JP2006208858A - Hologram processing apparatus and hologram processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hologram processing apparatus capable of processing in a short time with a simple configuration and capable of always performing satisfactory reproduction without causing deterioration of a recorded hologram even when reproduction is repeated. I will provide a.
During reproduction, the holographic memory 1 is irradiated with readout light and the phase conjugate mirror 2 is irradiated with excitation light. When the holographic memory 1 is irradiated with readout light, diffracted light from the recorded hologram is emitted. Further, by irradiating the phase conjugate mirror 2 with the diffracted light and the excitation light, a hologram that is phase conjugate with the hologram recorded in the holographic memory 1 is formed on the phase conjugate mirror 2. Then, when the holographic memory 1 is irradiated with feedback light based on this phase conjugate hologram, rewriting of the hologram is realized.
[Selection] Figure 1

Description

  The present invention relates to a hologram processing apparatus and a hologram processing method for using, for example, a photorefractive crystal as a holographic memory and recording / reproducing / erasing a hologram in the crystal.

  Holographic memory (hologram memory) applies technology (holography) that can record a three-dimensional stereoscopic image by illuminating an object with highly coherent light such as laser light. It is a storage device that can be recorded on a layer. Information reproduction is realized by reading an image with information by applying reference light for reading and writing information to the holographic memory.

  For example, conventional optical disc recording devices such as CDs and DVDs apply lasers to rotating optical discs to read and write data in bit units, while holographic memories read thousands of bits at the same time by applying a single laser. -Write. That is, the holographic memory is capable of high-speed data transfer as compared with a conventional optical disk recording apparatus.

  In addition, the holographic memory can multiplex-record a hologram of a two-dimensional image as information at the same location in the hologram medium. Therefore, the recording density of information can be made extremely high, so that a large capacity can be realized.

  As described above, the holographic memory has a high speed derived from a direct recording / reproducing system of two-dimensional data and a large capacity derived from multiplex recording at the same location. Attention has been paid.

  Many conventional holographic memories are in principle write-once (Write Once, data can be written only once in the same area of the medium, and the written data cannot be changed). On the other hand, in a holographic memory using a photorefractive medium (photorefractive crystal) as a hologram medium, a hologram once recorded can be erased, and application to a rewritable memory is expected.

The photorefractive medium refers to a group of media whose refractive index is changed by light irradiation. Lithium niobate (LiNbO ₃ : Fe) containing iron as an impurity and strontium barium niobate (Sr _x Ba ₁ ) containing cerium as an impurity. _{... x} Nb ₂ O ₆ : Ce) and many other media are known.

  On the other hand, as described above, a holographic memory in which data can be rewritten only by light even after data is recorded has a problem that data recorded by light for reading deteriorates or is erased. Have. In order to prevent such deterioration and erasure of recorded data due to reading light, a hologram fixing technique using an electric field or heat has also been proposed.

  However, when such a fixing technique is used, there arises a problem that real-time processing (hologram recording / reproduction in real time) becomes difficult. In addition, since an electronic circuit for performing the fixing process is required, the entire memory system becomes complicated, resulting in an increase in apparatus cost.

On the other hand, recently, in addition to recording light (wavelength l ₁ , reference light and object light) that forms a hologram, excitation light (wavelength l ₂ , l ₁ > l) that generates recording sensitivity at the recording wavelength (l ₁ ) ₂ ) The hologram is recorded by irradiating another light called simultaneously, and at the time of reproduction, only the readout light (wavelength l ₁ ) having the same wavelength as the recording light is emitted. Holographic memory has attracted attention. According to this method, it is possible to realize nondestructive reproduction of a hologram using only light.
L. Solymar, DJ Webb and A. Grunnet-jepsen; The Physics and Applications of Photorefractive Materials, Oxford University Press (1996). X. An, D. Psaltis and GW Burr; “Thermal fixing of 10,000 holograms in LiNbO3: Fe,” Appl. Opt. Vol.38, pp.386-393 (1999). B. Liu, L. Liu and L. Xu; “Characteristics of recording and thermal fixing in lithium niobate,” Appl. Opt. Vol.37, no.11, pp.2170-2176 (1998). J. Ma, T. Chang, J. Hong, R. Neurgaonkar, G. Barbastathis and D. Psaltis; “Electrical fixing of 1000 angle-multiplexed holograms in SBN: 75,” Opt. Lett. Vol.22, no.14 , pp.1116-1118 (1997). D. Kirillov and Jack Feinberg; “Fixable complementary gratings in photorefractive BaTiO3,” Opt. Lett. .Vol.16, no.19, pp.1520-1522 (1991). R. Matull and RA Rupp; “Microphotometric investigation of fixed holograms,” J. Phys. D: Appl. Phys. Vol.21. Pp.1556-1565 (1988). H. Guenther, G. Wittmann, RM Macfarlane, RR Neurgaonkar; “Intensity dependence and white-light gating of two-color photorefractive gratings in LiNbO3,” Opt. Lett. Vol.22, no.17, pp.1305-1307 ( 1997). Hideki Hatano, Satoru Tanaka, Takashi Yamaji, Yoshinao Ito, Hajime Matsushita; “Development of recording materials for digital hologram memory,” Pioneer R & D, vol.11, pp.73-82 (2001). Y. Liu, K. Kitamura, S. Takekawa, G. Ravi, M. Nakamura, H. Hatano, and T. Yamaji; “Nonvolatile two-color holography in Mn-doped near-stoichiometric lithium niobate,” Appl. Phys. Lett. Vol.81, pp.2686-2688 (2002). MD Ewbank, RA Vazquez, RR Neurgaonkar and Jack Feinberg, “Mutually pumped phase conjugation in photorefractive strontium bariumniobate: theory and experiment”, JOSA B, Volume 7, Issue 12, pp.2306-, December 1990.

However, in the case of a holographic memory using the two-photon writing / one-photon reading technique, in order to generate recording sensitivity at the recording light wavelength (l ₁ ) during recording, the light used for recording / reproduction is the wavelength. The problem is that different excitation lights are required, and intermediate excitation of electrons required to generate recording sensitivity at the wavelength of the recording light only when excitation light is irradiated, such as LiNbO ₃ crystal and LiTaO ₃ crystal There is a problem that it can be applied only when a specific inorganic photorefractive material having a level is used as a recording medium.

  In addition, when the data to be recorded is a binary digital image, the original image may contain some noise (noise), or the data may be deteriorated due to some noise being added during recording. Conceivable. Here, since the conventional holographic memory is basically subjected to analog recording, the deteriorated data is recorded and reproduced as it is.

  In order to avoid this, an optical signal containing data before recording is converted into an electrical signal, and after electronic processing is performed to compensate for noise removal and deterioration using an external processing device, the optical signal is converted again. Processing to convert and record in the optical memory is required. Further, with respect to noise and data degradation that occur during hologram reproduction, electronic processing is required to remove it after reproduction.

  From the above, as a major problem of the rewritable hologram memory at present, the data is erased (or deteriorated) by the reading light, and the data is deteriorated due to noise accompanying recording and reproduction. There are two.

  The present invention has been made in view of the above problems, and its purpose is to enable a short-time processing with a simple configuration, and even if reproduction is repeated, the recorded hologram is not deteriorated. An object of the present invention is to provide a hologram processing apparatus and a hologram processing method capable of always performing good reproduction without occurring.

  In order to solve the above problems, a hologram processing apparatus according to the present invention is a hologram processing apparatus for reproducing a hologram recorded in a hologram recording medium by irradiating the hologram recording medium with reading light, A phase conjugate hologram recording medium for recording a hologram, wherein the readout light is applied to the hologram recording medium, and diffracted light emitted from the hologram recording medium is applied to the phase conjugate hologram recording medium; Feedback light having a phase conjugate with the recorded hologram is irradiated from the phase conjugate hologram recording medium onto the hologram recording medium.

  The hologram processing method according to the present invention is a hologram processing method for reproducing a hologram recorded on the hologram recording medium by irradiating the hologram recording medium with the reading light, wherein the reading light is the hologram recording medium. Irradiating a phase conjugate hologram recording medium that records a hologram with diffracted light emitted from the hologram recording medium after being irradiated on the medium, and recorded on the hologram recording medium that is emitted from the phase conjugate hologram recording medium. And irradiating the hologram recording medium with phase-conjugate feedback light.

  In the above configuration and method, first, when reproducing a hologram recorded on the hologram recording medium, the hologram recording medium is irradiated with reading light. When reading light is irradiated onto the hologram recording medium, diffracted light from the recorded hologram is emitted. When this diffracted light is irradiated onto the phase conjugate hologram recording medium, feedback light that is phase conjugate with the hologram recorded on the hologram recording medium is irradiated onto the hologram recording medium.

  Here, when reproduction is performed simply by irradiating the hologram recording medium with readout light, the readout light causes an erasing effect of the hologram recorded on the hologram recording medium, and reproduction is repeated. Or a longer reproduction time causes a problem that the hologram recorded on the hologram recording medium deteriorates.

  On the other hand, according to the configuration and method described above, the readout light is irradiated to the hologram recording medium and the feedback light including image information phase conjugate with the recorded hologram is irradiated. Become. As a result, the rewriting effect by the feedback light is generated in the hologram recording medium so as to complement the erasing effect by the reading light, so that the recorded hologram is not deteriorated. Therefore, even if the reproduction is repeated, it is possible to provide a hologram processing apparatus capable of always performing good reproduction without causing deterioration of the recorded hologram.

  Further, since no fixing technique or the like is used, the time required for the reproduction process is short, real-time processing is possible, and a special electronic circuit or the like can be dispensed with.

  Further, since the light required for reproduction is only the readout light, for example, it is not necessary to provide a plurality of light sources for emitting light of different wavelengths, thereby reducing the apparatus cost and simplifying and downsizing the apparatus. be able to.

  The hologram processing apparatus according to the present invention further includes an excitation light source that irradiates the phase conjugate hologram recording medium with excitation light in the above-described configuration, and the hologram recording medium is irradiated with the readout light to emit the hologram. The diffracted light emitted from the recording medium and the diffracted light generated by the excitation light form a hologram that is phase-conjugated with the hologram recorded on the hologram recording medium in the phase-conjugate hologram recording medium. When the excitation light and the diffracted light are incident on the conjugate hologram recording medium, the phase conjugate hologram recording medium on which the mutual excitation type phase conjugate mirror is generated is incident on the excitation light and the diffracted light. In this case, a mutual excitation type phase conjugate mirror may be generated.

  In the above configuration, when excitation light and diffracted light are incident on the phase conjugate hologram recording medium, a mutual excitation type phase conjugate mirror is generated therein. As will be described in detail later, this mutual excitation type phase conjugate mirror is generated only when the ratio of the light intensity of the diffracted light to the light intensity of the excitation light is equal to or greater than a predetermined threshold value. Here, when noise is generated in the hologram recorded on the hologram recording medium, the light intensity of the diffracted light due to the noise may be smaller than the light intensity of the diffracted light from the part that is normally recorded. is expected. Therefore, if the ratio of the light intensity of the diffracted light due to the noise to the excitation light is smaller than the threshold value, the information about the noise is not reflected in the mutual excitation type phase conjugate mirror. In this case, noise information is not included in the feedback light, and noise generated in the hologram recorded on the hologram recording medium is erased by the feedback light.

  That is, according to the above configuration, it is possible to improve the quality of reproduced data by erasing noise generated in the hologram recorded on the hologram recording medium. For example, a noise removal function is provided for analog data reproduction, and the image quality of the recorded image can be improved. Digital data can be reproduced with a low bit error rate.

  In addition, since the excitation light can be realized by light having the same wavelength as the readout light, there is no need to provide a plurality of light sources for emitting light of different wavelengths, and the apparatus cost can be reduced and the apparatus can be simplified and miniaturized. Can do.

  Further, the hologram processing apparatus according to the present invention further includes a light modulation unit that performs spatial modulation on the light based on the two-dimensional image information and irradiates the hologram recording medium with light as object light in the above-described configuration, The hologram recording medium may be further subjected to a process of recording a hologram on the hologram recording medium by irradiating the hologram recording medium with object light and reference light.

  According to the above configuration, information can be recorded on the hologram recording medium. Therefore, it is possible to provide a hologram processing apparatus capable of holding recorded information for a long time without deterioration.

  The hologram processing apparatus according to the present invention further includes a control unit that controls irradiation of the readout light, the excitation light, the object light, and the reference light and controls the operation of the light modulation unit in the above configuration. The control unit controls the light modulation means to perform a light modulation operation according to information to be recorded at the time of recording, and irradiates the hologram recording medium with the object light and the reference light, A configuration may be adopted in which the readout light and the excitation light are controlled to irradiate the hologram recording medium and the phase conjugate hologram recording medium, respectively, during reproduction.

  According to said structure, a recording process and a reproduction | regeneration process are performed based on the above controls by a control part. Therefore, it is possible to provide a hologram processing apparatus capable of accurately controlling the recording process and the reproduction process.

The hologram processing apparatus according to the present invention may be configured such that, in the above configuration, the hologram recording medium is composed of a BaTiO ₃ crystal.

According to the above configuration, since the hologram recording medium is composed of BaTiO ₃ crystal, it is not necessary to use a material that takes time to write, such as LiNbO ₃ crystal. Therefore, the processing time can be reduced.

  As described above, the hologram processing apparatus according to the present invention includes the phase conjugate hologram recording medium for recording the hologram, and the diffracted light emitted from the hologram recording medium when the readout light is irradiated onto the hologram recording medium. Feedback light that is irradiated onto the phase conjugate hologram recording medium and is phase conjugate with the hologram recorded on the hologram recording medium is irradiated onto the hologram recording medium from the phase conjugate hologram recording medium.

  Thereby, even if the reproduction is repeated, there is an effect that it is possible to provide a hologram processing apparatus capable of always performing good reproduction without causing deterioration of the recorded hologram.

  An embodiment of the present invention will be described below with reference to FIGS.

(Configuration of holographic memory device)
FIG. 13 shows a schematic configuration of a holographic memory device (hologram processing device) according to an embodiment of the present invention. As shown in the figure, the holographic memory device includes a holographic memory (hologram recording medium) 1, a phase conjugate mirror (phase conjugate hologram recording medium) 2, a beam splitter 3, a first light source 4, a second light source 5, A third light source 6, an SLM (light modulation means) 7, a photodetector (light detection means) 8, and a control unit (control means) 9 are provided.

  The holographic memory 1 is a hologram recording unit that can be rewritten by light. The phase conjugate mirror 2 generates feedback light described later. Details of the holographic memory 1 and the phase conjugate mirror 2 will be described later.

  The first light source 4 is a light source for irradiating the SLM 7 with light. The SLM 7 modulates the light from the first light source 4 according to the data to be recorded, and makes it incident on the beam splitter 3 as object light. The beam splitter 3 guides the object light from the SLM 7 to the holographic memory 1 and guides the reproduction light incident from the holographic memory 1 to the photodetector 8. The photodetector 8 detects the reproduction light incident from the beam splitter 3 and reproduces data.

  The second light source 5 is a light source for irradiating the holographic memory 1 with reference light at the time of writing and read light at the time of reading. The reference light incident on the holographic memory 1 from the second light source 5 is coherent with the object light incident on the holographic memory 1 from the first light source 4 via the SLM 7 and the beam splitter 3. Controlled by. The third light source 6 is a light source for irradiating the phase conjugate mirror 2 with excitation light.

  The controller 9 controls the overall operation of the holographic memory device. Specifically, the control unit 9 controls the operations of the first light source 4, the second light source 5, the third light source 6, the SLM 7, and the photodetector 8. The following hologram recording / reproduction processing is performed based on the control of the control unit 9.

  In the above configuration, the first light source 4, the second light source 5, and the third light source 6 are provided as separate configurations. However, actually, light emitted from a single light source is converted into a beam. A configuration may be adopted in which the light is separated into three light beams using a splitter, a mirror, and the like, and functions as each light source. In this case, since a single light source can be used, the apparatus cost can be reduced and the configuration can be simplified.

  Further, in the configuration shown in FIG. 13, the configuration for multiplex recording of holograms is not considered, but such a configuration may be provided. For example, as an example of a hologram multiple recording method, when using a speckle multiple recording method that changes the wavefront of the reference light for each hologram, a configuration may be adopted in which ground glass is arranged on the optical path of the reference light described later. Also, as a wavelength multiplex recording method that changes the wavelength of the reference light for each hologram, an angle multiplex recording method that changes the angle of the reference light for each hologram, a phase code multiplex recording method that changes the phase of the reference light for each hologram, and reference light Other hologram multiplex recording methods such as a spherical reference light shift multiplex recording method in which a spherical wave is used and a recording medium (holographic memory) is moved by a minute distance for each hologram may be used.

  In the above configuration, both the configuration for recording the hologram and the configuration for reproducing the hologram are provided. However, only the hologram may be reproduced. In this case, the configuration of the first light source 4, the SLM 7, the beam splitter 3 and the like may be omitted.

  FIG. 1 shows a schematic configuration of a main part of the holographic memory device. As shown in the figure, the holographic memory device includes a holographic memory 1 that is rewritable by light, and a mutually pumped phase conjugate mirror (MPPCM) 2 as a feedback generation unit. (Hereinafter referred to as phase conjugate mirror 2) is provided. The holographic memory 1 is composed of a photorefractive medium. In this holographic memory device, data is written by object light (Object beam) and reference light (Reference beam). Further, reading of data from which noise has been removed and reading in a state where recording data is held are performed by reading light (Reading beam) and excitation light (Pump beam). With the above configuration, a rewritable holographic optical memory having a noise removal function and capable of nondestructive reading by light can be realized.

(Recording / rewriting operation)
Next, a data recording operation and a rewriting operation in the holographic memory device will be described with reference to FIG.

  The object beam transmitted through the beam splitter 3 is incident on the rewritable holographic memory 1 together with the reference beam. Data in the object light is recorded as a hologram (refractive index grating) H induced according to the interference fringes of these two light waves. Here, when there is a hologram H previously written in the holographic memory 1, the hologram H is erased, and the hologram H based on the newly incident object beam (Object beam) is written in the holographic memory 1. . Thereby, the rewriting operation is performed.

(Read operation)
Next, a data reading operation in the holographic memory device will be described with reference to FIG.

  When reading the data recorded in the holographic memory 1, the reading light (Reading beam) is irradiated on the holographic memory 1, and the excitation light (Pump beam) is incident on the phase conjugate mirror 2. The readout light enters the holographic memory 1 from the same direction as the reference light used during recording. The readout light is diffracted by the hologram H in the holographic memory 1, and the diffracted light (Diffraction beam) enters the phase conjugate mirror 2.

  In the phase conjugate mirror 2, mutual phase conjugation is performed via a mutual excitation type phase conjugation phenomenon caused by incidence of diffracted light and excitation light only when the intensity of diffracted light is larger than an intensity threshold determined by the intensity of excitation light. A hologram that generates light is generated. As a result, a feedback beam (Feedback beam) that is a phase conjugate of the diffracted light is generated, and automatically enters the phase matching condition of the hologram H recorded in the hologram memory 1.

  The feedback light transmitted through the hologram H in the holographic memory 1 is phase conjugate of the object light, and is output as output light (Output beam) via the beam splitter 3. Reproduction is performed by detecting this output light.

  In this reproduction process, a hologram erasing effect by beam exposure and two hologram rewritings as shown below occur simultaneously. One is rewriting due to interference between readout light and diffracted light, and the other is rewriting due to interference between feedback light and diffracted light of feedback light. By combining these two rewritings, under the optimum conditions, a rewriting effect exceeding the erasing effect by beam exposure can be obtained. Therefore, according to the above configuration, the hologram is held without performing processing such as heat fixing and electric field fixing, and all-optical nondestructive reproduction of recorded data is possible.

(Maintaining holograms by rewriting data during playback)
As described above, in the holographic memory device according to this embodiment, reading light and feedback light are simultaneously incident, that is, each rewrite obtained by simultaneously incident beams from both ends of the rewritable holographic memory 1. By combining the effects, it is possible to prevent the hologram H recorded in the holographic memory 1 from being deteriorated. Here, a case where a photorefractive crystal representative as a hologram material rewritable only by light is used as a recording medium of the holographic memory 1 will be described in detail.

  The rewriting effect of the hologram is that the readout light and the readout light diffracted by the hologram (the readout light has information on the recorded data, hereinafter referred to as diffracted light) form interference fringes, This is realized by recording the hologram having the information of the original object light again by the interference fringes. The strength of rewriting depends on the intensity ratio between the readout light and the diffracted light. When the intensity ratio is 1: 1, the contrast of the interference fringes due to the two light waves is the highest, so that writing is strongest.

  On the other hand, when a light wave having a distribution different from the light intensity distribution when the grating is recorded is incident on the hologram in the photorefractive crystal, the recorded hologram is erased.

  In FIG. 4, the propagation direction z of the light wave is the horizontal axis, the time elapsed from the start of the reading light irradiation is the vertical axis, and the mutual length defined as the length in the z direction of the region where the light waves interact via the photorefractive effect. The temporal and spatial changes of the hologram when the action length is L are shown. Here, when the reading light is incident on the hologram recorded in the photorefractive crystal from the direction of z = 0 in FIG. 4, the rewriting effect and the erasing effect generated in the photorefractive crystal are considered together. As shown in FIG. 4, only the erasing effect is produced at the point of z = 0, and the rewriting effect is produced at the point after z = 0 + dz.

  First, considering the point of z = 0 + dz, there are readout light and diffracted light from the hologram at the point of z = 0 at this point. Therefore, a hologram having the same information as the recorded hologram is written by the interference fringes between the diffracted light and the readout light. However, paying attention to the point where z = 0, there is no diffracted light here, and only readout light is incident on the hologram. Of course, since the light intensity distribution of the reading light is different from the light intensity distribution when the hologram is recorded, the hologram at the point of z = 0 is attenuated as time elapses, and is finally completely erased.

  As the above phenomenon continues in time and space, the hologram distribution is attenuated while moving in the positive z direction. When a region where the rewriting effect can be obtained reaches the crystal end face where z = L, there is no longer a region where rewriting is performed. For this reason, only a strong erasing effect remains, and the hologram in the crystal is completely erased as time elapses.

  In addition, when only the feedback light is incident from the direction opposite to the incident direction of the readout light, the same phenomenon as that when only the readout light is incident occurs in the negative direction of z which is the reverse direction. As a result, as in the case where only the readout light is incident, the hologram in the crystal is completely erased over time.

  However, when the reading light and the feedback light are simultaneously incident from both sides of the crystal, the hologram is maintained even after a lapse of time. In this case, as shown in FIG. 5, the following two hologram complementing actions occur. The first action is an action that complements the hologram in which the rewriting effect generated by the feedback light and its diffracted light is weakened by the erasing effect caused by the incidence of the reading light. The second effect is that the rewriting effect generated by the readout light and its diffracted light complements the hologram that is weakened by the erasing effect caused by the incidence of the feedback light. When the coupling strength of the photorefractive crystal is high, rewriting can be always repeated at a position where each rewriting effect exceeds the erasing effect even if time passes by these two actions. With the above mechanism, the hologram is maintained without being erased, and the recorded data can be reproduced nondestructively.

(Noise reduction using threshold characteristics of phase conjugate mirror)
The mutual excitation type phase conjugate mirror 2 is a phase conjugate mirror that can generate mutual phase conjugate light when two incoherent light waves are incident on each other. In order for the phase conjugate mirror 2 to function properly as a phase conjugate mirror as described above, that is, to generate a mutual excitation type phase conjugate mirror in the phase conjugate mirror 2, the intensity ratio of the two incident light waves is It becomes an important factor. If an appropriate intensity ratio is given, the phase conjugate mirror 2 can function properly as the phase conjugate mirror as described above. It cannot function properly as a conjugate mirror. Although the limit of the appropriate light intensity ratio will be described later in detail, it is determined by the bond strength, and the value of the bond strength at that time is referred to as a bond strength threshold. The formation process of the mutual excitation type phase conjugate mirror is shown in Non-Patent Document 10 described above.

FIG. 6 shows the relationship of the coupling intensity threshold value with respect to the incident intensity ratio in the phase conjugate mirror 2. Here, the intensity of the two light waves is I ₁ and I ₂ . When the incident intensity ratio is 1, that is, the intensity of the two incident light waves is equal, the minimum coupling intensity threshold value 2.0 is taken. As the incident intensity ratio increases or decreases, that is, the intensity difference between the two incident light waves increases, the coupling intensity threshold increases.

  FIG. 7 shows (a) and (b) a case where a light wave having almost no difference in incident intensity is incident on the medium of the phase conjugate mirror 2 and a case where a light wave having a large difference in incident intensity is incident on the medium. For example, when the coupling strength of the medium of the phase conjugate mirror 2 is 3.0, the states of (a) and (b) in FIG. 7 are the states of (a) and (b) shown in the graph of FIG. Corresponds to the incident intensity ratio. As shown in FIG. 7A, when there is almost no difference in incident intensity, the coupling intensity threshold obtained by the relationship of FIG. 6 is sufficiently smaller than the value of the coupling intensity in the medium of the phase conjugate mirror 2. Therefore, a mutual excitation type phase conjugate mirror is generated in the medium of the phase conjugate mirror 2 (the phase conjugate mirror 2 functions as the mutual excitation type phase conjugate mirror 2), and phase conjugate lights of two light waves are generated. .

  On the other hand, as shown in FIG. 7B, when the difference in incident intensity is large, the coupling intensity threshold obtained by the relationship of FIG. 6 becomes larger than the coupling intensity of the medium of the phase conjugate mirror 2. Therefore, a mutual excitation type phase conjugate mirror is not generated in the medium of the phase conjugate mirror 2 (the phase conjugate mirror 2 does not function as the mutual excitation type phase conjugate mirror 2). In FIG. 7, when a mutual excitation type phase conjugate mirror is generated in the medium of the phase conjugate mirror 2 (a), phase conjugate light is generated.

  Considering the above, when the phase conjugate mirror 2 is used as an optical feedback circuit as shown in FIG. 1 and the like, if the excitation light intensity (Pump beam in FIGS. 1 and 3) as the first light is constant, Even if diffracted light having a smaller intensity such as noise (Diffraction beam in FIGS. 1 and 3) is incident as the second light, a mutual excitation type phase conjugate mirror is not generated in the medium of the phase conjugate mirror 2. The feedback light (Feedback beam in FIGS. 1 and 3) is not generated.

  Since ordinary holographic memory performs analog recording, the noise component contained in the original data and the noise component generated during recording / reproduction are not removed, which causes degradation of the reproduced data as they are. . On the other hand, according to the configuration of the present embodiment, even if such a noise component occurs, a threshold sufficient to generate a mutual excitation type phase conjugate mirror whose intensity is sufficiently smaller than the excitation light intensity. If the value does not reach, feedback light due to these noise components does not occur and is not maintained during reproduction. Therefore, even if the hologram component is temporarily written by such a noise component and the recorded data is deteriorated, it is considered that it disappears due to the erasing action by the reading light during reproduction. With the mechanism as described above, it is possible to significantly reduce the possibility that the recorded hologram is degraded by the noise component, or that the noise component itself is erroneously recorded and the quality of the reproduction data is lowered.

(Example of numerical analysis)
Next, numerical analysis results on the nondestructive reading performance and noise removal performance according to the configuration of the present embodiment will be described. FIG. 8 schematically shows the flow of processing in performing this numerical analysis. In the figure, the vertical axis represents the passage of time. In the figure, OJ is an object beam, RD is a reading beam, RF is a reference beam, DF is a diffracted beam, and FB is a feedback beam. , And PU indicate excitation light (Pump Beam). The interaction of each light wave is expressed by the following coupled wave equation.

Here, I _2S (0) is the light intensity of the object light, I _1S (0) is the light intensity of the readout light or reference light, I _3M (L _M ) is the light intensity of the diffracted light, and I _3S (L _S ) is The light intensity of the feedback light, I _1M (0), represents the light intensity of the excitation light. A _jk (j = 1 to 4) represents the amplitude of each light wave, and the subscript k (k = S, M) represents the holographic memory 1 and the phase conjugate mirror 2, respectively. γ _k and τ _k are coupling coefficients and time constants in the interaction, and Q _k is a function proportional to the spatial electric field induced by the photorefractive effect. Further, e _jk (described as a vector e hat in _Equation 1) is a polarization unit vector, I _0k is the total light intensity, and L _k is an interaction length of the light wave. In this analysis, the time constant of the holographic memory 1 was set to 1 s with respect to the light intensity of 50 mW / cm ² and τ _M / τ _S = 0.05. In any analysis, as shown in FIG. 8, the object light and the reference light are incident on the holographic memory 1 for 50 seconds to record the hologram, and then the reproduction is performed by causing the reading light and the excitation light to enter. Suppose that the bond strength represented by the product of the coupling coefficient and the interaction length (γ _k L _k ) is γL _k .

(Non-destructive readout characteristics)
9, at the time of reproduction, in the case where the three values of binding strength GanmaL _S of the holographic memory 1 shows the time response characteristic of each of the output light intensity I _{3S (0).} Here, the object light intensity _I 2S (0), the read light intensity _I 1S (0) and the excitation light intensity _I 1M (0), respectively ^{10mW / cm 2, 10mW / cm} 2, and 20 mW / cm ^2, phase conjugation It was 3 the bonding strength GanmaL _M of the mirror 2. In the case of γL _S = 2.1, the output light intensity temporarily increases, but decreases immediately and disappears after about 30 seconds. On the other hand, in the case of γL _S = 2.4 and γL _S = 2.7, it can be confirmed that the output light continues to be generated for 200 seconds or more. From the above, it can be seen that nondestructive reading can be achieved by using a recording medium having a relatively large coupling coefficient as the holographic memory 1.

(Noise reduction characteristics)
FIG. 10 shows time response characteristics of the output light intensity I _3S (0) at the time of reading when the signal level of the object light used at the time of recording is set to four kinds of values. Here, the read light intensity _I 1S (0) and the excitation light intensity _I 1M (0), respectively 10 mW / cm ^2, and a 20 mW / cm ^2, binding of the holographic binding strength of the memory 1 γL _S and the phase conjugate mirror 2 The intensity γL _M was 2.4 and 3, respectively. When the signal level of the object light is 0 dB, −10 dB, and −20 dB, the output light intensity continues to be generated and takes a constant value of about 6.5 mW / cm ² . On the other hand, when the signal level of the object light is −30 dB, no output light is generated. From the above, the configuration of the present embodiment has an intensity threshold for recordable signal levels, and has a function of binarizing (digitizing) data with the intensity threshold as a boundary. Recognize.

  From the above, the holographic memory device according to the present embodiment has a noise removal function for analog data, and when used for recording / reproducing digital data, a low bit at the physical layer level. It can be seen that recording / reproduction with an error rate becomes possible.

(Experimental example)
Next, an experimental example in which recording / reproduction processing is actually performed using the holographic memory device according to the present embodiment will be described. In this experimental example, a BaTiO ₃ crystal (photorefractive crystal) was used as a recording medium constituting the holographic memory 1 and the phase conjugate mirror 2, and an argon ion laser having a wavelength of 514.5 nm was used as a laser light source. An outline of the experimental system in this case is shown in FIGS. 11 (a) to 11 (c). In addition, FIGS. 12A to 12C show temporal changes from the start of readout of reproduced images as experimental results, and FIG. 12D shows input images used in the experiments. In FIGS. 12A to 12C, the time indicated at the bottom of the reproduced image indicates the elapsed time from the start of reading.

  The experimental system shown in FIG. 11A shows a known holographic memory device system in which only the holographic memory 1 is provided and the phase conjugate mirror 2 is not provided. FIG. 12A shows the time change of the reproduced image by this system. As shown in the figure, the data is correctly reproduced at the start of reading, but it can be seen that most of the data disappears after 2 minutes and the data is almost completely erased after 5 minutes.

  The experimental system shown in FIG. 11C corresponds to the holographic memory device according to the present embodiment, and includes a holographic memory 1 and a phase conjugate mirror 2, and has a mutual excitation type phase. Feedback light from the conjugate mirror 2 is used. FIG. 12C shows the time change of the reproduced image by this system. As shown in the figure, it can be seen that even after 20 minutes have elapsed from the start of reading, the recorded data has not been erased and the nondestructive reading has succeeded. Further, it can be seen that even if long-time reading is performed, noise is not mixed in the image, and there is a noise removal effect.

  FIG. 11B shows a modified example of the holographic memory device according to the present embodiment, in which a self-excited phase conjugator (SPPCM (Self Pumped Phase Conjugate Mirror)) is provided as the phase conjugate mirror 2 ′. The structure using light is shown. FIG. 12B shows the time change of the reproduced image by this system. As shown in the figure, even in this system, the recorded data is not erased even after 20 minutes from the start of reading, and reading for a long time is possible. However, noise is generated in the reproduced image, and this noise is not removed over time, and an unclear reproduced image is observed. That is, it can be seen that this system has no noise removal effect. From the above, it can be understood that the configuration in which feedback is performed using the mutual excitation type phase conjugate mirror 2 is advantageous in terms of both image quality and nondestructive readout time.

  The distance between the holographic memory, the lens, and the phase conjugator is different between FIG. 11B and FIG. 11C for the convenience of the experimental equipment. In either system, the holographic memory, the lens, and the phase conjugate. It is possible to make the distances of the vessels equal. The difference between the two is that in SPPCM, it is not necessary to enter the excitation light into the phase conjugator, and the light beam generated at a light beam angle so that the light generated in the phase conjugator is totally reflected in the crystal. Incident. On the other hand, in MPPCM, it is necessary for the excitation light to be incident on the phase conjugator, and it is necessary that both incident waves to the phase conjugator be incident at an angle that efficiently generates beam fanning.

  The specific embodiments and examples described above are merely to clarify the technical contents of the present invention, and should not be interpreted in a narrow sense by limiting only to such specific examples. Rather, various modifications can be made within the spirit and scope of the present invention.

  The holographic memory device according to the present invention can be used as an information recording / reproducing device that is required to record a large amount of data and to perform a high data recording speed and a high data reading speed.

It is a figure which shows schematic structure of the principal part of the holographic memory device which concerns on one Embodiment of this invention. It is a figure which shows schematic structure of the principal part of the holographic memory device at the time of recording / rewriting operation | movement. It is a figure which shows schematic structure of the principal part of the holographic memory device at the time of reproduction | regeneration operation | movement. It is a figure which shows typically the temporal and spatial change of the hologram by rewriting and erasure | elimination accompanying incident of reading light. It is a figure which shows typically the complementation effect | action of two holograms accompanying incidence of reading light and feedback light. It is a graph which shows the coupling intensity threshold with respect to the incident intensity ratio in a mutual excitation type phase conjugate mirror. It is a figure which shows typically the incident light intensity ratio dependence of the production | generation of a mutual excitation type | mold phase conjugate mirror. It is a figure which shows typically the flow of a process in performing the numerical analysis about the nondestructive read-out performance and noise removal performance by the structure of this embodiment. It is a graph which shows the nondestructive read-out characteristic with respect to the coupling strength of a holographic memory. It is a graph which shows the time response characteristic of the output light intensity at the time of each reading when the signal level of object light is made into four kinds of values. The same figure (a)-the figure (c) are figures which show schematic structure of the principal part of an experiment system. FIGS. 7A to 7C are diagrams showing temporal changes from the start of readout of reproduced images as experimental results, and FIG. 9D is a diagram showing input images used in the experiments. . It is a block diagram which shows schematic structure of a holographic memory device.

Explanation of symbols

1 Holographic memory (hologram recording medium)
2 Phase conjugate mirror (phase conjugate hologram recording medium)
3 beam splitter 4 first light source 5 second light source 6 third light source (excitation light source)
7 SLM (light modulation means)
8 Light detector (light detection means)
9 Control unit (control means)

Claims (6)

A hologram processing apparatus for reproducing a hologram recorded on a hologram recording medium by irradiating the hologram recording medium with reading light,
A phase conjugate hologram recording medium for recording a hologram;
The readout light is applied to the hologram recording medium, the diffracted light emitted from the hologram recording medium is applied to the phase conjugate hologram recording medium, and feedback light that is phase conjugate with the hologram recorded on the hologram recording medium is generated. The hologram processing apparatus irradiates the hologram recording medium from the phase conjugate hologram recording medium. An excitation light source for irradiating the phase conjugate hologram recording medium with excitation light;
The readout light is applied to the hologram recording medium and is recorded on the hologram recording medium in the phase conjugate hologram recording medium by diffracted light generated by the diffracted light emitted from the hologram recording medium and the excitation light. A hologram that is phase conjugate with the hologram is formed,
2. The hologram processing apparatus according to claim 1, wherein when the excitation light and the diffracted light are incident on the phase conjugate hologram recording medium, a mutual excitation type phase conjugate mirror is generated. Light modulation means for performing spatial modulation on the light based on the two-dimensional image information and irradiating the hologram recording medium with light as object light;
2. The hologram processing apparatus according to claim 1, further comprising a process of recording a hologram on the hologram recording medium by irradiating the hologram recording medium with object light and reference light. A controller that controls irradiation of the readout light, the excitation light, the object light, and the reference light, and controls the operation of the light modulation unit;
The control unit controls the light modulation means to perform a light modulation operation according to information to be recorded at the time of recording, and controls to irradiate the hologram recording medium with the object light and the reference light. 4. The hologram processing apparatus according to claim 3, wherein the reading light and the excitation light are controlled to irradiate the hologram recording medium and the phase conjugate hologram recording medium, respectively. The hologram processing apparatus according to claim ₃ , wherein the hologram recording medium is composed of a BaTiO ₃ crystal. A hologram processing method for reproducing a hologram recorded on a hologram recording medium by irradiating the hologram recording medium with readout light,
Irradiating the phase conjugate hologram recording medium for recording the hologram with the diffracted light emitted from the hologram recording medium when the readout light is irradiated on the hologram recording medium;
A hologram processing method comprising: irradiating the hologram recording medium with a hologram recorded on the hologram recording medium and phase conjugate feedback light emitted from the phase conjugate hologram recording medium.

Images