JP4605007B2 - Hologram recording / reproducing method and apparatus - Google Patents

Description

  The present invention relates to a hologram recording / reproducing method and apparatus, and more particularly, hologram recording for recording a Fourier transform hologram in a state in which a zero-order component is removed from signal light, and restoring and reproducing the original signal light from the recorded hologram. The present invention relates to a reproduction method and apparatus.

  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 DC 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, it is possible to prevent waste of the dynamic range by suppressing unnecessary exposure due to the 0th-order component, and to reduce the recording area of each data page.
JP 2000-66565 A JP 2004-198816 A

  However, when 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 binary 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, binary digital data may not be correctly decoded.

  The present invention has been made to solve the above problems, and the object of the present invention is to provide an original signal from a recorded hologram even when a Fourier transform hologram is recorded in a state where a DC component is removed from the signal light. An object of the present invention is to provide a hologram recording / reproducing method and apparatus capable of restoring and reproducing light.

In order to achieve the above object, a hologram recording / reproducing method according to claim 1 is provided with a modulation region including a signal light region and a reference light region, and the incident light according to a pattern displayed in the modulation region is supplied to each pixel. When recording a hologram, a signal light pattern representing binary digital data as a light and dark image is displayed in the signal light area of the spatial light modulator and a reference is made to the reference light area. Displaying a light pattern, generating signal light and reference light coaxial with the signal light, Fourier transforming the signal light and the reference light generated by the spatial light modulator, and performing Fourier transform A step of removing a direct current component from the Fourier transform image of the signal light, a step of removing the direct current component from the Fourier transform image of the signal light, and a step of removing the direct current component from the Fourier transform image and removing the direct current component. By simultaneously and coaxially irradiating the optical recording medium with the reference light subjected to the Lie conversion, a step of recording a hologram on the optical recording medium, and a reference to the reference light area of the spatial light modulator when reproducing the hologram Displaying a light pattern and displaying a transmission pattern in the signal light region to generate a reference light for reading and a direct current component coaxial with the reference light; a reference light for reading and the direct current component; Irradiating the optical recording medium on which the hologram is recorded by Fourier transform with the same axis, and synthesizing the diffracted light by the recorded hologram and the direct current component Fourier-transformed to generate a synthesized light; and the diffracted light a step wherein the direct current component by performing inverse Fourier transform on combined light combined, to restore the signal light representing the digital data of the binary light and dark image and, It is characterized by comprising.

  In the hologram reproducing method of the present invention, the Fourier transform hologram is recorded in a state where the direct current component of the signal light is removed, but the synthesized light is generated by synthesizing the diffracted light and the direct current component by the recorded hologram, Since the signal light is restored by performing inverse Fourier transform on this, the original signal light pattern can be reproduced from the recorded hologram. Thereby, the digital data held in the signal light can be accurately decoded.

  In addition, the DC component of the reference light can be removed together with the DC component of the signal light.

  As described above, according to the present invention, even when a Fourier transform hologram is recorded in a state where a DC component is removed from the signal light, the original signal light can be restored from the recorded hologram. There is an effect that the held digital data can be accurately decoded.

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

(Signal light reproduction principle)
The principle of signal light reproduction will be briefly described. Here, the binary digital data “0, 1” is “bright, dark” and the signal light (digital pattern) digitalized is Fourier transformed, and the direct current component (0th order component) is obtained from the Fourier transform image of the signal light. The case where the interference pattern is recorded as a hologram by simultaneously irradiating the optical recording medium with the signal light and the reference light from which the direct current component is removed will be described.

  When a hologram is recorded using signal light from which the direct current component has been removed, when the recorded hologram is irradiated with reference light, diffracted light having the same component as the signal light from which the direct current component has been removed is reproduced. Therefore, at the time of reproducing the hologram, the signal light can be restored by supplementing the DC component removed by the reproduced diffracted light, and the original signal light pattern (digital pattern) can be reproduced from the recorded hologram. Thereby, the digital data held in the signal light can be accurately decoded.

  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. The role of the direct current component is to increase or decrease the value of the amplitude of the synthesized light generated by the diffracted light other than the zeroth order without changing the shape of the synthesized light. For example, when the phase of the direct current component is the same as that of the diffracted light, the positive amplitude of the combined light is increased, and the reproduced image that is the intensity pattern becomes a positive image similar to the signal light pattern. On the other hand, when the phase of the direct current component is opposite to that of the diffracted light, the positive amplitude of the combined light is reduced, and the reproduced image, which is an intensity pattern, becomes a negative image in which the brightness of the signal light pattern is inverted. Therefore, by appropriately selecting the phase difference between the diffracted light and the direct current component, the reproduced image can be a positive image or a negative image of the original signal light pattern.

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.

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 reconstructed image satisfying the above equations (1) to (4), the luminance of the image displayed on the spatial light modulator is set in consideration of this phase shift, and a DC component is generated and supplemented. do it. Thereby, a desired phase difference (| θ−φ |) can be realized.

  For example, in the case of a hologram based on refractive index modulation, the phase of diffracted light is shifted by π / 2 from the phase of reference light at the time of reproduction. On the other hand, since the DC component to be supplemented is not diffracted by the hologram, there is no phase change. For this reason, the phase difference between the diffracted light from the hologram and the supplemented DC component becomes zero by shifting the phase of the supplemented DC component by π / 2 from the phase of the reference light at the time of reproduction. Therefore, in order to satisfy the relationship of Expression (2) and maximize the positive amplitude of the combined light, the luminance of the image displayed on the spatial light modulator is set so that the phase of this DC component is realized. There is a need. Specifically, the relationship between the phase modulation amount of the DC component and the luminance of the display image is stored in advance, and the luminance of the display image is set so that a desired phase modulation amount is achieved based on this relationship. be able to.

(First embodiment)
FIG. 1 is a diagram showing a schematic configuration of a 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. Below, the case where a micromirror is used for the mask 38 is demonstrated.

  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. 2 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. 3, 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, pass through the polarization beam splitter 16, and are converted into an amplitude distribution of linearly polarized light (P-polarized light). 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.

  If reproduction processing is selected in step 100, in step 106, the driving device 34 is driven to insert the mask 38 into the optical path. In the next step 108, the luminance value of the display image for supplementing the DC component to the reproduced diffracted light is calculated, and in step 110, the laser beam is emitted from the light source 10 and the luminance value calculated from the personal computer 30 is predetermined. Is output at this timing, hologram reproduction processing is executed, and the routine is terminated.

  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 direct current component so that the amplitude of the interference wave (combined light) 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.

  As shown in FIG. 4, 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 same reference light pattern as that used during recording is displayed in the peripheral portion of the spatial light modulator 18. Is displayed. As a result, the laser light incident on the central portion of the spatial light modulator 18 passes through the polarization beam splitter 16 and a DC component of the signal light is generated. As explained as the signal light reproduction principle, the original signal light pattern is restored by setting the brightness of each pixel of the transmission pattern so that the amplitude of the interference wave between the generated DC component and diffracted light increases. The 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 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 depends on the type of hologram. Since the type of hologram is known depending on the recording material used, the amount of phase shift is known. Furthermore, the brightness of the pixel of the spatial light modulator and the amount of phase modulation to be generated are also known. Therefore, the luminance value of the transmission pattern is set so that the difference between the phase shift amount of the diffracted light and the phase modulation amount applied to the DC component to be supplemented satisfies the above equation (2).

  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 above formula (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 above formula (4) cannot be satisfied, the luminance value of the transmission pattern is set so as to satisfy the above formula (1) or the above formula (3).

Here, the hologram reproduction 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 the luminance value calculated from the personal computer 30 is input, a transmission pattern is generated according to the supplied luminance value in the pattern generator 32, synthesized with the reference light pattern, and displayed on the spatial light modulator 18. A display pattern is generated. In the spatial light modulator 18, the laser light is transmitted according to the transmission pattern to generate the DC component of the signal light, and the laser light is polarization-modulated according to the reference light pattern to generate the reference light.

  The reference light generated by the spatial light modulator 18 is applied to the polarization beam splitter 16, passes through the polarization beam splitter 16, and is converted into an amplitude distribution of linearly polarized light (P-polarized light). Thereafter, the light is converted into circularly polarized light by the quarter-wave plate 20 and Fourier-transformed by the lens 22. The reference light subjected to the Fourier transform is irradiated on the mask 38. The reference light that has not been blocked by the mask 38 is subjected to inverse Fourier transform by the lens 24, and is subjected to Fourier transform again by the lens 26, and is applied to the area where the hologram of the optical recording medium 28 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 ¼ wavelength plate 20, and incident on the polarization beam splitter 16. Reflected in the direction.

  On the other hand, the direct current component of the signal light generated by the spatial light modulator 18 is applied to the polarization beam splitter 16, passes through the polarization beam splitter 16, and is converted into circularly polarized light by the ¼ wavelength plate 20, and the lens 22. Fourier transform (condensed). The condensed DC component is reflected in the direction of the lens 22 by the mask 38 which is a micromirror. The direct current component reflected by the mask 38 is collimated by the lens 22, converted to S-polarized light by the quarter-wave plate 20, enters the polarization beam splitter 16, and is reflected in the direction of the photodetector 36.

  The light diffracted by the hologram and the direct current component of the signal light are incident on the photodetector 36. A reproduced image can be observed on the focal plane of the lens 22. In the present embodiment, the diffracted light is supplemented with the DC component of the signal light removed at the time of recording the hologram, so that the diffracted light having the same component as the original signal light is restored, and the original signal light pattern (light / dark image) Is played. This 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 reproduced image is input to the personal computer 30.

  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.

  Further, in this embodiment, when reproducing the hologram, the diffracted light obtained from the recorded hologram is supplemented with the direct current component of the signal light removed at the time of recording, so that the diffraction of the same component as the original signal light is performed. The light is restored and the original signal light pattern (light / dark image) is reproduced. Thereby, the digital data held in the signal light can be accurately decoded.

  In the above description, the mask 38, which is a micromirror, is movably disposed. However, the mask may be fixedly disposed. When the mask is fixedly arranged, a driving device for driving the mask is not necessary, and the configuration of the hologram recording / reproducing device is simplified.

(Second Embodiment)
In the second embodiment, the same hologram recording / reproducing apparatus as that in the first embodiment is used, and the direct current component of the signal light generated by the spatial light modulator is reflected by the reflection layer of the optical recording medium when reproducing the hologram. An example of entering the photodetector will be described. Since the method is the same as that of the first embodiment except that the method of reproducing the hologram is different, the description of the hologram recording process is omitted.

  Any mask may be used as the mask used in this embodiment as long as only the DC component of the Fourier transform image can be blocked. For example, a micromirror having a reflection function or a light absorption filter can be used.

FIG. 5 is a flowchart showing a processing routine of the recording / reproducing process.
In step 200, it is determined whether the recording process is selected or the reproduction process is selected. If the recording process is selected, in step 202, the driving device 34 is driven to insert the mask 38 into the optical path. In the next step 204, laser light is emitted from the light source 10 and digital data is output from the personal computer 30 at a predetermined timing, hologram recording processing is executed, and the routine is terminated. Note that the hologram recording processing method is the same as that in the first embodiment, and a description thereof will be omitted.

  If reproduction processing is selected in step 200, in step 206, the driving device 34 is driven to retract the mask 38 from the optical path. In the next step 208, the luminance value of the display image for supplementing the reproduced diffracted light with a direct current component is calculated. In step 210, the laser light is emitted from the light source 10 and the luminance value calculated from the personal computer 30 is determined in advance. Is output at this timing, hologram reproduction processing is executed, and the routine is terminated.

  Note that the DC component replenishment method (the method of calculating the luminance value of the transmission pattern) is the same as described in the first embodiment, and thus the description thereof is omitted.

Here, the hologram reproduction 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 the luminance value calculated from the personal computer 30 is input, a transmission pattern is generated according to the supplied luminance value in the pattern generator 32, synthesized with the reference light pattern, and displayed on the spatial light modulator 18. A display pattern is generated.

  As in the first embodiment, 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 during recording. A reference light pattern is displayed (see FIG. 4). As a result, the laser light incident on the central portion of the spatial light modulator 18 passes through the polarization beam splitter 16 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. Thereafter, the light is converted into linearly polarized amplitude distribution by passing through the polarization beam splitter 16.

  The direct current component of the signal light generated by the spatial light modulator 18 and the reference light are applied to the polarization beam splitter 16, transmitted through the polarization beam splitter 16, converted into circularly polarized light by the quarter wavelength plate 20, and the lens. 22 is Fourier transformed. The direct current component of the Fourier-transformed signal light and the reference light are subjected to inverse Fourier transform by the lens 24, and again subjected to Fourier transform by the lens 26, so that the hologram recording area of the optical recording medium 28 is irradiated.

  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 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 ¼ wavelength plate 20, and incident on the polarization beam splitter 16. Reflected in the direction.

  On the other hand, the direct current component of the signal light applied to 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 DC component of the reflected signal light is collimated 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, and the photodetector 36. Reflected in the direction of.

  The light diffracted by the hologram and the direct current component of the signal light are incident on the photodetector 36. A reproduced image can be observed on the focal plane of the lens 22. In the present embodiment, the diffracted light is supplemented with the DC component of the signal light removed at the time of recording the hologram, so that the diffracted light having the same component as the original signal light is restored, and the original signal light pattern (light / dark image) Is played. This 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 reproduced image is input to the personal computer 30.

  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.

  Further, in this embodiment, when reproducing the hologram, the diffracted light obtained from the recorded hologram is supplemented with the direct current component of the signal light removed at the time of recording, so that the diffraction of the same component as the original signal light is performed. The light is restored and the original signal light pattern (light / dark image) is reproduced. Thereby, the digital data held in the signal light can be accurately decoded.

  In addition, in the reproducing method of the present embodiment, as with the first embodiment, the supplemented DC component needs to propagate coaxially with the diffracted light from the hologram with high accuracy. Referring to FIG. 1, in the first embodiment, in order to realize this, it is necessary to align both the mask 38 having a reflection function and the optical recording medium 28. That is, the direct current component is reflected by the mask 38, and the diffracted light is reflected by the reflective layer 28 a of the optical recording medium 28. On the other hand, the present embodiment has an advantage that only the optical recording medium 28 needs to be aligned because the DC component to be supplemented and the diffracted light are reflected by the same reflective layer 28a.

(Third 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 third 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.

FIG. 6 is a diagram showing a schematic configuration of a hologram recording / reproducing apparatus according to the third 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 polarizing plate (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.

  As the mask 68, for example, a micromirror that reflects only the DC component of the Fourier transform image can be used. Also, a filter that absorbs only the DC 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 is transmitted through the optical recording medium 72. 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 third embodiment has a different configuration from the apparatus according to the second 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 200, it is determined whether the recording process is selected or the reproduction process is selected. If the recording process is selected, the driving device 70 is driven and the mask 68 is inserted into the optical path in step 202. To do. In the next step 204, the laser light is emitted from the light source 50, the digital data is output from the personal computer 56 at a predetermined timing, the hologram recording process is executed, and the routine is terminated.

  On the other hand, if reproduction processing is selected in step 200, in step 206, the driving device 70 is driven to retract the mask 68 from the optical path. In the next step 208, the brightness value of the display image for supplementing the reproduced diffracted light with a direct current component is calculated. In step 210, the laser light is emitted from the light source 50 and the brightness value calculated from the personal computer 56 is determined in advance. Is output at this timing, hologram reproduction processing is executed, and the routine is terminated.

  Note that the DC component replenishment method (the method of calculating the luminance value of the transmission pattern) is the same as described in the first embodiment, and thus the description thereof is omitted.

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 intensity of the laser light is 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. 3). 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.

  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.

Next, the hologram reproduction 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 is incident on the spatial light modulator 58. When the luminance value calculated from the personal computer 56 is input, a transmission pattern is generated according to the supplied luminance value in the pattern generator 60, synthesized with the reference light pattern, and displayed on the spatial light modulator 58. A display pattern is generated.

  Similar to the first embodiment, 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 peripheral portion of the spatial light modulator 58 is the same as during recording. A reference light pattern is displayed (see FIG. 4). 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. Thereafter, the direct current component and the reference light are transmitted through a polarizing plate (not shown) and converted into an amplitude distribution.

  The direct current component of the signal light and the reference light generated by the spatial light modulator 58 are Fourier transformed by the lens 62. The direct current component of the Fourier-transformed signal light and the reference light are subjected to inverse Fourier transform by the lens 64, and again subjected to Fourier transform by the lens 66, so that the hologram recording area of the optical recording medium 72 is irradiated.

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

  In the present embodiment, the diffracted light is supplemented with the DC component of the signal light removed at the time of recording the hologram, so that the diffracted light having the same component as the original signal light is restored, and the original signal light pattern (light / dark image) Is played. This 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 reproduced image is input to the personal computer 56.

  An experiment was performed using an apparatus having the same configuration as the hologram recording / reproducing apparatus described above. The spatial light modulator 58 displayed a pattern shown in FIG. In addition, a hologram was recorded by displaying the signal light pattern shown in FIG. 8B at the central portion of the spatial light modulator 58. As shown in FIG. 7, the signal light pattern uses only the central pixel of the pixel block represented by 3 × 3 pixels as a signal light component, and binary digital data “0, 1” is “bright (white pixel). ), Dark (black pixels) ”, and the pixels in the vicinity of 8 of the signal light component are white pixels, and the ratio of white pixels in this digital pattern is very high.

  At the time of recording a hologram, the direct current component of the signal light and the reference light is 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 high ratio of white pixels, and therefore, the proportion of the direct current component in all the Fourier components is very large (0.9 or more).

  The large DC component increases the amplitude of the signal that is originally “white pixel” and decreases the amplitude of the signal that is originally “black pixel”. However, since the DC component is removed by the mask 68, The amplitude of the signal that is “pixel” decreases, and the amplitude of the signal that is originally “black pixel” increases. 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.

  The recorded hologram was reproduced by (1) the method of irradiating only the reference light and (2) the method of irradiating the direct current component of the reference light and signal light described above, and the reproduced image was observed. When a digital pattern with a high ratio of white pixels is used, in the method (1), a signal light pattern at the time of recording (an inverted image (negative image) of the digital pattern displayed on the spatial light modulator 58) is reproduced ( In the methods of (2) and (2), 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) was reproduced (see FIG. 9B). reference).

  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.

  Further, in this embodiment, when reproducing the hologram, the diffracted light obtained from the recorded hologram is supplemented with the direct current component of the signal light removed at the time of recording, so that the diffraction of the same component as the original signal light is performed. The light is restored and the original signal light pattern (light / dark image) is reproduced. Thereby, the digital data held in the signal light can be accurately decoded.

  In order to optimize the recording conditions, it is preferable to perform defocusing by moving the recording medium to an area where the signal light and the reference light overlap. In the first and second embodiments, the focal position of the lens and the position of the reflective layer of the optical recording medium need to match, but since the optical recording medium is integrated with the reflective layer, optical recording The degree of freedom of medium defocusing is small. In contrast, the present embodiment uses a transmission type optical recording medium, and thus has a great degree of freedom in defocusing and easy design of recording conditions.

(Fourth embodiment)
In the first to third embodiments, the example in which the direct current component is removed from the Fourier transform image of the signal light using the reflection mirror that reflects the direct current component, the absorption filter that absorbs the direct current component, and the like as a mask has been described. In the fourth embodiment, an example in which a direct current component is removed from a Fourier transform image of signal light using a diffraction element that diffracts the direct current component will be described. It is to be noted that the signal light and the reference light can be irradiated on the optical recording medium coaxially as in the first to third embodiments.

  FIG. 10 is a diagram showing a schematic configuration of a hologram recording / reproducing apparatus according to the fourth embodiment. The same components as those of the apparatus according to the third embodiment are denoted by the same reference numerals, and description thereof is omitted. In this hologram recording / reproducing apparatus, there is a polarizing plate (not shown) on the light transmission side of the spatial light modulator 58, and the direct current component is diffracted from the Fourier transform image of the signal light by diffracting the direct current component in a predetermined direction on the light transmission side. The volume hologram element 80 for removing the light and the Fourier transform lens 66 for irradiating the optical recording medium 72 with the signal light and the reference light are arranged in this order. Further, the volume hologram element 80 is connected to the personal computer 56 via a driving device 82 and is driven by the driving device 82 so as to be inserted into and retracted from the optical path.

  As the volume hologram element 80, an element whose Bragg condition matches the DC component (0th-order component) of the signal light generated by the spatial light modulator 58 is used. According to the volume hologram element 80, only the DC component of the signal light generated by the spatial light modulator 58 is diffracted in a predetermined direction. On the other hand, components other than the DC component of the signal light pass through the volume hologram element 80 without being diffracted by the volume hologram element 80 because the Bragg conditions do not match. Thereby, the DC component is removed from the Fourier transform image of the signal light.

  The hologram recording / reproducing apparatus according to the fourth embodiment has a different configuration from that of the apparatus according to the second embodiment, and the hologram recording method and reproducing method are different, but the recording / reproducing process executed by the personal computer 56 is different. This processing routine is the same as the routine shown in FIG. However, in the fourth embodiment, the volume hologram element 80 is inserted into or retracted from the optical path by the driving device 82 instead of the mask 68.

  First, in step 200, it is determined whether the recording process is selected or the reproduction process is selected. If the recording process is selected, in step 202, the drive device 82 is driven to move the volume hologram element 80 to the optical path. Insert into. In the next step 204, the laser light is emitted from the light source 50, the digital data is output from the personal computer 56 at a predetermined timing, the hologram recording process is executed, and the routine is terminated.

  If reproduction processing is selected in step 200, in step 206, the drive device 82 is driven to retract the volume hologram element 80 from the optical path. In the next step 208, the brightness value of the display image for supplementing the reproduced diffracted light with a direct current component is calculated. In step 210, the laser light is emitted from the light source 50 and the brightness value calculated from the personal computer 56 is determined in advance. Is output at this timing, hologram reproduction processing is executed, and the routine is terminated.

  Note that the DC component replenishment method (the method of calculating the luminance value of the transmission pattern) is the same as described in the first embodiment, and thus the description thereof is omitted.

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. Thereafter, the signal light and the reference light are transmitted through a polarizing plate (not shown) and converted into an amplitude distribution.

  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. 3). 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. Then, it is converted into an amplitude distribution of linearly polarized light by passing through the polarizing plate.

  The signal light and the reference light generated by the spatial light modulator 58 are applied to the volume hologram element 80, and the DC component is removed from the Fourier transform image of the signal light and the reference light. The signal light and the reference light transmitted through the volume hologram element 80 are Fourier-transformed by the lens 66, and are 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.

Next, the hologram reproduction 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 is incident on the spatial light modulator 58. When the luminance value calculated from the personal computer 56 is input, a transmission pattern is generated according to the supplied luminance value in the pattern generator 60, synthesized with the reference light pattern, and displayed on the spatial light modulator 58. A display pattern is generated.

  Similar to the first embodiment, 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 peripheral portion of the spatial light modulator 58 is the same as during recording. A reference light pattern is displayed (see FIG. 4). 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. Thereafter, the direct current component and the reference light are transmitted through a polarizing plate (not shown) and converted into an amplitude distribution.

  The direct current component of the signal light and the reference light generated by the spatial light modulator 58 are Fourier-transformed by the lens 66 and applied to the area where the hologram of the optical recording medium 72 is recorded.

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

  In the present embodiment, the diffracted light is supplemented with the DC component of the signal light removed at the time of recording the hologram, so that the diffracted light having the same component as the original signal light is restored, and the original signal light pattern (light / dark image) Is played. This 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 reproduced image is input to the personal computer 56.

  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.

  Further, in this embodiment, when reproducing the hologram, the diffracted light obtained from the recorded hologram is supplemented with the direct current component of the signal light removed at the time of recording, so that the diffraction of the same component as the original signal light is performed. The light is restored and the original signal light pattern (light / dark image) is reproduced. Thereby, the digital data held in the signal light can be accurately decoded.

  In the first to third embodiments, two lenses are used to install a mask that removes a direct current component (lenses 22 and 24 in FIG. 1 and lenses 62 and 64 in FIG. 6). As a result, the optical system becomes large and the number of parts increases. On the other hand, the present embodiment has the advantage that the optical lens can be downsized and the number of components can be reduced because the two lenses and the mask can function as a single volume hologram element.

  In the first to fourth embodiments, the example in which the signal light and the reference light are irradiated coaxially to the optical recording medium has been described. However, the hologram is obtained by irradiating the signal light and the reference light from different directions. May be recorded. In this case, a mask or a volume hologram element is arranged on the optical path of the signal light, and the direct current component is removed from the Fourier transform image of the signal light.

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 the recording / reproducing process of 1st Embodiment. It is a figure which shows the display image of the spatial light modulator at the time of a recording process. It is a figure which shows the display image of the spatial light modulator at the time of reproduction | regeneration processing. It is a flowchart which shows the processing routine of the recording / reproducing process of 2nd Embodiment. It is a figure which shows schematic structure of the hologram recording / reproducing apparatus which concerns on 3rd Embodiment. It is a figure which shows the structure of the pixel block of signal light with a high white pixel ratio. (A) is a pattern displayed on the spatial light modulator in the third embodiment, and (B) is a signal light pattern displayed at the center of the pattern of (A). (A) is a figure showing the reproduction image when not giving a 0th-order component, (B) is a figure showing the reproduction image when a 0th-order component is provided. It is a figure which shows schematic structure of the hologram recording / reproducing apparatus which concerns on 4th Embodiment.

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 80 Volume hologram element 82 Driving device

Claims (17)

Using a spatial light modulator that includes a modulation region including a signal light region and a reference light region, and modulates incident light according to a pattern displayed in the modulation region for each pixel,
At the time of recording a hologram, a signal light pattern representing binary digital data as a bright and dark image is displayed in the signal light area of the spatial light modulator, and a reference light pattern is displayed in the reference light area. Generating the signal light and a coaxial reference light;
The signal light generated by the spatial light modulator and the reference light are Fourier-transformed, and a mask that blocks a DC component is disposed on the optical path of the Fourier-transformed signal light, and a Fourier-transformed image of the signal light Removing the DC component from
Irradiating the optical recording medium simultaneously and coaxially with the signal light subjected to Fourier transform and from which the direct current component has been removed and the reference light subjected to Fourier transform , to record a hologram on the optical recording medium;
When reproducing a hologram, a reference light pattern is displayed in the reference light region of the spatial light modulator and a transmission pattern is displayed in the signal light region, so that the reference light for reading and a direct current component coaxial with the reference light are displayed. Generating and
The reference beam for reading and the direct current component are Fourier transformed to irradiate the optical recording medium on which the hologram is recorded coaxially, and the diffracted light from the recorded hologram and the direct current component obtained by the Fourier transform are synthesized and synthesized. Generating light ;
Restoring the signal light representing the binary digital data as a bright and dark image by performing an inverse Fourier transform on the combined light obtained by combining the diffracted light and the DC component; and
Hologram recording / reproducing method comprising: At the time of recording the hologram, a mask for blocking the direct current component is arranged on the optical path of the Fourier transformed reference light, the direct current component is removed from the Fourier transformed image of the reference light, and the Fourier transformed and the direct current component is removed. The hologram recording / reproducing method according to claim 1, wherein the hologram is recorded on the optical recording medium by irradiating the optical recording medium simultaneously and coaxially with the signal light and the reference light . The hologram recording / reproducing method according to claim 1 , wherein the DC component to be synthesized corresponds to all or a part of the DC component removed from the Fourier transform image of the signal light.   4. The hologram recording / 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 increases.   The hologram recording / reproducing method according to claim 4, wherein the diffracted light and the direct current component are combined so that a positive amplitude of the combined light increases. 6. The hologram recording / reproducing method according to claim 5, wherein the diffracted light and the direct current component are combined so that the phase θ of the direct current component and the phase φ of the diffracted light satisfy the following formula (1).
0 ≦ | θ−φ | <π / 2 Formula (1) 7. The hologram recording / reproducing method according to claim 6, wherein the diffracted light and the direct current component are combined so that the phase θ of the direct current component and the phase φ of the diffracted light satisfy the following formula (2).
| Θ−φ | = 0 Formula (2)   5. The hologram recording / reproducing method according to claim 4, wherein the diffracted light and the direct current component are combined so that the negative amplitude of the combined light increases. 9. The hologram recording / reproducing method according to claim 8, wherein the diffracted light and the direct current component are synthesized so that the phase θ of the direct current component and the phase φ of the diffracted light satisfy the following formula (3).
π / 2 <| θ−φ | ≦ π Formula (3) The hologram recording / reproducing method according to claim 9, wherein the diffracted light and the direct current component are combined so that the phase θ of the direct current component and the phase φ of the diffracted light satisfy the following formula (4).
| Θ−φ | = π Formula (4) 11. The direct current component having a predetermined phase is generated by displaying a transmission pattern in the signal light region of the spatial light modulator and phase-modulating collimated light when reproducing a hologram. Hologram recording / reproducing method. The hologram recording / reproducing method according to claim 11 , wherein collimated light is phase-modulated by changing luminance of a pixel of a transmission pattern displayed in the signal light region of the spatial light modulator . A light source that emits coherent light;
A spatial light modulator comprising a modulation region including a signal light region and a reference light region, and modulating light incident from the light source for each pixel in accordance with a pattern displayed in the modulation region;
A first irradiating means disposed on the light output side of the spatial light modulator, Fourier-transforming the light incident from the spatial light modulator side, and irradiating the optical recording medium with the Fourier-transformed light;
A removal means that is arranged so as to be inserted into or retracted from the optical path of the Fourier transformed light and functions as a mask for blocking the direct current component to remove the direct current component from the Fourier transformed light;
A photodetector that is disposed on the light emitting side of the optical recording medium and detects light irradiated from the optical recording medium side;
Second irradiation, which is arranged between the optical recording medium and the photodetector, performs inverse Fourier transform on the light incident from the optical recording medium side, and irradiates the photodetector with the inverse Fourier transformed light. Means,
Drive control means for drivingly controlling each of the light source, the spatial light modulator, and the removal means so as to execute the following steps (1) to (6);
Hologram recording / reproducing apparatus.
(1) At the time of recording a hologram, a signal light pattern representing binary digital data as a bright and dark image is displayed in the signal light area of the spatial light modulator, and a reference light pattern is displayed in the reference light area, Generating signal light and reference light coaxial with the signal light
(2) Fourier transforming the signal light generated by the spatial light modulator and the reference light, and arranging a mask for blocking a direct current component on the optical path of the Fourier-transformed signal light, Step of removing DC component from Fourier transform image
(3) A step of recording a hologram on the optical recording medium by simultaneously and coaxially irradiating the optical recording medium with the signal light that has been subjected to Fourier transformation and from which the DC component has been removed, and the reference light that has undergone Fourier transformation.
(4) When reproducing a hologram, a reference light pattern is displayed in the reference light region of the spatial light modulator and a transmission pattern is displayed in the signal light region, so that the reference light for reading and the reference light are coaxial. To generate the DC component of
(5) The reference beam for reading and the direct current component are Fourier transformed to irradiate the optical recording medium on which the hologram is recorded coaxially, and the diffracted light from the recorded hologram and the direct current component obtained by the Fourier transform are synthesized. To generate synthetic light
(6) A step of restoring signal light representing the binary digital data as a light and dark image by performing inverse Fourier transform on the combined light obtained by combining the diffracted light and the DC component. The hologram recording / reproducing apparatus according to claim 13, wherein the spatial light modulator displays a transmission pattern in the signal light region and phase-modulates collimated light to generate the DC component having a predetermined phase when reproducing the hologram.   15. The hologram recording / reproducing apparatus according to claim 13, wherein the removing unit is a reflection mirror that selectively reflects the direct current component.   15. The hologram recording / reproducing apparatus according to claim 13, wherein the removing unit is an absorption filter that selectively absorbs the direct current component.   15. The hologram recording / reproducing apparatus according to claim 13, wherein the removing unit is a diffraction element that selectively diffracts the direct current component.