JP4992786B2 - Optical recording apparatus, optical recording method, optical recording / reproducing apparatus, and optical recording / reproducing method - Google Patents

Description

  The present invention relates to an optical recording apparatus, an optical recording method, an optical recording / reproducing apparatus, and an optical recording / reproducing method.

  Recently, as a recording / reproducing method for holographic memory, the optical system can be greatly simplified compared to the conventional two-beam interference method, and it has the advantages of being resistant to disturbances such as vibration and easy to introduce a servo mechanism. A recording method (collinear method) has been proposed. In this collinear method, the signal light and the reference light modulated by the spatial light modulator are collected by the same lens as a common optical axis, and interference is formed by interference between the signal light and the reference light. Stripes (diffraction gratings) are recorded as holograms on the optical recording medium. By displaying the signal light pattern obtained by two-dimensionally encoding the digital data on the spatial light modulator, the digital data is superimposed on the signal light.

  By irradiating the optical recording medium on which the hologram is recorded with the reference light as the readout light, the signal light is reproduced from the recorded hologram. The reproduced signal light can be detected by a photodetector, and the superimposed digital data can be decoded. There are various multiplexing methods as a method for multiplex recording of holograms. Among these, examples of using reference light whose phase is modulated include “phase code multiplexing” that gives a coded phase distribution, “speckle multiplexing” that gives a random phase using a diffusion plate, etc. Are known.

  The multiplexing method described in Patent Document 1 is called “correlation multiplexing (phase correlation multiplexing)”. Correlation multiplexing is a multiplex recording method similar to speckle multiplexing in that a randomized reference beam with little phase correlation is used. In the collinear method, which is the same principle as speckle multiplexing, the reference light is generated by being modulated by the spatial light modulator together with the signal light. Conventionally, the reference light pattern used is the same and the same display of the spatial light modulator is used. It was displayed at the position. Therefore, in order to perform multiplex recording, that is, to reduce the correlation with respect to the phase distribution of the reference light in which the adjacent hologram is recorded, the optical recording medium is moved. In “speckle multiplexing” in a two-beam system, a diffusion plate having a different frequency distribution for each multiplex recording can be used to change the phase of the reference light. However, in this case, there is also a drawback that the intensity ratio and overlap between the signal light and the reference light change as a result of modulating the phase of the reference light.

US Pat. No. 5,769,691

  An object of the present invention is to provide a novel optical recording apparatus, an optical recording apparatus, and a recording medium that perform recording and reproduction of a hologram using reference light whose phase is modulated according to the display position of a reference light pattern in a spatial light modulator in a collinear method. A recording method, an optical recording / reproducing apparatus, and an optical recording / reproducing method are provided.

In order to achieve the above object, an optical recording apparatus according to claim 1 is composed of a light source that emits coherent light, and a plurality of pixel units that are two-dimensionally arranged on an xy plane, and light incident from the light source. Is modulated for each pixel according to the pattern displayed for each page, and a spatial light modulator that generates recording light composed of signal light and reference light, and Fourier transform of the recording light generated by the spatial light modulator And a condensing optical system for concentrating the light on the optical recording medium coaxially, a signal light pattern is displayed in a fixed region of the spatial light modulator, and a reference light pattern including a plurality of spatial frequency components is not included in the fixed region. Drive control for driving and controlling each of the plurality of pixel portions of the spatial light modulator so that the center point of the variable area is located at a different location on the xy plane for each page while displaying in a ring-shaped variable area And equipped with The features.

  An optical recording apparatus according to a second aspect of the present invention is the optical recording apparatus according to the first aspect, wherein the condensing optical system uses a Fourier transform component obtained by Fourier transforming the recording light generated by the spatial light modulator. A removal element that removes the 0th-order diffraction component, a lens that performs inverse Fourier transform on the light from which the 0th-order diffraction component has been removed, and a lens that concentrically condenses on the optical recording medium by re-Fourier transforming the inverse Fourier transform And.

  According to a third aspect of the present invention, in the optical recording apparatus of the first aspect, the condensing optical system imparts a random phase distribution to at least the reference light among the recording light generated by the spatial light modulator. And a lens for concentrating the recording light with the phase distribution on the optical recording medium coaxially.

The optical recording method according to claim 4 is composed of a plurality of pixel units arranged two-dimensionally on the xy plane, and modulates light incident from the light source for each pixel according to a pattern displayed for each page. A spatial light modulator is used to generate a signal light by displaying a signal light pattern on a fixed region of the spatial light modulator, and a reference light pattern including a plurality of spatial frequency components is formed in a ring shape that does not include the fixed region . The reference light is generated by displaying in the variable region, the recording light composed of the signal light and the reference light generated by the spatial light modulator is Fourier-transformed and condensed coaxially on the optical recording medium, and the variable for each page. The center point of the area is moved to a different position on the xy plane, the display position of the reference light pattern is changed for each page, and the reference light beam having a different phase for each page is used to generate a hologram of a plurality of pages on the optical recording medium. Multiple recording, And wherein the door.

  The optical recording apparatus according to claim 5 is the optical recording apparatus according to claim 4, wherein the zero-order diffraction component is removed from the Fourier transform component obtained by Fourier transforming the recording light generated by the spatial light modulator. The light from which the 0th-order diffraction component has been removed is subjected to inverse Fourier transform, and the light subjected to inverse Fourier transform is re-Fourier transformed to be coaxially collected on an optical recording medium.

  An optical recording apparatus according to a sixth aspect provides the optical recording apparatus according to the fourth aspect, wherein a random phase distribution is given to at least the reference light among the recording lights generated by the spatial light modulator, and the phase distribution is given. The recorded light is Fourier-transformed and condensed coaxially on the optical recording medium.

The optical recording / reproducing apparatus according to claim 7 is composed of a light source that emits coherent light and a plurality of pixel units arranged two-dimensionally on an xy plane, and a signal light pattern is provided in a fixed region of the spatial light modulator. The reference light pattern that is displayed and includes a plurality of spatial frequency components is displayed in a ring-shaped variable area that does not include the fixed area, and light incident from the light source is modulated for each pixel according to the pattern displayed for each page. A spatial light modulator that generates recording light composed of signal light and reference light, and a condensing optical system that coaxially collects the recording light generated by the spatial light modulator on an optical recording medium by Fourier transform A photodetector for detecting a reproduced image reproduced from the optical recording medium and a reference beam pattern in a variable area whose central point is the same as that at the time of recording the page when reproducing a hologram for one page. Generated by a spatial light modulator The irradiated first reference light as reading light, and detects a first reproduced image at the photodetector, the page so that the DC component is applied and diffracted light from the hologram interference by reverse phase The reference light pattern is displayed in the variable area where the center point is moved to a position different from that at the time of recording, and the non-modulated pattern is displayed in the fixed area. The phase is different from the first reference light generated by the spatial light modulator. A drive control unit that drives and controls each of the light source, the spatial light modulator, and the photodetector so that the second reference light is emitted as readout light and the second reproduced image is detected by the photodetector. And.

  An optical recording / reproducing apparatus according to an eighth aspect of the present invention is the optical recording / reproducing apparatus according to the seventh aspect, wherein the reference light pattern has a matrix of bright spots of a Fourier transform image of the reference light generated by the spatial light modulator. As described above, it is a periodic pattern in which a bright part and a dark part are arranged at a predetermined period.

  The optical recording / reproducing apparatus according to claim 9 is the optical recording / reproducing apparatus according to claim 7 or 8, wherein the drive control unit displays a signal light pattern in the fixed area when recording a hologram for one page. And driving and controlling each of the light source and the spatial light modulator so as to display a reference light pattern in the variable region and generate recording light including signal light and reference light by the spatial light modulator. It is characterized by.

  An optical recording / reproducing apparatus according to a tenth aspect of the invention is the optical recording / reproducing apparatus according to any one of the seventh to ninth aspects, wherein the condensing optical system performs Fourier transform on the recording light generated by the spatial light modulator. Of the obtained Fourier transform components, a shield member that shields at least the second-order or higher diffraction component of the reference light, a lens that performs inverse Fourier transform on the first-order or lower diffraction component transmitted through the shield member, and an inverse Fourier transform. And a lens that concentrically collects the collected light on an optical recording medium by re-Fourier transform.

  An optical recording / reproducing apparatus according to an eleventh aspect is the optical recording / reproducing apparatus according to the tenth aspect, wherein the shielding member also shields a zero-order diffraction component.

  An optical recording / reproducing apparatus according to a twelfth aspect is the invention according to any one of the seventh to eleventh aspects, wherein a hologram for one page is reproduced using a digital micromirror device for the spatial light modulator. In this case, the swing period m (Hz) of the pixel portion of the fixed region displaying the signal light pattern and the swing cycle n (Hz) of the pixel portion of the variable region displaying the reference light pattern are k · n. = M (k is an integer of 1 or more).

The optical recording / reproducing method according to claim 13 is composed of a plurality of pixel units arranged two-dimensionally on an xy plane, and modulates light incident from a light source for each pixel according to a pattern displayed for each page. A spatial light modulator that generates a signal light by displaying a signal light pattern on a fixed region of the spatial light modulator and a reference light pattern including a plurality of spatial frequency components in a ring shape that does not include the fixed region. The reference light is generated by displaying in a variable area of the optical recording medium, and the recording light composed of the signal light and the reference light generated by the spatial light modulator is Fourier-transformed to be coaxially collected on the optical recording medium to record a hologram. When reproducing a hologram for one page, a reference light pattern is displayed in a variable area whose central point is the same as that at the time of recording the page, and the first reference light generated by the spatial light modulator is irradiated as readout light. The first reproduced image Detected, the page during recording and displaying a reference light pattern variable region center point is moved to different positions and fixed area to interfere with the diffracted light and the applied direct current component by reverse phase from the hologram An unmodulated pattern is displayed on the second reference light, and a second reproduced image is detected by irradiating a second reference light having a phase different from that of the first reference light generated by the spatial light modulator as a readout light. .

The optical recording / reproducing apparatus according to claim 14 includes a light source that emits coherent light and a plurality of pixel units that are two-dimensionally arranged in an xy plane, and the signal light pattern is provided in a fixed region of the spatial light modulator. The reference light pattern that is displayed and includes a plurality of spatial frequency components is displayed in a ring-shaped variable area that does not include the fixed area, and light incident from the light source is modulated for each pixel according to the pattern displayed for each page. Then, the spatial light modulator that generates the recording light composed of the signal light and the reference light, and the zero-order diffraction component is removed from the Fourier transform component obtained by Fourier transforming the recording light generated by the spatial light modulator. A collecting element, a lens that performs inverse Fourier transform on the light from which the 0th-order diffraction component has been removed, and a lens that re-Fourier transforms the light subjected to inverse Fourier transform and concentrically collects the light on an optical recording medium. With optical optics A photodetector for detecting a reproduction image reproduced from the optical recording medium, said when reproducing a hologram of one page, so that the diffracted light and the applied direct current component from the hologram interference by the same phase The reference light pattern is displayed in the variable area that has been moved to the first position, which is different from the center point during page recording, and the unmodulated pattern is displayed in the fixed area, and the first reference light generated by the spatial light modulator is read out. Irradiated as light, the first reproduced image is detected by the photodetector, and the central point is different from that at the time of recording the page so that the diffracted light from the hologram interferes with the applied DC component due to the opposite phase. The reference light pattern is displayed in the variable region moved to the second position and the non-modulated pattern is displayed in the fixed region, and the second reference light having a phase different from that of the first reference light generated by the spatial light modulator is read out. Shine as light And a drive control unit that drives and controls each of the light source, the spatial light modulator, and the photodetector so that a second reproduced image is detected by the photodetector. .

  An optical recording / reproducing apparatus according to a fifteenth aspect is the optical recording / reproducing apparatus according to the fourteenth aspect, wherein the reference light pattern has a matrix of bright spots of a Fourier transform image of the reference light generated by the spatial light modulator. As described above, it is a periodic pattern in which a bright part and a dark part are arranged at a predetermined period.

  An optical recording / reproducing apparatus according to a sixteenth aspect is the invention according to the fourteenth or fifteenth aspect, wherein the drive control unit displays a signal light pattern in the fixed area when recording a hologram for one page. And driving and controlling each of the light source and the spatial light modulator so as to display a reference light pattern in the variable region and generate recording light including signal light and reference light by the spatial light modulator. It is characterized by.

The optical recording / reproducing method according to claim 17 is composed of a plurality of pixel units arranged two-dimensionally on an xy plane, and modulates light incident from a light source for each pixel according to a pattern displayed for each page. A spatial light modulator that generates a signal light by displaying a signal light pattern on a fixed region of the spatial light modulator and a reference light pattern including a plurality of spatial frequency components in a ring shape that does not include the fixed region. The reference light is generated by displaying in a variable area of the optical recording medium, and the recording light composed of the signal light and the reference light generated by the spatial light modulator is Fourier-transformed to be coaxially collected on the optical recording medium to record a hologram. When reproducing a hologram for one page, the variable area moved to the first position where the center point is different from that at the time of recording so that the diffracted light from the hologram interferes with the applied DC component due to the same phase To reference light Display unmodulated pattern turn displayed and a fixed area, by irradiating a first reference light generated by the spatial light modulator as a reading light, to detect a first reproduced image, applying diffracted light from the hologram The reference light pattern is displayed in the variable area moved to the second position where the center point is different from that during the page recording so that the direct current component interferes with the opposite phase , and the unmodulated pattern is displayed in the fixed area. The second reproduction image is detected by irradiating the second reference light having a phase different from that of the first reference light generated by the spatial light modulator as the readout light.

  According to the first aspect of the present invention, in the collinear method, the hologram is used without moving the optical recording medium by using the reference light whose phase is modulated in accordance with the display position of the reference light pattern in the spatial light modulator. Can be multiplexed and recorded.

  According to the second and third aspects of the invention, crosstalk due to the 0th-order diffraction component common to the reference light can be prevented, and the signal light-to-noise ratio (SNR) is improved.

  According to the invention described in claim 4, in the collinear method, the hologram is used without moving the optical recording medium by using the reference light whose phase is modulated in accordance with the display position of the reference light pattern in the spatial light modulator. Can be multiplexed and recorded.

  According to the fifth and sixth aspects of the invention, crosstalk due to the 0th-order diffraction component common to the reference light can be prevented, and the signal light-to-noise ratio (SNR) is improved.

  According to the invention described in claim 7, in the collinear method, the liquid crystal type spatial light modulator is used by using the reference light whose phase is modulated in accordance with the display position of the reference light pattern in the spatial light modulator. Without being limited to the case, there is an effect that a plurality of reproduced images can be obtained from one hologram.

  According to the eighth aspect of the invention, there is an effect that the phase control of the reference light becomes easy.

  According to the invention described in claim 9, in the collinear method, the liquid crystal type spatial light modulator is used by using the reference light whose phase is modulated in accordance with the display position of the reference light pattern in the spatial light modulator. Without being limited thereto, there is an effect that the hologram can be recorded and reproduced so that a plurality of reproduced images can be obtained from one hologram.

  According to the tenth aspect of the invention, there are the effects that the phase control of the reference light becomes easy and the recording area can be miniaturized.

  According to the eleventh aspect of the invention, crosstalk due to the 0th-order diffraction component common to the reference light can be prevented, and the signal light-to-noise ratio (SNR) is improved.

  According to the twelfth aspect of the present invention, there is an effect that a reproduced image with high contrast can be obtained by using a digital micromirror device for the spatial light modulator.

  According to the invention described in claim 13, in the collinear method, the liquid crystal type spatial light modulator is used by using the reference light whose phase is modulated in accordance with the display position of the reference light pattern in the spatial light modulator. Without being limited to the case, there is an effect that a plurality of reproduced images can be obtained from one hologram.

  According to the fourteenth aspect of the present invention, in the collinear method, the liquid crystal type spatial light modulator is used by using the reference light whose phase is modulated in accordance with the display position of the reference light pattern in the spatial light modulator. Without being limited to the case, there is an effect that a plurality of reproduced images can be obtained from one hologram.

  According to the fifteenth aspect of the invention, there is an effect that the phase control of the reference light becomes easy.

  According to the sixteenth aspect of the present invention, in the collinear method, the liquid crystal type spatial light modulator is used by using the reference light whose phase is modulated in accordance with the display position of the reference light pattern in the spatial light modulator. Without being limited thereto, there is an effect that the hologram can be recorded and reproduced so that a plurality of reproduced images can be obtained from one hologram.

  According to the invention described in claim 17, in the collinear method, the liquid crystal type spatial light modulator is used by using the reference light whose phase is modulated in accordance with the display position of the reference light pattern in the spatial light modulator. Without being limited thereto, there is an effect that the hologram can be recorded and reproduced so that a plurality of reproduced images can be obtained from one hologram.

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

<First Embodiment>
In the first embodiment, in the collinear method, the optical recording medium is moved relative to the recording light by using the reference light whose phase is modulated according to the display position of the reference light pattern in the spatial light modulator. A multiplex recording method for multiplex recording of holograms without any movement (without moving the medium) will be described. Hereinafter, this multiplex recording method is referred to as “reference light pattern slide multiplexing”.

(Hologram recording / reproducing device)
FIG. 1 is a schematic diagram showing the configuration of the hologram recording / reproducing apparatus according to the first embodiment. This hologram recording / reproducing apparatus irradiates an optical recording medium with a signal beam having a common optical axis and a reference light as a single beam of recording light from the same direction as a “coaxial recording method (collinear method)” hologram recording / reproducing apparatus. It is. In the present embodiment, a “coaxial transmission type” hologram recording / reproducing apparatus using a transmission type spatial light modulator (SLM) and a transmission type optical recording medium will be described.

  The hologram recording / reproducing apparatus is provided with a light source 10 that oscillates laser light that is coherent light. As the light source 10, for example, a laser light source that oscillates green laser light having an oscillation wavelength of 532 nm is used. On the light exit side of the light source 10, a shutter 12 that can be inserted into or retracted from the optical path (open / close), a half-wave plate 14 that gives a half-wave optical path difference between orthogonal linearly polarized light components, and a predetermined polarization A polarizing plate 16 that transmits light in a direction and a beam expander 18 that is an expansion / collimating optical system are arranged in this order along the optical path from the light source 10 side.

  The light source 10 is driven to oscillate by a driving device 40 connected to the control device 36. The driving device 40 is connected to a control device 36 constituted by a personal computer having a CPU, ROM, RAM, storage device and the like, and is controlled by the control device 36. The shutter 12 is opened and closed by being driven by a driving device 38 connected to the control device 36.

  A transmissive spatial light modulator 20 that modulates incident light for each pixel is disposed on the light transmission side of the beam expander 18. As the transmissive spatial light modulator 20, a liquid crystal spatial light modulator in which transparent electrodes are formed on both surfaces of an electro-optic conversion material such as liquid crystal can be used. The spatial light modulator 20 is connected to the control device 36 via a pattern generator (not shown).

  The pattern generator represents the digital data supplied from the control device 36 as a bright and dark image, and generates a signal light pattern to be displayed on the spatial light modulator 20. The signal light pattern is, for example, a digital pattern in which binary digital data “0, 1” is expressed as “dark (black pixels), light (white pixels)”. In addition to the signal light pattern, the spatial light modulator 20 also displays a reference light pattern. The reference light pattern is a pattern having a plurality of spatial frequency components such as a random pattern and a checker pattern. The spatial light modulator 20 modulates the incident laser light according to the displayed signal light pattern or reference light pattern to generate signal light or reference light. The spatial light modulator 20 emits the generated signal light and reference light to the lens 22 side.

  FIG. 2A is a diagram showing an example of a recording pattern displayed on the spatial light modulator 20 during recording. As shown in FIG. 2A, the recording pattern 42 includes a signal light pattern that generates signal light and a reference light pattern that generates reference light. The signal light pattern is displayed in the central portion of the spatial light modulator 20. The reference light pattern is displayed on the peripheral portion of the spatial light modulator 20 so as to surround the signal light pattern.

  FIG. 2B is a plan view of the spatial light modulator 20. The spatial light modulator 20 includes a display surface 20A having a rectangular shape in plan view. On the display surface 20A, a plurality of pixel portions are two-dimensionally arranged on the xy plane. The display surface 20A is provided with a rectangular signal light region 20S for displaying the signal light pattern and a ring-shaped reference light region 20R for displaying the reference light pattern. In the present embodiment, the signal light region 20S is a fixed region that is fixedly arranged. On the other hand, the reference light region 20R is a variable region whose position is translated (slided) to a different position on the xy plane for each page. The shape of the signal light region 20S and the shape of the reference light region 20R can be changed as appropriate according to the recording pattern.

  On the light transmission side of the spatial light modulator 20, a pair of lenses 22 and 26, and a Fourier transform lens 28 that Fourier-transforms incident light and condenses it on the optical recording medium 30, the optical path from the spatial light modulator 20 side. Are arranged in this order. The signal light and reference light generated by the spatial light modulator 20 are incident on the lens 22. Between the lens 22 and the lens 26, a light shielding plate 24 having an opening (aperture) 24A is disposed in the vicinity of the beam waist. In the present embodiment, the light shielding plate 24 is not essential and can be omitted as appropriate.

  A holding stage (not shown) for holding the optical recording medium 30 is provided on the light emitting side of the Fourier transform lens 28. The holding stage is driven by a driving device (not shown) connected to the control device 36 and moves in the optical axis direction or a surface direction perpendicular to the optical axis. For example, the holding stage holds the optical recording medium 30 at a position where the vicinity of the surface of the optical recording medium 30 is a focal position of the Fourier transform lens 28.

  The optical recording medium 30 is an optical recording medium capable of recording a hologram by a change in refractive index due to light irradiation. Examples of such an optical recording medium 30 include an optical recording medium using a recording material such as a photopolymer material, a photorefractive material, or a silver salt photosensitive material.

  On the light transmission side of the optical recording medium 30, a Fourier transform lens 32 and a sensor array 34 as a photodetector are arranged in this order along the optical path from the optical recording medium 30 side. The Fourier transform lens 32 is a Fourier transform lens that forms an image on the sensor array 34 by performing inverse Fourier transform on incident light. The sensor array 34 is composed of an image sensor such as a CCD or a CMOS array, converts the received reproduction light (diffracted light) into an electrical signal and outputs it. The sensor array 34 is connected to the control device 36. The sensor array 34 captures a reproduced image formed on the light receiving surface and outputs the obtained image data to the control device 36.

(Hologram recording operation)
Next, the recording operation of the hologram recording / reproducing apparatus shown in FIG. 1 will be briefly described.
When recording a hologram, the shutter 12 is opened and laser light is emitted from the light source 10. At the same time, digital data is output from the control device 36 at a predetermined timing, and a recording pattern (see FIG. 2) is displayed on the spatial light modulator 20. The laser light oscillated from the light source 10 passes through the shutter 12, and the light intensity and the polarization direction are adjusted by the half-wave plate 14 and the polarizing plate 16. The light that has passed through the polarizing plate 16 is converted into a large-diameter parallel light by the beam expander 18 and irradiated to the spatial light modulator 20.

  In the spatial light modulator 20, the laser light is modulated in accordance with the displayed pattern, and signal light and reference light are generated. The recording light modulated by the spatial light modulator 20, that is, the signal light and the reference light are collected by the lens 22 and applied to the light shielding plate 24. In the recording light condensed by the lens 22, unnecessary frequency components are cut by the light shielding plate 24, and the remaining part passes through the aperture 24A. The recording light that has passed through the aperture 24A is converted into parallel light by the lens 26, is subjected to Fourier transform by the Fourier transform lens 28, is condensed, and is irradiated onto the optical recording medium 30 simultaneously and coaxially. Interference fringes formed by the interference between the signal light and the reference light are recorded on the optical recording medium 30 as a hologram at a position where the signal light and the reference light are collected.

  3A to 3C are diagrams illustrating an example of a recording pattern displayed for each page when a plurality of pages are recorded. In the “reference light pattern slide multiplexing” of the present embodiment, the reference light pattern is displayed by changing the position of the reference light region 20R for each page to be recorded. That is, the same reference light pattern moves on the xy plane and is displayed at different positions on the xy plane for each page to be recorded. The phase of the reference light is modulated according to the display position of the reference light pattern. By using reference light having a different phase distribution for each page, a plurality of pages of holograms can be multiplexed and recorded at the same position. Therefore, unlike “speckle multiplexing”, it is not necessary to change the reference light pattern in order to modulate the phase of the reference light, and the intensity ratio between the signal light and the reference light is kept constant. Further, unlike “shift multiplexing”, it is not necessary to move the optical recording medium relative to the recording light for each page to be recorded.

  For example, as shown in FIG. 3A, when the first page is recorded, the signal light pattern is displayed in the signal light region 20S that is fixedly arranged and is symmetric with respect to the center point of the display surface 20A. Thus, the ring-shaped reference light region 20R is arranged, and the reference light pattern is displayed on the reference light region 20R. As shown in FIG. 3B, when recording the second page, the signal light pattern is displayed in the fixedly arranged signal light region 20S, and the ring-shaped reference light region 20R is drawn with respect to the center point. The reference light pattern is displayed in the reference light region 20R that has been slid so as to move to the lower left and moved to the lower left. As shown in FIG. 3C, when recording the third page, the signal light pattern is displayed in the fixedly arranged signal light region 20S, and the ring-shaped reference light region 20R is drawn with respect to the center point. The reference light pattern is displayed in the reference light region 20R that has been slid so as to move to the upper right and moved to the upper right.

  Further, in “reference light pattern slide multiplexing”, the hologram can be recorded / reproduced without moving the optical recording medium relative to the recording light (without moving the medium) every time one page is processed during reproduction or recording. It is also possible to aim for faster playback and recording.

  Here, with reference to FIGS. 4A and 4B, the reason why the phase of the reference light is modulated in accordance with the display position of the reference light pattern will be described. As shown in FIG. 4A, it is assumed that the reference light region 20R is moved a distance a in the x-axis direction and a distance b in the y-axis direction while the position of the signal light region 20S is fixed. Accordingly, the reference light pattern r (x, y) is shifted by the distance a in the x-axis direction and is shifted by the distance b in the y-axis direction.

As shown in FIG. 4B, the reference light generated based on the reference light pattern is Fourier transformed by the Fourier transform lens 28. A Fourier transform image is formed on the Fourier transform plane represented by the ω _x ω _y plane. For example, when a periodic pattern such as a checker pattern is used as the reference light pattern, the Fourier transform image of the reference light becomes a diffraction pattern due to the periodicity of the reference light, and the 0th to nth order diffraction components have. The order here is ω _x axis and ω _y axis when the focal length of the Fourier transform lens is f _s , the recording wavelength is λ, and the pixel pitch of the spatial light modulator that generated the signal light is d / 2. The order of bright spots that appear at every distance of ζ = f _s λ / d from the 0th order, which is the intersection of.

A Fourier transform image F [r (x, y)] obtained by subjecting the reference light of the pattern r (x, y) to Fourier transform has the following in which the spatial frequency in the x-axis direction is μ and the spatial frequency in the y-axis direction is ν: It is represented by Formula (1).

As described above, the shift amount (shift amount) of the display position of the reference light pattern is a in the x-axis direction and b in the y-axis direction. According to the Fourier transform shift theorem that the shift in the real space is equivalent to the phase shift in the frequency space, the following relational expression (2) is established in the case of a Fourier transform image having 0th to nth diffraction components. To do. There are a plurality of spatial frequencies μ and ν depending on the order. In this case, the spatial frequency in the x-axis direction is _{n from} μ ₁ to μ n, and the spatial frequency in the y-axis direction is _{n from} ν _{1 to} ν _n . As the randomness of the reference light pattern increases, higher-order diffraction components increase.

In the case of a Fourier transform image having 0th to 1st diffraction components (i = 1), when the reference light pattern is deviated by a in the x-axis direction and b in the y-axis direction, the reference light is derived from the above equation (2). It can be seen that the phase of the first-order diffraction component of is shifted by | 2π (μ ₁ a + ν ₁ b) |. The amount of phase shift depends on the spatial frequencies μ ₁ and ν ₁ as well as the distances a and b. Therefore, when there are a plurality of spatial frequency components included in the reference light pattern, even if the shift amounts a and b are constant, the phase amount to be changed differs depending on the spatial frequency components. In the 0th-order diffraction component of the reference light, since μ ₀ = 0 and ν ₀ = 0, the phase does not change regardless of the shift amounts a and b. The intensity distribution is represented by the square of the amplitude component R (μ, ν) and is substantially constant.

  As a result, when the display position of the reference light pattern is shifted, another reference light having a low phase correlation with the reference light generated from the reference light pattern before the display position is shifted is generated. Using this reference light having a low phase correlation, a plurality of pages of holograms can be multiplexed and recorded at the same position on the optical recording medium with sufficiently small crosstalk. In FIG. 4B, the 0th to 2nd diffraction components are shown. The 0th to 1st diffraction components exist in the range surrounded by the dotted line.

(Reproduction operation of hologram)
When reproducing the signal light from the hologram recorded on the optical recording medium 30, the shutter 12 is opened and the laser light is emitted from the light source 10. At the same time, digital data for displaying the reference light pattern is output from the control device 36 at a predetermined timing, and the optical recording medium 30 is irradiated with only the reference light, which is the same as when recording with the spatial light modulator 20. The reference light region 20R is arranged at the position, and the same reference light pattern as that during recording is displayed on the reference light region 20R.

  FIGS. 5A to 5C are diagrams illustrating an example of a reproduction pattern displayed when each page recorded by “reference light pattern slide multiplexing” is reproduced. As shown in FIG. 3A, when recording the first page, a ring-shaped reference light region 20R is arranged so as to be symmetric with respect to the center point of the display surface 20A, and this reference light region. The reference light pattern was displayed on 20R. When the first page recorded using the reference light generated from the reference light pattern is reproduced, as shown in FIG. 5A, the reference light region 20R is arranged at the same position as in FIG. Then, the same reference light pattern as that during recording is displayed in the reference light region 20R.

  Similarly, as shown in FIG. 5B, when reproducing the second page, the reference light region 20R is arranged at the same position as in FIG. 3B, and the recording is performed in this reference light region 20R. The same reference light pattern is displayed. Further, as shown in FIG. 5C, when reproducing the third page, the reference light region 20R is arranged at the same position as in FIG. 3C, and the same as when recording in the reference light region 20R. Display the reference light pattern. In this way, each of a plurality of pages that are multiplexed and recorded by “reference light pattern slide multiplexing” can be reproduced separately.

  The laser light oscillated from the light source 10 enters the spatial light modulator 20 in the same manner as in recording. In the spatial light modulator 20, the laser light is modulated in accordance with the displayed pattern, and reference light is generated. The generated reference light is applied to the area where the hologram of the optical recording medium 30 is recorded, as in the case of recording. That is, only the reference light is irradiated to the optical recording medium 30 as readout light.

  The irradiated reference light is diffracted by the hologram when passing through the optical recording medium 30, and transmitted diffracted light (reproduced signal light) is emitted to the Fourier transform lens 32 side. Some reference light passes through the optical recording medium 30 without being diffracted. The reproduction signal light and the transmitted reference light emitted from the optical recording medium 30 are subjected to inverse Fourier transform by the Fourier transform lens 32 and enter the sensor array 34.

  The sensor array 34 converts the received reproduction light (diffracted light) into an electrical signal and outputs it. That is, the sensor array 34 captures a reproduced image formed on the light receiving surface and outputs the obtained image data to the control device 36. In the control device 36, the digital data superimposed on the signal light is decoded based on the input image data. The sensor array 34 preferably performs oversampling in which one pixel of the signal light data is received by a plurality of light receiving elements. For example, 1-bit data is received by four (2 × 2) light receiving elements.

<Second Embodiment>
In the second embodiment, the direct current component is removed from the reference light whose phase is modulated in accordance with the display position of the reference light pattern, and multiple-page holograms are multiplexed and recorded by “reference light pattern slide multiplexing”. explain.

(Hologram recording / reproducing device)
FIG. 6 is a schematic diagram showing a configuration of a hologram recording / reproducing apparatus according to the second embodiment. This hologram recording / reproducing apparatus is the same as the hologram recording / reproducing apparatus according to the first embodiment shown in FIG. 1 except that a mask 44 is provided as a “removal element” for removing the direct current component of the reference light in the frequency space. Since it is a structure, the same code | symbol is attached | subjected to the same component and description is abbreviate | omitted.

  The mask 44 is disposed at a position where the reference light is collected by the lens 22. In the example shown in FIG. 6, the mask 44 is disposed in the vicinity of the aperture 24 </ b> A of the light shielding plate 24. Further, the mask 44 is driven by a driving device 46 connected to the control device 36, and is configured so that it can be inserted into or retracted from the optical path of the reference light. As the mask 44, a reflection mirror that selectively reflects a direct current component, an absorbing member that selectively absorbs the direct current component, an ND filter that selectively attenuates the direct current component, or the like can be used.

(Hologram recording operation)
When recording a hologram, the control device 36 controls the drive device 46 to drive the mask 44 and insert the mask 44 into the optical path of the reference light. Further, the shutter 12 is opened and the laser light is emitted from the light source 10. At the same time, digital data is output from the control device 36 at a predetermined timing, and a recording pattern (see FIG. 2) is displayed on the spatial light modulator 20.

  The laser light oscillated from the light source 10 enters the spatial light modulator 20 in the same manner as in the first embodiment. In the spatial light modulator 20, the laser light is modulated in accordance with the displayed pattern, and signal light and reference light are generated. The recording light modulated by the spatial light modulator 20, that is, the signal light and the reference light are collected by the lens 22 and irradiated to the light shielding plate 24 and the mask 44. The recording light collected by the lens 22 has an unnecessary frequency component cut by the light shielding plate 24, a direct current component is removed by the mask 44, and the remaining portion passes through the aperture 24A.

  The recording light that has passed through the aperture 24A is converted into parallel light by the lens 26, is subjected to Fourier transform by the Fourier transform lens 28, is condensed, and is irradiated onto the optical recording medium 30 simultaneously and coaxially. Interference fringes formed by the interference between the signal light and the reference light are recorded on the optical recording medium 30 as a hologram at a position where the signal light and the reference light are collected.

  The reference lights having different phases obtained by changing the display position of the reference light pattern have a common DC component because the zero-order diffraction component does not shift in phase. This common DC component causes crosstalk. If there is crosstalk between the reference beams, a noise grating is formed during recording. Further, the S / N of the reproduced image is deteriorated during reproduction. Therefore, in order to suppress this crosstalk, it is desirable to remove the direct current component of the reference light during recording.

(Reproduction operation of hologram)
When the original signal light is reproduced from the hologram recorded on the optical recording medium 30, the control device 36 controls the driving device 46 to drive the mask 44 and retract the mask 44 from the optical path of the reference light. In addition, the luminance value of the display image for supplementing the reproduced diffracted light with a DC component is calculated in advance and stored in a storage unit (not shown) of the control device 36. At the time of reproduction, the shutter 12 is opened and laser light is emitted from the light source 10.

  At the time of reproduction, the shutter 12 is opened and laser light is emitted from the light source 10. At the same time, digital data for displaying the reference light pattern is output from the control device 36 at a predetermined timing. Further, the luminance value calculated from the control device 36 is output at a predetermined timing. The reference light region 20R is arranged at the same position as that during recording of the spatial light modulator 20, and the same reference light pattern as that during recording is displayed in the reference light region 20R. Further, the signal light region 20S is arranged at the same position as when recording by the spatial light modulator 20, and a transmission pattern having the calculated luminance value is displayed in the signal light region 20S. In the transmission pattern, the luminance value of each pixel is constant.

  For example, as shown in FIG. 7A, at the time of recording, a signal light pattern is displayed in a fixedly arranged signal light region 20S, and the ring-shaped reference light is symmetric with respect to the center point of the display surface 20A. An area 20R is arranged, and a reference light pattern is displayed on the reference light area 20R (pattern example 1 at the time of recording). In this case, at the time of reproduction, as shown in FIG. 7B, a transmission pattern is displayed in the signal light region 20S that is fixedly arranged, and a ring-shaped reference light region 20R is arranged at the same position as in recording, The same reference light pattern as that during recording is displayed in this reference light region 20R (pattern example 1 during reproduction).

  Further, as shown in FIG. 7C, during recording, the signal light pattern is displayed in the signal light region 20S fixedly arranged, and the ring-shaped reference light region 20R is moved to the upper right in the drawing with respect to the center point. The reference light pattern is displayed on the reference light region 20R moved to the upper right (pattern example 2 during recording). In this case, at the time of reproduction, as shown in FIG. 7D, a transmission pattern is displayed in the signal light area 20S that is fixedly arranged, and a ring-shaped reference light area 20R is arranged at the same position as in recording, The same reference light pattern as that during recording is displayed in the reference light region 20R (pattern example 2 during reproduction).

  The laser light oscillated from the light source 10 enters the spatial light modulator 20 in the same manner as in recording. In the spatial light modulator 20, the laser light is modulated in accordance with the displayed pattern, and the reference light and the direct-current component to be applied are generated. The generated reference light and the applied direct current component are applied to the area where the hologram of the optical recording medium 30 is recorded, as in the case of recording. That is, the optical recording medium 30 is irradiated with the reference light and the applied direct current component as readout light.

  The irradiated reference light is diffracted by the hologram when passing through the optical recording medium 30, and the transmitted diffracted light is emitted to the Fourier transform lens 32 side. The applied DC component passes through the optical recording medium 30 without being diffracted. The reproduction of the original signal light pattern is realized as a result of interference between the diffracted light from the hologram and the applied DC component. That is, the diffracted light and the direct current are increased so that the amplitude of the interference wave (synthetic light) increases or the phase of the diffracted light from the hologram matches the phase of the applied direct current component (phase difference = 0 or π). By setting the phase difference with the component, an image reflecting the original signal light pattern is reproduced. When a positive reproduction image is obtained, the phase difference is set to 0. When a negative reproduction image is obtained, the phase difference is set to π. The phase of the direct current component can be set by appropriately changing the luminance value of the signal light pixel of the spatial light modulator 20.

  The diffracted light from the hologram and the applied direct current component are subjected to inverse Fourier transform by the Fourier transform lens 32 and are incident on the sensor array 34. A reproduced image can be observed on the focal plane of the Fourier transform lens 32. In this embodiment, since the direct current component removed during hologram recording is added to the diffracted light, the diffracted light having the same component as the original signal light is restored, and the original signal light pattern (positive image) is reproduced. The

  The calculation of the luminance value of the transmission pattern is performed according to the following procedure. 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 determined by 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 luminance of the pixel of the spatial light modulator 20 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 applied DC component satisfies the following condition.

  That is, the reproduced image from the hologram is detected by the sensor array 34. 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 sensor array 34. 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 (restore the original signal light), 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 becomes positive, that is, the following equation (3) is satisfied.

  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 maximum amplitude of the direct current component, and the positive amplitude of the combined light is maximized.

<Third Embodiment>
In the third embodiment, a direct-current component included in reference light whose phase is modulated according to the display position of the reference light pattern is diffused, and holograms of a plurality of pages are multiplexed and recorded by “reference light pattern slide multiplexing”. The case will be described.

(Hologram recording / reproducing device)
FIG. 8 is a schematic diagram showing the configuration of a hologram recording / reproducing apparatus according to the third embodiment. This hologram recording / reproducing apparatus is the first embodiment shown in FIG. 1 except that a phase mask 48 is provided as a “phase adjusting element” that diffuses a direct current component by imparting a phase distribution to reference light in real space. Since the configuration is the same as that of the hologram recording / reproducing apparatus according to FIG.

  The phase mask 48 is disposed on the focal plane (focal plane) between the lens 26 and the Fourier transform lens 28. As the phase mask 48, a phase modulation filter such as a random phase mask that causes a spatially random phase shift can be used.

(Hologram recording operation)
When recording a hologram, the shutter 12 is opened and laser light is emitted from the light source 10. At the same time, digital data is output from the control device 36 at a predetermined timing, and a recording pattern is displayed on the spatial light modulator 20.

  The laser light oscillated from the light source 10 enters the spatial light modulator 20 in the same manner as in the first embodiment. In the spatial light modulator 20, the laser light is modulated in accordance with the displayed pattern, and signal light and reference light are generated. The recording light modulated by the spatial light modulator 20, that is, the signal light and the reference light are collected by the lens 22 and applied to the light shielding plate 24. In the recording light condensed by the lens 22, unnecessary frequency components are cut by the light shielding plate 24, and the remaining part passes through the aperture 24A.

  The recording light that has passed through the aperture 24A is converted into parallel light by the lens 26 and is phase-modulated through the phase mask 48. The recording light that has passed through the phase mask 48, that is, the signal light and the reference light, is subjected to Fourier transform by the Fourier transform lens 28 and collected, and is irradiated onto the optical recording medium 30 simultaneously and coaxially. Interference fringes formed by the interference between the signal light and the reference light are recorded on the optical recording medium 30 as a hologram at a position where the signal light and the reference light are collected.

  In “reference light pattern slide multiplexing”, since the display position of the reference light pattern on the spatial light modulator 20 is different for each page to be recorded, the position of the phase mask 48 through which the reference light passes is also different for each page. Therefore, the direct current component diffused by the phase mask 48 also varies from page to page. Therefore, by diffusing the DC component with the phase mask 48, the position where the optical recording medium 30 is irradiated with the 0th-order diffraction component also changes, and multiplex recording can be realized with sufficiently low crosstalk. It is not necessary to remove the DC component during recording. Further, the phase modulation by the phase mask 48 improves the overlap between the signal light and the reference light, makes the light intensity distribution of the recording light uniform, and can effectively use the dynamic range of the optical recording medium.

(Reproduction operation of hologram)
When reproducing the signal light from the hologram recorded on the optical recording medium 30, the shutter 12 is opened and the laser light is emitted from the light source 10. At the same time, digital data for displaying the reference light pattern is output from the control device 36 at a predetermined timing, and the optical recording medium 30 is irradiated with only the reference light, which is the same as when recording with the spatial light modulator 20. The reference light region 20R is arranged at the position, and the same reference light pattern as that during recording is displayed on the reference light region 20R.

  The laser light oscillated from the light source 10 enters the spatial light modulator 20 in the same manner as in the first embodiment. In the spatial light modulator 20, the laser light is modulated in accordance with the displayed pattern, and reference light is generated. The generated reference light is collected by the lens 22 and irradiated to the light shielding plate 24. In the reference light collected by the lens 22, unnecessary frequency components are cut by the light shielding plate 24, and the remaining part passes through the aperture 24A.

  The reference light that has passed through the aperture 24A is converted into parallel light by the lens 26 and phase-modulated by the phase mask 48. The light phase-modulated by the phase mask 48 is subjected to Fourier transform by the Fourier transform lens 28 and collected, and is irradiated onto the area where the hologram of the optical recording medium 30 is recorded, as in the first embodiment. . That is, only the reference light is irradiated to the optical recording medium 30 as readout light.

  The irradiated reference light is diffracted by the hologram when passing through the optical recording medium 30, and transmitted diffracted light (reproduced light) is emitted to the Fourier transform lens 32 side. The reproduction light emitted from the optical recording medium 30 is subjected to inverse Fourier transform by the Fourier transform lens 32 and enters the sensor array 34. In this way, the reproduced image from the hologram is detected by the sensor array 34.

  In the first to third embodiments, preferred embodiments in which “reference light pattern slide multiplexing” is performed without moving the medium will be described. However, the display position of the reference light pattern is changed, and the recording light A plurality of pages of holograms may be multiplex-recorded at different positions by relatively moving the recording medium.

<Fourth embodiment>
In the fourth embodiment, in the collinear method, a liquid crystal type spatial light modulator is used by using reference light whose phase is modulated in accordance with the display position of the reference light pattern in the spatial light modulator. Without limitation, a recording / reproducing method for recording and reproducing a hologram so that two reproduced images of a positive image and a negative image can be obtained from one hologram will be described.

(Hologram recording / reproducing device)
The hologram recording / reproducing apparatus according to the fourth embodiment has the same configuration as the hologram recording / reproducing apparatus according to the first embodiment shown in FIG. 1 except that the aperture diameter of the aperture 24A of the light shielding plate 24 is controlled. Therefore, the description is omitted except for the different components.

  The aperture 24A of the light shielding plate 24 transmits the 0th-order and 1st-order diffraction components (portions surrounded by dotted lines) in the Fourier transform image shown in FIG. 4B and shields the second-order and higher-order diffraction components. Thus, it is formed with a predetermined opening diameter. By shielding the second and higher order diffraction components, the optical recording medium 30 is irradiated with only the zeroth and first order diffraction components. The spatial frequency μ in the x-axis direction and the spatial frequency ν in the y-axis direction are each one, and as described later, the phase difference between the reference light and the applied DC component can be easily controlled. In addition, the recording area is reduced, and the recording density is improved.

(Hologram recording operation)
When recording a hologram, a recording pattern is displayed on the spatial light modulator 20. For example, as shown in FIG. 9A, the signal light pattern is displayed in the fixedly arranged signal light region 20S, and the ring-shaped reference light region 20R is made symmetrical with respect to the center point of the display surface 20A. The reference light pattern is displayed in the reference light region 20R. Since only the 0th and 1st order diffraction components of the reference light are used, it is preferable to use a periodic pattern such as a checker pattern as the reference light pattern.

  As in the first embodiment, the spatial light modulator 20 is irradiated with laser light. In the spatial light modulator 20, the laser light is modulated in accordance with the displayed recording pattern, and signal light and reference light are generated. The recording light modulated by the spatial light modulator 20, that is, the signal light and the reference light are collected by the lens 22 and applied to the light shielding plate 24. The recording light collected by the lens 22 has second-order or higher-order diffracted light components cut by the light-shielding plate 24, and the zeroth-order and first-order diffracted components pass through the aperture 24A.

  The recording light that has passed through the aperture 24A is converted into parallel light by the lens 26, is subjected to Fourier transform by the Fourier transform lens 28, is condensed, and is irradiated onto the optical recording medium 30 simultaneously and coaxially. Interference fringes formed by the interference between the signal light and the reference light are recorded on the optical recording medium 30 as a hologram at a position where the signal light and the reference light are collected.

(Reproduction operation of hologram)
In the present embodiment, two reproduced images, a positive image and a negative image, are detected from one recorded Fourier transform hologram. First, when the same positive image as the original signal light pattern is reproduced, the shutter 12 is opened and the light source 10 emits laser light. At the same time, digital data for displaying the reference light pattern is output from the control device 36 at a predetermined timing, and as shown in FIG. 9B, only the reference light is irradiated onto the optical recording medium 30. The reference light region 20R is arranged at the same position as that during recording of the spatial light modulator 20, and the same reference light pattern as that during recording is displayed in the reference light region 20R.

  The laser light oscillated from the light source 10 enters the spatial light modulator 20 in the same manner as in the first embodiment. In the spatial light modulator 20, the laser light is modulated in accordance with the displayed pattern, and reference light is generated. The reference light generated by being modulated by the spatial light modulator 20 is collected by the lens 22, and only the 0th and 1st order diffraction components pass through the aperture 24 </ b> A and are converted into parallel light by the lens 26. The reference light converted into parallel light by the lens 26 is subjected to Fourier transform by the Fourier transform lens 28, collected, and applied to the optical recording medium 30. That is, only the reference light is irradiated to the optical recording medium 30 as readout light.

  The reference light applied to the optical recording medium 30 is diffracted by the hologram when passing through the optical recording medium 30, and transmitted diffracted light (reproduced light) is emitted to the lens 32 side. The reproduction light emitted from the optical recording medium 30 is converted into parallel light by the lens 32 and imaged on the surface of the sensor array 34. The original signal light pattern displayed on the spatial light modulator 20 during recording, that is, a positive image of the original signal light pattern (hereinafter referred to as “positive image”) is formed on the sensor array 34. The sensor array 34 detects this reproduction light, and a first reproduction image (positive image) is acquired.

  Next, when reproducing a negative image in which the original signal light pattern is reversed, the shutter 12 is opened and the laser beam is emitted from the light source 10. At the same time, digital data for displaying the reference light pattern is output from the control device 36 at a predetermined timing, and as shown in FIG. 9C, the ring-shaped reference light region 20R of the spatial light modulator 20 is centered. The point is slid so as to move to the upper right on the drawing, and the reference light pattern is displayed in the reference light region 20R moved to the upper right.

  For example, the optical recording medium 30 is made of a photopolymer, and the shift amount (shift amount) of the display position of the reference light pattern is set to a in the x-axis direction and b in the y-axis direction, as shown in FIG. The case will be described. In this case, the display position of the reference light pattern is shifted so that the phase shift amount with respect to the reference light used for reproducing the positive image is | 2π (μa + νb) | = π / 2. At the same time, a transmission pattern having a predetermined luminance value is displayed in the signal light region 20S fixedly arranged. In the transmission pattern, the luminance value of each pixel is constant.

  The laser light oscillated from the light source 10 enters the spatial light modulator 20 in the same manner as in the first embodiment. In the spatial light modulator 20, the laser light is modulated in accordance with the displayed pattern, and the reference light and the direct-current component to be applied are generated. The generated reference light and the applied direct current component are collected by the lens 22, and only the 0th and 1st order diffraction components pass through the aperture 24 </ b> A and are converted into parallel light by the lens 26. The reference light converted into parallel light by the lens 26 is subjected to Fourier transform by the Fourier transform lens 28 and collected, and is irradiated onto the area where the hologram of the optical recording medium 30 is recorded. That is, the optical recording medium 30 is irradiated with the reference light and the applied direct current component as readout light.

  The reference light applied to the optical recording medium 30 is diffracted by the hologram when passing through the optical recording medium 30, and transmitted diffracted light (reproduced light) is emitted to the Fourier transform lens 32 side. The applied DC component passes through the optical recording medium 30 without being diffracted. Reproduction of the inverted image of the original signal light pattern is realized as a result of interference due to the antiphase between the diffracted light from the hologram and the applied DC component. That is, an inverted image of the original signal light pattern is reproduced by setting the phase difference between the diffracted light and the direct current component so that the diffracted light from the hologram and the applied direct current component interfere with each other in opposite phases. . The phase of the direct current component can be set by appropriately changing the luminance value of the signal light pixel of the spatial light modulator 20.

  The diffracted light from the hologram and the applied direct current component are subjected to inverse Fourier transform by the Fourier transform lens 32 and are incident on the sensor array 34. An inverted pattern of the original signal light pattern displayed on the spatial light modulator 20 during recording, that is, a negative image of the original signal light pattern (hereinafter referred to as “negative image”) is formed on the sensor array 34. . The sensor array 34 detects this reproduction light, and a second reproduction image (negative image) is acquired.

  As described above, in this embodiment, two reproduced images of a positive image and a negative image can be detected from one Fourier transform hologram. The sensor array 34 converts the detected reproduction signal light into an electrical signal and outputs it to the control device 36. That is, the sensor array 34 performs A / D conversion on the detected analog data, and outputs the image data of the first reproduced image and the image data of the second reproduced image to the control device 36. The image data of the first reproduced image and the image data of the second reproduced image are held in a storage unit such as a RAM of the control device 36.

(Principle of negative image generation)
Here, as shown in FIG. 9C, the ring-shaped reference light region 20R of the spatial light modulator 20 is moved, and the reference light pattern is displayed on the moved reference light region 20R and fixedly arranged. The reason why a negative image can be obtained by displaying a transmission pattern having a predetermined luminance value in the signal light region 20S will be described.

  The phase difference between the DC component to be applied and the reference light can be controlled by changing the display position of the reference light pattern. In the present embodiment, the spatial frequency μ in the x-axis direction and the spatial frequency ν in the y-axis direction are each one type. As shown in FIG. 4A, the reference light pattern used to reproduce the positive image when the reference light pattern display position shift amount (shift amount) is a in the x-axis direction and b in the y-axis direction. The amount of phase shift from the above becomes | 2π (μa + νb) |.

  On the other hand, the position where the applied DC component is displayed is unchanged. Further, the phase of the DC component is constant. Therefore, by changing the display position of the reference light pattern, the phase difference between the applied DC component and the reference light can be controlled. This means the following: That is, the phase difference between the direct-current component and the higher-order component included in the light emitted from the hologram forming the reproduced image can be controlled by the display position of the reference light pattern.

  As described above, the phase of the diffracted light from the hologram deviates from the phase of the reference light used during reproduction. 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. Hereinafter, a case where a photopolymer is used for the optical recording medium 30 will be described.

  Assume that the display position of the reference light pattern is changed so that the phase shift amount with respect to the reference light used for reproducing the positive image satisfies | 2π (μa + νb) | = π / 2. Due to this reference light, higher-order diffracted light is emitted from the hologram. When a photopolymer is used, according to the coupled wave theory, a phase shift occurs by π / 2 in the diffracted light when diffracting from the hologram. On the other hand, the applied DC component only transmits the hologram. Therefore, the phase difference between the applied DC component and the higher order component diffracted from the hologram is π. That is, the applied DC component and the higher-order component diffracted from the hologram are in opposite phases. As a result, the reproduced image becomes a reverse image.

  In the above example, since a photopolymer is used for the optical recording medium 30, the phase shift imparted by diffraction is π / 2. However, for any type of hologram, from the imparted DC component and the hologram By selecting the display position of the reference light pattern so that the phase difference from the diffracted higher-order component becomes π, the inverted image can be reproduced.

(Data decryption process)
Next, a “decoding process” for decoding digital data superimposed on the reproduced signal light from the image data of the first reproduced image and the second reproduced image will be described. This process is executed by the control device 36. As described above, the control device 36 receives and holds the image data of the first reproduced image and the image data of the second reproduced image. Using these two image data, a luminance difference is calculated for each pixel of the signal light pattern. By calculating the difference, noise common to both image data is canceled. Based on the calculated difference value (positive or negative), the sign of each pixel is accurately determined, and the original digital data is decoded with a high SNR. Also, when the original digital data is decoded with a high SNR, the bit error rate (BER) also decreases.

  For example, when the binary digital data “1, 0” is a signal light pattern expressed in “bright, dark”, the second reproduced image (negative image) is obtained from the image data of the first reproduced image (positive image). ) Is subtracted to calculate a luminance difference for each pixel of the signal light pattern (bright and dark image). The second reproduced image (negative image) is an inverted image of the original light / dark image, and the bright portion of the original light / dark image is dark and the dark portion is bright. Therefore, the difference value when the luminance of the second reproduced image (negative image) is subtracted from the luminance of the first reproduced image (positive image) is positive in the bright part of the original bright and dark image, and in the dark part of the original bright and dark image. Become negative. When the code “positive, negative” of the calculated difference value is decoded as “1, 0”, the original digital data can be decoded with a high SNR.

  Further, the image data of the second reproduction image (negative image) is subtracted from the image data of the first reproduction image (positive image) to generate a third reproduction image (positive image), and this third reproduction image is used. Decoding processing can also be performed. When the third reproduced image (positive image) is generated by setting the sign “positive, negative” of the calculated difference value as “bright, dark”, the first reproduced image, The contrast is emphasized more than each of the second reproduced images. If this third reproduced image (positive image) is used, the original digital data can be decoded with a high SNR.

  In the above example, the example in which the image data of the second reproduced image (negative image) is subtracted from the image data of the first reproduced image (positive image) has been described. It is also possible to calculate the difference value of the luminance by subtracting the image data of the first reproduced image (positive image) from the image data of the negative image). The luminance difference value is negative in the bright part of the original bright / dark image and positive in the dark part of the original bright / dark image. When the code “positive, negative” of the calculated difference value is decoded as “0, 1”, the original digital data can be decoded with a high SNR. Alternatively, a third reproduced image (positive image) is generated with the calculated difference value code “positive, negative” set to “dark, light”, and the original digital data is decoded with a high SNR using the third reproduced image. can do.

  In addition, when the above-described oversampling is performed, one pixel of the signal light data corresponds to a plurality of pixels (a plurality of light receiving elements) of the sensor array. The luminance difference value is calculated. That is, the average value of the luminance difference values is calculated for a plurality of pixels of the corresponding sensor array.

<Fifth embodiment>
In the fifth embodiment, a reflective spatial light modulator such as a reflective liquid crystal panel (LCOS) or a digital micromirror device (DMD) is used in the collinear method, and this spatial light modulation is performed. Using the reference light whose phase is modulated according to the display position of the reference light pattern in the device, the hologram is recorded and reproduced so that two reproduced images of a positive image and a negative image can be obtained from one hologram A recording / reproducing method will be described.

(Hologram recording / reproducing device)
FIG. 10 is a schematic diagram showing the configuration of a hologram recording / reproducing apparatus according to the fifth embodiment. This hologram recording / reproducing apparatus irradiates an optical recording medium with a signal beam having a common optical axis and a reference light as a single beam of recording light from the same direction as a “coaxial recording method (collinear method)” hologram recording / reproducing apparatus. It is. In the present embodiment, a “coaxial reflection type” hologram recording / reproducing apparatus using a reflection type spatial light modulator (SLM) and a reflection type optical recording medium will be described.

  The hologram recording / reproducing apparatus is provided with a light source 50 that oscillates laser light that is S-polarized coherent light. On the laser beam emission side of the light source 50, a shutter 52 that can be inserted into or retracted from the optical path (open / close), a half-wave plate 54 that gives a half-wavelength optical path difference between orthogonal linearly polarized light components, a predetermined wavelength A polarizing plate 56 that transmits light in the polarization direction, a beam expander 58 that is an expansion / collimating optical system, and a polarizing beam splitter 60 that transmits only polarized light in a predetermined direction and reflects other polarized light are provided on the light source 50 side. Are arranged in this order along the optical path. In the following description, it is assumed that the polarizing beam splitter 60 transmits P-polarized light and reflects S-polarized light.

  The light source 50 is driven by the driving device 84 and oscillates. The drive device 84 is connected to a control device 82 configured by a personal computer having a CPU, ROM, RAM, storage device, and the like, and is controlled by the control device 82. The shutter 52 is driven by a drive device 84 connected to the control device 82 to open and close.

  When viewed from the beam expander 58, a reflective spatial light modulator 62 that modulates incident light for each pixel is disposed on the light reflection side of the polarization beam splitter 60. As the reflective spatial light modulator 62, LCOS, DMD, or the like can be used. FIG. 10 illustrates a case where LCOS is used as the spatial light modulator 62. When a DMD is used as the spatial light modulator, light is incident from an oblique direction of the DMD reflecting surface, and the reflected light is directly incident on the lens 66. Therefore, when the DMD is used, it is not necessary to arrange the polarizing beam splitter 60 or the quarter wavelength plate 64 described later. The spatial light modulator 62 is connected to the control device 82 via a pattern generator (not shown).

  The pattern generator represents the digital data supplied from the control device 82 as a bright and dark image, and generates a signal light pattern and a reference light pattern to be displayed on the spatial light modulator 62. The signal light pattern is, for example, a digital pattern in which binary digital data “0, 1” is expressed as “dark (black pixels), light (white pixels)”. The reference light pattern is a pattern having a plurality of spatial frequency components such as a random pattern and a checker pattern. The spatial light modulator 62 modulates the incident laser light according to the displayed signal light pattern or reference light pattern, and generates signal light or reference light.

  The spatial light modulator 62 includes a display surface 20A having a rectangular shape in plan view, like the transmissive spatial light modulator 20 of the first embodiment shown in FIG. On the display surface 20A, a plurality of pixel portions are two-dimensionally arranged on the xy plane. The display surface 20A is provided with a rectangular signal light region 20S for displaying the signal light pattern and a ring-shaped reference light region 20R for displaying the reference light pattern. Also in the present embodiment, the signal light region 20S is a fixed region that is fixedly arranged as in the first embodiment. On the other hand, the reference light region 20R is a variable region whose position is translated (slided) to a different position on the xy plane for each page.

  The signal light and the reference light generated by the spatial light modulator 62 are reflected in the direction of the polarization beam splitter 60 and pass through the polarization beam splitter 60. When viewed from the spatial light modulator 62, on the light transmission side of the polarization beam splitter 60, a quarter-wave plate 64 for converting linearly polarized light into circularly polarized light or circularly polarized light into linearly polarized light, lenses 66 and 70, and Fourier transform Lenses 72 are arranged in this order along the optical path. Further, between the lens 66 and the lens 70, a light shielding plate 68 having an opening (aperture) 68A is disposed in the vicinity of the beam waist.

  The aperture 68A of the light shielding plate 68 transmits the 0th-order and 1st-order diffraction components (portions surrounded by dotted lines) in the Fourier transform image shown in FIG. 4B and shields the second-order and higher-order diffraction components. Thus, it is formed with a predetermined opening diameter. By shielding the second and higher order diffraction components, the optical recording medium 74 is irradiated with only the zeroth and first order diffraction components. The spatial frequency μ in the x-axis direction and the spatial frequency ν in the y-axis direction are each one, and the control of the phase difference between the reference light and the direct-current component to be applied becomes easy. In addition, the recording area is reduced, and the recording density is improved.

  A holding stage (not shown) for holding the optical recording medium 74 is provided on the light emitting side of the Fourier transform lens 72. The optical recording medium 74 is an optical recording medium including a reflective layer 74A and an optical recording layer 74B capable of recording a hologram by a change in refractive index due to light irradiation. The reflective layer 74A is composed of a thin film having a high reflectance obtained by vapor-depositing a metal or a semimetal. For the optical recording layer 74B, for example, a recording material such as a photopolymer material, a photorefractive material, or a silver salt photosensitive material is used.

  The holding stage is driven by a driving device (not shown) connected to the control device 82 and moves in the optical axis direction or a plane direction perpendicular to the optical axis. For example, the holding stage holds the optical recording medium 74 at a position where the interface between the reflective layer 74 </ b> A and the optical recording layer 74 </ b> B of the optical recording medium 74 is the focal position of the Fourier transform lens 72. The Fourier transform lens 72 performs Fourier transform on the incident light and collects it on the optical recording medium 74.

  When viewed from the quarter-wave plate 64, the light reflecting side of the polarization beam splitter 60 is constituted by an imaging device such as a CCD or a CMOS array, and converts the received reproduction light (diffracted light) into an electrical signal for output. A sensor array 80 as a photodetector is arranged. The sensor array 80 is connected to the control device 82. The sensor array 80 captures a reproduced image formed on the light receiving surface and outputs the obtained image data to the control device 82.

(Hologram recording operation)
When recording a hologram, a recording pattern is displayed on the spatial light modulator 20. For example, as shown in FIG. 9A, the signal light pattern is displayed in the fixedly arranged signal light region 20S, and the ring-shaped reference light region 20R is made symmetrical with respect to the center point of the display surface 20A. The reference light pattern is displayed in the reference light region 20R. Since only the 0th and 1st order diffraction components of the reference light are used, it is preferable to use a periodic pattern such as a checker pattern as the reference light pattern.

  When recording a hologram, the shutter 52 is opened and laser light is emitted from the light source 50. At the same time, digital data is output from the control device 82 at a predetermined timing, and a recording pattern (see FIG. 9A) is displayed on the spatial light modulator 62. The laser light oscillated from the light source 50 passes through the shutter 52, and the light intensity and the polarization direction are adjusted by the half-wave plate 54 and the polarizing plate 56. The light (S-polarized light) that has passed through the polarizing plate 56 is converted into large-diameter parallel light by the beam expander 58, reflected by the polarizing beam splitter 60, and applied to the spatial light modulator 62.

  In the spatial light modulator 62, the laser light is modulated in accordance with the displayed pattern, and signal light and reference light are generated. The recording light modulated by the spatial light modulator 62, that is, the signal light and the reference light are reflected to the polarization beam splitter 60 side, converted into P-polarized light, and incident on the polarization beam splitter 60. The P-polarized signal light and reference light are transmitted through the polarization beam splitter 60, converted into circularly polarized light by the ¼ wavelength plate 64, condensed by the lens 66, and applied to the light shielding plate 68.

  In the recording light condensed by the lens 66, the second and higher order diffracted light components are cut by the light shielding plate 68, and the zeroth and first order diffracted components pass through the aperture 68A. The recording light (signal light and reference light) that has passed through the aperture 68A is converted into parallel light by the lens 70, is subjected to Fourier transform by the Fourier transform lens 72, is condensed, and is irradiated onto the optical recording medium 74 simultaneously and coaxially. . Interference fringes formed by the interference between the signal light and the reference light are recorded on the optical recording medium 74 as a hologram at a position where the signal light and the reference light are collected.

(Reproduction operation of hologram)
In the present embodiment, two reproduced images, a positive image and a negative image, are detected from one recorded Fourier transform hologram. First, when reproducing the same positive image as the original signal light pattern, the shutter 52 is opened and laser light is emitted from the light source 50. At the same time, digital data for displaying the reference light pattern is output from the control device 82 at a predetermined timing, so that only the reference light is irradiated onto the optical recording medium 74 as shown in FIG. The reference light region 20R is arranged at the same position as that during recording of the spatial light modulator 62, and the same reference light pattern as that during recording is displayed in the reference light region 20R.

  The laser light oscillated from the light source 50 enters the spatial light modulator 62 in the same manner as at the time of recording. In the spatial light modulator 62, the laser light is modulated in accordance with the displayed pattern, and reference light is generated. The reference light generated by being modulated by the spatial light modulator 62 is reflected to the polarization beam splitter 60 side, converted into P-polarized light, and enters the polarization beam splitter 60. The P-polarized signal light and reference light are transmitted through the polarization beam splitter 60, converted into circularly polarized light by the ¼ wavelength plate 64, condensed by the lens 66, and applied to the light shielding plate 68.

  In the reference light collected by the lens 66, only the 0th-order and 1st-order diffraction components pass through the aperture 68A and are converted into parallel light by the lens. The reference light converted into parallel light by the lens 70 is subjected to Fourier transform by the Fourier transform lens 72, collected, and applied to the optical recording medium 74. That is, only the reference light is irradiated to the optical recording medium 74 as readout light.

  The reference light applied to the optical recording medium 74 is diffracted by the hologram when passing through the optical recording layer 74B, and the transmitted diffracted light (reproduced light) is reflected by the reflective layer 74A and emitted to the lens 72 side. . The reproduction light emitted from the optical recording medium 74 is subjected to inverse Fourier transform by the lens 72, relayed by the lenses 70 and 66, converted to linearly polarized light (S-polarized light) by the quarter wavelength plate 64, and the polarization beam splitter 60. Is incident on.

  The reproduction light (S-polarized light) incident on the polarization beam splitter 60 is reflected by the polarization beam splitter 60 toward the sensor array 80 and forms an image on the surface of the sensor array 80. The original signal light pattern displayed on the spatial light modulator 62 at the time of recording, that is, a positive image (hereinafter referred to as “positive image”) of the original signal light pattern is formed on the sensor array 80. The sensor array 80 detects this reproduction light, and a first reproduction image (positive image) is acquired.

  Next, when reproducing a negative image in which the original signal light pattern is reversed, the shutter 52 is opened and the laser light is emitted from the light source 50. At the same time, digital data for displaying the reference light pattern is output from the control device 82 at a predetermined timing, and the ring-shaped reference light region 20R of the spatial light modulator 62 is centered as shown in FIG. 9C. The point is slid so as to move to the upper right on the drawing, and the reference light pattern is displayed in the reference light region 20R moved to the upper right. At the same time, a transmission pattern having a predetermined luminance value is displayed in the signal light region 20S fixedly arranged.

  The laser light oscillated from the light source 50 enters the spatial light modulator 62 in the same manner as at the time of recording. In the spatial light modulator 62, the laser light is modulated according to the displayed pattern, and the reference light and the direct-current component to be applied are generated. The generated reference light and the direct-current component to be imparted are reflected to the polarization beam splitter 60 side, converted into P-polarized light, and enter the polarization beam splitter 60. The P-polarized reference light and the direct-current component imparted are transmitted through the polarization beam splitter 60, converted into circularly polarized light by the quarter-wave plate 64, condensed by the lens 66, and irradiated to the light shielding plate 68. .

  Of the reference light collected by the lens 66 and the direct-current component to be applied, only the 0th-order and 1st-order diffraction components pass through the aperture 68A and are converted into parallel light by the lens 70. The reference light converted into parallel light by the lens 70 and the applied direct current component are subjected to Fourier transform by the Fourier transform lens 72 to be condensed and irradiated onto the optical recording medium 74. That is, the optical recording medium 74 is irradiated with the reference light and the applied direct current component as readout light.

  The reference light applied to the optical recording medium 74 is diffracted by the hologram when passing through the optical recording layer 74B, and the transmitted diffracted light (reproduced light) is reflected by the reflective layer 74A and emitted to the Fourier transform lens 72 side. Is done. The applied direct current component is reflected by the reflection layer 74A without being diffracted. Reproduction of the inverted image of the original signal light pattern is realized as a result of interference between the diffracted light from the hologram and the applied DC component. That is, an inverted image of the original signal light pattern is reproduced by setting the phase difference between the diffracted light and the direct current component so that the diffracted light from the hologram and the applied direct current component interfere with each other in opposite phases. . This phase difference can be set by the amount of deviation of the display position of the reference light pattern in the spatial light modulator 62.

  The diffracted light from the hologram and the applied DC component are inverse Fourier transformed by the Fourier transform lens 72, relayed by the lenses 70 and 66, and converted to linearly polarized light (S polarized light) by the quarter wavelength plate 64. , Enters the polarization beam splitter 60. The reproduction light (S-polarized light) incident on the polarization beam splitter 60 is reflected by the polarization beam splitter 60 toward the sensor array 80 and forms an image on the surface of the sensor array 80. An inverted pattern of the original signal light pattern displayed on the spatial light modulator 62 during recording, that is, a negative image of the original signal light pattern (hereinafter referred to as “negative image”) is formed on the sensor array 80. . The sensor array 80 detects this reproduction light, and a second reproduction image (negative image) is acquired.

  Note that the reason why a negative image is obtained is as described in the fourth embodiment. When DMD is used as the reflective spatial light modulator 62, when reproducing a negative image of the hologram, each of the interferences due to the antiphase between the diffracted light from the hologram and the applied DC component is increased. It is preferable that the vibration period of the pixel portion is synchronized. For example, the swing period m (Hz) of the pixel portion of the signal light region 20S that displays the signal light pattern and the swing cycle n (Hz) of the pixel portion of the reference light region 20R that displays the reference light pattern are expressed as k · n. = M (k is an integer of 1 or more).

  Further, in the present embodiment, as in the fourth embodiment, two reproduced images of a positive image and a negative image can be detected from one Fourier transform hologram. The sensor array 80 outputs the image data of the first reproduced image and the image data of the second reproduced image to the control device 82. The image data of the first reproduced image and the image data of the second reproduced image are held in a storage unit such as a RAM of the control device 82.

  Also in the present embodiment, as in the fourth embodiment, the luminance of each pixel of the signal light pattern using the image data of the first reproduced image and the image data of the second reproduced image, and these two image data. The difference between is calculated. By calculating the difference, noise common to both image data is canceled. Based on the calculated difference value (positive or negative), the sign of each pixel is accurately determined, and the original digital data that has been two-dimensionally encoded is decoded with a high SNR. Also, when the original digital data is decoded with a high SNR, the bit error rate (BER) also decreases.

<Sixth Embodiment>
In the sixth embodiment, in the collinear method, a liquid crystal type spatial light modulator is used by using reference light whose phase is modulated in accordance with the display position of the reference light pattern in the spatial light modulator. Without limitation, a recording / reproducing method for recording and reproducing a hologram so that two reproduced images of a positive image and a negative image can be obtained from one hologram will be described.

(Hologram recording / reproducing device)
The hologram recording / reproducing apparatus according to the sixth embodiment has the same configuration as the hologram recording / reproducing apparatus according to the second embodiment shown in FIG. 6 except that the aperture diameter of the aperture 24A of the light shielding plate 24 is controlled. Therefore, the description is omitted except for the different components.

  The aperture 24A of the light shielding plate 24 transmits the 0th-order and 1st-order diffraction components (portions surrounded by dotted lines) in the Fourier transform image shown in FIG. 4B and shields the second-order and higher-order diffraction components. Thus, it is formed with a predetermined opening diameter. By shielding the second and higher order diffraction components, the optical recording medium 30 is irradiated with only the zeroth and first order diffraction components. The spatial frequency μ in the x-axis direction and the spatial frequency ν in the y-axis direction are each one, and as described later, the phase difference between the reference light and the applied DC component can be easily controlled. In addition, the recording area is reduced, and the recording density is improved.

(Hologram recording operation)
When recording a hologram, the control device 36 controls the drive device 46 to drive the mask 44 and insert the mask 44 into the optical path of the reference light. Further, the shutter 12 is opened and the laser light is emitted from the light source 10. At the same time, digital data is output from the control device 36 at a predetermined timing, and a recording pattern is displayed on the spatial light modulator 20.

  For example, as shown in FIG. 11A, the signal light pattern is displayed in the fixedly arranged signal light region 20S, and the ring-shaped reference light region 20R is symmetric with respect to the center point of the display surface 20A. The reference light pattern is displayed in the reference light region 20R. Since only the 0th and 1st order diffraction components of the reference light are used, it is preferable to use a periodic pattern such as a checker pattern as the reference light pattern.

  The laser light oscillated from the light source 10 enters the spatial light modulator 20 in the same manner as in the first embodiment. In the spatial light modulator 20, the laser light is modulated in accordance with the displayed pattern, and signal light and reference light are generated. The recording light modulated by the spatial light modulator 20, that is, the signal light and the reference light are collected by the lens 22 and irradiated to the light shielding plate 24 and the mask 44.

  In the recording light condensed by the lens 22, the second and higher order diffracted light components are cut by the light shielding plate 24, the zeroth order diffracted components are removed by the mask 44, and only the first order diffracted components pass through the aperture 24A. . The recording light (signal light and reference light) that has passed through the aperture 24A is converted into parallel light by the lens 26, is Fourier-transformed by the Fourier transform lens 28, is condensed, and is irradiated onto the optical recording medium 30 simultaneously and coaxially. . Interference fringes formed by the interference between the signal light and the reference light are recorded on the optical recording medium 30 as a hologram at a position where the signal light and the reference light are collected.

  In the present embodiment, as in the second embodiment, “reference light pattern slide multiplexing” that displays the reference light pattern by changing the position of the reference light region 20R for each page to be recorded can be performed. . That is, the same reference light pattern moves on the xy plane and is displayed at different positions on the xy plane for each page to be recorded. The phase of the reference light is modulated according to the display position of the reference light pattern. By using reference light having a different phase distribution for each page, a plurality of pages of holograms can be multiplexed and recorded at the same position.

  The reference lights having different phases obtained by changing the display position of the reference light pattern have a common DC component because the zero-order diffraction component does not shift in phase. This common DC component causes crosstalk. If there is crosstalk between the reference beams, a noise grating is formed during recording. Further, the S / N of the reproduced image is deteriorated during reproduction. In the present embodiment, since the direct current component of the reference light is removed during recording, this crosstalk can be suppressed.

(Reproduction operation of hologram)
In the present embodiment, two reproduced images, a positive image and a negative image, are detected from one recorded Fourier transform hologram. First, when the same positive image as the original signal light pattern is reproduced, the shutter 12 is opened and the light source 10 emits laser light. At the same time, digital data for displaying the reference light pattern is output from the control device 36 at a predetermined timing, and the ring-shaped reference light region 20R of the spatial light modulator 20 is centered as shown in FIG. The reference light pattern is displayed in the reference light region 20R moved to the lower left by sliding the point so as to move to the lower left on the drawing. At the same time, a transmission pattern having a predetermined luminance value is displayed in the signal light region 20S fixedly arranged.

  For example, the optical recording medium 30 is made of a photopolymer, and the shift amount (shift amount) of the display position of the reference light pattern is −a, y in the x-axis direction, contrary to that shown in FIG. A case where −b is set in the axial direction will be described. In this case, the amount of phase shift from the reference light used for recording is | 2π (μa + νb) | = π / 2, and the magnitude is the same as in FIG. The display position of the reference light pattern is shifted so that the sign of the case value is reversed.

  The laser light oscillated from the light source 10 enters the spatial light modulator 20 in the same manner as in the first embodiment. In the spatial light modulator 20, the laser light is modulated in accordance with the displayed pattern, and the reference light and the direct-current component to be applied are generated. The reference light modulated by the spatial light modulator 20 and the direct-current component to be applied are condensed by the lens 22, and only the first-order diffraction component passes through the aperture 24 </ b> A and is converted into parallel light by the lens 26. Converted. The reference light converted into parallel light by the lens 26 and the applied direct current component are subjected to Fourier transform by the Fourier transform lens 28, collected, and irradiated onto the optical recording medium 30. That is, the optical recording medium 30 is irradiated with the reference light and the applied DC component as readout light.

  The reference light applied to the optical recording medium 30 is diffracted by the hologram when passing through the optical recording medium 30, and transmitted diffracted light (reproduced light) is emitted to the Fourier transform lens 32 side. The applied DC component passes through the optical recording medium 30 without being diffracted. The restoration of the original signal light pattern is realized as a result of interference between the diffracted light from the hologram and the applied DC component. That is, the same image as the original signal light pattern is reproduced by setting the phase difference between the diffracted light and the DC component so that the diffracted light from the hologram interferes with the applied DC component in the same phase. . This phase difference can be set by the amount of shift of the display position of the reference light pattern in the spatial light modulator 20.

  The diffracted light from the hologram and the applied direct current component are subjected to inverse Fourier transform by the Fourier transform lens 32 and are incident on the sensor array 34. The same pattern as the original signal light pattern displayed on the spatial light modulator 20 during recording, that is, a positive image of the original signal light pattern is formed on the sensor array 34. The sensor array 34 detects this reproduction light, and a first reproduction image (positive image) is acquired.

  Next, when reproducing a negative image in which the original signal light pattern is reversed, the shutter 12 is opened and the laser beam is emitted from the light source 10. At the same time, digital data for displaying the reference light pattern is output from the control device 36 at a predetermined timing, and the ring-shaped reference light region 20R of the spatial light modulator 20 is centered as shown in FIG. The point is slid so as to move to the upper right on the drawing, and the reference light pattern is displayed in the reference light region 20R moved to the upper right. At the same time, a transmission pattern having a predetermined luminance value is displayed in the signal light region 20S fixedly arranged.

  For example, the optical recording medium 30 is made of a photopolymer, and the shift amount (shift amount) of the display position of the reference light pattern is set to a in the x-axis direction and b in the y-axis direction, as shown in FIG. The case will be described. In this case, the display position of the reference light pattern is shifted so that the phase shift amount with respect to the reference light used at the time of recording becomes | 2π (μa + νb) | = π / 2.

  The laser light oscillated from the light source 10 enters the spatial light modulator 20 in the same manner as in the case of a positive image. In the spatial light modulator 20, the laser light is modulated in accordance with the displayed pattern, and the reference light and the direct-current component to be applied are generated. The generated reference light and the applied direct current component are condensed by the lens 22, and only the first-order diffraction component passes through the aperture 24 </ b> A, is converted into parallel light by the lens 26, and is Fourier transformed by the Fourier transform lens 28. The light is converted and collected, and is irradiated onto the area where the hologram of the optical recording medium 30 is recorded. That is, the optical recording medium 30 is irradiated with the reference light and the applied direct current component as readout light.

  The reference light applied to the optical recording medium 30 is diffracted by the hologram when passing through the optical recording medium 30, and transmitted diffracted light (reproduced light) is emitted to the Fourier transform lens 32 side. The applied DC component passes through the optical recording medium 30 without being diffracted. Reproduction of the inverted image of the original signal light pattern is realized as a result of interference between the diffracted light from the hologram and the applied DC component. That is, an inverted image of the original signal light pattern is reproduced by setting the phase difference between the diffracted light and the direct current component so that the diffracted light from the hologram and the applied direct current component interfere with each other in opposite phases. . This phase difference can be set by the amount of shift of the display position of the reference light pattern in the spatial light modulator 20.

  The diffracted light from the hologram and the applied direct current component are subjected to inverse Fourier transform by the Fourier transform lens 32 and are incident on the sensor array 34. On the sensor array 34, an inverted pattern of the original signal light pattern displayed on the spatial light modulator 20 during recording, that is, a negative image of the original signal light pattern is formed. The sensor array 34 detects this reproduction light, and a second reproduction image (negative image) is acquired.

  Further, in the present embodiment, as in the fourth embodiment, two reproduced images of a positive image and a negative image can be detected from one Fourier transform hologram. The sensor array 34 outputs the image data of the first reproduced image and the image data of the second reproduced image to the control device 36. The image data of the first reproduced image and the image data of the second reproduced image are held in a storage unit such as a RAM of the control device 36.

  Also in the present embodiment, as in the fourth embodiment, the luminance of each pixel of the signal light pattern using the image data of the first reproduced image and the image data of the second reproduced image, and these two image data. The difference between is calculated. By calculating the difference, noise common to both image data is canceled. Based on the calculated difference value (positive or negative), the sign of each pixel is accurately determined, and the original digital data that has been two-dimensionally encoded is decoded with a high SNR. Also, when the original digital data is decoded with a high SNR, the bit error rate (BER) also decreases.

  In the fourth to sixth embodiments, since the phase of the reference light is modulated by changing the display position of the reference light pattern, it is 1 at the time of reproduction or recording, as in “reference light pattern slide multiplexing”. Since the hologram can be recorded / reproduced without moving the optical recording medium relative to the recording light every time page processing is performed (without moving the medium), it is possible to aim at high speed during reproduction or recording.

It is the schematic which shows the structure of the hologram recording / reproducing apparatus which concerns on 1st Embodiment. (A) is a figure which shows an example of the pattern for recording displayed on a spatial light modulator at the time of recording. (B) is a plan view of the spatial light modulator. (A)-(C) are figures which show an example of the pattern for recording displayed when recording a several page. (A) And (B) is a figure explaining the reason for which the phase of a reference light is modulated according to the display position of a reference light pattern. (A)-(C) is a figure which shows an example of the pattern for a reproduction | regeneration displayed when reproducing | regenerating each page recorded by "reference light pattern slide multiplexing". It is the schematic which shows the structure of the hologram recording / reproducing apparatus which concerns on 2nd Embodiment. (A)-(D) are figures which show an example of the pattern for recording and the pattern for reproduction | regeneration displayed when recording a several page. It is the schematic which shows the structure of the hologram recording / reproducing apparatus which concerns on 3rd Embodiment. (A)-(C) are figures which show an example of the pattern for recording and the pattern for reproduction | regeneration displayed when obtaining a some reproduced image from one hologram. It is the schematic which shows the structure of the hologram recording / reproducing apparatus which concerns on 5th Embodiment. (A)-(C) is a figure which shows another example of the pattern for recording and the pattern for reproduction | regeneration displayed when obtaining a some reproduced image from one hologram.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Light source 12 Shutter 14 1/2 wavelength plate 16 Polarizing plate 18 Beam expander 20 Spatial light modulator 20R Reference light area 20S Signal light area 20A Display surface 22 Lens 24A Aperture 24 Light shielding plate 26 Lens 28 Fourier transform lens 30 Optical recording medium 32 Fourier transform lens 32 Lens 34 Sensor array 36 Control device 38 Drive device 40 Drive device 42 Recording pattern 44 Mask 46 Drive device 48 Phase mask 50 Light source 52 Shutter 54 Half-wave plate 56 Polarizer 58 Beam expander 60 Polarized beam Splitter 62 Spatial light modulator 64 1/4 wavelength plate 66 Lens 68A Aperture 68 Light shielding plate 70 Lens 72 Fourier transform lens 72 Lens 74 Optical recording medium 74A Reflective layer 74B Optical recording layer 80 Sensor array 82 Controller 84 Driving device

Claims (17)

A light source that emits coherent light;
A recording comprising signal light and reference light, which is composed of a plurality of pixel portions arranged two-dimensionally on an xy plane, and modulates light incident from the light source for each pixel according to a pattern displayed for each page. A spatial light modulator that generates light;
A condensing optical system that coaxially condenses the recording light generated by the spatial light modulator on an optical recording medium by Fourier transform;
A signal light pattern is displayed in a fixed area of the spatial light modulator, and a reference light pattern including a plurality of spatial frequency components is displayed in a ring-shaped variable area that does not include the fixed area, and the variable area is displayed for each page . A drive control unit that drives and controls each of the plurality of pixel units of the spatial light modulator such that the center point is located at a different location on the xy plane;
An optical recording apparatus comprising: The condensing optical system is
A removing element for removing a zero-order diffraction component from a Fourier transform component obtained by Fourier transforming the recording light generated by the spatial light modulator;
A lens that performs inverse Fourier transform on the light from which the zero-order diffraction component has been removed;
A lens that re-Fourier-transforms the inverse Fourier transformed light and concentrically collects it on the optical recording medium;
The optical recording apparatus according to claim 1, further comprising: The condensing optical system is
A phase adjusting element that imparts a random phase distribution to at least the reference light among the recording light generated by the spatial light modulator;
A lens for concentrating the recording light with phase distribution on the optical recording medium by Fourier transform;
The optical recording apparatus according to claim 1, further comprising: Using a spatial light modulator that is composed of a plurality of pixel portions arranged two-dimensionally on the xy plane and modulates light incident from a light source for each pixel according to a pattern displayed for each page,
A signal light pattern is displayed on a fixed area of the spatial light modulator to generate signal light, and a reference light pattern including a plurality of spatial frequency components is displayed on a ring-shaped variable area that does not include the fixed area for reference. Produce light,
The recording light consisting of the signal light and the reference light generated by the spatial light modulator is Fourier-transformed and condensed coaxially on the optical recording medium,
The center point of the variable region is moved to a different position on the xy plane for each page, the display position of the reference light pattern is changed for each page, and the reference light having a different phase is used for each page, and the optical recording medium is used. Multiplex recording of multiple page holograms,
An optical recording method.   The zero-order diffraction component is removed from the Fourier transform component obtained by Fourier transforming the recording light generated by the spatial light modulator, and the light from which the zero-order diffraction component is removed is inverse Fourier transformed and inverse Fourier transformed. 5. The optical recording method according to claim 4, wherein the collected light is re-Fourier transformed and concentrically focused on the optical recording medium.   A random phase distribution is imparted to at least the reference light of the recording light generated by the spatial light modulator, and the recording light to which the phase distribution is imparted is Fourier-transformed and condensed coaxially on the optical recording medium. The optical recording method according to claim 4, wherein: A light source that emits coherent light;
A reference light pattern that includes a plurality of pixel portions arranged two-dimensionally on the xy plane, displays a signal light pattern in a fixed region of the spatial light modulator, and does not include the fixed region. A spatial light modulator that displays in a ring-shaped variable region, modulates light incident from the light source for each pixel according to a pattern displayed for each page, and generates recording light composed of signal light and reference light; ,
A condensing optical system that coaxially condenses the recording light generated by the spatial light modulator on an optical recording medium by Fourier transform;
A photodetector for detecting a reproduced image reproduced from the optical recording medium;
When reproducing a hologram for one page, a reference light pattern is displayed in a variable area whose central point is the same as that at the time of recording the page, and the first reference light generated by the spatial light modulator is irradiated as readout light. Then, the first reproduced image is detected by the photodetector, and the center point is moved to a position different from that at the time of page recording so that the diffracted light from the hologram interferes with the applied DC component due to the opposite phase. Displaying the reference light pattern in the variable region and displaying the non-modulated pattern in the fixed region, irradiating the second reference light having a phase different from that of the first reference light generated by the spatial light modulator as readout light, A drive control unit that drives and controls each of the light source, the spatial light modulator, and the photodetector so that a second reproduced image is detected by the photodetector;
An optical recording / reproducing apparatus comprising:   The reference light pattern is a periodic pattern in which bright portions and dark portions are arranged at a predetermined period so that bright spots of Fourier transform images of the reference light generated by the spatial light modulator are distributed in a matrix. The optical recording / reproducing apparatus according to claim 7.   The drive control unit displays a signal light pattern in the fixed area and a reference light pattern in the variable area when recording one page of hologram, and the spatial light modulator displays the signal light and reference. 9. The optical recording / reproducing apparatus according to claim 7, wherein each of the light source and the spatial light modulator is driven and controlled so as to generate recording light composed of light. The condensing optical system is
A shielding member that shields at least a second-order or higher diffraction component of the reference light among the Fourier transform components obtained by Fourier transforming the recording light generated by the spatial light modulator;
A lens that performs inverse Fourier transform on a first-order or lower diffraction component transmitted through the shielding member;
A lens that re-Fourier-transforms the inverse Fourier transformed light and concentrically collects it on the optical recording medium;
The optical recording / reproducing apparatus according to claim 7, further comprising:   The optical recording / reproducing apparatus according to claim 10, wherein the shielding member also shields a zero-order diffraction component.   When a hologram for one page is reproduced using a digital micromirror device for the spatial light modulator, a swing period m (Hz) of the pixel portion of the fixed region for displaying a signal light pattern, and a reference light pattern 12. The swing period n (Hz) of the pixel portion of the variable region for displaying the image satisfies a relationship of k · n = m (k is an integer of 1 or more). The optical recording / reproducing apparatus according to any one of the above. Using a spatial light modulator that is composed of a plurality of pixel portions arranged two-dimensionally on the xy plane and modulates light incident from a light source for each pixel according to a pattern displayed for each page,
A signal light pattern is displayed on a fixed area of the spatial light modulator to generate signal light, and a reference light pattern including a plurality of spatial frequency components is displayed on a ring-shaped variable area that does not include the fixed area for reference. Produce light,
The recording light consisting of the signal light and the reference light generated by the spatial light modulator is Fourier-transformed and concentrically focused on an optical recording medium to record a hologram,
When reproducing a hologram for one page, a reference light pattern is displayed in a variable area whose central point is the same as that at the time of recording the page, and the first reference light generated by the spatial light modulator is irradiated as readout light. Detecting the first reproduced image,
The reference beam pattern is displayed in the variable area where the center point has been moved to a position different from that at the time of page recording so that the diffracted light from the hologram interferes with the applied DC component due to the opposite phase, and is not modulated in the fixed area Displaying a pattern, irradiating a second reference light having a phase different from that of the first reference light generated by the spatial light modulator as a readout light, and detecting a second reproduced image;
An optical recording / reproducing method. A light source that emits coherent light;
A reference light pattern that includes a plurality of pixel portions arranged two-dimensionally on the xy plane, displays a signal light pattern in a fixed region of the spatial light modulator, and does not include the fixed region. A spatial light modulator that displays in a ring-shaped variable region, modulates light incident from the light source for each pixel according to a pattern displayed for each page, and generates recording light composed of signal light and reference light; ,
A removal element that removes the zero-order diffraction component from the Fourier transform component obtained by Fourier transforming the recording light generated by the spatial light modulator, and a lens that performs inverse Fourier transform on the light from which the zero-order diffraction component has been removed. A condensing optical system comprising: a lens for re-Fourier transforming the light subjected to inverse Fourier transform and concentrating on the optical recording medium coaxially;
A photodetector for detecting a reproduced image reproduced from the optical recording medium;
When reproducing a hologram for one page, the variable region moved to the first position where the center point is different from that at the time of page recording so that the diffracted light from the hologram interferes with the applied DC component due to the same phase. The reference light pattern is displayed on the fixed area and the non-modulated pattern is displayed on the fixed region. The first reference light generated by the spatial light modulator is irradiated as the readout light, and the first reproduced image is detected by the photodetector. At the same time, the reference light pattern is displayed and fixed in the variable area moved to the second position where the center point is different from that at the time of the page recording so that the diffracted light from the hologram interferes with the applied DC component due to the opposite phase. An unmodulated pattern is displayed in the region, and a second reference light having a phase different from that of the first reference light generated by the spatial light modulator is irradiated as readout light, and the second reproduced image is detected by the photodetector. So that the light source And the spatial light modulator, and a drive control unit for driving and controlling each of said optical detector,
An optical recording / reproducing apparatus comprising:   The reference light pattern is a periodic pattern in which bright portions and dark portions are arranged at a predetermined period so that bright spots of Fourier transform images of the reference light generated by the spatial light modulator are distributed in a matrix. The optical recording / reproducing apparatus according to claim 14.   The drive control unit displays a signal light pattern in the fixed area and a reference light pattern in the variable area when recording one page of hologram, and the spatial light modulator displays the signal light and reference. 16. The optical recording / reproducing apparatus according to claim 14, wherein each of the light source and the spatial light modulator is driven and controlled so as to generate recording light composed of light. Using a spatial light modulator that is composed of a plurality of pixel portions arranged two-dimensionally on the xy plane and modulates light incident from a light source for each pixel according to a pattern displayed for each page,
A signal light pattern is displayed on a fixed area of the spatial light modulator to generate signal light, and a reference light pattern including a plurality of spatial frequency components is displayed on a ring-shaped variable area that does not include the fixed area for reference. Produce light,
The recording light consisting of the signal light and the reference light generated by the spatial light modulator is Fourier-transformed and concentrically focused on an optical recording medium to record a hologram,
When reproducing a hologram for one page, the variable area is moved to the first position where the center point is different from that at the time of recording so that the diffracted light from the hologram interferes with the applied DC component due to the same phase. Displaying a reference light pattern and displaying an unmodulated pattern in a fixed region, irradiating the first reference light generated by the spatial light modulator as readout light, and detecting a first reproduced image;
The reference light pattern is displayed in the variable area moved to the second position where the center point is different from that at the time of page recording so that the diffracted light from the hologram interferes with the applied DC component due to the opposite phase, and the fixed area is displayed on the fixed area. An unmodulated pattern is displayed, and a second reproduced image is detected by irradiating with a second reference light having a phase different from that of the first reference light generated by the spatial light modulator, as a readout light.
An optical recording / reproducing method.