JP2005121855A - Hologram recording and reproducing apparatus, hologram recording and reproducing method and phase modulation method for light beam - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform phase modulation of reference light capable of efficiently increasing multiplicity without increasing optical load. <P>SOLUTION: Reference light 200 is phase-modulated by a two-dimensional phase modulator 22, the phase-modulated reference light 200 is applied on a hologram recording medium 14, and interference fringes of the phase-modulated reference light 200 and signal light 100 applied from a lens 12 are recorded on the recording medium 14 by phase modulation multiple recording. Though the size of the phase modulator 22 is increased in accordance with increase of multiplicity, increase of the lateral size can be extremely reduced as compared with a one-dimensional phase modulator, accordingly reference light 200 of uniform brightness can be produced without increasing optical load and multiplicity can be efficiently increased. By tiling and displaying a phase-modulated pattern on the two-dimensional phase modulator 22, optical load is made small and multiplicity can be increased furthermore. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a hologram recording / reproducing apparatus and a hologram recording / reproducing method for recording / reproducing a hologram by a phase modulation multiplexing method, and more particularly to a phase modulation method of a light beam when optical phase modulation of reference light is performed by a phase modulation pattern.

  In recent years, holographic technology has been rapidly developed for the practical use of holographic memory, which is attracting attention as a candidate for powerful storage that competes with next-generation and next-generation optical discs. Thus, a hologram recording / reproducing system for recording / reproducing a large amount of data has been proposed.

  In this hologram storage recording / reproducing system, as shown in FIG. 9, the coherent laser light emitted from the laser light source 2 enters the beam splitter 6 through the shutter 4 and is branched into the signal light 100 and the reference light 200. The The signal light 100 is incident on the spatial light modulator 10 via the mirror 8, and the signal light is intensity-modulated by the spatial light modulator 10 displaying the data page. The modulated signal light is focused on the hologram recording medium 14 by the lens 12. On the other hand, since the reference light 200 irradiates the hologram recording medium 14 via the mirror 16, the signal light 100 and the reference light 200 are overlapped in the hologram recording medium 14, and the interference fringes formed as a result are holograms. It is recorded as a fine density pattern on the recording medium 14.

  In order to reproduce the data recorded on the hologram recording medium 14, the same reproduction light as the reference light 200 is incident on the hologram recording medium 14, thereby diffracted light corresponding to the interference fringes recorded on the hologram recording medium 14. And the diffracted light is focused on an image sensor 20 such as a CCD or CMOS by a lens (inverse Fourier lens) 18. The image sensor 20 photoelectrically converts the received diffracted light, and the obtained light reception signal is analyzed and reproduced as image data.

  By the way, the storage capacity of the holographic memory is determined by the volume recording density while the storage capacity of the optical disk is determined by the surface recording density. When recording data in a holographic manner, recording data is not directly recorded on the hologram recording medium 14, but interference fringes between the signal light and the reference light are recorded. Therefore, 1 Mbyte is recorded in one hologram (data page). It is also possible to allocate the data. When the multiplex recording of this data page is repeated using the volume of the hologram recording medium, a large capacity exceeding several hundreds of Gbytes can be realized. Normal volume hologram multiplexing does not fix the reference beam for each data page, records some interference fringes at the same location or almost the same location with some change, and between different reference beams (reproduced beams) This is realized by giving Bragg selectivity. Typical multiplexing methods include angle multiplexing, shift multiplexing, wavelength multiplexing, phase modulation multiplexing, and the like.

  Focusing on the phase modulation multiplexing method among the above multiplexing methods, this phase modulation multiplexing method is greatly different from other methods, such as mechanically changing the angle of the reference beam in the optical system, Rather than shifting the recording medium, all the optical components remain fixed, and reference light having various phases is generated by optical phase modulation of the reference light with a spatial light modulator. This is a method of recording multiple data in the same recording area (spot) of the hologram recording medium by interfering with the signal light. By using this light phase characteristic, the S / N ratio of the reproduction data is lowered. There is an advantage that the crosstalk, which is one of the causes, can be greatly reduced.

  All the literatures related to the hologram memory using the phase modulation multiplexing method reported so far are only about the case where a one-dimensional spatial light phase modulator is used. A report of 180 multiplexing in an experiment using a one-dimensional phase modulator was made by Denz et al. In 1998 (see Non-Patent Document 1, for example).

  In the phase modulation multiplex system, the condition imposed on the actual optical system to realize multiplex recording without crosstalk is that the reference light has a uniform intensity and is generated in accordance with a certain law as described later. It is subject to phase modulation that accurately reproduces the modulation code. This condition does not change whether the spatial light phase modulator is one-dimensional or two-dimensional, but in any case, the uniform irradiation with the reference light becomes more difficult as the size of the spatial light phase modulator increases. In addition, since the parallelism of the light wave varies depending on the region of the reference light due to the aberration of the lens, the code orthogonality is not satisfied.

The advantage of making the spatial light phase modulator two-dimensional is that the multiplicity can be increased efficiently. When displaying a phase modulation pattern (phase address pattern) on a spatial light phase modulator, even if the pattern is displayed with a finer pitch than a certain resolution, the selectivity does not work between those patterns, so different data pages Cannot be played independently. The limit of resolution is determined by the Bragg condition, but what is actually recorded as a hologram is a speckle pattern formed by a diffuser incorporated in the optical system of the reference light. Therefore, it actually depends on the resolution of the spatial light phase modulator. Therefore, once the resolution is determined, the size of the spatial light phase modulator must be increased in order to increase the multiplicity. For example, in order to ensure the same multiplicity, 1 is required. The horizontal dimension (width) of the two-dimensional spatial light phase modulator is a square multiple of the two-dimensional case. As described above, the larger the display area of the spatial light phase modulator, the more optically unfavorable conditions increase. Therefore, it is clear that the display with a two-dimensional pattern is more advantageous.
C. Denz, Kai-Oliver Muller, Thorsten Heimann, and Theo Tschudi, “Volume Holographic Storage Demonstrator Based on Phase-Coded Multiplexing”, IEEE J. Sel. Top. Quantum Electron. 4, 832 (1998)

  As described above, in the phase modulation multiplexing method, the phase of the light must be modulated by the spatial light modulator. However, as the degree of multiplexing increases, the size of the spatial light phase modulator needs to be increased. . For that purpose, the two-dimensional spatial light phase modulator using the two-dimensional spatial light phase modulator can reduce the lateral size of the spatial light phase modulator when the multiplicity is the same. Uniform irradiation of light is facilitated and optically advantageous. However, at present, there is no example in which a hologram recording apparatus using a phase modulation multiplexing method is realized at a practical level using a two-dimensional spatial light phase modulator.

  The present invention has been devised in view of the above circumstances, and an object of the present invention is to provide a hologram recording capable of efficiently increasing the recording multiplicity by the phase modulation multiplexing method without increasing the optical burden. It is an object of the present invention to provide a reproducing apparatus, a hologram recording / reproducing method, and a light beam phase modulation method.

  In order to achieve the above object, the present invention provides a hologram recording apparatus for recording on a hologram recording medium interference fringes generated by causing a first light beam spatially modulated by recording data to interfere with a second light beam. , Comprising a two-dimensional phase modulation means for phase-modulating the second light beam, the second light beam optically phase-modulated by the two-dimensional phase modulation means and the spatial light-modulated first light. The beam interference fringes are multiplexed and recorded in the same recording area of the hologram recording medium.

  As described above, in the hologram recording apparatus of the present invention, since the second light beam (reference light) is optically phase-modulated by the two-dimensional phase modulation means, the same resolution can be obtained when the multiplicity when recording data is increased. In order to ensure, the size of the phase modulator that displays the phase modulation pattern must also be increased, but the two-dimensional phase modulation means increases the size of the lateral dimension compared to the one-dimensional phase modulation means. Since the second light beam that irradiates the light beam can be extremely reduced and the spread of the second light beam can be reduced, the second light beam with uniform brightness can be generated without increasing the loss of laser light. Thus, the recording multiplicity by the phase modulation multiplexing method can be increased efficiently without increasing the optical burden.

  Further, the present invention is a hologram reproducing apparatus for reproducing data by reading out diffracted light generated by irradiating a light beam onto a hologram recording medium on which interference fringes are recorded, by receiving the light with a light receiving element, Two-dimensional phase modulation means for optical phase modulation of the light beam, and a recording region of the hologram recording medium is irradiated with the light beam modulated by the two-dimensional phase modulation means.

  As described above, in the hologram reproducing apparatus of the present invention, the light beam (reproduced light) is optically phase-modulated by the two-dimensional phase modulation means, but the two-dimensional phase modulation means having the same size as that when the reference light is optically phase-modulated. Therefore, for the same reason as the optical phase modulation of the reference light, the two-dimensional phase modulation means when the multiplicity is increased has a lateral size compared to the one-dimensional phase modulation means. The amount of increase can be extremely small, and the spread of the light beam that irradiates it can be reduced. Therefore, a light beam with uniform brightness can be generated without increasing the loss of laser light. It is possible to efficiently increase the recording multiplicity by the phase modulation multiplexing method without increasing the burden.

  The present invention is also a hologram recording method for recording interference fringes generated by causing a first light beam spatially modulated by recording data to interfere with a second light beam on a hologram recording medium, The optical beam is optically phase-modulated with a two-dimensional phase modulation pattern, and the optical phase-modulated second optical beam is caused to interfere with the spatial light-modulated first optical beam.

  As described above, in the hologram recording method of the present invention, in order to optically modulate the second light beam (reference light) with the two-dimensional phase modulation pattern, a two-dimensional phase modulator that displays the two-dimensional phase modulation pattern is used. In order to ensure the same resolution when the multiplicity when recording data is increased, the size of the phase modulator must be increased, but a two-dimensional phase modulator is a one-dimensional Compared with the phase modulator, since the increase in the size of the lateral dimension can be extremely reduced, the spread of the second light beam that irradiates this can be reduced. Since the light beam can be generated without increasing the loss of the laser beam, the recording multiplicity by the phase modulation multiplexing method can be efficiently increased without increasing the optical burden.

  Further, the present invention is a hologram reproducing method for reproducing data by reading out the diffracted light generated by irradiating the hologram recording medium on which the interference fringes are recorded with a light receiving element, and reproducing the data, The optical beam is optically phase-modulated with a two-dimensional phase modulation pattern, and the recording region of the hologram recording medium is irradiated with the optical phase-modulated light beam.

  As described above, in the hologram reproducing method of the present invention, the light beam (reproduced light) is optically phase-modulated by the two-dimensional phase modulation means, but with a two-dimensional phase modulation pattern having the same size as that when the reference light is optically phase-modulated. Since optical phase modulation requires the use of a two-dimensional phase modulator that displays this two-dimensional phase modulation pattern, when the multiplicity increases for the same reason as the optical phase modulation of the reference light Compared with a one-dimensional phase modulator, the two-dimensional phase modulator can greatly increase the size of the lateral dimension, and the spread of the light beam that irradiates this can be reduced. Since a light beam with uniform brightness can be generated without increasing the loss of laser light, it is possible to efficiently increase the recording multiplicity by the phase modulation multiplexing method without increasing the optical burden.

  Further, the present invention provides a phase modulation pattern corresponding to the recording data in advance in a phase modulation method of a light beam when a light beam is optically phase modulated by a phase modulation means displaying a phase modulation pattern, A plurality of phase modulation patterns are displayed on the phase modulation means.

  As described above, in the light beam phase modulation method of the present invention, the light beam is optically phase-modulated by a plurality of phase modulation patterns displayed so as to spread over the phase modulation means. Even if the brightness is non-uniform over the entire surface, the non-uniformity of the brightness of the light beam is alleviated within the area occupied by one of the modulation patterns displayed, and the entire phase modulation pattern Since the light beam is modulated, the overall non-uniformity of the brightness of the light beam is reduced, and the light beam is almost the same as when the phase modulation means is irradiated with the light beam and the light beam is optically phase modulated. An effect can be obtained. Therefore, when this light beam is used as the reference light, and the interference fringe between the reference light and the separately generated signal light is phase-modulated and multiplexed on the hologram recording medium, the crosstalk of the reproduced data is reduced, and the S / N The ratio can be improved.

As described above in detail, according to the present invention, since the reference light and the reproduction light are generated by optical phase modulation by the two-dimensional phase modulation means, when the recording multiplicity by the phase modulation multiplexing method is increased, The two-dimensional phase modulation means can greatly reduce the increase in the size of the lateral dimension as compared with the one-dimensional phase modulation means, and therefore, the spread of the reference light and the reproduction light that irradiates this can be reduced. Therefore, it is possible to generate the reference light and reproduction light with uniform brightness without increasing the loss of laser light, so that the recording multiplicity by the phase modulation multiplexing method is increased without increasing the optical burden. Can be increased efficiently.
Further, the phase modulation pattern is displayed so as to be spread over the phase modulation means, and the reference light and the reproduction light irradiated to the phase modulation means are modulated by optical phase modulation of the reference light and the reproduction light by the plurality of phase modulation patterns. Even if the brightness is somewhat non-uniform, the non-uniformity of the brightness of the reference light and the reproduction light is alleviated in the area occupied by one of the plurality of displayed modulation patterns, and a plurality of phase modulation patterns Since the reference light and the reproduction light are modulated by the set of light beams, the non-uniform brightness of the reference light and the reproduction light is reduced, and the reference light and the reproduction light are modulated by irradiating the phase modulation means with a more uniform light beam. It is possible to obtain substantially the same effect as when Accordingly, the degree of uniformity of the reference light and the reproduction light can be moderated and the loss of laser light can be reduced, so that the optical burden can be further reduced to increase the phase modulation in hologram recording. The severity can be increased efficiently. In other words, even if the phase modulation multiplicity is increased and the size of the phase modulation means is increased, the uniformity of the brightness of the reference light and the reproduction light that irradiates the phase modulation means can be moderated. The phase modulation multiplicity can be made considerably larger than before without increasing the burden so much.

  For the purpose of efficiently increasing the multiplicity without increasing the optical burden, phase modulation multiplex recording / reproduction of the hologram is performed by phase-modulating the reference light and the reproduction light with a two-dimensional phase modulator, and further required Accordingly, it is realized by reducing and displaying a plurality of phase modulation patterns to be displayed on the two-dimensional phase modulator.

  FIG. 1 is a block diagram showing a configuration of a hologram recording / reproducing apparatus according to the first embodiment of the present invention. However, the same parts as in the conventional example will be described using the same reference numerals. The hologram recording / reproducing apparatus includes a laser light source 2, a shutter 4, a beam splitter 6, a mirror 8, a spatial light modulator 10, a signal light lens (Fourier lens) 10, a hologram recording medium 14, a mirror 16, and a lens (inverse Fourier lens). 18. An image pickup device 20 such as a CCD, a two-dimensional spatial light phase modulator (hereinafter simply referred to as a phase modulator) 22, a lenslet array 24, and a reference light (or reproduction light) lens (Fourier lens) 26. Configured. However, the claimed hologram recording medium corresponds to the hologram recording medium 14, the two-dimensional phase modulation means corresponds to the spatial light phase modulator 22, and the light receiving element corresponds to the image sensor 20.

  As the two-dimensional phase modulator 22, a PPMX8267 manufactured by Hamamatsu Photonics is used, and the performance is such that the input signal is an XGA level, the number of control pixels is about 590000 (pixels), the effective screen size is 20 × 20 mm, and the phase modulation amount. 2π or more.

  FIG. 2 is a block diagram showing a configuration of a signal processing system of the hologram recording / reproducing apparatus shown in FIG. In the signal processing system, a control unit 28 such as a personal computer controls opening / closing of the shutter 4, display control of a data page of the spatial light modulator 10, and a two-dimensional phase modulator 22 having a phase modulation pattern corresponding to a phase address code described later. Display control and control for reproducing image data from the light reception signal of the image sensor 20.

  Next, the operation of the present embodiment will be described. First, the control unit 28 displays a data page to be recorded on the spatial light modulator 10 and displays a phase modulation pattern on the two-dimensional phase modulator 22 and then opens the shutter 4. As a result, the coherent laser light emitted from the laser light source 2 enters the beam splitter 6 through the shutter 4 and branches into the signal light (first light beam) 100 and the reference light (second light beam) 200. Is done. The signal light 100 is incident on the spatial light modulator 10 via the mirror 8, and the signal light is intensity-modulated by the spatial light modulator 10 displaying the data page. The intensity-modulated signal light is condensed on the hologram recording medium 14 by the lens 12.

  On the other hand, the reference beam 200 is incident on the two-dimensional phase modulator 22 via the mirror 16 and is optically phase-modulated (hereinafter simply referred to as phase modulation), and then the hologram via the lenslet array 24 and the lens 26. The light is condensed on the recording medium 14 so as to overlap the signal light 100. As a result, the signal light 100 and the reference light 200 are overlapped in the recording area of the hologram recording medium 14, and the interference fringes formed as a result are recorded on the hologram recording medium 14 as a fine dense pattern.

  In order to reproduce the data recorded on the hologram recording medium 14, the same reproduction light as the reference light 200 is incident on the hologram recording medium 14, thereby diffracted light corresponding to the interference fringes recorded on the hologram recording medium 14. And the diffracted light is focused on the image sensor 20 such as a CCD or CMOS by the lens 18. The image sensor 20 photoelectrically converts the received diffracted light, and the received light signal obtained is analyzed by the control unit 28 and reproduced as image data.

  Here, the phase modulation pattern displayed on the two-dimensional phase modulator 22 that modulates the reference light 200 will be described in more detail with reference to FIG. The phase modulator 22 displays a phase modulation pattern for optical phase modulation of the reference light 200 as shown in FIG. The reference beam 200 enters the phase modulator 22 as a plane wave, and is optically phase-modulated according to the recording multiplicity by the phase modulation pattern.

  Thereafter, the reference light 200 having the respective phases is incident on the same region of the hologram recording medium 14 by the lens 26, undergoes intensity modulation by the spatial light modulator 10, and interferes with the signal light 100 incident by the lens 12. Form stripes. When reproducing a data page that has been phase-modulated and multiplexed, the reproduction light that is optically phase-modulated with the same phase modulation pattern as that used to record each data page is reproduced at the same angle as the recording. When incident on the recording medium 14, the corresponding data page can be reproduced independently from the same recording area.

  By the way, when data pages are multiplexed and recorded in one recording area of the hologram recording medium 14 by the phase modulation multiplexing method, the situation where there is no crosstalk between the data pages is expressed by the following equation (1).

  In this equation (1), the phase address of the reference beam 200 used for recording the i-th data page is indicated by a χ · χ * unitary matrix. N columns (or rows) which are orthogonal to each other are obtained from the matrix of the equation (1), and the matrix element corresponds to the phase term of the phase address code, which is a phase address code capable of maximum multiplexing (N multiplexing) and Become.

  This phase address code is generally generated using a Walsh-Hadamard matrix (H-matrix). This H-matrix includes the same number of 1s and -1 except for elements on two sides of the matrix, and the H-matrix is generated by the following algorithm.

  Here, if 1 of each element of the H-matrix represented by Expression (2) is associated with modulation 0 and −1 is associated with modulation π, a phase orthogonal address code satisfying Expression (1) is obtained.

  In the case of the phase modulation multiplexing method, as described above, among the light waves diffracted according to the Bragg condition, extra light waves contributing to crosstalk are weakened by interference and disappear. By this principle, multiple recording of data is possible, and the S / N ratio can be dramatically improved as compared with other multiplexing systems.

  Next, the correspondence relationship between the orthogonal code (address code) obtained by the H-matrix and the phase modulation pattern displayed by the phase modulator 22 will be described. The simplest method for associating the orthogonal code with the display pattern of the phase modulator 22 is to divide the reference beam 200 into a two-dimensional lattice and to divide the multiplicity into N, and to associate one element of the orthogonal code with each cell. is there. In the case of H-matrix H (4), it is expressed as shown in equation (3), and the phase address code is (φi1, φi2, φi3, φi4).

  A phase modulation pattern is created corresponding to the address codes φ1, φ2, φ3, and φ4 shown on the right side of the equation (3). With these phase modulation patterns, the phase address codes are allocated in a two-dimensional plane. Therefore, as shown in FIG. 4A, the phase modulation pattern is obtained by dividing the display surface of the phase modulator 22 into four, and each divided portion has a pattern in which 0 corresponds to −π. FIG. 4B shows an actual phase modulation pattern, where 0 is black and −π is white. In FIG. 4B, the phase modulation patterns corresponding to the address codes φ1, φ2, φ3, and φ4 are indicated by P1, P2, P3, and P4, respectively.

  Therefore, the control unit 28 displays the phase modulation pattern P1 shown in FIG. 4B on the phase modulator 22, and also displays the data page data D1 (not shown) on the spatial light modulator 10, which is described above. As described above, the data page data D1 is hologram-recorded in the recording area of the hologram recording medium 14. Next, the control unit 28 displays the phase modulation pattern P2 shown in FIG. 4B on the phase modulator 22, and also displays the data page data D2 (not shown) on the spatial light modulator 10, Data page data D2 is hologram-recorded in the same recording area of the hologram recording medium 14 as described above. By repeating this, four pages of data from the data page data D1 to D4 are phase-modulated and multiplexed recorded in the same recording area of the hologram recording medium 14.

  At the time of reproduction, phase modulation patterns P1 to P2 are sequentially displayed on the phase modulator 22 to generate reproduction light that is the same as the reference light 200, and irradiating the reproduction light onto the recording area of the hologram recording medium 14 The data page data D1 to D4 can be read and reproduced sequentially.

  According to the present embodiment, since the two-dimensional phase modulator 22 is used to phase-modulate the reference beam 200, the multiplicity can be the same as compared with the case where a one-dimensional phase modulator is used. For example, since the lateral dimension of the phase modulator 22 can be shortened, the spread of the reference light (reproduced light) 200 can be suppressed correspondingly, and therefore, the reference light (reproduced light) having a uniform intensity. 200 can be generated without increasing the output of the laser light source 2 so much (the loss of laser light emitted from the laser light source 2 is reduced), so that hologram recording by the phase modulation multiplexing method can be performed without imposing a burden on the optical system. It can be carried out.

  Further, when the data multiplicity is increased, the number of divisions of the display surface of the phase modulator 22 is increased. Therefore, in order to maintain the same resolution, the size of the phase modulator 22 must be increased. However, if the two-dimensional phase modulator 22 is used as the phase modulator as in the present embodiment, the increase in the lateral dimension can be made extremely small compared to the case where the one-dimensional phase modulator is used. Therefore, it is not necessary to increase the output of the laser light source 2 so much in order to generate the reference light 200 having a uniform intensity, and the multiplicity can be increased efficiently.

  Here, in an actual optical system, it is difficult for the reference light 200 to irradiate the entire surface of the phase modulator 22 with completely uniform brightness. Even if the reference beam 200 is made uniform using a spatial filter, the intensity tends to decrease from the center to the periphery of the beam. The reason why multiple recording without crosstalk is realized by using a phase address code (orthogonal code) is when the phase modulator 22 performs modulation with binary values of 0 or π, but the phase modulator 22 If there is unevenness in the brightness of the reference light that irradiates the displayed phase modulation pattern, modulation will not be performed with binary values of 0 or π, and orthogonality will be lost and crosstalk will increase. In addition, there remains a problem that the power efficiency of the laser beam decreases as the reference beam becomes uniform.

  FIG. 5 is an explanatory diagram for explaining how to display the phase modulation pattern on the phase modulator of the hologram recording / reproducing apparatus according to the second embodiment of the present invention. However, since the configuration of this example is the same as that of the first embodiment described above, the description of the configuration operation of each unit having the same configuration is omitted, and the characteristic part of the operation is described below. .

  The hologram recording / reproducing apparatus of the present embodiment has the same configuration and operation except that the method of displaying the phase modulation pattern displayed on the phase modulator 22 is different from that of the first embodiment shown in FIG. . FIG. 5 shows a phase modulation pattern corresponding to a phase address code generated by the same method as in the first embodiment for multiplexing four. In the present embodiment, these phase modulation patterns are not displayed using the entire display surface of the phase modulator 22, but the phase modulation patterns are repeatedly displayed vertically and horizontally as shown in FIG.

  In the example of FIG. 6, a plurality of phase modulation patterns P1 are repeatedly displayed on the display surface of the phase modulator 22, and the phase modulation pattern P1 is complete and fills the entire display surface of the phase modulator 22. The so-called phase modulation pattern is tiled. In the example of FIG. 7, a plurality of phase modulation patterns P <b> 2 are repeatedly displayed on the display surface of the phase modulator 22. However, in FIG. 6 and FIG. 7, d is a length extending from 4 to 8 pixels of the pixels constituting the phase modulator 22, for example. The phase modulation pattern is tiled on the display surface of the phase modulator 22 by the control unit 28 and displayed.

  When a plurality of phase modulation patterns are repeatedly displayed on the display surface of the phase modulator 22 in this way, the display area occupied by one phase modulation pattern becomes a part of the display surface of the phase modulator 22. Even if the brightness of the reference light 200 applied to the entire display surface is uneven, the uneven brightness of the reference light 200 that irradiates the display area occupied by one phase modulation pattern is alleviated. The orthogonality at the time of optical phase modulation is maintained, the amount of crosstalk can be reduced, and the S / N ratio of reproduced data can be improved.

  The same applies to the case where the same reproduction light as the reference light 200 is generated.

  FIG. 8 shows a case in which the phase modulation pattern is tiled and displayed on the phase modulator 22 as shown in FIG. 6 to reproduce the multiplexed recording (50 multiplexed in this example), and the phase modulation pattern is typed. It is the figure which compared the experimental result at the time of reproducing | regenerating the data which were displayed like 1st Embodiment without ringing and were multiplexed-recorded (50 multiplexing in this example). In this example, however, the phase address code from which the phase modulation pattern is generated is created from a 64 × 64 H matrix.

  FIG. 8A shows a case where the phase modulation pattern is displayed without tiling on the phase modulator 22, although the target data is data including only the top one white, An image of other data due to crosstalk is displayed, and the S / N ratio is 4.64 (dB). However, FIG. 8B shows a case where the phase modulation pattern is tiled and displayed on the phase modulator 22, and only the target data including only one white is displayed, the crosstalk component disappears, and the S / N is displayed. The ratio is 10.77 (dB), and it can be seen that the S / N ratio is improved.

  Note that the number of the phase modulation patterns that are tiled and displayed on the display surface of the phase modulator 22 is determined by the relationship with the non-uniformity of the brightness of the reference light 200, and as the non-uniformity increases, the display surface becomes larger. By increasing the number of phase modulation patterns to be displayed, the deterioration of the S / N ratio of the reproduced image tends to be prevented. However, the maximum number of phase modulation patterns that can be displayed is determined by the performance of the phase modulator 22.

  According to the present embodiment, even if the brightness of the reference light 200 is not uniform, the phase modulation pattern displayed on the phase modulator 22 is reduced and a plurality of reduced phase modulation patterns are spread on the display surface, that is, By tiling and displaying the phase modulation pattern, it is possible to reduce crosstalk when reproducing multiplex recorded data and improve the S / N ratio.

  Therefore, the phase modulation pattern is tiled and displayed on the phase modulator 22, thereby preventing the S / N ratio of the reproduction data from deteriorating even if the brightness of the reference light 200 is somewhat uneven. Therefore, the burden on the optical system can be reduced, and laser beam power efficient multiplexing can be performed. Further, even if the phase modulation multiplicity is increased and the size of the phase modulator 22 is increased, the uniformity of the brightness of the reference light and the reproduction light that irradiates the phase modulator 22 can be moderate. The phase modulation multiplicity can be increased more easily than before without increasing the load so much.

  In addition, although the above-mentioned effect was described for the reference light 200, it is needless to say that the same is true for the reproduction light that is the same as the reference light 200.

  In addition, the present invention is not limited to the above-described embodiment, and can be implemented in various other forms in specific configurations, functions, operations, and effects without departing from the gist thereof.

1 is a block diagram showing a configuration of a hologram recording / reproducing apparatus according to a first embodiment of the present invention. FIG. 2 is a block diagram showing a configuration of a signal processing system of the hologram recording / reproducing apparatus shown in FIG. 1. FIG. 2 is a perspective view illustrating an operation of a two-dimensional phase modulator that optically modulates the reference light illustrated in FIG. 1. It is explanatory drawing which showed the production | generation method of the phase modulation pattern displayed on the two-dimensional phase modulator shown in FIG. It is explanatory drawing explaining the method of the display of the phase modulation pattern to the phase modulator of the hologram recording / reproducing apparatus which concerns on the 2nd Embodiment of this invention. FIG. 6 is a diagram showing a method of tiling and displaying the phase modulation pattern shown in FIG. 5 on a phase modulator. FIG. 6 is a diagram showing how to display the other phase modulation patterns shown in FIG. 5 by tiling them on the phase modulator. It is a figure showing the difference in the S / N ratio of the reproduced image when the multiplex recording is performed by tiling and displaying the phase modulation pattern on the phase modulator and when the multiplex recording is performed by directly displaying the phase modulation pattern. It is the schematic which showed the structural example of the recording / reproducing system of the conventional hologram storage.

Explanation of symbols

  2 ... Laser light source, 4 ... Shutter, 6 ... Beam splitter, 8, 16 ... Mirror, 10 ... Spatial light modulator, 12 ... Signal light lens (Fourier lens), 14 ... Hologram recording medium , 18... Lens (inverse Fourier lens), 20... Imaging element, 22... Spatial light phase modulator, 24... Lenslet array, 26. .

Claims (12)

A hologram recording apparatus that records interference fringes generated by causing a first light beam spatially modulated by recording data to interfere with a second light beam on a hologram recording medium,
Comprising two-dimensional phase modulation means for optical phase modulation of the second light beam;
The interference fringes of the second light beam optically phase-modulated by the two-dimensional phase modulation means and the first light beam spatially modulated are multiplexed and recorded in the same recording area of the hologram recording medium. Hologram recording device.   The two-dimensional phase modulation means displays a plurality of one phase modulation patterns of the second light beam arranged in advance on a two-dimensional plane, corresponding to one recording data of the first light beam. The hologram recording apparatus according to claim 1, wherein:   The two-dimensional phase modulation means includes a two-dimensional phase modulator for displaying a phase modulation pattern for optical phase modulation of the second light beam, and a second optical phase modulated by the two-dimensional phase modulator. 2. The hologram recording apparatus according to claim 1, comprising a lenslet array for diffusing a light beam. A hologram reproducing apparatus for reproducing data by reading out a diffracted light generated by irradiating a light beam onto a hologram recording medium on which interference fringes are recorded, by receiving the light with a light receiving element,
Two-dimensional phase modulation means for optical phase modulation of the light beam;
A hologram reproducing apparatus for irradiating a recording region of the hologram recording medium with a light beam optically modulated by the two-dimensional phase modulation means.   5. The hologram reproducing apparatus according to claim 4, wherein the two-dimensional phase modulation means displays a plurality of phase modulation patterns for optical phase modulation of the light beam arranged on a two-dimensional plane.   The two-dimensional phase modulation means includes a two-dimensional phase modulator for displaying a phase modulation pattern for optical phase modulation of the light beam, and a lens for diffusing the light beam modulated by the two-dimensional phase modulator. The hologram reproducing apparatus according to claim 4, comprising a let array. A hologram recording method for recording interference fringes generated by causing a first light beam spatially modulated by recording data to interfere with a second light beam on a hologram recording medium,
The second light beam is optically phase-modulated by a two-dimensional phase modulation pattern, and the second light beam that has been optically phase modulated is caused to interfere with the first light beam that has been spatially modulated. Hologram recording method.   A plurality of phase modulation patterns for optical phase modulation of the second light beam are arranged on a two-dimensional plane, and the second light beam is optically phase modulated by the plurality of arranged phase modulation patterns. The hologram recording method according to claim 7. A hologram reproduction method for reproducing data by reading out diffracted light generated by irradiating a light beam onto a hologram recording medium on which interference fringes are recorded, by receiving the light with a light receiving element,
A hologram reproducing method, wherein the light beam is optically phase-modulated by a two-dimensional phase modulation pattern, and the recording region of the hologram recording medium is irradiated with the light phase-modulated light beam.   10. The hologram reproducing method according to claim 9, wherein a plurality of the two-dimensional phase modulation patterns are arranged on the two-dimensional plane, and the light beam is optically phase-modulated by the plurality of arranged phase modulation patterns. . In the phase modulation method of the light beam when the light beam is optically phase modulated by the phase modulation means displaying the phase modulation pattern,
A phase modulation method for a light beam, characterized in that the phase modulation pattern corresponding to the recording data is prepared in advance, and a plurality of the phase modulation patterns are displayed on the phase modulation means. 12. The light beam phase modulation method according to claim 11, wherein the phase modulation means includes a phase modulator for displaying a two-dimensional phase modulation pattern.

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