JP2005135479A - Hologram recording/reproducing device, hologram recording/reproducing method, phase modulation pattern display method, and phase modulation pattern creation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform light phase modulation precisely with a target amount of modulation at any part regardless of the spatial frequency of a partial pattern for forming a phase modulation pattern, when performing the multiplex recording of data by a phase modulation multiplex method. <P>SOLUTION: A control section 28 increases a display signal level at a part having a higher spatial frequency, and reduces the display signal level at a part having a low spatial frequency, according to the spatial frequency of a pattern at each part for forming a phase modulation pattern created from a phase address code for giving light phase modulation information, for displaying the phase modulation pattern in a phase modulator 22. As a result, the phase modulator 22 corrects deviation from the amount of target modulation by the level of a spatial frequency and precisely performs the light phase modulation of reference light or reproduction light. In addition, crosstalk is suppressed, thus performing recording/reproduction having improved S/N ratio. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a hologram recording / reproducing apparatus and method for recording / reproducing a hologram by a phase modulation multiplexing method, and more particularly to a phase modulation pattern used when spatial light phase modulation (hereinafter simply referred to as optical phase modulation) of reference light or reproduction light is performed. The present invention relates to a display method and a phase modulation pattern creation method.

  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. 11, 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, 1M is recorded in one hologram spot (data page). It is also possible to allocate byte 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 optical components remain fixed, and the reference light is optically phase-modulated with a spatial light modulator to generate reference light having various phases. This is a method of multiplex recording data in the same recording area (spot) of a hologram recording medium by interfering with light. One of the causes of reducing the S / N ratio of reproduced data by using this light phase characteristic. There is an advantage that a certain crosstalk 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 conditions imposed on the actual optical system to realize multiplex recording without crosstalk are that the reference light is first modulated with uniform brightness and accurately modulated to 0 or π as described later. Is Rukoto. 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 lens aberration, the 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 system, it is advantageous to use a two-dimensional spatial light phase modulator with less optical burden. In order to optically modulate the reference light with the two-dimensional spatial light phase modulator, a phase modulation pattern corresponding to a phase address code giving phase modulation information is created, and the spatial light phase modulator on which the phase modulation pattern is displayed Thus, the reference light is optically phase modulated. As a result, reference light that is optically phase-modulated for each phase modulation pattern is generated, and interference fringes of each reference light and signal light are multiplexed and recorded in the same recording area. At this time, the phase address code is different in pattern and frequency in orthogonal codes. For this reason, a portion of the phase modulation pattern displayed on the spatial light phase modulator has a low spatial frequency, so the optical phase is modulated with good accuracy, but another portion of the pattern has a high spatial frequency. In other words, the optical phase modulation deviates from the target modulation amount. When spots occur in the optical phase modulation by such a spatial light phase modulator, there arises a problem that crosstalk increases and the S / N ratio of the reproduction signal deteriorates.

  The present invention has been devised in view of the above circumstances, and the object of the present invention is to determine which of the spatial frequencies of the partial pattern forming the phase modulation pattern when data is multiplexed and recorded by the phase modulation multiplexing method. An object of the present invention is to provide a hologram recording / reproducing apparatus, a hologram recording / reproducing method, a method for displaying a phase modulation pattern, and a method for creating a phase modulation pattern capable of accurately performing optical phase modulation with a target modulation amount even at a portion.

  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. A phase modulation pattern creating means for creating a phase modulation pattern from a phase address code giving optical phase modulation information, and changing a display signal level according to a spatial frequency of each pattern forming the created phase modulation pattern Optical phase modulation means for displaying the phase modulation pattern and optically phase-modulating the second light beam by the displayed phase modulation pattern, and the optical phase-modulated second light beam and the space The interference fringes of the light-modulated first light beam are multiplexed and recorded in the same area of the hologram recording medium.

  As described above, in the hologram recording apparatus of the present invention, the display signal level (corresponding to the luminance level) is set in the portion where the spatial frequency is high, according to the spatial frequency of the pattern of each part forming the phase modulation pattern created from the phase address code. The phase modulation pattern is displayed on the spatial light phase modulator by increasing the display signal level and decreasing the display signal level in the lower part, thereby correcting the deviation from the target modulation amount due to the spatial frequency level. 2 light beam (reference light) can be optically phase-modulated with high accuracy, crosstalk can be suppressed, and recording with a good S / N ratio can be performed.

  The present invention also relates to a hologram reproducing apparatus for reproducing data by reading out a diffracted light generated by irradiating a hologram recording medium on which interference fringes are recorded with a light receiving element, and reproducing the data. Phase modulation pattern creating means for creating a phase modulation pattern from a phase address code giving phase modulation information, and changing the display signal level according to the spatial frequency of the pattern of each part forming the created phase modulation pattern An optical phase modulation means for displaying a modulation pattern and optically phase-modulating the light beam with the displayed phase modulation pattern, and irradiating the hologram recording medium with the optical phase-modulated light beam It is characterized by obtaining light.

  As described above, in the hologram reproducing apparatus of the present invention, the display signal level is increased in the portion where the spatial frequency is high and the display signal level is increased in the portion where the spatial frequency is low, according to the spatial frequency of the pattern of each portion forming the phase modulation pattern created from the phase address code. By changing the display signal level to be small and displaying the phase modulation pattern on the spatial light phase modulator, the deviation from the target modulation amount is corrected by the spatial frequency level, and the light beam (reproduced light) is corrected. Optical phase modulation can be performed with high accuracy, and a reproduction signal having a good S / N ratio can be obtained from the diffracted light while suppressing crosstalk.

  The present invention also relates to 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, wherein the optical phase modulation is performed. A step of creating a phase modulation pattern from a phase address code providing information, and a step of displaying the phase modulation pattern by changing a display signal level according to a spatial frequency of a pattern of each part forming the created phase modulation pattern And optical phase modulating the second light beam with the displayed phase modulation pattern, the optical phase modulated second light beam and the spatial light modulated first light beam. The interference fringes are multiplexed and recorded in the same area of the hologram recording medium.

  As described above, in the hologram recording method of the present invention, the display signal level is increased and reduced in the portion where the spatial frequency is high, for example, in accordance with the spatial frequency of the pattern of each portion forming the phase modulation pattern created from the phase address code. In the part, the phase modulation pattern is displayed on the spatial light phase modulator by changing the display signal level to be small, so that the deviation from the target modulation amount is corrected by the spatial frequency level, and the second light beam is corrected. (Reference light) can be optically phase-modulated with high accuracy, crosstalk can be suppressed, and recording with a good S / N ratio can be performed.

  The present invention also provides a hologram reproducing 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, and receiving the light by a light receiving element. A step of creating a phase modulation pattern from a phase address code giving phase modulation information, and displaying the phase modulation pattern by changing the display signal level according to the spatial frequency of the pattern of each part forming the created phase modulation pattern And optical phase modulating the light beam with the displayed phase modulation pattern, and irradiating the hologram recording medium with the optical phase modulated light beam to obtain the diffracted light Features.

  As described above, in the hologram reproducing method of the present invention, the display signal level is increased and lowered in the portion where the spatial frequency is high, for example, in accordance with the spatial frequency of the pattern of each portion forming the phase modulation pattern created from the phase address code. In the part, the phase modulation pattern is displayed on the spatial light phase modulator by changing the display signal level to be small, so that the deviation from the target modulation amount is corrected by the spatial frequency level, and the light beam (reproduced light) is corrected. ) Can be accurately modulated, and crosstalk can be suppressed, and a reproduced signal having a good S / N ratio can be obtained from the diffracted light.

  The present invention also provides a method for displaying a phase modulation pattern displayed on a spatial light phase modulator when optical phase modulation of a light beam is performed, wherein a display signal is displayed in accordance with the spatial frequency of each pattern forming the phase modulation pattern. And changing the level to display the phase modulation pattern on the spatial light phase modulator.

  Thus, in the phase modulation pattern display method of the present invention, according to the spatial frequency of the pattern of each part forming the phase modulation pattern created from the phase address code, the display signal level is increased in the portion where the spatial frequency is high, By displaying the phase modulation pattern on the spatial light phase modulator by changing the display signal level to be lower in the low part, the deviation from the target modulation amount is corrected by the level of the spatial frequency and the light beam is accurate. Optical phase modulation can be performed well.

  The present invention also relates to a method for creating a phase modulation pattern to be displayed on the spatial light phase modulator when the light beam is optically phase modulated by the spatial light phase modulator, from a phase address code that provides optical phase modulation information A step of creating a phase modulation pattern; a step of measuring a spatial frequency of a pattern of each part forming the created phase modulation pattern; and a display signal level corresponding to the measured spatial frequency for each pattern of the part The step of setting, and the step of linking the set display signal level to the created phase modulation pattern.

  As described above, in the phase modulation pattern creating method of the present invention, the spatial frequency of the pattern of each part forming the phase modulation pattern created from the phase address code is measured, and the display signal level corresponding to the measured spatial frequency is This is set for each pattern of each part, and since this set display signal level is linked to the phase modulation pattern, this phase modulation pattern is displayed on the spatial light phase modulator according to the linked display signal level. The optical beam can be optically phase-modulated with high accuracy by correcting the deviation from the target modulation amount due to the level of the spatial frequency.

According to the present invention, a portion having a high spatial frequency is displayed on a spatial light phase modulator in accordance with the spatial frequency of each part pattern forming a phase modulation pattern created from a phase address code that provides optical phase modulation information. The signal level is increased and the signal level is changed so that the signal level is reduced in the lower part, so that the deviation from the target modulation amount is corrected by the high and low spatial frequencies, and the reference light and reproduction light are accurately emitted. Phase modulation can be performed, thereby suppressing crosstalk and performing recording / reproduction with a good S / N ratio.
Further, even if the multiplicity of phase modulation multiplex recording is increased due to the above effect, a reproduction signal with a low S / N ratio with little crosstalk can be obtained, so that the recording capacity of the hologram recording medium can be increased.

  When the data is multiplexed and recorded by the phase modulation multiplexing method, it is created from the phase address code for the purpose of optical phase modulation with the target modulation amount with high accuracy regardless of the spatial frequency of the partial pattern forming the phase modulation pattern. By changing the display signal level according to the spatial frequency of the pattern of each part forming the phase modulation pattern and displaying the phase modulation pattern on the spatial light phase modulator, the level of the spatial frequency is changed from the target modulation amount. This was realized by correcting the deviation and optically phase-modulating the reference light and the reproduction light with high accuracy.

  FIG. 1 is a schematic diagram showing a configuration of a hologram recording / reproducing apparatus according to an 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 hologram recording medium in the claims corresponds to the hologram recording medium 14, the optical phase modulation means corresponds to the control unit 28 (see FIG. 2) and the spatial light phase modulator 22, and the light receiving element corresponds to the imaging element 20. Equivalent to.

  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. However, the phase modulation pattern creating means in the claims corresponds to the control unit 28.

  FIG. 3 is a perspective view showing the configuration of the phase modulator 22 shown in FIG. The phase modulator 22 is configured such that a liquid crystal layer 223 is sandwiched between a liquid crystal display unit (LCD) 221 and a transparent electrode unit 222, and a voltage is applied to the surface of the liquid crystal display unit 221 on the liquid crystal layer 223 side due to light intensity. A changing transparent photoconductive layer (not shown) is applied. When a phase modulation pattern, which will be described later, is displayed on the liquid crystal display unit 221, the transmissivity of the writing light 91 is different between the black (0 modulation) and the white (π modulation) portions, so that the voltage of the photoconductive layer corresponds to the modulation degree. Change. For this reason, the voltage applied to the liquid crystal layer 223 between the transparent electrode portion 222 to which a constant voltage is applied changes in accordance with the modulation degree, and the refractive index of the liquid crystal layer 223 in between corresponds to the modulation degree. Will be controlled. Thereby, the writing light (reference light 200 or reproduction light) 91 incident from behind the liquid crystal display unit 221 is phase-modulated when passing through the liquid crystal layer 223 according to the phase modulation pattern displayed on the liquid crystal display unit 221. The phase-modulated readout light (phase-modulated reference light 200 or reproduction light) 92 is emitted from the transparent electrode portion 222.

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

  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 100 is intensity-modulated by the spatial light modulator 10 displaying the data page. The intensity-modulated signal light 100 is focused 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. A phase modulation pattern for optical phase modulation of the reference light 200 is displayed on the phase modulator 22 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), χ · χ * is a unitary matrix and indicates the phase address of the reference beam 200 used for recording the i-th data page. 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 (phase 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).

  Phase modulation patterns are created corresponding to the phase address codes φ1, φ2, φ3, and φ4 shown on the right side of the equation (3). With these phase modulation patterns, phase address codes are allocated on a two-dimensional plane. Therefore, as shown in FIG. 5A, the phase modulation pattern is obtained by dividing the display surface of the phase modulator 22 into four parts, and each divided part has a pattern corresponding to 0 or π. FIG. 5B shows an actual phase modulation pattern, where 0 is black and π is white. In FIG. 5B, the phase modulation patterns corresponding to the phase 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. 5B on the phase modulator 22, and also displays the data page D1 (not shown) on the spatial light modulator 10, which has been described above. As described above, the data page 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. 5B on the phase modulator 22, and displays the data page D2 on the spatial light modulator 10, so that the hologram recording medium 14 is The data page D2 is hologram-recorded in the same recording area. By repeating this, four pages of data pages 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 identical to the reference light 200, and by irradiating the reproduction light onto the recording area of the hologram recording medium 14, Sequentially, the data pages D1 to D4 can be read and reproduced.

  Incidentally, it is necessary to increase the number of divisions of the pixel array of the phase modulator 22 in proportion to the multiplicity of the hologram. FIG. 6A shows an example of a phase modulation pattern with a low degree of multiplicity, and FIG. 6B shows an example of a phase modulation pattern with a high degree of multiplicity. The higher the multiplicity, the higher the spatial frequency of the pattern. Thus, the number of divisions of the pixel array of the phase modulator 22 is also increasing. In addition, as shown in FIG. 6B, in the phase address pattern with the increased number of divisions, there is a part in which the repetition of the modulation areas of 0 and π is very sparse in the pattern of one page. There is also a dense part.

  Some phase modulators that have been realized so far exist that can display an XGA signal with a pixel size of 26 μm (for example, manufactured by Hamamatsu Photonics). The feature of this phase modulator (P-SLM module) is that the light utilization efficiency is higher than that of a general electric signal input type LCD. This is because the P-SLM module uses an optical image transmission element. This is because the non-pixel structure is realized by combining the electric address type LCD and the phase modulation SLM. Although this structure has an advantage of high light utilization efficiency, adjacent pixel information leaks due to the limit of MTF (modulation transfer function). Since this leakage affects the orthogonality of the phase modulation code, there is an inconvenience that a phase address code that easily causes crosstalk is generated. In the present embodiment, it is assumed that a phase modulator having the same structure as the phase modulator manufactured by Hamamatsu Photonics Co. is used as the phase modulator 22, and thus the above inconvenience is solved by the method described below.

  The pixel information in the P-SLM module indicates a gray scale signal level (a signal level (display signal level) applied to the liquid crystal display unit 221 that displays a phase modulation pattern), and the light phase modulation is performed on this signal level. The amount corresponds. Assuming that MTF = 1, if the signal level corresponding to the modulation amounts 0 and π is alternately applied to each pixel of the P-SLM as shown in FIG. 7A, the light is accurately set to 0 for each pixel. Modulated to π. However, if MTF ≠ 1, even if a signal level corresponding to 0 and π is given to each pixel as shown in FIG. 7B, crosstalk occurs between those signals, so that a binary signal is originally generated. The value changes continuously, and the amount of phase modulation also becomes a continuous value corresponding to it.

  The phase address code is different in pattern and frequency in the orthogonal code, and a certain pattern can be phase-modulated with good accuracy because the frequency is low, and phase modulation is performed from the target modulation amount in the high frequency part of another pattern. It will shift. As described above, when the orthogonality of the phase address code is not satisfied, that is, when the modulation amount deviates from the target value, crosstalk of the holographically recorded hologram occurs.

  When the relationship between the frequency of the phase modulation pattern generated from the phase address code and the noise due to crosstalk is experimentally examined, the higher the spatial frequency of the pattern, the higher the π modulation region displayed on the phase modulator 22 If the signal level is increased, the orthogonal condition is satisfied and the noise can be minimized. This is because even if binary modulation cannot be performed completely accurately with a high-frequency pattern, crosstalk can be suppressed to an extent that does not affect information readout by controlling the signal level according to the frequency of the pattern. Means.

  Therefore, the control unit 28 according to the present embodiment creates a phase modulation pattern and a signal level (PPM input signal) to be assigned to the phase modulation pattern as shown in the flowchart of FIG. 8, and generates a phase modulator according to the spatial frequency of the phase modulation pattern. Control to change the signal level at the time of display on the screen 22 is performed. First, when a matrix size m corresponding to the maximum multiplicity at the time of phase modulation multiplex recording is designated and inputted (step S1), an m × m Hadamard matrix H (m) is generated (step S2) and created. The elements in the first column are removed from the Hadamard matrix H (m) thus obtained (step S3). The second column, the third column,..., The nth column of the Hadamard matrix H (m) from which the elements of the first column are removed are address codes φ1, φ2,..., Φm−1, and the phase address codes (φ1, φ2,. ..., φm-1) is created (step S4).

  Next, m−1 two-dimensional phase modulation patterns corresponding to φ1, φ2,..., Φm−1 of the phase modulation address code are created (step S5). The spatial frequency of each created phase modulation pattern is counted (step S6), and a signal level corresponding to the spatial frequency of the phase modulation pattern is assigned (step S7).

  Here, the assignment of the signal level corresponding to the spatial frequency of the phase modulation pattern will be described in detail. For example, in the case where the phase modulation pattern is in the form as shown in FIG. 9, the Q1 portion having a low spatial frequency is referred to the spatial frequency (corresponding to the number of pixels) and π modulation signal level correspondence table shown in FIG. Since it is a part having 64 or more pixels, 100 is assigned as the signal level. Since the portion indicated by Q2 in FIG. 9 is a portion corresponding to 16 to 32 pixels, 108 is assigned as the signal level. However, the size of the lattice constituting the pattern is proportional to the number of pixels, and the smaller the number of pixels, the higher the spatial frequency. Further, since the black portion of the pattern indicated by Q3 is modulation 0, the signal levels are all zero regardless of the frequency.

  The control unit 28 stores the phase modulation pattern in which the signal level at the time of display is assigned for each spatial frequency in the built-in memory (step S8). Thereafter, the control unit 28 reads the phase modulation pattern from the memory, applies the signal level assigned for each spatial frequency to the phase modulator 22, and displays the phase modulation pattern on the phase modulator 22 (step S9).

  Since the phase modulator 22 displays the phase modulation pattern by changing the signal level according to the spatial frequency as described above, the target phase modulation amount in all the pattern portions regardless of the spatial frequency of the phase modulation pattern. Thus, the optical phase modulation is performed, and the spatial spatial phase modulation of the reference beam 200 can be performed with good accuracy. Therefore, it is possible to reduce the crosstalk and obtain a reproduction signal with a good S / N ratio.

  Therefore, even if the multiplicity of phase modulation multiplex recording is increased, a reproduction signal with a small crosstalk and a good S / N ratio can be obtained, so that the recording capacity of the hologram recording medium 14 can be increased.

  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. In the above embodiment, the present invention has been described with reference to an example in which the present invention is applied to a hologram recording / reproducing apparatus. However, the present invention is applied to any apparatus that performs optical phase spatial modulation of a light beam using a spatial light phase modulator. An effect of performing optical phase modulation with high accuracy without deviating from the target modulation amount can be obtained.

It is the schematic which showed the structure of the hologram recording / reproducing apparatus which concerns on one embodiment of this 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 a configuration example of a spatial light phase modulator illustrated 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 the figure which showed the example of the phase modulation pattern according to the degree of multiplicity. FIG. 4 is a waveform diagram showing signal levels applied to the liquid crystal display unit shown in FIG. 3 according to the degree of modulation. FIG. 3 is a flowchart showing a procedure for creating a phase modulation code and a phase modulation pattern of the control unit shown in FIG. 2. FIG. It is the figure which showed the signal level allocated to a phase modulation pattern and its each part. FIG. 3 is a table showing a correspondence relationship between a spatial frequency and a signal level referred to when setting a signal level assigned to a phase modulation pattern by a control unit shown in FIG. 2. 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 device, 22... Spatial light phase modulator, 24... Lenslet array, 26. 221... Liquid crystal display (LCD), 222... Transparent electrode, 223.

Claims (13)

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,
Phase modulation pattern creating means for creating a phase modulation pattern from a phase address code giving optical phase modulation information;
The display signal level is changed according to the spatial frequency of the pattern of each part forming the created phase modulation pattern to display the phase modulation pattern, and the second light beam is emitted by the displayed phase modulation pattern. Optical phase modulation means for phase modulation,
A hologram recording apparatus, wherein interference fringes of the optical phase-modulated second light beam and the spatial light-modulated first light beam are multiplexed and recorded in the same region of the hologram recording medium.   The optical phase modulation means measures a spatial frequency of each part pattern forming the phase modulation pattern, and sets a display signal level corresponding to the measured spatial frequency for each part pattern. 1. The hologram recording apparatus according to 1.   2. The hologram recording apparatus according to claim 1, wherein the optical phase modulation means includes a spatial light phase modulator that displays a two-dimensional phase modulation pattern. 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,
Phase modulation pattern creating means for creating a phase modulation pattern from a phase address code giving optical phase modulation information;
The display signal level is changed according to the spatial frequency of the pattern of each part forming the created phase modulation pattern to display the phase modulation pattern, and the optical beam is optically phase modulated by the displayed phase modulation pattern. Optical phase modulation means,
A hologram reproducing apparatus, wherein the hologram recording medium is irradiated with the light beam subjected to optical phase modulation to obtain the diffracted light.   The optical phase modulation means measures a spatial frequency of each part pattern forming the phase modulation pattern, and sets a display signal level corresponding to the measured spatial frequency for each part pattern. 5. The hologram reproducing device according to 4.   5. The hologram reproducing apparatus according to claim 4, wherein the optical phase modulation means includes a spatial light phase modulator that displays a two-dimensional phase modulation pattern. 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,
Creating a phase modulation pattern from a phase address code that provides optical phase modulation information;
Changing the display signal level according to the spatial frequency of the pattern of each part forming the created phase modulation pattern and displaying the phase modulation pattern;
Optical phase modulating the second light beam with the displayed phase modulation pattern,
A hologram recording method, wherein interference fringes of the optical phase-modulated second light beam and the spatial light-modulated first light beam are multiplexed and recorded in the same region of the hologram recording medium.   Measuring a spatial frequency of a pattern of each part forming the phase modulation pattern; and setting a display signal level corresponding to the measured spatial frequency for each pattern of each part. 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,
Creating a phase modulation pattern from a phase address code that provides optical phase modulation information;
Changing the display signal level according to the spatial frequency of the pattern of each part forming the created phase modulation pattern and displaying the phase modulation pattern;
Optical phase modulating the light beam with the displayed phase modulation pattern,
A hologram reproducing method, wherein the diffracted light is obtained by irradiating the hologram recording medium with the optical phase-modulated light beam.   Measuring a spatial frequency of a pattern of each part forming the phase modulation pattern; and setting a display signal level corresponding to the measured spatial frequency for each pattern of each part. The hologram reproducing method according to claim 9. A phase modulation pattern display method for displaying on a spatial light phase modulator when optical phase modulation of a light beam is performed,
A method of displaying a phase modulation pattern, comprising: displaying a phase modulation pattern on the spatial light phase modulator by changing a display signal level according to a spatial frequency of a pattern of each part forming the phase modulation pattern .   Measuring a spatial frequency of a pattern of each part forming the phase modulation pattern; and setting a display signal level corresponding to the measured spatial frequency for each pattern of each part. The phase modulation pattern display method according to claim 11. A method of creating a phase modulation pattern to be displayed on the spatial light phase modulator when the light beam is optically phase modulated by the spatial light phase modulator,
Creating a phase modulation pattern from a phase address code that provides optical phase modulation information;
Measuring the spatial frequency of the pattern of each part forming the created phase modulation pattern;
Setting a display signal level corresponding to the measured spatial frequency for each pattern of each part;
Linking the set display signal level to the created phase modulation pattern;
A method of creating a phase modulation pattern, comprising:

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