JP2007200517A - Hologram reproducing method and device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hologram reproducing method and a device capable of reproducing digital data held in a signal light with sufficient accuracy even when the hologram recording is performed in a state where a zero order component is removed from a Fourier transformation image of the signal light. <P>SOLUTION: An optical recording medium is irradiated with a read reference light, an inverse Fourier transformation image of the diffracted light by the recorded hologram is detected and a first reproduced image is acquired. The diffracted light and direct current component of the Fourier transformation image for the signal light are combined, and the combined light is generated. An inverse Fourier transformation image of this combined light is detected, and a second reproduction image is acquired. Difference between brightness of the second reproduction image and the brightness of the first reproduction image is calculated for each of the identical pixels. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a hologram reproducing method and apparatus, and more particularly to a hologram reproducing method and apparatus for accurately reproducing recorded digital data when digital data is recorded as a Fourier transform hologram.

  In the holographic data storage, binary digital data “0, 1” is converted into a digital image (signal light) as “bright, dark”, and the signal light is Fourier-transformed by a lens and applied to an optical recording medium. . A Fourier transform image is recorded as a hologram on the optical recording medium. However, since the Fourier transform image of digital data has an extremely strong peak intensity in the 0th order, in the holographic data storage, the dynamic range of the optical recording medium is wasted due to this 0th order component (DC component). If an attempt is made to increase the multiplicity (the number of holograms that are multiplexed and recorded), the S / N (signal-noise ratio) of the reproduced image is greatly reduced. Therefore, there is a problem that the multiplicity cannot be increased.

In order to solve this problem, various methods for blocking the zero-order component of the Fourier transform image of the signal light have been proposed (Patent Documents 1 and 2). For example, in the method described in Patent Document 1, a light shield is disposed between a lens and an optical recording medium, and the 0th-order component of signal light is blocked by this light shield. Then, only the components of the specific order of the signal light pass through the aperture formed in the light shielding body, and the image edge portion of the signal light is recorded as a hologram. According to this method, waste of the dynamic range can be prevented by suppressing useless exposure due to the zeroth-order component, and the recording area of each data page can be reduced.
JP 2000-66565 A JP 2004-198816 A

  However, if the zero-order component is removed from the Fourier transform image of the signal light, there is a problem that an intensity pattern different from the original digital image appears in the reproduced image, and the digital data cannot be accurately decoded. That is, when the hologram is recorded and reproduced by removing the zero-order component from the Fourier transform image of the signal light, the intensity pattern of the reproduced image is different from the intensity pattern generated by the spatial light modulator at the time of recording. . For example, in the method described in Patent Document 1, only the image edge portion is reproduced. In this case, there is a possibility that the digital data cannot be correctly decoded.

  The present invention has been made to solve the above problems, and the object of the present invention is to record a hologram from a recorded hologram even when the hologram is recorded in a state where the zero-order component is removed from the Fourier transform image of the signal light. Another object of the present invention is to provide a hologram reproducing method and apparatus capable of accurately reproducing digital data held in signal light.

  In order to achieve the above object, the hologram reproduction method of the present invention is a hologram reproduction method in which digital data is represented by a bright and dark image and signal light from which a direct current component has been removed and reference light are Fourier-transformed and simultaneously irradiated onto an optical recording medium. Irradiating the recording medium with the reference light for reading to generate the diffracted light by the recorded hologram, and combining the diffracted light and the direct current component of the signal light, 1 or 2 The step of generating the combined light and detecting the diffracted light and the inverse Fourier transform image of the combined light, respectively, or detecting the two types of the inverse Fourier transform images of the combined light, and detecting the two types of image data And subtracting the other from either one of the two types of image data for each pixel of the light and dark image.

  In the hologram reproducing method of the present invention, when a hologram is recorded with the DC component removed from the signal light, the diffracted light from the recorded hologram and the combined light in which the DC component of the signal light is superimposed on the diffracted light. And two types of image data are acquired from the diffracted light and the combined light or from the two types of combined light. By performing a subtraction process that subtracts the other from one of these two types of image data and calculating a luminance difference for each pixel, noise is canceled and binary digital data can be accurately reproduced. .

  The hologram reproducing method of the present invention is the optical recording in which a hologram is recorded by digitally representing a digital image as a bright and dark image and Fourier transforming the signal light from which the direct current component has been removed and the reference light and simultaneously irradiating the optical recording medium. The present invention can also be applied to the case where a recorded hologram is read from a medium and digital data is reproduced. Moreover, it is preferable that the present invention further includes a step of decoding the digital data based on the value obtained by subtraction.

  As described above, according to the present invention, even when hologram recording is performed in a state where the zero-order component is removed from the Fourier transform image of the signal light, the digital data held in the signal light is accurately reproduced. There is an effect that can be.

  Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings.

(Decoding principle of digital data)
FIG. 1 is a diagram for explaining the principle of decoding digital data.
Here, the binary digital data “0, 1” is subjected to Fourier transform on the signal light (digital pattern) that is digitally imaged as “dark (black pixel), light (white pixel)”, and a Fourier transform image of the signal light. A case will be described in which the direct current component (zeroth order component) is removed from the optical recording medium and the optical recording medium is simultaneously irradiated with the signal light and the reference light from which the direct current component has been removed to record the interference pattern as a hologram.

  When a hologram is recorded using signal light from which the DC component has been removed, when the recorded hologram is irradiated with reference light, the same diffracted light as the signal light from which the DC component has been removed is reproduced. In the inverse Fourier transform image (first reproduced image) of the diffracted light, the bright part of the original bright and dark image tends to be dark and the dark part tends to be bright. As a result, the contrast between white pixels and black pixels (or S / N). Ratio) tends to be worse. The reason why the contrast deteriorates in the first reproduced image can be explained as follows.

  That is, removing the DC component is equivalent to causing a plane wave having the same overall intensity as the signal light and having a phase difference π to interfere with the signal light. For example, in the case where the signal light is a plane wave composed entirely of DC components, the signal light becomes zero when the plane wave having the same intensity and phase difference π is caused to interfere. Therefore, when removing the direct current component of the signal light modulated with digital data, it is equal to the result of interference of the plane wave having the same total intensity and the phase difference π with the original signal light.

  In this case, since the electric field amplitude becomes small due to negative interference in the bright part, the intensity becomes small and dark. On the other hand, a light wave is added to the dark part and the amplitude of the electric field increases, so that the intensity increases and the image becomes brighter. As a result, the intensity of the bright part and the dark part becomes comparable, and the contrast deteriorates.

  Note that the above description of contrast degradation is the case where the number of white pixels and the number of black pixels in the digital pattern of signal light are approximately the same. In general, in the digital pattern, the number of white pixels and the number of black pixels are substantially equal. When the number of white pixels is much larger than the number of black pixels, as will be described later, the first reproduced image is an inverted image obtained by inverting the contrast of the digital pattern of the signal light. However, it is common that the bright part of the original bright and dark image tends to be dark and the dark part tends to be bright.

  When the reproduced diffracted light is supplemented with a DC component having the same phase, synthesized light close to the original signal light is generated. That is, when the phase of the DC component to be supplemented is the same as the phase of the diffracted light from the hologram, the second reproduced image is a bright and dark positive image (positive image) similar to the original signal light. In the inverse Fourier transform image (second reproduced image) of the combined light, the bright part of the original bright and dark image is bright and the dark part is dark. Accordingly, the difference when the luminance of the first reproduced image is subtracted from the luminance of the second reproduced image is positive in the bright part of the original bright and dark image and negative in the dark part of the original bright and dark image.

  As described above, for example, if binary digital data “0, 1” is defined as “dark (black pixel), bright (white pixel)”, a pixel with a positive difference is “1” and a difference is negative. The sign of each pixel can be accurately determined by the sign of the difference, such as “0”, and binary digital data can be accurately reproduced.

  On the other hand, when the phase of the DC component to be supplemented is opposite to the phase of the diffracted light from the hologram, the second reproduced image is an inverted image (negative image) in which the contrast of the original signal light is inverted. In this case, the difference when the luminance of the positive image obtained by supplementing the reproduced diffracted light with the DC component in phase is reduced from the luminance of the second reproduced image as described above. It is negative in the bright part and positive in the dark part of the original light / dark image.

  Similarly to the above example, if binary digital data “0, 1” is defined as “dark (black pixel), bright (white pixel)”, a negative pixel is “1” and a difference is The sign of each pixel can be accurately determined based on the sign of the difference, such as “0” for positive pixels, and binary digital data can be accurately reproduced.

  Next, a method and effect of performing a decoding process using an image obtained by subtracting one of two types of reproduced images (a negative image and a positive image) will be described. Normally, a reproduced image is affected by noise light caused by a recording medium or an optical system at the time of reading, and the S / N ratio is deteriorated. Therefore, the negative image and the positive image are images including a signal component and a noise component, and the difference in brightness (luminance difference) between the white pixel and the black pixel is reduced. This phenomenon becomes more prominent as the multiplicity increases in multiplex recording. Therefore, when there are a lot of noise components, if one of the negative image and the positive image is used for decoding, the bit error increases and it is difficult to correctly restore the recorded data.

  On the other hand, when the subtraction process for subtracting the luminance of the first reproduction image (negative image) from the luminance of the second reproduction image (positive image), the third reproduction image obtained by the subtraction process is S / N. The contrast is improved and the image is large. The reason why the S / N ratio is improved is that the first reproduced image and the second reproduced image reproduced from the same hologram contain a common noise component, so that the first reproduced image is derived from the luminance of the second reproduced image. This is because the common noise component is canceled by subtracting the luminance of. By performing decoding using these images with an improved S / N ratio, as described above, the sign of each pixel can be accurately determined based on the sign of the difference, and binary digital data is reproduced accurately. It can be done.

  In addition, the method described in the above three examples may subtract the negative image luminance from the positive image luminance, and conversely, subtract the positive image luminance from the negative image luminance. Can be decrypted.

  The reproduction of the original signal light pattern is realized as a result of interference between the diffracted light from the hologram and the supplemented DC component. That is, the original signal light pattern is reproduced by setting the phase difference between the diffracted light and the DC component so that the amplitude of the interference wave (synthetic wave) increases.

  The phase of the direct current component can be set by appropriately changing the luminance of the signal light pixel of the spatial light modulator. The spatial light modulator modulates the polarization of incident light and emits it. The polarization modulation is performed by phase-modulating incident light. That is, phase modulation can be performed by polarization modulation. The magnitude of polarization modulation depends on the luminance of the image displayed on the spatial light modulator. Therefore, the phase of the DC component can be set by setting the brightness of the image.

Next, a method for setting the phase difference between the diffracted light from the hologram and the DC component will be described.
The phase of the diffracted light from the recorded hologram is shifted from the phase of the reference light at the time of reproduction. The degree of phase change depends on the type of hologram. For example, the phase of diffracted light is shifted by π / 2 and π in a hologram based on refractive index modulation and a hologram based on absorptance modulation, respectively. Therefore, in order to generate a reproduced image, the luminance of the image displayed on the spatial light modulator is set in consideration of this phase shift, and a direct current component is generated to supplement the diffracted light. Thereby, a desired phase difference can be realized.

The relationship between the phase difference and the reproduced image and the appropriate phase difference condition will be described below.
A reproduced image from the hologram is detected by a photodetector. Let φ be the phase at a certain time t at which the amplitude of the diffracted light reaches a maximum at a position r on the imaging plane of the photodetector. The amplitude and phase of the DC component at time t and position r are A and θ, respectively. In order to obtain a positive image as a reproduced image, the amplitude of the combined light may be moved in the positive direction. For this purpose, θ is set so that the amplitude A of the DC component is positive, that is, the following equation (1) is satisfied.

                    0 ≦ | θ−φ | <π / 2 Formula (1)

  It is more desirable to set θ so as to satisfy the following formula (2). Under this condition, the maximum amplitude of the diffracted light coincides with the maximum amplitude of the direct current component, and the positive amplitude of the combined light is maximized.

                    | Θ−φ | = 0 Formula (2)

  On the other hand, in order to obtain a negative image as a reproduced image, the amplitude of the combined light may be moved in the negative direction. For that purpose, θ is set so that the amplitude A of the DC component becomes negative, that is, the following equation (3) is satisfied.

                    π / 2 <| θ−φ | ≦ π Formula (3)

  It is more desirable to set θ so as to satisfy the following formula (4). Under this condition, the maximum amplitude of the diffracted light coincides with the minimum amplitude of the direct current component, and the negative amplitude of the combined light is maximized.

                    | Θ−φ | = π Formula (4)

  The phase of the direct current component as described above can be set by appropriately changing the luminance of the signal light pixel of the spatial light modulator. The spatial light modulator modulates the polarization of incident light and emits it. The polarization modulation is performed by phase-modulating incident light. That is, phase modulation can be performed by polarization modulation. The magnitude of polarization modulation depends on the luminance of the image displayed on the spatial light modulator. Therefore, the phase of the DC component can be set by setting the brightness of the image.

  2A and 2B are graphs showing the results of a computer experiment. The horizontal axis represents the luminance value expressed in 256 gradations, and the vertical axis represents the pixel detection frequency. ● is a plot of black pixels, and ○ is a plot of white pixels.

  In the computer experiment, a signal light data image was created as follows. That is, binary digital data “0, 1” is set as a black pixel (luminance 0) and a white pixel (luminance 255). Further, in order to add the intensity distribution of irradiation light and the influence of noise to the entire data image, the black pixel The average luminance is 10 and the variance is 10, and the distribution of white pixels is the average luminance is 220 and the variance is 10. A simulation was performed using this data image, the signal light (data image) was Fourier transformed, the intensity of the direct current component (0th order component) of the Fourier transform image of the signal light was modulated, and the inverse Fourier transform was performed.

  Then, a histogram of the inverse Fourier transform image was generated and evaluated. Here, the intensity modulation of the 0th order component corresponds to changing the ratio of the 0th order component supplemented at the time of reproduction to the 0th order component included in the original signal light. An inverse Fourier transform image corresponds to a reproduced image from a hologram. In this computer experiment, the diffracted light from the hologram and the supplemented zeroth-order component are in phase. Therefore, the second reproduced image is a positive image similar to the contrast of the original signal light.

  As can be seen from the experimental results, when the 0th order component is added (0th order light ratio: 0.2, FIG. 2B), when the 0th order component is removed (0th order light ratio: 0, FIG. Compared with A)), the luminance of the black pixel decreases, the luminance of the white pixel increases, and the contrast increases. That is, the luminance distribution of the reproduced image from the hologram can be controlled by removing or adding the zero-order component. Therefore, as described above, binary digital data can be accurately reproduced by subtracting the luminance of two reproduced images having different 0th-order light ratios.

(First embodiment)
FIG. 3 is a diagram showing a schematic configuration of the hologram recording / reproducing apparatus according to the first embodiment. As shown in the figure, the recording / reproducing apparatus can irradiate the optical recording medium with the signal light and the reference light coaxially.

  The hologram recording / reproducing apparatus is provided with a light source 10 that oscillates laser light that is coherent light. A beam expander 15 including lenses 12 and 14 is disposed on the laser light irradiation side of the light source 10. On the light transmission side of the beam expander 15, a polarization beam splitter 16 that transmits only polarized light in a predetermined direction and reflects other polarized light is disposed. In the following description, it is assumed that the polarizing beam splitter 16 transmits P-polarized light and reflects S-polarized light.

  A reflective spatial light modulator 18 is disposed on the light reflection side of the polarization beam splitter 16. The spatial light modulator 18 is connected to the personal computer 30 via the pattern generator 32. The pattern generator 32 generates a pattern to be displayed on the spatial light modulator 18 according to the digital data supplied from the personal computer 30, and the spatial light modulator 18 modulates the incident laser light according to the display pattern. A digital image (signal light) and reference light for each page are generated. The generated signal light and reference light are reflected in the direction of the polarization beam splitter 16 and pass through the polarization beam splitter 16.

  On the signal light transmitting side of the polarizing beam splitter 16, a quarter wavelength plate 20, lenses 22, 24, and a Fourier transform lens 26 are arranged in this order along the optical path. Further, a mask 38 for removing a direct current component from the Fourier transform images of the signal light and the reference light is disposed between the lens 22 and the lens 24 so as to be able to be inserted into and retracted from the optical path. The mask 38 is connected to the personal computer 30 via a driving device 34 that drives the mask 38. As the mask 38, for example, a micromirror that reflects only the DC component of the Fourier transform image can be used.

  When reproducing the hologram, when the optical recording medium 28 is irradiated with the reference light, the irradiated reference light is diffracted by the hologram, and the diffracted light is reflected by the reflection layer 28a of the optical recording medium 28 in the direction of the Fourier transform lens 26. The The reflected diffracted light enters the polarization beam splitter 16. On the diffracted light reflecting side of the polarization beam splitter 16, a photodetector 36 is arranged which is composed of an image sensor such as a CCD or CMOS array and converts the received reproduction light (diffracted light) into an electric signal and outputs it. . The photodetector 36 is connected to the personal computer 30.

  Next, a processing routine of recording / reproducing processing executed by the personal computer 30 will be described. FIG. 4 is a flowchart showing the processing routine of the recording / reproducing process. First, the user operates an input device (not shown) and selects recording processing or reproduction processing. When digital data is recorded as a hologram, the digital data to be recorded is input to a personal computer in advance.

  In step 100, it is determined whether the recording process is selected or the reproduction process is selected. If the recording process is selected, in step 102, the driving device 34 is driven to insert the mask 38 into the optical path. In the next step 104, laser light is emitted from the light source 10, digital data is output from the personal computer 30 at a predetermined timing, hologram recording processing is executed, and the routine is terminated.

Here, the hologram recording process will be described.
The laser light oscillated from the light source 10 is collimated into a large-diameter beam by the beam expander 15, enters the polarization beam splitter 16, and is reflected in the direction of the spatial light modulator 18. When digital data is input from the personal computer 30, the pattern generator 32 generates a signal light pattern according to the supplied digital data, combines it with the reference light pattern, and displays it on the spatial light modulator 18. A pattern is generated. In the spatial light modulator 18, the laser light is polarization-modulated according to the displayed pattern, and signal light and reference light are generated.

  For example, as shown in FIG. 5, the central portion of the spatial light modulator 18 is used for data display (for signal light), and the peripheral portion of the spatial light modulator 18 is used for reference light. The laser light incident on the central portion of the spatial light modulator 18 is polarization-modulated according to the display pattern to generate signal light. On the other hand, the laser light incident on the peripheral portion of the spatial light modulator 18 is polarization-modulated according to the display pattern to generate reference light.

  The signal light and the reference light that have been polarization-modulated by the spatial light modulator 18 are applied to the polarization beam splitter 16 and transmitted through the polarization beam splitter 16 to be converted into linearly polarized amplitude distribution. Thereafter, the light is converted into circularly polarized light by the quarter-wave plate 20 and Fourier-transformed by the lens 22. The signal light and the reference light subjected to the Fourier transform are irradiated onto the mask 38, and the DC component is removed from the Fourier transform images of the signal light and the reference light.

  The signal light and the reference light that are not blocked by the mask 38 are subjected to inverse Fourier transform by the lens 24, subjected to Fourier transform again by the lens 26, and irradiated onto the optical recording medium 28 simultaneously and coaxially. As a result, the signal light and the reference light interfere with each other in the optical recording medium 28, and the interference pattern is recorded as a hologram.

  In the present embodiment, two types of reproduction images, a first reproduction image and a second reproduction image, are acquired, and digital data is reproduced using these reproduction images. Therefore, when the reproduction process is selected in step 100 in FIG. 4, the first reproduction image acquisition process is started in step 106. That is, the driving device 34 is driven to retract the mask 38 from the optical path. In the next step 108, laser light is emitted from the light source 10, and the first reproduction image acquisition process is executed.

Here, the acquisition process of a 1st reproduction image is demonstrated.
As shown in FIG. 6A, a light-shielding pattern (all black pixels) is displayed in the central portion of the spatial light modulator 18, and the same reference light pattern as in recording is displayed in the peripheral portion of the spatial light modulator 18. To do. As a result, only the laser light incident on the peripheral portion of the spatial light modulator 18 is polarized and modulated to generate reference light, which is transmitted through the polarization beam splitter 16 and converted into an amplitude distribution, and then the hologram of the optical recording medium 28. Only the reference light is irradiated to the area where the is recorded.

  The irradiated reference light is diffracted by the hologram, and the diffracted light is reflected in the direction of the lens 26 by the reflection layer 28 a of the optical recording medium 28. The reflected diffracted light is subjected to inverse Fourier transform from the lens 26, relayed by the lenses 24 and 22, converted to S-polarized light by the quarter-wave plate 20, and incident on the polarization beam splitter 16. Reflected in the direction. A reproduced image can be observed on the focal plane of the lens 22. This reproduced image (first reproduced image) is detected by the photodetector 36. The detected analog data is A / D converted by the photodetector 36, and the image data of the first reproduced image is input to the personal computer 30 and held in the RAM (not shown).

  Next, in step 110, the luminance value of the display image for supplementing the DC component to the reproduced diffracted light is calculated, the laser light is emitted from the light source 10, and the luminance value calculated from the personal computer 30 is calculated at a predetermined timing. The second reproduction image acquisition process is executed.

Here, the acquisition process of a 2nd reproduction image is demonstrated.
As described above, the reproduction of the original signal light pattern is realized as a result of interference between the diffracted light from the hologram and the supplemented DC component. That is, the original signal light pattern is reproduced by setting the phase difference between the diffracted light and the DC component so that the amplitude of the interference wave (synthetic wave) increases. The phase of the direct current component can be set by appropriately changing the luminance of the signal light pixel of the spatial light modulator.

  In the present embodiment, the case where the phase of the DC component to be supplemented is the same as the phase of the diffracted light from the hologram will be described. In this case, in the inverse Fourier transform image (second reproduced image) of the synthesized light, the bright part of the original bright and dark image is bright and the dark part is dark.

  As shown in FIG. 6B, a transmission pattern (pixels having the same luminance other than 0) is displayed in the central portion of the spatial light modulator 18, and the peripheral portion of the spatial light modulator 18 is the same as at the time of recording. Display the reference light pattern. Thereby, the laser beam incident on the central portion of the spatial light modulator 18 is transmitted, and a DC component of the signal light is generated. On the other hand, the laser light incident on the peripheral portion of the spatial light modulator 18 is polarization-modulated according to the display pattern to generate reference light. Then, after being transmitted through the polarization beam splitter 16 and converted into an amplitude distribution, the direct current component of the generated signal light and the reference light are irradiated onto the area where the hologram of the optical recording medium 28 is recorded.

The calculation of the luminance value of the transmission pattern is performed according to the following procedure.
As described above, the phase of the diffracted light from the hologram is deviated from the phase of the reference light during reproduction, and the amount of deviation is a predetermined value depending on the type of hologram. Since the type of hologram is determined by the recording material to be used, the phase shift amount is a known value. Furthermore, the correspondence between the luminance of the pixel of the spatial light modulator and the generated phase modulation amount can be obtained in advance. Accordingly, the luminance value of the transmission pattern is set so that the difference | θ−φ | between the phase shift amount θ of the diffracted light and the phase modulation amount φ applied to the supplemented DC component satisfies the above equation (2). be able to.

  However, depending on the specifications of the spatial light modulator, a desired phase modulation amount may not be given to the DC component. In that case, the luminance value of the transmission pattern is set so as to satisfy the equation (4). As the reproduced image in this case, an inverted image obtained by inverting the brightness of the original signal light pattern is obtained. When the condition of the expression (4) cannot be satisfied, the luminance value of the transmission pattern is set so as to satisfy the expression (1) or the expression (3).

  In the following description, it is assumed that the second reproduced image is acquired under the condition satisfying the expression (2). In this case, the second reproduced image is the same positive image as the image displayed on the spatial light modulator during recording.

  The reference light applied to the optical recording medium 28 is diffracted by the hologram, and the diffracted light is reflected by the reflection layer 28 a of the optical recording medium 28 in the direction of the lens 26. The direct current component of the signal light irradiated on the optical recording medium 28 is reflected in the direction of the lens 26 by the reflection layer 28 a of the optical recording medium 28. The reflected diffracted light and the direct current component of the signal light are subjected to inverse Fourier transform by the lens 26, relayed by the lenses 24 and 22, converted to S-polarized light by the ¼ wavelength plate 20, and incident on the polarization beam splitter 16. Then, it is reflected in the direction of the photodetector 36. A reproduced image can be observed on the focal plane of the lens 22.

  This reproduced image (second reproduced image) is detected by the photodetector 36. The detected analog data is A / D converted by the photodetector 36, and the image data of the second reproduced image is input to the personal computer 30 and held in the RAM (not shown).

  When the acquisition process of the second reproduction image is completed, the process proceeds to the next step 112, where the image data of the first reproduction image and the image data of the second reproduction image held in the RAM are read out from the image data of the second reproduction image. The image data of the first reproduced image is subtracted to calculate a luminance difference for each pixel of the digital image (signal light). In the inverse Fourier transform image (second reproduced image) of the synthesized light, the bright part of the original bright and dark image is bright and the dark part is dark. Accordingly, the difference when the luminance of the first reproduced image is subtracted from the luminance of the second reproduced image is positive in the bright part of the original bright and dark image and negative in the dark part of the original bright and dark image.

  When one pixel of the signal light data corresponds to a plurality of pixels of the photodetector 36, a difference in average value of luminance is calculated for the plurality of pixels of the photodetector 36.

  Next, in step 114, the sign of each pixel is determined based on whether the calculated difference is positive or negative, decoded into binary digital data, and the routine ends. Thereby, the digital data held in the signal light is accurately decoded.

  As described above, in the present embodiment, since the recording of the hologram is performed in a state where the direct current component is removed from the Fourier transform image of the signal light, unnecessary exposure due to the direct current component of the signal light is prevented during recording of the hologram, The dynamic range can be used effectively, and the multiplicity is improved.

  In this embodiment, the hologram is recorded in a state where the DC component is removed from the Fourier transform image of the signal light. The first reproduced image is acquired from the diffracted light by the recorded hologram, and the diffraction is performed. The second reproduced image is acquired from the combined light obtained by combining the direct current component of the Fourier transform image of the signal light with the light, and the image data of the first reproduced image is subtracted from the image data of the second reproduced image, and the luminance for each pixel And the sign of each pixel is determined based on whether the calculated difference is positive or negative, so that binary digital data can be accurately reproduced.

  In addition, according to the encoding method described above, one bit can be expressed by one pixel, so that a high recording density can be realized.

(Second Embodiment)
In the second embodiment, the ratio of white pixels in the binary digital image (signal light) is increased to obtain an inverted image (negative image) of the original light and dark image as the first reproduced image, and the second reproduced image. An example of acquiring a positive image will be described.

  As a result of various investigations, the inventors have found that when a DC component is removed in the case of a binary digital image (signal light) having the same black-and-white ratio, the S / N is reduced during reproduction. As a result of various studies on the structure of the data page based on this knowledge, the ratio of the signal light component to each pixel of the digital image or the ratio of the white pixel to each pixel block (hereinafter referred to as “the white ratio of the image”) is increased. Thus, it was found that a reversed image (negative image) of the signal light can be recorded / reproduced even if the DC component is removed.

  Therefore, as shown in FIG. 7A, only the central pixel of the pixel block represented by 3 × 3 pixels is used as the signal light component, and the binary digital data “0, 1” is expressed as “bright (white pixel). ), Dark (black pixels) ". The surrounding 8 pixels are white pixels. In this case, as shown in FIG. 7B, the first reproduced image is a negative image in which the brightness of the center pixel of the pixel block is inverted, and the second reproduced image is as shown in FIG. The positive image is the same as the original digital pattern.

  As described above, since the first reproduction image and the second reproduction image are in a negative and positive relationship, the image data of the first reproduction image is subtracted from the image data of the second reproduction image, and a luminance difference is obtained for each pixel. , The sign of the calculated difference clearly appears. Therefore, it is possible to accurately determine the code of each pixel, and the decoding accuracy of digital data is further improved.

  FIG. 8 is a diagram showing a schematic configuration of a hologram recording / reproducing apparatus according to the second embodiment. In the first embodiment, a recording / reproducing apparatus using a reflective spatial light modulator and a reflective optical recording medium has been described. In the second embodiment, a transmissive spatial light modulator and a transmissive spatial light modulator are used. A recording / reproducing apparatus using an optical recording medium of a type will be described. Note that the signal light and the reference light can be coaxially irradiated onto the optical recording medium in the same manner as in the first embodiment.

  The hologram recording / reproducing apparatus is provided with a light source 50 that oscillates laser light that is coherent light. A beam expander 55 including lenses 52 and 54 is disposed on the laser light irradiation side of the light source 50. A transmissive spatial light modulator 58 is disposed on the light transmission side of the beam expander 55. The spatial light modulator 58 is connected to the personal computer 56 via the pattern generator 60.

  The pattern generator 60 generates a pattern to be displayed on the spatial light modulator 58 according to the digital data supplied from the personal computer 56, and the spatial light modulator 58 modulates the incident laser light according to the display pattern. A digital image (signal light) and reference light for each page are generated.

  On the light transmission side of the spatial light modulator 58, a polarization transforming lens (not shown), lenses 62 and 64, and a Fourier transform lens 66 for irradiating the optical recording medium 72 with signal light and reference light are arranged in this order along the optical path. . Further, a mask 68 for removing a direct current component from the Fourier transform image of the signal light and the reference light is disposed between the lens 62 and the lens 64 so as to be able to be inserted into and retracted from the optical path. The mask 68 is connected to the personal computer 56 via a driving device 70 that drives the mask 68.

  Any mask 68 can be used as long as it can remove the direct current component of the Fourier transform image. For example, a filter that absorbs only the direct current component of the Fourier transform image can be used.

  When the optical recording medium 72 is irradiated with the reference light during hologram reproduction, the irradiated reference light is diffracted by the hologram, and the diffracted light passes through the optical recording medium 72 and is emitted in the direction of the Fourier transform lens 74. The On the diffracted light emission side of the optical recording medium 72, a photodetector comprising a Fourier transform lens 74 and an image sensor such as a CCD or CMOS array, which converts the received reproduction light (diffracted light) into an electrical signal and outputs it. 76 is arranged. The photodetector 76 is connected to the personal computer 56.

  The hologram recording / reproducing apparatus according to the second embodiment has a different configuration from the apparatus according to the first embodiment, and the hologram recording method and reproducing method are different. This processing routine is the same as the routine shown in FIG.

  First, in step 100, it is determined whether the recording process is selected or the reproduction process is selected. If the recording process is selected, in step 102, the driving device 70 is driven to insert the mask 68 into the optical path. To do. In the next step 104, laser light is emitted from the light source 50, digital data is output from the personal computer 56 at a predetermined timing, hologram recording processing is executed, and the routine is terminated.

Here, the hologram recording process will be described.
The laser light oscillated from the light source 50 is collimated into a large-diameter beam by the beam expander 55 and irradiated to the spatial light modulator 58. When digital data is input from the personal computer 56, a signal light pattern is generated according to the supplied digital data in the pattern generator 60, synthesized with the reference light pattern, and displayed on the spatial light modulator 58. A pattern is generated. In the spatial light modulator 58, the laser light is polarized and modulated in accordance with the displayed pattern, and signal light and reference light are generated.

  As in the first embodiment, the central portion of the spatial light modulator 58 is used for data display (for signal light), and the peripheral portion of the spatial light modulator 58 is used for reference light (FIG. 5). reference). The laser light incident on the central portion of the spatial light modulator 58 is polarization-modulated according to the display pattern, and signal light is generated. On the other hand, the laser light incident on the peripheral portion of the spatial light modulator 58 is polarization-modulated according to the display pattern to generate reference light. Thereafter, the signal light and the reference light are transmitted through a polarizing plate (not shown) and converted into an amplitude distribution.

  In the present embodiment, the pattern shown in FIG. 9A is displayed on the spatial light modulator 58. In the central portion of the spatial light modulator 58, as shown in FIG. 9B, only the central pixel of a pixel block represented by 3 × 3 pixels is used as a signal light component, and binary digital data “0” is used. , 1 ”is displayed as“ bright (white pixel), dark (black pixel) ”. Since pixels in the vicinity of 8 of the signal light component are white pixels, this digital pattern has a very high ratio of white pixels.

  The signal light and the reference light generated by the spatial light modulator 58 are Fourier transformed by the lens 62. The Fourier-transformed signal light and reference light are applied to the mask 68, and the DC component is removed from the Fourier-transformed images of the signal light and reference light. The signal light and the reference light that are not blocked by the mask 68 are subjected to inverse Fourier transform by the lens 64, subjected to Fourier transform again by the lens 66, and irradiated onto the optical recording medium 72 simultaneously and coaxially. Thereby, the signal light and the reference light interfere in the optical recording medium 72, and the interference pattern is recorded as a hologram.

  If reproduction processing is selected in step 100 of FIG. 4, in step 106, the driving device 34 is driven to retract the mask 38 from the optical path. In the next step 108, laser light is emitted from the light source 10, and the first reproduction image acquisition process is executed.

  In the first reproduction image acquisition process, as shown in FIG. 6A, a light-shielding pattern (all black pixels) is displayed in the central portion of the spatial light modulator 58, and in the peripheral portion of the spatial light modulator 58. The same reference beam pattern as that during recording is displayed. As a result, only the laser light incident on the peripheral portion of the spatial light modulator 58 is polarized and modulated to generate reference light, which is converted into an amplitude distribution by a polarizing plate (not shown), and then passed through the lenses 62, 64, and 66. Only the reference light is irradiated to the area where the hologram of the optical recording medium 72 is recorded.

  The irradiated reference light is diffracted by the hologram, and the diffracted light is transmitted through the optical recording medium 72 and emitted. The emitted diffracted light is subjected to inverse Fourier transform by the lens 74 and is incident on the photodetector 76. A reproduced image can be observed on the focal plane of the lens 74.

  This reproduced image (first reproduced image) is detected by the photodetector 76. The detected analog data is A / D converted by the photodetector 76, and the image data of the first reproduced image is input to the personal computer 56 and held in the RAM (not shown). In this embodiment, since signal light having a high white ratio is used, the first reproduced image is a negative image of the original light and dark image as shown in FIG.

  Next, in step 110 of FIG. 4, the brightness value of the display image for supplementing the reproduced diffracted light with a direct current component is calculated, the laser light is emitted from the light source 50, and the brightness value calculated from the personal computer 56 is predetermined. The second reproduction image acquisition process is executed at this timing.

  In the second reproduction image acquisition process, as shown in FIG. 6B, a transmission pattern (pixels having the same luminance other than 0) is displayed in the central portion of the spatial light modulator 58, and the spatial light modulator 58 is displayed. The same reference beam pattern as that at the time of recording is displayed in the peripheral portion of. As a result, the laser light incident on the central portion of the spatial light modulator 58 is transmitted and a DC component of the signal light is generated. On the other hand, the laser light incident on the peripheral portion of the spatial light modulator 58 is polarization-modulated according to the display pattern to generate reference light. Then, after being converted into an amplitude distribution by a polarizing plate (not shown), the direct current component of the generated signal light and the reference light are generated in the region where the hologram of the optical recording medium 72 is recorded via the lenses 62, 64 and 66. Is irradiated.

  The irradiated reference light is diffracted by the hologram, and the diffracted light is transmitted through the optical recording medium 72 and emitted. Further, the direct current component of the irradiated signal light is transmitted through the optical recording medium 72. The transmitted diffracted light and the direct current component of the signal light are subjected to inverse Fourier transform by the lens 74 and enter the photodetector 76. A reproduced image can be observed on the focal plane of the lens 74.

  This reproduced image (second reproduced image) is detected by the photodetector 76. The detected analog data is A / D converted by the photodetector 76, and the image data of the second reproduced image is input to the personal computer 56 and held in a RAM (not shown). In the present embodiment, since signal light having a high white ratio is used, the second reproduced image is a positive image of the original light and dark image as shown in FIG. 10B.

  Next, as a supplement, the principle that the positive image and the negative image are reproduced in the present embodiment will be described. At the time of recording, the direct current components of the signal light and the reference light are removed by the mask 68 and irradiated onto the optical recording medium 72. At this time, the signal light pattern irradiated on the optical recording medium 72 is an inverted image of the pattern displayed on the spatial light modulator 58. This is because the signal light pattern displayed on the spatial light modulator 58 has a large number of white pixels, and therefore the proportion of the direct current component in the total Fourier components is very large.

  Due to the large DC component, the amplitude of the electric field of the signal originally “white pixel” is large and the amplitude of the electric field of the signal originally “black pixel” is small, but the DC component is removed by the mask 68. In addition, the amplitude of the electric field of the signal originally “white pixel” is reduced, and the amplitude of the electric field of the signal originally “black pixel” is increased. As a result, the intensity distribution is an image in which “bright” and “dark” are reversed. That is, the signal light pattern irradiated on the optical recording medium 72 is a reverse image of the pattern displayed on the spatial light modulator 58.

  Therefore, in the acquisition of the first reproduced image in step 108 in FIG. 4, the signal light pattern at the time of recording (the inverted image (negative image) of the digital pattern displayed on the spatial light modulator 58) is reproduced. On the other hand, in the acquisition of the second reproduced image in step 110, an inverted image of the signal light pattern at the time of recording (the same positive image as the digital pattern displayed on the spatial light modulator 58) is reproduced.

  When the acquisition process of the second reproduction image is completed in step 110 of FIG. 4, the process proceeds to the next step 112, where the image data of the first reproduction image and the image data of the second reproduction image stored in the RAM are read out. The image data of the first reproduction image is subtracted from the image data of the two reproduction images, and the luminance difference is calculated for each pixel of the digital image (signal light). FIG. 10C shows a light and dark image generated based on the image data after the subtraction process. It can be seen that in the image after the subtraction process, the contrast is enhanced more than in the first reproduction image and the second reproduction image.

  Next, in step 114, the sign of each pixel is determined based on whether the calculated difference is positive or negative, decoded into binary digital data, and the routine ends. Thereby, the digital data held in the signal light is accurately decoded.

  As described above, in the present embodiment, since the recording of the hologram is performed in a state where the direct current component is removed from the Fourier transform image of the signal light, unnecessary exposure due to the direct current component of the signal light is prevented during recording of the hologram, The dynamic range can be used effectively, and the multiplicity is improved.

  In the present embodiment, hologram recording is performed with the DC component removed from the Fourier transform image of the signal light and the reference light. However, since signal light with a high white ratio is used, the signal light is negative. A first reproduced image that is an image and a second reproduced image that is a positive image of signal light can be acquired. When subtracting the image data of the first reproduction image from the image data of the second reproduction image and calculating the luminance difference for each pixel, the sign of the calculated difference appears clearly. Can be accurately performed, and the decoding accuracy of digital data is further improved.

  In addition, according to the encoding method described above, one bit can be expressed by one pixel, so that a high recording density can be realized.

  In the second embodiment, the case where the white ratio of the image in the digital image of the signal light is large has been described, but the present invention is not limited to that case. As described above, by controlling the phase difference between the diffracted light from the hologram and the direct current component, it is possible to acquire a reproduced image of a positive image and a negative image from one hologram. The decoding accuracy of digital data can also be improved by subtracting these images.

  In the above embodiment, the example in which the luminance difference is calculated by subtracting the image data of the first reproduction image from the image data of the second reproduction image has been described, but the first difference is calculated from the image data of the first reproduction image. The difference in luminance may be calculated by subtracting the image data of the two reproduced images. When the two reproduced images are a positive image and a negative image, it is preferable to calculate a difference in luminance by subtracting the image data of the negative image from the image data of the positive image. The noise removal efficiency is improved by subtracting the image data of the negative image from the image data of the positive image.

  In the above embodiment, the example in which the signal light and the reference light are coaxially irradiated onto the recording medium has been described, but the present invention is not limited thereto. In other words, an optical system may be used in which the signal light and the reference light propagate along different optical axes and are irradiated onto the recording medium to record the hologram.

  In the above embodiment, an example in which the direct current component of the signal light is not supplemented when the first reproduced image is obtained has been described. However, the first reproduced image may be obtained by supplementing the direct current component of the signal light. In this case, the direct current component of the signal light to be supplemented is selected so that the first reproduced image and the second reproduced image are inverted images in which the brightness is inverted. For example, when acquiring the first reproduction image, the DC component of the signal light to be supplemented is selected so as to satisfy the expression (1) or the expression (2), and when the second reproduction image is acquired, The direct current component of the signal light to be supplemented is selected so as to satisfy the formula (3) or the formula (4).

  In the above embodiment, the case where binary digital data is recorded as a hologram has been described. However, even when multi-value digital data is used, the luminance of two reproduced images reproduced from the same hologram is described. By calculating the difference and calculating the value, the digital data can be accurately reproduced.

  In the above embodiment, an example is described in which the difference in luminance is calculated by subtracting the image data of the first reproduction image and the image data of the second reproduction image, and the sign of each pixel is determined based on whether the calculated difference is positive or negative. As described above, it is also possible to use the image data (third reproduced image) newly obtained by the subtraction process to decode the digital data. In the third reproduced image, the S / N is improved because the noise component included in both the first reproduced image and the second reproduced image is reduced by the subtraction process. Therefore, digital data can be reproduced with high accuracy.

  For example, an encoding method (black and white = 0, black and white = 1, etc.) that expresses one bit with two pixels is known (Science 265, 749-752 (1994)). When such an encoding method is used, the original digital data is restored (decoded) from the third reproduced image obtained by the subtraction process.

It is a figure for demonstrating the decoding principle of digital data. (A) And (B) is a graph which shows the result of a computer experiment. It is a figure which shows schematic structure of the hologram recording / reproducing apparatus which concerns on 1st Embodiment. It is a flowchart which shows the processing routine of a recording / reproducing process. It is a figure which shows the display image of a spatial light modulator. (A) is a figure which shows the display image of a spatial light modulator when acquiring a 1st reproduction image, (B) is a figure which shows the display image of a spatial light modulator when acquiring a 2nd reproduction image. is there. (A) is a figure which shows the structure of the pixel block of the signal light with a high white ratio, (B) is a negative image of (A), (C) is a positive image of (A). It is a figure which shows schematic structure of the hologram recording / reproducing apparatus which concerns on 2nd Embodiment. (A) is a pattern displayed on the spatial light modulator in the second embodiment, and (B) is a signal light pattern displayed at the center of the pattern of (A). (A) is a 1st reproduction image acquired in 2nd Embodiment, (B) is a 2nd reproduction image, (C) is an image after a subtraction process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Light source 12, 14 Lens 15 Beam expander 16 Polarizing beam splitter 18 Spatial light modulator 20 1/4 wavelength plate 22 Lens 24 Lens 26 Fourier transform lens 28 Optical recording medium 28a Reflective layer 30 Personal computer 32 Pattern generator 34 Drive device 36 Photo detector 38 Mask 50 Light source 52, 54 Lens 55 Beam expander 56 Personal computer 58 Spatial light modulator 60 Pattern generator 62 Lens 64 Lens 66 Fourier transform lens 66 Lens 68 Mask 70 Drive device 72 Optical recording medium 74 Fourier transform lens 74 Lens 76 Photodetector

Claims (17)

The reference light for reading is recorded on the optical recording medium on which the hologram is recorded by Fourier-transforming the signal light and the reference light from which the digital data is represented by a bright and dark image and from which the direct current component has been removed and simultaneously irradiating the optical recording medium. And generating diffracted light by a recorded hologram;
Combining the diffracted light and the DC component of the signal light to generate one or more combined lights;
Detecting each of the inverse Fourier transform images of the diffracted light and the combined light, or detecting each of the two types of inverse Fourier transform images of the combined light to obtain two types of image data;
Subtracting the other from either one of the two types of image data for each pixel of the bright and dark image;
A hologram reproducing method comprising: The optical recording medium on which the hologram is recorded by digitally representing the digital data as a bright and dark image and Fourier-transforming the signal light from which the DC component is removed and the reference light from which the DC component is removed and irradiating the optical recording medium simultaneously. Irradiating a reference light for reading and generating diffracted light by a recorded hologram;
Combining the diffracted light and the DC component of the signal light to generate one or more combined lights;
Detecting each of the inverse Fourier transform images of the diffracted light and the combined light, or detecting each of the two types of inverse Fourier transform images of the combined light to obtain two types of image data;
Subtracting the other from either one of the two types of image data for each pixel of the bright and dark image;
Decoding the digital data based on the value obtained by subtraction;
A hologram reproducing method comprising:   The hologram reproduction method according to claim 1, further comprising a step of decoding digital data based on a value obtained by subtraction.   4. The first image data is acquired by detecting an inverse Fourier transform image of the diffracted light, and the second image data is acquired by detecting an inverse Fourier transform image of the combined light. 5. The hologram reproducing method as described in 2.   The hologram reproducing method according to claim 4, wherein a ratio of a bright part of the bright and dark image is increased so that the first image data and the second image data are mutually inverted images.   The diffracted light and the first DC component are combined to generate a first combined light, and the diffracted light and the second DC component having a phase different from that of the first DC component are combined to generate a second The hologram reproduction method according to claim 1, wherein synthetic light is generated.   7. The first image data is acquired by detecting an inverse Fourier transform image of the first composite light, and the second image data is acquired by detecting an inverse Fourier transform image of the second composite light. The hologram reproducing method as described.   The phase of the first DC component and the phase of the second DC component are different from each other so that the first image data and the second image data are mutually inverted images. Hologram reproduction method.   The hologram reproducing method according to claim 1, wherein all or part of a direct current component of signal light is combined with the diffracted light.   The hologram reproducing method according to claim 1, wherein the diffracted light and the direct current component are combined so that the contrast of the intensity of the combined light is increased.   The hologram reproducing method according to claim 1, wherein the diffracted light and the direct current component are combined so that a positive amplitude of the combined light is increased.   The hologram reproduction method according to any one of claims 1 to 10, wherein the diffracted light and the direct current component are combined so that a negative amplitude of the combined light increases.   When binary digital data 1 is associated with a bright part and 0 is associated with a dark part, the calculated difference is determined as a positive pixel sign, and the calculated difference is a negative pixel sign. The hologram reproduction method according to claim 1, wherein 0 is determined to be zero.   The hologram reproducing method according to claim 1, wherein collimated light is phase-modulated by a spatial light modulator to generate the DC component having a predetermined phase. The hologram reproducing method according to claim 14, wherein the phase of the collimated light is modulated by changing the luminance of a pixel displayed on the spatial light modulator. The recorded hologram is recorded from the optical recording medium on which the hologram is recorded by Fourier-transforming the signal light from which the digital data is expressed as a bright and dark image and the direct current component is removed and the reference light, and simultaneously irradiating the optical recording medium. A hologram reproducing device for reproducing read digital data,
Reference light irradiating means for irradiating the optical recording medium with reference light for reading;
A synthesized light generating means for generating a synthesized light by synthesizing the DC component of the signal light with the diffracted light by the recorded hologram;
Image data acquisition means for acquiring reproduced image data by detecting an inverse Fourier transform image of the diffracted light or the combined light;
The optical recording medium is irradiated with a reference light for reading to generate diffracted light by a recorded hologram, and a synthesized light is generated by synthesizing a DC component of signal light with the diffracted light, and the diffracted light and The reference light irradiating means, the synthetic light generating means, and the two kinds of inverse Fourier transformed images of the synthetic light or two types of inverse Fourier transformed images of the synthetic light are detected and two types of image data are acquired. Control means for controlling the image data acquisition means;
A computing means for subtracting the other from either one of the two types of image data for each pixel of the light and dark image;
A hologram reproducing apparatus comprising:   The hologram reproducing apparatus according to claim 16, further comprising decoding means for decoding digital data based on a value obtained by subtraction by the calculating means.