JP2006126642A - Hologram memory device and method for erasing hologram - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To selectively erase a multiple hologram with high accuracy by a small/simple and inexpensive optical system and to realize a small/simple and inexpensive hologram memory device. <P>SOLUTION: The device is equipped with a double Mach-Zehnder interference optical system 25 using light made incident onto both surfaces of a beam splitter 14 as an optical system to irradiate a hologram memory 11 with object light 19 and reference light 20. A multiple hologram is selectively erased by irradiating the hologram with object light 19 and reference light used for recording the hologram by using light made incident onto other surface of the beam splitter 14 in the optical system 25, the surface different from the surface used for irradiation of the hologram with the object light 19 and the reference light 20 on recording. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a hologram memory device that uses, for example, a photorefractive crystal as a hologram memory, and records, reproduces, and erases a hologram in the crystal, and in particular, an erasing method that enables selective erasure of multiple holograms, And a hologram memory device.

  The hologram memory (holographic memory) records and reproduces two-dimensional data in the form of a hologram. Therefore, high-speed data transfer is possible as compared with a conventional optical memory, and a large capacity can be realized by multiplex recording of holograms at the same location in the hologram medium. In particular, in a hologram memory using a photorefractive medium (photorefractive crystal) as a hologram medium, it is possible to erase a once recorded hologram, 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.

  Holograms (multiple holograms) recorded in a multiple manner in such a photorefractive medium can be erased collectively by irradiation with uniform incoherent light or heat treatment. However, in a hologram memory that multiplex-records 1 terabyte of data in a recording area of about 1 cm square, it is practically important to selectively erase only one hologram in the multiplex hologram in addition to batch erasure. .

  Holograms are recorded in the photorefractive medium as interference fringes of object light and reference light to which two-dimensional data is added. It is known that the hologram can be erased at a much higher speed than when the hologram is irradiated with uniform incoherent erasing light by overwriting the fringe with the interference fringe whose phase is spatially displaced by π. ing. An interference fringe whose phase is spatially displaced by π from the interference fringe at the time of recording can be obtained by π-displacing either the phase of the object light or the reference light by a phase shifter.

  In the following, with reference to FIG. 9, multiple recording of holograms on a photorefractive medium and selective erasure of multiple holograms by overwriting the interference fringes at the time of recording described above with interference fringes whose phase is spatially displaced by π will be described. To do.

  In FIG. 9, when the first light wave 51 is the reference light and the second light wave 52 is the object light having the two-dimensional image data, the hologram is recorded in the photorefractive medium by the interference fringes 54 of the reference light and the object light. . Here, the phase shifter 53 arranged on the optical path of the reference light switches and changes the phase of the reference light according to 0 or π, but the phase shift amount during recording (amount to change the phase) Is 0 and does not change the phase of the reference beam. Further, multiplex recording of holograms is usually performed by changing the angle, wavelength, phase distribution, or the like of reference light for each hologram.

  In the selective erasure of the multiplex hologram recorded in this way, the same angle and the same angle as the recording of the hologram to be erased with respect to the hologram to be erased (hologram to be erased) of the multiplex recorded holograms. Reference light having a wavelength or the same phase distribution (in other words, reference light satisfying the reproduction condition of the erasure-symmetric hologram) and the same object light as that at the time of recording are made incident on the photorefractive medium. Here, the phase of the reference light is displaced by p with respect to the phase at the time of recording by the phase shifter 53 arranged on the optical path of the reference light, and the interference fringe is spatially displaced by π from the time of recording. 54 'is overwritten on the hologram to be erased, and the hologram to be erased is erased.

  In this case, for other multiplex holograms, these selective erasure lights (object light and reference light displaced by p) work as incoherent erasure lights. However, as described above, the speed of selective erasure is much faster than that of incoherent erasure, so that only the hologram to be erased is selectively erased, and other multiple holograms are not erased simultaneously.

Conventionally, as the phase shifter 53, a configuration in which a liquid crystal phase modulator is disposed on the optical path of the reference light and the phase of the reference light is delayed by π by electronic control thereof, or a piezo mirror is used as the reference light or object. There is a configuration in which the phase of the interference fringes is displaced by arranging on the optical path of light and creating a half-wavelength optical path difference by mechanical movement (for example, Non-Patent Documents 1 and 2).
H. Sasaki, J. Ma, Y. Fainman, and S. Lee, “Fast update of dynamic holographic memory,” Opt. Lett. 17, 20, pp.1468-1470, 1992. JV Alvarez-Bravo, L. Arizmendi, “Coherent erasure and updating of holograms in LiNbO3,” Opt. Mater. 4, pp.419-422, 1995. P. Yeh, Introduction to Photorefractive Nonlinear Optics, John Wiley & Sons, Inc., New York, 1993.

  However, the configuration of a conventional hologram memory device using electronic control using a liquid crystal phase modulator or mechanical control using a piezoelectric mirror as the phase shifter 53 has the following problems.

  First, in a phase shifter using a liquid crystal phase modulator or a piezo mirror, the optical system becomes large and complicated, and the liquid crystal phase modulator and the piezo mirror are expensive elements. This leads to a price increase.

  In addition, in a phase shifter using a liquid crystal phase modulator or a piezo mirror, an error occurs in the amount of phase shift due to a voltage error of the liquid crystal phase modulator or backlash of the piezo mirror, thereby selectively erasing multiple holograms. May degrade the performance.

  Furthermore, the phase shift amount used for selective erasure of the multiple holograms is only 0 at the time of recording or reproduction and π at the time of erasure, but with a phase shifter using a liquid crystal phase modulator or a piezoelectric mirror. Since a continuous intermediate value which is unnecessary for this application can also be taken, this also causes an error in the amount of phase shift, and there is a possibility that the performance of selective erasure of multiple holograms is deteriorated.

  Regardless of the erasure of multiple holograms, the erasure method by overwriting the interference fringes at the time of recording and the interference fringes whose phase is spatially shifted by π is also used for erasing holograms that are not multiplex recorded. This is useful because it can be erased at a higher speed than coherent erasing light, but has the same problem as described above.

  The present invention has been made in view of the above problems, and its object is to avoid the increase in size and complexity of the hologram memory device caused by the use of a phase shifter by electronic or mechanical control, and to reduce the size, simplicity, and An inexpensive optical system is capable of erasing a hologram with high accuracy, and thus a small, simple and inexpensive hologram memory device is realized.

  In order to solve the above problems, the hologram erasing method of the present invention irradiates a hologram memory with object light and reference light, and records a hologram among the holograms recorded in multiple by these interference fringes. A hologram erasing method for erasing an object beam and a reference beam at the time of recording by irradiating the hologram memory so that these interference fringes are displaced by π from the interference fringes at the time of recording. As the optical system for irradiating the object light and the reference light, a double Mach-Zehnder-interference optical system using incident light from both surfaces of the beam splitter is used, and the object is recorded when the hologram to be erased is recorded in the beam splitter of the optical system. By generating object light and reference light at the time of erasure with incident light from a surface different from the one used to generate light and reference light It is characterized by obtaining the interference fringe and π only displaced interference fringe during recording.

  The double Mach-Zehnder-interference optical system, which will be described in detail later, is an interference fringe formed in a crossing region by a light wave obtained by demultiplexing a light wave incident from one surface of the beam splitter, and the other surface of the beam splitter. There is a characteristic that there is a spatially p phase difference between the light wave incident from the light wave and the interference fringe formed in the intersection region by propagating along the same optical path as the light wave.

  Therefore, as described above, by using the double Mach-Zehnder-interference optical system, the incidence of the light wave serving as the reference light and the object light is switched between the recording time and the erasing time, and the interference fringes at the time of recording are displaced by π. Since interference fringes can be obtained, a multi-hologram can be selectively erased, but a phase shifter is not necessary.

  Therefore, by using this erasing method, multiple holograms can be selectively erased with high precision, and the device configuration that realizes this method can be realized in a small, simple and inexpensive manner, so it is adopted in a hologram memory device. Thus, a small, simple and inexpensive hologram memory device can be realized.

  In order to solve the above problem, the hologram erasing method of the present invention irradiates the hologram memory with object light and reference light, and records the hologram recorded by these interference fringes as object light at the time of recording the hologram. A hologram erasing method for erasing the reference light by irradiating the hologram memory so that these interference fringes are displaced by π from the interference fringes at the time of recording. Is used to generate object light and reference light when recording a hologram in the beam splitter of the optical system, using a double Mach-Zehnder interference optical system that uses incident light from both sides of the beam splitter. By generating the object light and reference light at the time of erasing with incident light from a different surface, the interference fringes at the time of recording and the interference fringes displaced by π It is characterized by obtaining.

  Similar to the selective erasure of multiple holograms, the double Mach-Zehnder-interference optical system is also used for holograms that are not multiple-recorded, and the incidence of light waves as reference light and object light is different between recording and erasing. By simply switching, an interference fringe at the time of recording and an interference fringe displaced by π can be obtained, so that a hologram can be erased at high speed, but a phase shifter is not necessary.

  Therefore, by using the erasing method, the hologram can be erased at high speed and with high accuracy, and the device configuration for realizing the method can be realized in a small, simple and inexpensive manner, and therefore, it can be adopted in a hologram memory device. Thus, a small, simple and inexpensive hologram memory device can be realized.

  In order to solve the above-mentioned problem, the hologram memory device of the present invention irradiates the hologram memory with object light and reference light, records a hologram by these interference fringes, and has a plurality of holograms at the same location. In a hologram memory device comprising a recording means for recording in a multiplexed manner and a reproducing means for selectively reproducing any hologram among the holograms recorded in a multiplexed manner by irradiating with a reference light, the object light and the reference light are As an optical system for irradiating the hologram memory, a double Mach-Zehnder-interference optical system using incident light from both surfaces of the beam splitter is provided.

  More preferably, it comprises an erasing means for selectively erasing an arbitrary hologram among the holograms recorded in a multiplexed manner in the hologram memory, the erasing means using the optical system, and the beam splitter in the optical system. The recording means irradiates the object light and the reference light at the time of recording the hologram from the light incident from the other surface different from the surface used for the irradiation of the object light and the reference light at the time of recording the hologram to be erased. It is to be erased.

  As described above, by using the double Mach-Zehnder-interference optical system, multiple holograms can be selectively erased without the need for a phase shifter. Compared to the configuration in which liquid crystal phase modulators and piezo mirrors are arranged as phase shifters and electronic or mechanical control is used, multiple holograms can be selected with a very small, simple and inexpensive optical system. Erasure becomes possible.

  Also, in the configuration using a conventional phase shifter such as backlash when using a piezo mirror and voltage error when using a liquid crystal phase modulator, the interference fringes when recording a hologram are spatially accurate. There is a possibility that the hologram erasing performance is deteriorated by overwriting without causing interference fringes whose phase is shifted by π. However, in the present invention, the element used for phase shifting at the time of erasing is only a beam splitter, and an error is caused. Can be selectively erased with high accuracy and little deterioration with time.

  Further, the phase displacement of the interference fringes by the double Mach-Zehnder-interference optical system cannot take a continuous intermediate value that is unnecessary for erasing the hologram, so that it does not cause an error in the phase shift amount. Therefore, it is possible to realize selective erasure of a highly accurate multiplex hologram with little error.

  In order to solve the above-described problem, the hologram memory device of the present invention irradiates the hologram memory with object light and reference light, records the hologram by these interference fringes, and applies the reference light to the recorded hologram. In a hologram memory device provided with reproducing means for irradiating and reproducing, a double beam that uses incident light from both surfaces of the beam splitter as an optical system for irradiating the hologram memory with the object beam and the reference beam. A Mach-Zehnder interference optical system is provided.

  In this case as well, more preferably, an erasing unit for erasing the hologram recorded in the hologram memory is used, and the erasing unit uses the optical system, and the recording unit of the beam splitter in the optical system uses the hologram. Erasing by irradiating the object light and the reference light at the time of recording the hologram from the light incident from the other surface different from the surface used for the irradiation of the object light and the reference light at the time of recording It is.

  As described above, by using the double Mach-Zehnder-interference optical system, the hologram can be erased at high speed without requiring a kind of phase shifter. A liquid crystal phase modulator or piezo mirror is arranged as a phase shifter, and holograms can be erased at high speed with an optical system that is extremely small, simple, and inexpensive compared to the configuration that has been performed through electronic or mechanical control. Become.

  Also, in the configuration using a conventional phase shifter such as backlash when using a piezo mirror and voltage error when using a liquid crystal phase modulator, the interference fringes when recording a hologram are spatially accurate. There is a possibility that the hologram erasing performance to be overwritten is deteriorated without being an interference fringe whose phase is shifted by π, but in the present invention, the element used for the phase displacement at the time of erasing is only a beam splitter, High-speed erasure of few high-precision holograms can be realized, and deterioration with time does not occur.

  Further, the phase shift of the interference fringes by the double Mach-Zehnder-interference optical system cannot take a continuous intermediate value that is unnecessary for the selective erasure of the hologram, so that it does not cause an error in the amount of phase shift. This also realizes high-speed erasure of a highly accurate hologram with few errors.

  In each of the above-described hologram memory devices of the present invention, for example, a hologram memory containing a photorefractive medium can be used as the hologram memory.

  In the hologram erasing method of the present invention, as described above, the hologram memory is irradiated with the object light and the reference light, and a hologram among the holograms recorded in multiple by these interference fringes is recorded at the time of recording the hologram. The hologram erasing method erases the object light and the reference light by irradiating the hologram memory so that these interference fringes are displaced by π from the interference fringes at the time of recording. As the optical system for irradiating the reference beam and the reference beam, a double Mach-Zehnder-interference optical system that uses incident light from both sides of the beam splitter is used. The object light and the reference light at the time of erasure are generated by the incident light from the side different from the one used for the generation of the light. It is intended to obtain the interference fringes displaced by stripes and [pi.

  As a result, it is possible to obtain interference fringes at the time of recording and interference fringes displaced by π simply by switching the incidence of light waves as reference light and object light between recording and erasing. This eliminates the need for a phase shifter.

  Therefore, if a hologram memory device is configured using the erasing method, a hologram memory device capable of selectively erasing multiple holograms can be realized without using a phase shifter as a component.

  In other words, the hologram memory device, which is caused by the use of a phase shifter by electronic or mechanical control, is avoided in size and complexity, and a highly accurate hologram can be erased by a small, simple and inexpensive optical system. A small, simple and inexpensive hologram memory device can be realized.

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

  The present invention utilizes a double Mach-Zehnder-interference optical system composed of a beam splitter and a mirror. FIG. 2 shows a double Mach-Zehnder-interference optical system. Unlike a normal Mach-Zehnder-interference optical system, incident light from both surfaces of the beam splitter 5 is used, so that it is called a double Mach-Zehnder-interference optical system.

  In the optical system, a light wave (shown by a solid line) 10 incident from the first port 1 is incident on one surface of the beam splitter 5 and a light wave (shown by a solid line) 11 entered from the second port 2. Enters the other surface of the beam splitter 5. These light waves 10 and 11 are respectively demultiplexed by the beam splitter 5 into light waves 10a and 10b and light waves 11a and 11b. The light waves 10a and 10b obtained by demultiplexing the light wave 10 intersect via the second mirror 4 and the first mirror 3, and similarly, the light waves 11a and 11b obtained by demultiplexing the light wave 11 are also separated from the demultiplexed lights 10a and 10b. Propagates the same optical path and intersects.

  At this time, an interference fringe (shown by a solid line) 6 formed in the intersection region by the light waves 10a and 10b obtained by demultiplexing the light wave 10 incident from the first port 1 and the light wave incident from the second port 2 The phase difference of p is spatially between the light waves 11a and 11b obtained by demultiplexing the light 11 and the interference fringes (shown by broken lines) 7 formed in the intersection region by propagating on the same optical path as the light waves 10a and 10b. It is known that there exists (for example, nonpatent literature 3).

  The inventors of the present application pay attention to such characteristics, dispose a photorefractive medium in the intersecting region of the optical system, and record a hologram recorded by incident light from one port due to the above phase difference to a different port. The present invention has been completed by finding that multiple recorded holograms can be selectively erased by overwriting with interference fringes formed by incident light from.

  FIG. 1 shows a hologram memory device according to an embodiment of the present invention using a double Mach-Zehnder-interference optical system. The basic configuration is that the first and second light sources 12 and 13, the first and second mirrors 16 and 17 constituting the double Mach-Zehnder-interference optical system 25, and a beam splitter (in the figure: BS) 14, a photorefractive medium (photorefractive crystal) 11 that is a hologram memory, a shutter 15, a spatial light modulator (SLM in the figure: SLM) 18, a ground glass 21, and a photodetector 24.

  Here, the first and second light sources 12 and 13 satisfy the positional relationship between the first and second input ports 1 and 2 in the double Mach-Zehnder-interference optical system shown in FIG. As described above, the double Mach-Zehnder-interference optical system 25 is disposed.

  In FIG. 1, as an example of the hologram multiple recording method, a speckle multiple recording method is used in which the wavefront of the reference light is changed for each hologram. Therefore, the ground glass 21 is disposed on the optical path of the reference light, which will be described later. However, the wavelength multiplex recording method for changing the wavelength of the reference light for each hologram, the angle multiplex recording method for changing the angle of the reference light for each hologram, and each hologram. Other holographic multiplex recording methods, such as phase code multiplex recording, which changes the phase of the reference beam, and spherical reference beam shift multiplex recording, which uses a spherical wave as the reference beam and moves the recording medium (hologram memory) for each hologram by a minute distance Can be used.

  Hereinafter, each procedure of recording / reproducing / selectively erasing a hologram in the hologram memory device will be described with reference to FIGS.

  At the time of recording the hologram, only the light wave from the first light source 12 is incident as shown in FIG. This light wave is split into two by the beam splitter 14, one of which is an object beam 19 and one of which is a reference beam 20 that interferes in the photorefractive medium 11 and records a hologram 22. Here, the recording data is given to the object light 19 by the SLM 18. When multiplex recording a plurality of holograms (# 1, # 2,... #N), the wave front of the reference beam 20 is changed by moving the ground glass 21 by a minute distance, and this is used as a new reference beam 20 to be a different hologram. Are recorded in the same location of the photorefractive medium 11 (operation of the recording means).

  Next, at the time of reproducing the hologram, as shown in FIG. 4, the light wave is incident only from the first light source 12, and further, the shutter 15 disposed on the optical path of the object light 19 is closed, whereby the photorefractive medium 11 is obtained. Only the reference beam 20 is incident to reproduce the hologram. Of the holographically recorded holograms, diffracted light is generated only from the hologram whose position of the ground glass 21 at the time of reproduction coincides with the position at the time of recording, and this is detected by the photodetector 24 as output light 23 (operation of the reproducing means) ).

  When the hologram is selectively erased, as shown in FIG. 5, light waves are incident only from the second light source 13 that is not used during recording and reproduction. At this time, the ground glass 21 on the optical path of the reference beam 20 is arranged so as to satisfy the reproduction condition for the hologram to be erased, and the input image corresponding to the hologram to be erased is given to the object beam 19 by the SLM 18. As a result, this hologram is overwritten by the interference fringes displaced by exactly p at the time of recording, and is selectively erased (operation of the erasing means).

  In the above procedure, the uses of the first light source 12 and the second light source 13 are interchangeable. That is, it is also possible to perform multiplex recording and reproduction of the hologram with the light wave from the second light source 13 and to selectively erase the hologram with the light wave from the first light source 12.

  Hologram recording and selective erasure of holograms must be performed with light waves from different light sources, but reproduction can be performed with light waves from either light sources for recording or selective erasure. is there. Therefore, here, the reproduction is performed by the light wave from the first light source 12, but the reproduction may be performed by the light wave from the second light source 13.

  Further, in the hologram memory device of FIG. 1, two light sources are provided as the first and second light sources 12 and 13, but the light wave from one light source is divided into two, which are divided into the first light source 12 and It can also be used as the second light source 13.

  As described above, in the hologram memory device according to the present embodiment, the light fringes incident from the first surface of the beam splitter are demultiplexed to form the interference fringes in the intersection region, and the light is incident from the second surface of the beam splitter. There is a characteristic that there is a spatial phase difference of p between the light wave and the interference fringes formed in the intersection region by propagating on the same optical path as the light wave from which the incident light from the first surface is split. The photorefractive medium 11 is irradiated with reference light and object light using a double Mach-Zehnder-interference optical system.

  Therefore, by switching the incidence of the light wave that becomes the reference light and the object light between the recording time and the selective erasing, the phase shift of the light wave at the time of selective erasing is shifted on the optical path of the reference light or the object light. There is no need to arrange a phase shifter, and a liquid crystal phase modulator or piezo mirror is arranged as a phase shifter, which is very small, simple, and inexpensive compared to a configuration that is performed through electronic or mechanical control. The hologram can be selectively erased by the optical system.

  In addition, in the configuration using a conventional phase shifter such as backlash when using a piezo mirror and voltage error when using a liquid crystal phase modulator, the performance of selective erasure of hologram may be deteriorated. In the present invention, the element used for the phase displacement at the time of selective erasure is only a beam splitter, so that selective erasure of a highly accurate hologram with little error can be realized, and deterioration with time does not occur.

  Moreover, the phase shift of the interference fringes by the double Mach-Zehnder-interference optical system cannot take a continuous intermediate value that is unnecessary for the selective erasure of the hologram, so that it does not cause an error in the amount of phase shift. This also makes it possible to realize selective erasure of a highly accurate hologram with little error.

  As described above, the selective erasing method of the multiplex hologram in the present invention can be applied to other multiplex recording methods such as an angle multiplex recording method, a spherical reference light shift multiplex recording method, and a phase code multiplex recording method. It is.

Hereinafter, the present invention will be further described in Examples.
[Example 1]
First, an embodiment experimentally confirmed that selective erasure of a hologram recorded in a multiplexed manner by the double Mach-Zehnder-interference optical system shown in FIG. 2 will be described.

6, lithium niobic acid (LiNbO ₃ : Fe) crystal containing iron as an impurity is arranged as a photorefractive medium in the intersecting region of the light waves 10a, 10b, 11a, 11b in FIG. The normalized diffraction efficiency when the hologram recorded with the interference fringe 6 by the incident light is overwritten with the interference fringe 7 by the incident light from the second port 2 is shown.

  Here, the hologram is phase-conjugate reproduced with a light wave having an intensity of about 1/10 of the hologram recording light, and the diffraction efficiency is measured. The phase conjugate reproduction is a reproduction of a hologram from a normal opposite side by a light wave propagating opposite to the reference light, and thus it is possible to observe the dynamic behavior of the diffraction efficiency of the hologram in the recording process. For comparison, the normalized diffraction efficiency when the crystal is irradiated with incoherent erasing light having the same intensity as the incident light from the second port 2 is also plotted.

From FIG. 6, it can be seen that hologram erasure (selective erasure) by incident light (selective erasure light) from the second port 2 is much faster than incoherent erasure. Further, since the selective erasing light acts as an incoherent erasing light for other multiple holograms, this property enables selective erasing of the hologram. In FIG. 6, in the case of selective erasure, the normalized diffraction efficiency takes a minimum value in about 260 seconds. When the normalized diffraction efficiency takes the minimum value, the overwriting of the interference fringes 7 by the light wave from the second port 2 is completed, thereby realizing the selective erasure of the hologram.
[Example 2]
Next, a lithium niobic acid (LiNbO ₃ : Fe) crystal containing iron as an impurity is used as a photorefractive medium, and a hologram is actually recorded in this crystal, and one of the multiple holograms is selectively erased. Examples that have been experimentally confirmed to be able to do are described.

FIG. 7 shows the system used in the experiment. Here, an M wave mirror, BS is a beam splitter, HWP is a half-wave plate, PBS is a polarization beam splitter, and L is a lens. In this experiment, a lithium niobic acid (LiNbO ₃ : Fe) crystal 33 containing 7 × 7 × 5 [mm ³ ] iron as an impurity was used as a photorefractive crystal, and an Ar ⁺ laser having a wavelength of 514 nm was used as a light source.

The laser light from the Ar ⁺ laser is split into two by the polarization beam splitter PBS1 via the mirror M1 and the half-wave plate HWP1, and one of the lights is first transmitted to the beam splitter BS via the mirror M2 and the mirror M3. The other light is guided as a light wave from the second port 32 to the beam splitter BS via the half-wave plate HWP2, the polarization beam splitter PBS2, and the mirror M4. In the figure, reference numerals S1 and S2 denote shutters for selectively controlling the incidence of light waves from the first port 31 and the second port 32.

  Using such an experimental system, multiple recording of holograms, subsequent selective erasure of holograms, and additional writing to selective erasure portions were performed. FIG. 8 shows a reproduced image after initial recording, after selective erasure, and after additional recording.

First, a light wave entered from the first port 31 was split by the beam splitter BS, and a hologram was formed in the crystal 33 as object light and reference light, respectively. Specifically, out of the demultiplexed light, the one irradiated to the mirror M5 has image data added as object light by the input mask 34, and is condensed into the LiNbO ₃ : Fe crystal 33 through the lens L1, The one irradiated on the mirror M6 is condensed as reference light into the LiNbO ₃ : Fe crystal 33 through the ground glass 35 and the lens L2. Here, three holograms were speckle multiplexed and recorded by moving the ground glass 35 in the horizontal direction. Each recorded image data of the three holograms is shown as a recorded image in FIG.

  Next, a light wave was incident from the second port 32, and only the alphabet "C" which is a part of the third hologram was selectively erased. Here, when selectively erasing the hologram, the ground glass 35 on the optical path of the reference light is arranged so as to satisfy the reproduction condition for the hologram to be erased (here, the third piece), and for the object light, An input mask 34 (here, “C”) corresponding to the hologram to be erased (here, the third sheet) is arranged at the same position as during recording.

  After that, a light wave is incident from the first port 31 and an “E” is newly added to the “C” position of the erased third hologram by using the input mask 34 having the image data “C”. Added.

  From FIG. 8, it can be seen that an arbitrary part in an arbitrary hologram can be selectively erased / updated by this method.

  In the above-described embodiments and examples, the description has been given as the selective erasure of the multiplexed hologram. However, even for holograms that are not multiplexed recording, by using such a double Mach-Zehnder-interference optical system, a type of phase shifter can be used. The hologram can be erased at high speed without the necessity.

  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.

  It can be used as a large capacity optical memory.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1, showing an embodiment of the present invention, is a drawing showing a main configuration of a hologram memory device. It is drawing which shows the structure of a double Mach-Zehnder-interference optical system. It is drawing which shows the procedure at the time of recording in the hologram memory apparatus of FIG. It is drawing which shows the procedure at the time of reproduction | regeneration in the hologram memory apparatus of FIG. 2 is a diagram showing a procedure at the time of selective erasure in the hologram memory device of FIG. 1. FIG. 3 shows an embodiment of the present invention, in which a LiNbO ₃ : Fe crystal is arranged as a photorefractive medium in an intersecting region in the double Mach-Zehnder-interference optical system of FIG. 2, and interference fringes due to incident light from the first port 6 is a diagram illustrating normalized diffraction efficiency when the hologram recorded in (1) is overwritten with interference fringes by incident light from a second port. FIG. 4 shows another embodiment of the present invention, in which a LiNbO ₃ : Fe crystal is used as a photorefractive medium, a hologram is actually recorded in the crystal, and one of the multiplexed holograms can be selectively erased. It is drawing which shows the structure of the experimental system which confirmed this. It is drawing which shows the reproduction | regeneration image of the hologram in each process which recorded three holograms in the experimental system of FIG. 7, selectively erased one of them, and additionally recorded in the erased part. 6 is a diagram for explaining selective erasure of multiple holograms by generating interference fringes having a phase shifted by π spatially and overwriting the interference fringes during recording using a conventional phase shifter.

Explanation of symbols

11 Photorefractive medium (hologram memory)
12 First light source 13 Second light source 14 Beam splitter 15 Shutter 16 First mirror 17 Second mirror 18 Spatial light modulator 19 Object light 20 Reference light 21 Ground glass 22 Hologram 24 Photo detector 25 Double Mach-Zehnder interference optical system

Claims (7)

The hologram memory is irradiated with object light and reference light, and a hologram is recorded among the holograms recorded in multiples by these interference fringes. These interference fringes record the object light and reference light at the time of recording the hologram. A method for erasing a hologram that erases by irradiating the hologram memory so as to be displaced by an interference fringe and π at the time,
A double Mach-Zehnder-interference optical system using incident light from both surfaces of the beam splitter is used as an optical system for irradiating the hologram memory with object light and reference light, and a hologram to be erased in the beam splitter of the optical system. By generating the object light and the reference light at the time of erasing with the incident light from the surface different from the one used for the generation of the object light and the reference light at the time of recording, the interference fringes and π A method for erasing a hologram, characterized by obtaining an interference fringe displaced by a distance. The hologram memory is irradiated with object light and reference light, and the hologram recorded by these interference fringes is recorded as the object light and reference light at the time of recording the hologram. A hologram erasing method for erasing by irradiating the hologram memory so as to be displaced,
As an optical system for irradiating the hologram memory with object light and reference light, a double Mach-Zehnder-interference optical system using incident light from both surfaces of the beam splitter is used, and at the time of recording a hologram in the beam splitter of the optical system By generating the object light and the reference light at the time of erasing with the incident light from the surface different from the one used for the generation of the object light and the reference light, the interference fringes and π at the time of recording are displaced. A method for erasing a hologram, comprising obtaining interference fringes. Irradiating the hologram memory with object light and reference light and recording holograms by these interference fringes, and a recording means for recording a plurality of holograms in the same location, In a hologram memory device comprising a reproducing means for selectively reproducing an arbitrary hologram by irradiating a reference beam,
A hologram memory comprising a double Mach-Zehnder-interference optical system that uses incident light from both surfaces of a beam splitter as an optical system for irradiating the hologram memory with the object light and the reference light. apparatus. An erasing means for selectively erasing an arbitrary hologram among the holograms recorded in a multiplexed manner in the hologram memory;
The erasing means uses the optical system, and the recording means of the beam splitter in the optical system is incident from the other surface different from the surface used for irradiating the object light and the reference light when recording the hologram to be erased. A hologram memory device characterized by being erased by irradiating object light and reference light at the time of recording the hologram from light. In a hologram memory device comprising a recording means for irradiating a hologram memory with object light and reference light, and recording a hologram by these interference fringes, and a reproducing means for reproducing the recorded hologram by irradiating the reference light,
A hologram memory comprising a double Mach-Zehnder-interference optical system that uses incident light from both surfaces of a beam splitter as an optical system for irradiating the hologram memory with the object light and the reference light. apparatus. An erasing unit for erasing the hologram recorded in the hologram memory;
The erasing means uses the optical system, and the recording means of the beam splitter in the optical system uses light incident from the other surface different from the surface used for irradiating the object light and the reference light when recording the hologram. A hologram memory device, wherein the hologram memory device is adapted to be erased by irradiating with object light and reference light at the time of recording the hologram.   The hologram memory device according to claim 3, wherein the hologram memory includes a photorefractive medium.

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