JP2007178458A - Hologram recording method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hologram recording method and a hologram recording apparatus for recording a hologram by irradiation with signal light and reference light in a coaxial state and achieving high-density recording in a volume hologram with less light loss. <P>SOLUTION: An optical recording medium is irradiated with Fourier transformed signal light and first reference light in a coaxial state from the same side to record the information in the signal light as a transmissive hologram. The first reference light transmitting the optical recording medium is modulated to generate second reference light, and the optical recording medium is irradiated with Fourier transformed signal light and the second reference light in a coaxial state from different sides from each other so as to record information of the signal light as a reflective hologram in the defocus position of the signal light and the first reference light. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a hologram recording method and apparatus, and more particularly to a hologram recording method and apparatus for recording a hologram by irradiating signal light and reference light coaxially.

  There has been proposed a method of producing a transmission hologram by irradiating signal light and reference light coaxially from the same surface side of a recording medium (Patent Document 1). In this method, the signal light generated from spatially different positions of the spatial light modulator and the reference light are Fourier transformed by a lens. Since the Fourier-transformed signal light and the reference light overlap in the vicinity of the Fourier transform plane, a hologram can be recorded by placing a recording medium at this position. In this method, since the signal light and the reference light are irradiated coaxially, the optical system is simplified and the recording apparatus can be made compact.

  However, the region where the signal light and the reference light overlap becomes smaller as the distance from the Fourier transform plane increases in the optical axis direction. Therefore, in the coaxial recording of the transmission hologram, there is a serious problem that if the film thickness of the recording material is increased, the hologram cannot be recorded in the entire optical axis direction (thickness direction) of the recording material.

  In general, there are two methods for improving the recording density: (1) increasing the film thickness of the recording medium, and (2) shortening the focal length of the Fourier transform lens.

  That is, in a volume hologram in which interference fringes are recorded three-dimensionally using the thickness direction of the recording medium, the Bragg condition becomes stricter and the dynamic range can be increased as the material thickness increases. . In addition, as the Bragg conditions are stricter and the dynamic range is larger, more holograms can be recorded in a multiplexed manner. Therefore, the larger the material thickness, the more holographic recordings can be made. In order to increase the recording density, it is necessary to reduce the recording area. In order to reduce the recording area, it is desirable to shorten the focal length of the Fourier transform lens.

  However, it is difficult to achieve both a thick film of the recording medium and a short focus of the Fourier transform lens. This is because when the thickness of the recording medium increases, the recording material exists at a position away from the Fourier transform plane in the optical axis direction. The shorter the focal length of the Fourier transform lens, the smaller the Fourier transform plane from the optical axis direction. This is because the region where the signal light and the reference light overlap is small even at a distant position. Therefore, recording of the hologram becomes difficult at that position.

  In order to achieve such a high recording density, a thick recording material and a Fourier transform lens with a short focal length are required. However, in the coaxial recording of a transmission hologram, the optical recording medium is made thicker and the Fourier transform lens Since it is difficult to achieve both short focal length, it is difficult to realize high density recording.

In order to solve this problem, a recording method for generating speckle-shaped reference light by displaying a random pattern in the reference light area of the spatial light modulator in a coaxial recording system has been proposed (Patent Document 2). ). In this recording method, the reference light is diffused by generating speckle-shaped reference light, and the region where the signal light and the reference light overlap (the defocus amount from the Fourier plane) can be increased. In addition, a technique (patent document 3) for diffusing reference light by arranging a phase mask close to the spatial light modulator and a technique (patent document 4) for generating reference light using a diffusion plate are proposed. .
Japanese Patent No. 3452113 JP-A-2005-241684 JP 2005-241773 A US Pat. No. 6,108,110

  However, in the recording methods described in Patent Documents 2 to 4, the overlap between the signal light and the reference light at the defocus position is not sufficient, and as the film thickness of the optical recording medium increases, a hologram is formed in the entire film thickness direction. It becomes difficult to record. Further, since the reference light is diffused, there is a problem that the amount of light not involved in recording is large and the loss of light amount is large.

  The present invention has been made to solve the above problems, and an object of the present invention is to record holograms by irradiating signal light and reference light on the same axis, and to perform high-density recording with volume holograms. It is an object of the present invention to provide a hologram recording method and apparatus that enable the above. Another object of the present invention is to provide a hologram recording method and apparatus with little light loss.

  In order to achieve the above object, the hologram recording method of the present invention irradiates an optical recording medium with Fourier-transformed signal light and first reference light coaxially from the same side, and information on the signal light is recorded in the optical recording medium. Recording as a transmission hologram on a medium, modulating the first reference light transmitted through the optical recording medium to generate a second reference light, and Fourier-transforming the signal light and the first reference light onto the optical recording medium The reference light of 2 is irradiated coaxially from different sides, and the information of the signal light is recorded on the optical recording medium as a reflection hologram.

  According to the hologram recording method of the present invention, the information light of the signal light is recorded as a transmission hologram by irradiating the optical recording medium with the Fourier transformed signal light and the first reference light coaxially from the same side. Can do. In addition, the first reference light transmitted through the optical recording medium is modulated to generate the second reference light, and the optically-transformed signal light and the second reference light are irradiated coaxially from different sides to the optical recording medium. As a result, signal light information can be further recorded as a reflection hologram at a position where the signal light and the first reference light do not sufficiently overlap within the optical recording medium.

  Thus, since the transmission hologram and the reflection hologram can be recorded at different positions on one optical recording medium, the hologram can be recorded in the entire optical axis direction (thickness direction) of the optical recording material. . As a result, it is possible to achieve both a thick film of the optical recording medium and a short focus of the Fourier transform lens, thereby realizing high-density recording.

  In addition, since the second reference light is generated by modulating the first reference light transmitted through the optical recording medium, there is an advantage that the light amount loss is small.

As described above, according to the present invention, the hologram can be recorded by irradiating the signal light and the reference light on the same axis, and there is an effect that high-density recording with a volume hologram becomes possible.
In addition, there is an effect that light loss is reduced.

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

(First embodiment)
FIG. 1 is a diagram showing a schematic configuration of a hologram recording / reproducing apparatus according to the first embodiment. As shown in the figure, the recording / reproducing apparatus can irradiate the optical recording medium with the signal light and the reference light coaxially.

  The hologram recording / reproducing apparatus is provided with a light source 10 that oscillates laser light that is coherent light. A beam expander 12 including a pair of lenses is disposed on the laser light irradiation side of the light source 10. A transmissive spatial light modulator 14 is disposed on the light transmitting side of the beam expander 12.

  The spatial light modulator 14 is connected to the personal computer 30 via the pattern generator 32. The pattern generator 32 generates a pattern to be displayed on the spatial light modulator 14 according to the digital data supplied from the personal computer 30, and the spatial light modulator 14 modulates the incident laser light according to the display pattern. Then, a digital image (signal light) and reference light for each page are generated.

  On the light transmission side of the spatial light modulator 14, a glass plate 16 that increases the optical path length of the reference light and a Fourier transform lens 18 that irradiates the optical recording medium 20 with the signal light and the reference light are provided along the optical path. Arranged in this order. The glass plate 16 is installed only in the region where the reference light propagates. That is, an opening 16a through which signal light passes is provided at the center of the glass plate, and only the reference light passes through the glass plate. A Fourier transform lens 22 is disposed on the signal light transmission side of the optical recording medium 20. A flat mirror 26 that reflects the reference light toward the optical recording medium 20 is provided on the optical path of the reference light that passes through the optical recording medium 20 and is inverse Fourier transformed by the Fourier transform lens 22.

  When the optical recording medium 20 is irradiated with the reference light during hologram reproduction, the irradiated reference light is diffracted by the hologram, and the diffracted light is emitted in the direction of the Fourier transform lens 22 and is inverse Fourier transformed by the Fourier transform lens 22. The On the diffracted light emitting side of the optical recording medium 20, there is disposed a photodetector (camera) 24 that is composed of an image sensor such as a CCD or CMOS array and converts the received reproduction light (diffracted light) into an electrical signal and outputs it. Has been. The photodetector 24 is connected to the personal computer 30.

  The purpose of providing the glass plate 16 is to increase the optical path length of the reference light so that the position of the imaging surface of the reference light pattern is on the front side of the photodetector 24 (on the optical recording medium 20 side). By doing so, the area of the plane mirror 26 that reflects the reference light pattern can be reduced, and the physical difficulty of juxtaposing the photodetector 24 and the plane mirror 26 can be eliminated. However, the glass plate 16 is not an essential element for satisfying the advantages of the present invention and can be omitted.

  Next, a processing routine of recording / reproducing processing executed by the personal computer 30 will be described. FIG. 2 is a flowchart showing the processing routine of the recording / reproducing process. First, the user operates an input device (not shown) and selects recording processing or reproduction processing. When digital data is recorded as a hologram, the digital data to be recorded is input to a personal computer in advance.

  In step 100, it is determined whether the recording process is selected or the reproduction process is selected. If the recording process is selected, the laser light is emitted from the light source 10 and the digital data is received from the personal computer 30 in step 102. Is output at a predetermined timing, hologram recording processing is executed, and the routine is terminated.

Here, the hologram recording process will be described.
The laser light oscillated from the light source 10 is collimated into a large-diameter beam by the beam expander 12 and applied to the spatial light modulator 14. When digital data is input from the personal computer 30, the pattern generator 32 generates a signal light pattern (page data) according to the supplied digital data, and synthesizes it with the reference light pattern. A pattern to be displayed is generated. In the spatial light modulator 14, the laser light is amplitude-modulated according to the displayed pattern, and the signal light S and the reference light R1 are generated.

  For example, as shown in FIG. 3, the central portion of the spatial light modulator 14 is used for data display (for signal light), and the peripheral portion of the spatial light modulator 14 is used for reference light. The laser light incident on the central portion of the spatial light modulator 14 is polarization-modulated according to the display pattern, and the signal light S is generated. On the other hand, the laser light incident on the peripheral portion of the spatial light modulator 14 is amplitude-modulated according to the display pattern, and the reference light R1 is generated.

  The signal light S and the reference light R1 amplitude-modulated by the spatial light modulator 14 are Fourier transformed by the lens 18. Before that, only the reference light passes through the glass plate 16, thereby increasing the optical path length. The Fourier-transformed signal light S and reference light R1 are simultaneously and coaxially applied to the optical recording medium 20. As a result, the signal light S and the reference light R1 interfere with each other in the optical recording medium 20, and the interference pattern is recorded as a hologram.

  In the present embodiment, the Fourier transform lens 18 is disposed so that the signal light S and the reference light R1 are focused on the emission-side end face 20b of the optical recording medium 20. The Fourier-transformed signal light S and the reference light R1 overlap in the vicinity of the exit side end face 20b (Fourier transform plane), and a transmission hologram is recorded on the exit side end face 20b.

  The reference light R1 that has passed through the optical recording medium 20 is subjected to inverse Fourier transform by the lens 22, irradiated onto the plane mirror 26, and reflected as the reference light R2 in the direction of the optical recording medium 20. The reflected reference light R 2 is Fourier transformed by the lens 22. The Fourier-transformed reference light R2 and signal light S are simultaneously and coaxially irradiated onto the optical recording medium 20 from different sides. As a result, the signal light S and the reference light R2 interfere with each other in the optical recording medium 20, and the interference pattern is recorded as a hologram.

  In the present embodiment, the reference light R1 is reflected by the plane mirror 26 so that the reference light R2 has an inclination with respect to the optical axis. Therefore, when the reference light R2 passes through the Fourier transform lens 22, it is condensed at a position different from the condensing point of the reference light R1 within the exit side surface 20b of the optical recording medium 20. Thereafter, the wavefront of the reference light R2 spreads as it propagates in the direction of the incident side end face 20a. The spread is inclined with respect to the optical axis. Accordingly, the signal light S and the reference light R2 overlap in the vicinity of the incident side end face 20a where the signal light S and the reference light R1 do not sufficiently overlap, and a reflection hologram is recorded in the optical recording medium 20. That is, the reflection hologram is recorded at a position different from the transmission hologram.

  If reproduction processing is selected in step 100 of FIG. 2, in step 104, laser light is emitted from the light source 10, reproduction image acquisition processing is executed, and the routine is terminated.

Here, a reproduction image acquisition process will be described.
As shown in FIG. 4, a light shielding pattern (all black pixels) is displayed in the central portion of the spatial light modulator 14, and the same reference light pattern as that during recording is displayed in the peripheral portion of the spatial light modulator 14. As a result, only the laser light incident on the peripheral portion of the spatial light modulator 14 is amplitude-modulated to generate the reference light, and only the reference light is applied to the area where the hologram of the optical recording medium 20 is recorded.

  The irradiated reference light is diffracted by the hologram, and the diffracted light is emitted from the optical recording medium 20. The emitted diffracted light is subjected to inverse Fourier transform by the lens 22 and is incident on the photodetector 24. A reproduced image can be observed on the focal plane of the lens 22.

  In the present embodiment, the reference light R 1 generated by the spatial light modulator 14 passes through the glass plate 16 and is Fourier transformed by the lens 18. The Fourier-transformed reference light R1 is applied to the optical recording medium 20. Thereby, the Fourier transform image of the signal light S is reproduced from the transmission hologram recorded in the vicinity of the emission side end face 20b.

  Further, the reference light R1 transmitted through the optical recording medium 20 is subjected to inverse Fourier transform by the lens 22, irradiated onto the mirror 26, and reflected as the reference light R2 in the direction of the optical recording medium 20. The reflected reference light R 2 is Fourier transformed by the lens 22. The Fourier-transformed reference light R2 is applied to the optical recording medium 20. As a result, the Fourier transform image of the signal light S is reproduced from the reflection hologram recorded in the optical recording medium 20.

  Since the information recorded in the transmission hologram and the reflection hologram is one type of information (signal light S), the photodetector 24 also detects one type of information (signal light S). The detected analog data is A / D converted by the photodetector 24, and the image data of the reproduced image is input to the personal computer 30 and held in a RAM (not shown). The luminance value (image data) detected at each pixel of the photodetector 24 is input to the personal computer 30 in association with each pixel of the spatial light modulator 14.

  As described above, in the present embodiment, a transmission hologram is recorded in the vicinity of the Fourier transform plane of the signal light by irradiating the optical recording medium with the Fourier transform signal light and the reference light coaxially from the same side. be able to. In addition, the reference light transmitted through the optical recording medium is modulated to generate another reference light, and this reference light is Fourier transformed to irradiate the optical recording medium. A hologram can be recorded.

  Thus, since the transmission hologram and the reflection hologram can be recorded at different positions on one optical recording medium, the hologram can be recorded in the entire optical axis direction (thickness direction) of the optical recording material. . As a result, it is possible to achieve both a thick film of the optical recording medium and a short focus of the Fourier transform lens, thereby realizing high-density recording.

  Further, since the reference light transmitted through the optical recording medium is modulated to generate another reference light, the light amount loss is small.

(Second Embodiment)
FIG. 5 is a diagram showing a schematic configuration of a hologram recording / reproducing apparatus according to the second embodiment. A convex mirror 28 is provided in place of the plane mirror 26, and the zero-order component of the reference light R1 transmitted through the Fourier transform lens 22 is converted into a reference light R2 that is a spherical wave, and the reference light R2 transmitted through the Fourier transform lens 22 Since the configuration is the same as that of the hologram recording / reproducing apparatus according to the first embodiment except that the optical recording medium 20 is arranged so as to be focused on the incident side end surface 20a, the same components are denoted by the same reference numerals. Description is omitted.

  In the first embodiment, the 0th order component of the reference light R2 is mainly used as the reference light R2 for recording the reflection hologram. For this reason, the influence of the 0th-order component of the reference light is large, and the selectivity (shift selectivity) with respect to the shift amount of the diffracted light intensity in shift multiplex recording is limited.

  On the other hand, in the present embodiment, by using the convex mirror 28, the wavefront of the reference light R2 reflected by the convex mirror 28 is condensed on the incident side end face 20a of the optical recording medium 20, and a diffraction pattern in the vicinity thereof. Becomes complicated. Therefore, the shift selectivity of the reflection hologram recorded with the reference light R2 and the signal light S is improved. That is, by arranging the Fourier transform lens 22 so that the reference light R2 is condensed on the incident side end surface 20a of the optical recording medium 20, the diffraction pattern of the reference light R2 and the Fresnel diffraction pattern of the Fourier-transformed signal light S are obtained. And overlap in the vicinity of the incident side end face 20a, and a reflection hologram is recorded on the incident side end face 20a. Since the reference light wavefront becomes more complex in the region closer to the focal point of the reference light, a hologram can be recorded with the reference light having a more complicated wavefront. Therefore, shift selectivity can be improved as compared with the first embodiment.

  As described above, in this embodiment, the influence of the 0th-order component of the reference light can be reduced by complicating the wavefront of the reference light R2 in the region where the reflection hologram is recorded as described above. Selectivity can be improved.

(Third embodiment)
FIG. 6 is a diagram showing a schematic configuration of a hologram recording / reproducing apparatus according to the third embodiment. Between the spatial light modulator 14 and the Fourier transform lens 18, a glass plate 16, a lens 34, a filter 36 for removing a direct current component, and a lens 38 are arranged in this order, and the thickness direction of the optical recording medium 20 is set. Since the configuration is the same as that of the hologram recording / reproducing apparatus according to the first embodiment except that the center is arranged on the Fourier transform plane of the Fourier transform lens 18, the same components are denoted by the same reference numerals and description thereof is omitted. To do. The surface 40 that includes the center in the thickness direction of the optical recording medium 20 and is orthogonal to the optical axis is hereinafter referred to as a “center surface”.

  The reason why the center plane 40 of the optical recording medium 20 is disposed on the Fourier transform plane of the Fourier transform lens 18 is to cause the reference beams R1 and R2 having complex wavefronts and the signal beam S to interfere with each other. The wavefronts (diffracted lights) of the reference beams R1 and R2 in the vicinity of the Fourier transform plane are complicated, and the shift selectivity can be improved by recording a hologram with the complex wavefront and the signal beam S.

  As the filter 36, for example, as shown in FIG. 7, a Fourier transform such as a light shielding plate provided with a light shielding part 42 that blocks the zeroth order component of the Fourier transform image and an opening 44 that transmits a component other than the zeroth order. A filter that blocks only the zeroth order component of the image can be used.

  In this apparatus, at the time of hologram recording, the laser light oscillated from the light source 10 is collimated into a large-diameter beam by the beam expander 12 and irradiated to the spatial light modulator 14. In the spatial light modulator 14, the laser light is polarization-modulated in accordance with the displayed pattern, and signal light and reference light are generated. The signal light S and the reference light R1 amplitude-modulated by the spatial light modulator 14 are Fourier transformed by the lens 34. Before that, only the reference light passes through the glass plate 16, thereby increasing the optical path length.

  The Fourier-transformed signal light and reference light are applied to the filter 36, and the DC component is removed from the Fourier transform images of the signal light and reference light. The signal light and the reference light not blocked by the filter 36 are subjected to inverse Fourier transform by the lens 38, subjected to Fourier transform again by the lens 18, and irradiated onto the optical recording medium 20 simultaneously and coaxially. Thereby, the signal light and the reference light interfere with each other in the optical recording medium 20, and the interference pattern is recorded as a hologram. By removing the direct current component from the Fourier transform image by the filter 36, useless exposure due to the zeroth-order component can be suppressed, and wasted dynamic range can be prevented.

  In the present embodiment, the Fourier transform lens 18 is arranged so that the signal light S and the reference light R1 are collected on the central plane 40 of the optical recording medium 20. The Fourier-transformed signal light S and reference light R1 overlap in the vicinity of the center plane 40 (Fourier transform plane), and a transmission hologram is recorded in the vicinity of the center plane 40.

  The reference light R1 that has passed through the optical recording medium 20 is subjected to inverse Fourier transform by the lens 22, irradiated onto the plane mirror 26, and reflected as the reference light R2 in the direction of the optical recording medium 20. The reflected reference light R 2 is Fourier transformed by the lens 22. The Fourier-transformed reference light R2 and signal light S are applied to the optical recording medium 20 from different sides. As a result, the signal light S and the reference light R2 interfere with each other in the optical recording medium 20, and the interference pattern is recorded as a hologram. This hologram is a reflection hologram.

  In the present embodiment, since the reference beams R1 and R2 are collected on the center plane 40 (Fourier transform plane), the wavefronts of the reference beams R1 and R2 near the center plane have a complicated shape. Therefore, it is possible to improve the shift selectivity of both the transmission hologram recorded with the reference light R1 and the signal light S and the reflection hologram recorded with the reference light R2 and the signal light S. As a result, the shift selectivity of the entire hologram can be improved.

  As described above, in the present embodiment, since the Fourier transform lens is arranged so that the signal light S and the reference light R1 are focused on the center plane of the optical recording medium, the defocus distance is shortened. Further, by removing the DC component from the Fourier transform image by the filter, it is possible to suppress unnecessary exposure due to the 0th-order component and prevent the dynamic range from being wasted.

It is a figure which shows schematic structure of the hologram recording / reproducing apparatus which concerns on 1st Embodiment. It is a flowchart which shows the processing routine of a recording / reproducing process. It is a figure which shows the display image of a spatial light modulator. It is a figure which shows the display image of a spatial light modulator in the case of acquiring a reproduced image. It is a figure which shows schematic structure of the hologram recording / reproducing apparatus which concerns on 2nd Embodiment. It is a figure which shows schematic structure of the hologram recording / reproducing apparatus which concerns on 3rd Embodiment. It is a top view of the filter for removing a direct current component

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Light source 12 Beam expander 14 Spatial light modulator 16a Opening part 16 Glass plate 18 Fourier-transform lens 20 Optical recording medium 20a Incident side end surface 20b Outgoing side end surface 22 Fourier transform lens 24 Photo detector 26 Plane mirror 28 Convex mirror 30 Personal computer 32 Pattern generator 34 Lens 36 Filter 38 Lens 40 Center plane 42 Light-shielding portion 44 Opening portion

Claims (6)

The optical recording medium is irradiated with the Fourier transform signal light and the first reference light coaxially from the same side, and the information of the signal light is recorded on the optical recording medium as a transmission hologram,
The first reference light transmitted through the optical recording medium is modulated to generate a second reference light, and the signal light Fourier-transformed to the optical recording medium and the second reference light are coaxial from different sides. And recording the information of the signal light on the optical recording medium as a reflection hologram,
Hologram recording method.   The hologram recording method according to claim 1, wherein the signal light, the first reference light, and the second reference light are collected on the same surface of the optical recording medium.   2. The signal light and the first reference light are condensed on one surface of the optical recording medium, and the second reference light is condensed on the other surface of the optical recording medium. Hologram recording method.   4. The direct-current component is removed from the Fourier-transformed signal light and the first reference light, and the optical recording medium is irradiated with the signal light from which the direct-current component is removed and the first reference light. The hologram recording method according to claim 1.   5. The hologram recording method according to claim 1, wherein the first reference light that is transmitted through the optical recording medium is reflected toward the optical recording medium to modulate the first reference light. 6. A light source that emits laser light;
A spatial light modulator that spatially modulates laser light emitted from the light source to generate signal light and reference light;
A first lens for Fourier transforming the signal light and the reference light generated by the spatial light modulator to irradiate the optical recording medium;
A second lens that performs inverse Fourier transform on the signal light and the reference light transmitted through the optical recording medium;
A reflection mirror that reflects the reference light that is transmitted through the optical recording medium and inverse Fourier transformed by a second lens toward the optical recording medium;
A hologram recording apparatus comprising:

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