximately equal to the refractive index. Therefore, when no electric field is applied to the liquid crystal, even if a part of the signal light 4 reaching the diffraction efficiency variable layer 116 satisfies the plug condition, it is transmitted through the diffraction efficiency variable layer 116 without being diffracted. . Further, since the light component that does not satisfy the Bragg condition is not diffracted, it only passes through the diffraction efficiency variable layer 116 without depending on the state of the liquid crystal.

When an electric field is applied to the liquid crystal forming the second region 118, the liquid crystal molecules move and change their orientation, and the refractive index of the liquid crystal region where light is felt under the plug conditions is that of the surrounding polymer resin (first region 117). The light is diffracted because of the difference in refractive index. If the refractive index of the second region 118 changes and the difference from the refractive index of the surrounding first region 117 decreases, the diffraction efficiency decreases. Conversely, the refractive index of the second region 118 changes and the surrounding first region 118 changes. If the difference from the refractive index of the first region 117 is increased, the diffraction efficiency is increased.
Therefore, when an electric field is applied to the diffraction efficiency variable layer 116, only specific light that satisfies the Bragg condition is diffracted at the portion where the transparent electrodes 114 and 115 serving as optical paths are provided.
Therefore, in a state where no electric field is applied to the transparent electrodes 114 and 115, the signal light beam 4 transmitted through the transparent electrode 115 is transmitted through the diffraction efficiency variable layer 116 and the transparent electrode 114 as indicated by a left-pointing arrow A shown in FIG. In a state where an electric field is not applied to the transparent electrodes 114 and 115, the signal light beam 4 transmitted through the transparent electrode 115 is diffracted as indicated by an upward arrow B shown in FIG.
The above spatial modulator switching means 112, 113 to 142, 143 and each optical element are all optimized by the laser wavelength to be used and the incident angle to each element in order to reduce the loss of light quantity due to surface reflection. An antireflection coating (so-called AR coating) is preferably applied to the surface.

  The control means controls the spatial light modulator switching means 112, 113 to 152, 153 so as to sequentially switch the expanded signal light 4 to the spatial light modulation means 110 to 150, and these spatial light modulation means 110 to 150. The two spatial data modulators 111 to 15 are controlled to display the two-dimensional digital data.

  FIG. 4 is a time chart showing the operating state of the spatial light modulator and the recording state on the recording medium in the present embodiment. In this figure, the spatial light modulator 111 in the spatial light modulator 110 is indicated by SLM1, the spatial light modulator switching means 112 and 113 are indicated by ESBG1, and the recording medium 10 is indicated by RM. The spatial light modulators 120 and 130 are shown in the same manner as the spatial light modulator 110, but the spatial light modulator 140 is not shown. In the spatial light modulators 111 to 113, the display transition state is represented as on. In the recording medium 10, the spatial movement for changing the recording location or the angle setting of the reference light beam is set to the operation state (on), and the stop (recording) state is set to (off).

As shown in FIG. 4, the control means starts with the spatial light modulation means 110.
The display transition state (on) is started for the first time when the spatial light modulator 111 is made to display the two-dimensional digital data at time t0, and the movement (setting) of the writing position in the recording medium 10 is started (on).
-The movement of the writing position in the recording medium 10 is ended (off) at time t1,
A display saturation state (off) in which the spatial light modulator 111 is in a display state that can sufficiently perform spatial modulation at time t2,
At time t3, the spatial light modulator switching means 112 and 113 are switched so that the signal light beam enters the spatial light modulator 111 (on), and writing in the recording medium 10 is performed (off).
At the time t4, the writing on the recording medium 10 is completed and the writing position starts to be moved (on), the display saturation state in the spatial light modulator 111 is switched to the display transition state (on) to the next display, and The spatial light modulator switching means 112 and 113 are switched so that the signal light beam does not enter the spatial light modulator 111 (off),
At the time t5, the movement of the writing position in the recording medium 10 is ended (off).
As described above, one cycle S corresponds to the display transition state S1 from time t0 to time t2 and the display saturation state S2 from time t2 to time t4, and this one cycle S is changed from time t0 to time t4 ( From the time t1 to the time t5), the two-dimensional digital data displayed on the spatial light modulator 110 is written to the recording medium 10 once during this time, and at time t12, this one cycle S is performed. The display cycle of the spatial light modulator 111 and the transmission cycle of the spatial light modulator switching means 112 and 113 in the spatial light modulation means 110 are used.

Similarly, in the spatial light modulator 120 that repeats such a display cycle and a transmission cycle,
The display transition state (on) is not started until the spatial light modulator 121 displays two-dimensional digital data.
A display saturation state (off) in which the spatial light modulator 121 is in a display state in which sufficient spatial modulation can be performed at time t3;
At time t6, the spatial light modulator switching means 122, 123 are switched so that the signal light beam enters the spatial light modulator 121 (on), and writing on the recording medium 10 is performed (off).
At the time t7, the writing on the recording medium 10 is finished and the writing position starts to be moved (on), the display saturation state in the spatial light modulator 121 is switched to the display transition state (on) to the next display, The spatial light modulator switching means 122 and 123 are switched so that the signal light beam does not enter the spatial light modulator 121 (off),
At the time t8, the movement of the writing position in the recording medium 10 is ended (off).
As described above, in the spatial light modulator 120, the cycle S having the same length is repeated in a state where the cycle from the time t2 to the time t3 is shifted from the cycle S of the spatial light modulator 110.

Similarly, in the spatial light modulator 130 that repeats such a display cycle and a transmission cycle,
Only when two-dimensional digital data is displayed on the spatial light modulator 131 is in the display transition state (on),
A display saturation state (off) in which the spatial light modulator 131 is in a display state in which sufficient spatial modulation can be performed at time t6,
At time t9, the spatial light modulator switching means 132 and 133 are switched so that the signal light beam enters the spatial light modulator 131 (on), and writing on the recording medium 10 is performed (off).
At the time t10, the writing on the recording medium 10 is completed and the writing position starts to be moved (on), the display saturation state in the spatial light modulator 121 is switched to the display transition state (on) to the next display, The spatial light modulator switching means 122 and 123 are switched so that the signal light beam does not enter the spatial light modulator 121 (off),
At the time t11, the movement of the writing position in the recording medium 10 is ended (off).
As described above, the spatial light modulator 130 has the same length with respect to the cycle S of the spatial light modulator 120 while being shifted by a time from time t3 to time t6 which is equal to time t3. Cycle S is repeated.

Further, similarly, the spatial light modulator 140 repeats the cycle S having the same length while being shifted from the time t2 by a time equal to the time t3 with respect to the cycle S of the spatial light modulator 130. In addition, the cycle S having the same length is repeated in the spatial light modulator 150 while being shifted from the time t2 by a time equal to the time t3 with respect to the cycle S of the spatial light modulator 140.
Next, in the spatial light modulation unit 110, the cycle S having the same length is repeated with the cycle S of the spatial light modulation unit 150 being shifted by a time from time t2 to time t3. ing.

  As described above, the control unit controls the switching so that the switching of the plurality of spatial light modulation units 110 to 150 is sequentially repeated. As a result, it becomes possible to perform writing corresponding to the high data rate on the recording medium 10 as in the writing cycle S3.

  In the holographic memory device of this embodiment, the digital signal to be recorded is input to the controller of the control means, and after processing such as error correction code addition and binary encoding is performed by this controller, the signal light control driver is supplied. The image data is converted into a page image sequence signal, and data for each page is sent as a page image to light transmissive spatial light modulators 111 to 140 such as LCDs in the spatial light modulators 110 to 140 to form image data. Is done. The controller of the control means controls the time for irradiating the recording medium of both light beams while there is image data via the spatial light modulation means 110-150. The controller of the control means can also control the time for irradiating the recording medium of both light beams while there is image data via a shutter control driver that automatically shutters the laser light source device.

  At the same time, the controller moves the pager reflecting mirror 12 via the reference light control light driver and changes the angular position of the reflecting mirror corresponding to the image data, thereby changing the reference light set to a predetermined incident angle (θ). Is irradiated for a predetermined time to write a hologram. Thereafter, the procedure of sending the page image, setting the incident angle of the reference light, and hologram recording is sequentially repeated. One page of information is stored corresponding to the setting of one angle.

  In the present embodiment, a plurality of spatial light modulators 110 to 150 are provided in the optical path of the signal light beam optical system, and the spatial light modulators 110 to 150 can be sequentially switched, and the plurality of spaces The two-dimensional digital data is sequentially displayed on the light modulation means 110 to 150, and the signal light beams are switched to sequentially enter the respective spatial light modulation means 110 to 150 corresponding to this display and recorded (written) on the recording medium 10. Thus, even if the speed (display speed) for displaying the two-dimensional digital data in each of the spatial light modulation means is about several tens of Hz to 200 Hz, the switching of the signal light beam to the spatial light modulation means is performed several times. By performing the operation at about one hundred to several kHz, it becomes possible to perform writing corresponding to a high-speed data rate.

  Each of the spatial light modulators 110 to 150 includes spatial light modulators 111 to 151 and a pair of spatial light modulator switching means 112 for selectively irradiating the spatial light modulators 111 to 151 with a signal light beam. , 113 to 152, and 153, the spatial light modulators 111 to 151 on which the signal light beam is incident are sequentially selected by the respective spatial light modulator switching means 112, 113 to 152, and 153. Thus, the signal light beam can be sequentially spatially modulated in accordance with the two-dimensional digital data displayed in the plurality of spatial light modulators 111 to 151. Therefore, even if the spatial light modulator is a liquid crystal panel or the like and the display speed is about several tens Hz to 200 Hz, the spatial light switching means switches the incidence switching of the signal light beam to the spatial light modulator from several hundred to Writing at a high data rate can be performed by modulating the signal light with a spatial light modulator that is in a display state sufficient for modulation of the display signal light beam.

  The spatial light modulator switching means 112, 113 to 152, 153 is formed by sandwiching a diffraction efficiency variable layer 116, whose diffraction efficiency is changed by applying an electric field, between a pair of transparent electrodes 114, 115,. By changing the diffraction efficiency of the diffraction efficiency variable layer 116 by applying an electric field to the electrodes 114, 115 of the spatial light modulator switching means 112, 113 to 152, 153 according to the display state in the spatial light modulator. The spatial light modulators 111 to 151 can be switched so that the signal light beam is incident, and the spatial light modulators 111 to 151 can spatially modulate the signal light beam, thereby performing spatial modulation of the signal light beam. The spatial light modulators 111 and 151 can be switched at about several hundred to several kHz.

  The control means of this embodiment repeats the display cycle in which the spatial light modulator 111 is in the display transition state and the display saturation state, and the spatial light modulator switching means 112 and 113 are configured to transmit the signal light beam in the display saturation state. A transmission cycle for switching the diffraction state and the transmission state corresponding to the display cycle is repeated so as to irradiate the spatial light modulator 111, and these cycles S are sequentially switched in the plurality of spatial light modulators 110 to 150. By performing recording on the recording medium 10 as described above, it is possible to perform writing on the recording medium 10 by the writing cycle S3 shorter than the display or transmission cycle S as described above. As a result, it becomes possible to perform writing corresponding to a high data rate by a writing cycle of about several hundreds to several kHz with respect to a display cycle S of about several tens Hz to 200 Hz defined by a liquid crystal panel or the like.

  In the present embodiment, four examples of the spatial light modulation means are described. However, corresponding to the writing rate of the recording medium, that is, corresponding to the possible number of writing cycles S3 in one display cycle S, A number of spatial light modulation means can be provided.

As an application example of the present invention, in other holographic memory devices, a plurality of spatial light modulation means having a diffraction efficiency variable layer may be provided.

FIG. 1 is a schematic view showing an embodiment of a holographic memory device according to the present invention. FIG. 2 is a schematic sectional view showing the spatial light modulator switching means of the holographic memory device according to the present invention. FIG. 3 is an enlarged sectional view schematically showing a part of the diffraction efficiency variable layer of the spatial light modulator switching means according to the present invention. FIG. 4 is a time chart showing the operating state of the spatial light modulator and the recording state on the recording medium in an embodiment of the holographic memory device according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser light source device 3 Translucent mirror 4 Signal light beam 5 Reference light beam 6 Reflective mirror 7 Light beam expander 9 Fourier transform lens 10 Recording medium 11 Light beam expander 12 Pager reflector 13 Fourier transform lens 20 Inverse Fourier transform lens 21 Two-dimensional photodetector array 110-150 Spatial light modulators 111-151 Spatial light modulators 112-152, 113-153 Spatial light modulator switching means 114, 115 Transparent electrode 116 Diffraction efficiency variable layer 117 First region 118 First 2 areas

Claims (5)

A recording medium that records therein a three-dimensional optical interference pattern of at least two coherent lights as a spatial change in the refractive index thereof can be mounted.
A signal light beam optical system for injecting a coherent signal light beam into the recording medium via a Fourier transform lens; and
A reference light beam optical system for injecting a coherent reference light beam into the recording medium;
Means for intersecting the reference light beam with the signal light beam inside the recording medium, and a volume holographic memory device comprising:
A holographic memory device characterized in that a plurality of spatial light modulation means are provided in the optical path of the signal light beam optical system, and these spatial light modulation means are sequentially switchable. The spatial light modulation means includes a spatial light modulator and a pair of spatial light modulator switching means for selectively irradiating the spatial light modulator with a signal light beam. The holographic memory device according to claim 1. 3. The holographic memory device according to claim 1, wherein the spatial light modulator switching unit includes a diffraction efficiency variable layer whose diffraction efficiency changes by applying an electric field, and is sandwiched between a pair of transparent electrodes. . The diffraction efficiency variable layer is provided in parallel with the transparent electrode, and incident light incident through the transparent electrode layer can be switched between reflection and transmission by an electric field strength applied between the transparent electrodes. The holographic memory device according to claim 3. When the spatial light modulator repeats a display cycle consisting of a display transition state and a display saturation state,
The diffractive state and the transmissive state correspond to the display cycle so that the spatial light modulator switching unit corresponding to the spatial light modulator irradiates the spatial light modulator with the signal light beam in the saturated display state. And repeat the permeation cycle to switch,
5. The holographic memory according to claim 3, further comprising a control unit that sequentially switches the display cycle and the transmission cycle in the plurality of spatial light modulation units to perform recording on the recording medium. apparatus.

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