JP4218574B2 - Hologram recording method and apparatus - Google Patents

Description

  The present invention relates to a hologram recording method and apparatus, and more particularly to a hologram recording medium that is a holographic storage, and more particularly to a hologram recording method and apparatus that overwrites and records data in a digital holographic storage.

An optical memory typified by a DVD is widely used as a memory capable of realizing a large capacity and high density. Research and development of an optical memory using a blue-violet laser as a light source has been actively conducted with the aim of further increasing the capacity of the optical memory.
Hitherto, high density optical memories have been realized by reducing the laser spot used for data recording / reproduction by shortening the wavelength of light used and increasing the NA. In the method of shortening the wavelength of light, there is no appropriate optical material in the ultraviolet region, and there is no appropriate optical material used for recording media and lenses, so the use of blue-violet laser is limited. It is considered difficult to shorten the wavelength further.

  On the other hand, as a method of increasing the NA, a method of increasing the numerical aperture by the refractive index multiple of the prism by using a SIL (Solid Immersion Lens) that uses a circular prism having a high refractive index to reduce the focused spot is proposed. Yes. In this method, since a minute condensing spot is formed using the evanescent wave formed on the prism bottom surface, it is necessary to make the distance between the prism bottom surface and the recording medium sufficiently small. Therefore, there are problems such as distance control between the prism and the substrate, establishment of portability of the optical disk, and the like. Further, since the refractive index of the prism material is at most about 2, the improvement in recording density is at most about 4 times. In order to record 50 GB or more on an optical disc, volume type recording is required for recording in the depth direction of the recording medium.

  A holographic memory is known as one of volume type recordings. The holographic memory uses a three-dimensional recording area and can perform large-capacity recording, and also has a high speed because it is a two-dimensional batch recording / reproducing method. In this method, a plurality of data pages can be recorded by being multiplexed in the same volume, and the data can be read collectively for each page.

Also, digital information can be recorded / reproduced by digitizing binary digital data “0, 1” as “bright, dark” instead of an analog image and performing hologram recording / reproduction. Recently, from a more engineering point of view, such as the S / N ratio and bit error rate evaluation or two-dimensional encoding based on the specific optical system and volume multiplex recording system of this digital holographic memory, the influence of aberration of the optical system, etc. Research is progressing.
FIG. 6 is a diagram for explaining an angle multiplex recording method which is one of the hologram multiplex recording methods. In the case of this angle multiplex recording method, a digital data page (signal light) 2 is collected by a lens 4 and a hologram is recorded by simultaneously irradiating reference light into the same volume of the hologram recording medium 6. In the case of multiplex recording, the signal light is recorded by changing the incident angle of the reference light. At the time of data reading (reproduction), the target hologram can be accessed by addressing with the reference light used for recording, and the data page can be reproduced.

  7A and 7B are diagrams for explaining the shift multiplex recording method. In this method, a light wave whose wavefront changes sharply such as a spherical wave or a speckle pattern is used as the reference light. When such reference light is used, the Bragg condition for reproduction can be removed by slightly shifting the position of the recording medium from the recording spot by the shift amount δ (FIG. 7B). New holograms can be recorded. That is, holograms can be multiplex-recorded in substantially the same volume by recording while slightly moving the recording medium.

  In addition, the angle between the signal light and the reference light is kept constant, and the signal light and the reference light are simultaneously irradiated onto the recording medium while changing the wavelengths of the signal light and the reference light, so that the information of the signal light is converted into a hologram of multiple pages. A wavelength multiplex recording method for performing multiplex recording on a recording medium is also known. In this method, when reproducing a certain page of hologram, there is a wavelength at which the information of the signal light is not reproduced if the wavelength of the reference light is changed. A new hologram can be recorded at the same location on the recording medium using the signal light of this wavelength and the reference light. Thus, by changing the wavelengths of the signal light and the reference light, a plurality of holograms can be multiplexed and recorded at the same location on the recording medium.

  As described above, the digital holographic storage can simultaneously realize high-speed transfer by two-dimensional batch recording / reproduction and large capacity by volume recording.

  As recording media materials, photopolymer materials, photorefractive materials, photochromic materials such as azopolymer materials, and the like have been actively researched and developed. Of these, photorefractive materials and photochromic materials such as azo polymer materials can be rewritten. When the rewritable material is used, it is possible to erase the recorded data and record new data. For this reason, since the rewritable media can be used repeatedly, it is highly expected to be used as a backup memory such as a hard disk in addition to storing a large amount of information. A write-once (writeable only once) holographic memory is not rewritable and does not require a rewriting function.

  On the other hand, in a rewritable holographic memory, it is necessary to obtain reproduction light that can be rewritten at high speed and has a low noise ratio and a large noise ratio. When erasing data recorded for rewriting, it is most common to erase the recorded hologram by irradiating the entire recording area with uniform light. In addition, with photochromic materials such as photorefractive materials and azo polymer materials, holograms can be erased collectively by once maintaining the recording medium at a high temperature.

  When erasing the recorded data, if all information is erased and overwritten with new information, two steps, an erasing step and a recording step, are required. For example, a large-capacity recording medium exceeding 100 GB is required. In some cases, a great deal of time is required. For this reason, in the shift multiplex recording method, the signal light with the same polarization state and the reference as when the already recorded hologram was recorded at the same position as the recording position of each page of the hologram recorded without performing the erasing process It is practically desirable to erase unnecessary data by irradiating light and overwriting a new hologram.

  However, even if the new hologram is overwritten, the already recorded hologram cannot be completely erased. Therefore, when reproducing the overwritten new hologram, both the already recorded hologram and the newly recorded hologram are used. There is a problem that the diffracted light from the mixture is mixed and the quality of the reproduced light is deteriorated.

  The present invention has been made to solve the above-described problems, and provides a hologram recording method and apparatus that can improve the quality of reproduction light and perform high-speed rewrite recording without performing an erasing process. With the goal.

In order to achieve the above object, the hologram recording method of the present invention irradiates a signal recording medium and a reference light simultaneously on a rewritable optical recording medium while changing the angle formed by the signal light and the reference light. By using a hologram recording method in which the information of the signal light is multiplexed and recorded on the optical recording medium as a hologram of a plurality of pages, a hologram is newly recorded by overwriting in a recording area in which the hologram of a plurality of pages is multiplexed and recorded. A hologram recording method for overwriting a recording angle when newly recording a hologram so that the intensity of diffracted light from a plurality of pages of holograms that are multiplexed and recorded in the recording area to be overwritten is minimized. The hologram is newly recorded at a recording angle different from the recording angle when the hologram to be recorded is recorded.

In another hologram recording method of the present invention, the angle between the signal light and the reference light is made constant, and the signal light and the reference light are moved while relatively moving at least one of the signal light, the reference light, and the optical recording medium. Simultaneously irradiating a rewritable optical recording medium , the recording position is changed, and a hologram recording system is used to multiplex-record the signal light information as a multi-page hologram on the optical recording medium. A hologram recording method for newly recording a hologram by overwriting in a multiplex-recorded recording area, wherein a recording position when newly recording a hologram is recorded on a plurality of pages that are multiplex-recorded in the overwritten recording area as intensity of diffracted light from the hologram is minimized, as different recording position and the recording position when recording a hologram to be overwritten, Wherein the newly recorded program.

Still another hologram recording method of the present invention provides an optical recording medium in which the angle formed by the signal light and the reference light is constant and the signal light and the reference light can be rewritten simultaneously while changing the wavelengths of the reference light and the signal light. Irradiate and record a new hologram by overwriting in a recording area where multiple pages of holograms are multiplexed and recorded, using a hologram recording method in which information of the signal light is multiplexed and recorded on the optical recording medium as multiple pages of holograms A hologram recording method, wherein the diffracted light intensity from a hologram of a plurality of pages that are multiplexed and recorded in the recording area to be overwritten with the wavelengths of the reference light and the signal light when newly recording a hologram is minimized. to, as a wavelength different from the wavelength of the reference light and the signal light when recording a hologram to be overwritten, and characterized in that newly recorded holograms That.

  In each of the above inventions, since each hologram page is newly recorded under a condition different from the condition at the time of recording each hologram page without performing an erasing process, the quality of the reproduction light is reduced. This makes it possible to perform rewriting and recording at high speed.

  It is effective that the recording angle for newly recording each page of the hologram of the angle multiplex recording method is an angle at which the light intensity of the reproduction light from each page of the hologram recorded on the optical recording medium is minimized. It is effective that the recording position for newly recording each page of the hologram of the shift multiplex recording method is a position where the light intensity of the reproduction light from each page of the hologram recorded on the optical recording medium is minimized. In the wavelength multiplexing recording method, each page of the hologram is newly recorded using reference light and signal light having a wavelength at which the light intensity of the reproduction light from each page of the hologram recorded on the optical recording medium is minimized. Is effective.

  That is, under the condition that the light intensity of the reproduction light from each page of the hologram recorded on the optical recording medium is minimized, the condition deviates from the Bragg condition of the reproduction light from each page of the hologram recorded on the optical recording medium. Therefore, by recording as described above, the Bragg condition of the reproduction light from each page of the recorded hologram does not match the Bragg condition of the reproduction light from each page of the newly recorded hologram. When reproducing each page of a newly recorded hologram, reproduction light from each page of the already recorded hologram is not mixed and can be reproduced with high quality.

  In each of the above inventions, the polarization state of the signal light or reference light when recording each page of the hologram recorded on the optical recording medium, and the signal light or new light when recording each page of the hologram It is more preferable to make the polarization state of the reference light different. In order to change the polarization condition, for example, when the polarization direction of the signal light and the polarization direction of the reference light are parallel when recording each page of the hologram recorded on the optical recording medium, each page of the hologram The polarization direction of the signal light and the polarization direction of the reference light at the time of newly recording are orthogonalized, and the polarization direction of the signal light and the reference light at the time of recording each page of the hologram recorded on the optical recording medium When the polarization direction is orthogonal, the polarization direction of the signal light and the polarization direction of the reference light at the time of newly recording each page of the hologram may be made parallel.

  In this way, by recording differently the polarization state at the time of recording each page of the already recorded hologram and the polarization condition at the time of newly recording each page of the hologram, When reproducing each page, reproduction light from each page of a hologram already recorded is not mixed, and can be reproduced with high quality.

  As the optical recording medium, one having a photorefractive material, one having a photochromic material, or one having a polarization sensitive material can be used. The polarization-sensitive material is at least one polymer selected from the polyester group, and preferably has an azobenzene skeleton in the side chain.

  The hologram recording apparatus of the present invention uses the hologram recording method described above to multiplex-record signal light information as a plurality of pages of holograms on an optical recording medium.

For example, a hologram recording apparatus to which a shift multiplex recording method is applied includes a light source that emits coherent light, a stage that rotates or moves an optical recording medium, and the coherent light is separated into light for reference light and light for signal light. After that, the reference light and the signal light are irradiated to the rewritable optical recording medium at the same time , and the optical path of the separated light is changed so that the angle formed by the reference light and the signal light is changed. And signal light arranged in the optical path of the signal light and modulating the signal light according to the supplied recording signal for each page and recording each page of the hologram a spatial light modulator for generating, in the case of recording the first hologram, to irradiation while changing the angle between the signal light and the reference light and the signal light and the reference light simultaneously in a rewritable optical recording medium A recording signal for each page is supplied to the spatial light modulator so that the information of the signal light is multiplexed and recorded on the optical recording medium as a hologram of a plurality of pages by changing the recording angle. When the hologram is newly recorded by overwriting in the recording area in which the stage and the light separation optical path changing unit are controlled and the hologram of a plurality of pages is multiplexed and recorded, the recording angle when the hologram is newly recorded the diffracted light intensity from the hologram of a plurality of pages that are multi-recorded in the recording area to be overwritten so is minimized, as different recording angles the recording angle when recording the hologram to be overwritten, the hologram as newly recorded, and supplies the recording signal for each page in the spatial light modulator, the light source, the stage, and the light separation optical It can be configured to include a control means for controlling the changing means.

  As described above, the recording position for newly recording each page of the hologram can be a position where the light intensity of the reproduction light from each page of the hologram recorded on the optical recording medium is minimized.

  The hologram recording apparatus includes an analyzer that transmits a component in a predetermined polarization direction of diffracted light from each page of a hologram recorded on the optical recording medium, and a detection that detects the intensity of transmitted light that has passed through the analyzer. Can be further provided. By providing the analyzer in this way, necessary components can be selected and reproduced.

  In this hologram recording apparatus, the polarization state of signal light or reference light when recording each page of the hologram recorded on the optical recording medium, and signal light or reference when newly recording each page of the hologram It is effective to make the polarization state of light different.

  In order to change the polarization state, when the polarization direction of the signal light and the polarization direction of the reference light at the time of recording each page of the hologram recorded on the optical recording medium are parallel, each page of the hologram is renewed. The polarization direction of the signal light and the polarization direction of the reference light when recording each page of the hologram recorded on the optical recording medium by making the polarization direction of the signal light and the polarization direction of the reference light orthogonal to each other Are orthogonal to each other, the polarization direction of the signal light and the polarization direction of the reference light at the time of newly recording each page of the hologram may be made parallel to each other.

  As described above, according to the present invention, it is possible to obtain the effects that the erasing process is not performed, the quality of the reproduction light is improved, and rewriting and recording can be performed at high speed.

  Hereinafter, a hologram recording / reproducing apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings. The hologram recording / reproducing apparatus of the present embodiment applies the shift multiplex recording method of the present invention.

As shown in FIG. 1, the hologram recording / reproducing apparatus of the present embodiment is provided with a laser oscillator 10 using, for example, an Nd: YVO ₄ crystal. Laser light having a wavelength of 532 nm, which is coherent light, is oscillated and irradiated from the laser oscillator 10.

  On the laser beam irradiation side of the laser oscillator 10, a polarization beam splitter that transmits the P-polarized light and reflects the S-polarized light, thereby separating the laser light into two light beams, a reference light beam and a signal light beam. 16 is arranged.

  On the light reflection side of the polarization beam splitter 16, the reflection mirror 18 that reflects the laser light for reference light and changes the optical path in the direction of the hologram recording medium, and the laser light for reference light is condensed to generate a spherical reference wave. The objective lenses 20 that generate the reference light are arranged in order. On the laser beam condensing side of the objective lens 20, an xz stage 22 having a stepping motor for rotating a hologram recording medium 24 formed in a disk shape in the z plane is provided. The objective lens 20 irradiates the hologram recording medium 24 with S-polarized light, which is a spherical reference wave, as reference light.

  On the light transmitting side of the polarizing beam splitter 16, a shutter 12 for blocking the P-polarized light transmitted through the polarizing beam splitter 16 is disposed so as to be inserted into and retracted from the optical path. Further, on the light transmission side of the polarization beam splitter 16, a reflection mirror 28 for reflecting the signal light laser beam at a reflection angle of 45 ° and changing the optical path toward the hologram recording medium, and lenses 30, 32, 34 are configured. These lens systems are arranged in order. Between the lens 32 and the lens 34, a liquid crystal display element or the like is used. The laser light for signal light is modulated in accordance with the supplied recording signal for each page, and each hologram page is recorded. A transmissive spatial light modulation element 36 that generates signal light is disposed. On the light transmission side of the spatial light modulation element 36, an optical rotator 26 whose polarization plane is rotated by 90 degrees is disposed so as to be inserted into and retracted from the optical path. The spatial light modulator 36 emits P-polarized incident light as S-polarized light. On the light transmission side of the spatial light modulator 36, an optical rotator 26 that emits S-polarized incident light as P-polarized light is disposed.

  The lenses 30 and 32 collimate the laser light into a large-diameter beam and irradiate the spatial light modulator 36, and the lens 34 uses the P-polarized light modulated and transmitted by the spatial light modulator 36 and the optical rotator 26 as signal light. The light is condensed on the hologram recording medium 24. At this time, the condensing spot of the signal light is condensed so as to be smaller than the condensing spot of the reference light, and the hologram recording medium 24 is irradiated with the signal light and the reference light simultaneously. In addition, since the P-polarized light is used as the signal light and the S-polarized light is used as the reference light, the polarization direction of the signal light and the polarization direction of the reference light when recording each page of the hologram are orthogonal. Note that S-polarized light may be used as signal light, P-polarized light may be used as reference light, signal light having a parallel polarization plane and reference light may be used, and circularly polarized light that rotates in different directions is used as signal light and reference light, respectively. May be.

  On the reproducing light transmission side of the hologram recording medium 24, a lens 38 and an analyzer that selectively transmits light (for example, 0 ° polarization component, 45 ° polarization component, or 90 ° polarization component) from the reproduction light. 44 and an image sensor such as a CCD, and a detector 40 for converting the received reproduction light into an electric signal and outputting it is disposed. The detector 40 is connected to a personal computer 42.

  The personal computer 42 is connected to the spatial light modulator 36 via a pattern generator 46 that generates a pattern in accordance with a recording signal supplied from the personal computer at a predetermined timing. Further, the personal computer 42 is driven to insert the shutter 12 and the optical rotator 26 into the optical path separately, and to drive the shutter 12 or the optical rotator 26 inserted into the optical path separately from the optical path. A device 48 is connected. The personal computer 42 is connected to a driving device 50 that drives the xz stage 22.

  FIG. 2 shows a configuration of the hologram recording medium (optical recording medium) 24. Note that the hologram recording medium of the present embodiment is formed in a disk shape, but in the figure, a part cut out in a rectangular shape is shown. As shown in FIG. 2A, the hologram recording medium 24 is composed of an optical recording layer 23 formed into a thick film having a thickness of, for example, 100 μm or more. When the strength of the optical recording layer alone is insufficient, as shown in FIG. 2B or 2C, a substrate 25 made of a plate-like transparent medium such as quartz or plastic is provided on one side or both sides.

The optical recording layer, i.e., the photosensitive layer, exhibits photoinduced refractive index change or photoinduced dichroism, and photorefractive material, photochromic material, polarization sensitive, in which photoinduced refractive index change or photoinduced dichroism is maintained at room temperature. Any material can be used as long as it is a polymer having a photoisomerizable group in the side chain, for example, at least one polymer selected from the group of polyesters. A material having a photoisomerizable group, for example, an azobenzene skeleton is preferable.
The principle of light-induced birefringence will be described using azobenzene as an example. Azobenzene repeats a trans-cis-trans isomerization cycle upon irradiation with light. Before the light irradiation, a lot of trans azobenzene molecules exist in the optical recording layer. These molecules are randomly oriented and are isotropic in macro. When irradiated with linearly polarized light, azobenzene molecules having an absorption axis in the same direction as the polarization direction are selectively trans-cis isomerized. Molecules relaxed in a trans form having an absorption axis perpendicular to the polarization direction no longer absorb light and are fixed in that state. As a result, anisotropy of absorption coefficient and refractive index, that is, dichroism and birefringence are induced macroscopically. A polymer containing such a photoisomerization group can change the orientation of the polymer itself by photoisomerization and induce a large birefringence. The birefringence induced in this way is stable below the glass transition temperature of the polymer and is suitable for recording each page of a hologram.

  Polyester having cyanoazobenzene in the side chain represented by the following chemical formula (see JP-A-10-340479) is suitable as a material for recording a hologram by the above-described mechanism. This polyester can record the polarization direction of signal light as a hologram due to photo-induced anisotropy due to photoisomerization of side chain cyanoazobenzene, and can record holograms at room temperature. Is held semi-permanently.

  Next, the recording / playback processing routine executed by the personal computer 42 will be described with reference to FIG. First, the operator operates an operating device (not shown) to select a recording process for each page of the hologram or a hologram reproducing process. When recording each page of a hologram, recording information is input to a personal computer in advance and a recording signal is generated.

  In step 100, it is determined whether the recording process for each page of the hologram is selected or the reproduction process for the hologram is selected. If the recording process for each page of the hologram is selected, the hologram is recorded in step 102. Recording to a non-hologram recording medium (whether it is the first recording process of a hologram), or a new recording of each page of a hologram to a hologram recording medium on which a hologram has already been recorded (whether it is the second recording process of a hologram) ).

  Whether or not it is the second recording process of the hologram can be determined by, for example, recording the number of times of writing on the hologram recording medium in advance and irradiating the reproducing light and reading the number of times of writing as described later. It should be noted that at the time of recording, the operator may instruct the hologram recording / reproducing apparatus to determine whether the recording process is the first recording process or the second recording process, and may make a determination based on this instruction.

  In the case of the first hologram recording process, the driving device 48 is driven in steps 105 and 106, the optical rotator 26 is inserted into the optical path, and the shutter 12 is retracted from the optical path so that the laser beam can pass. At the same time, the driving device 50 drives the stepping motor of the xz stage 22 to rotate the hologram recording medium at a predetermined rotational speed.

  In the next step 108, laser light is irradiated and a recording signal for each page is output from the personal computer 42 at a predetermined timing from the recording start position to execute a shift multiple recording process of the hologram on the hologram recording medium.

In the shift multiplex recording method of the present embodiment, shift multiplex recording is performed by using a spherical wave as reference light, making the hologram recording material into a disk shape, and rotating a disk-shaped hologram recording medium (disk). In this shift multiplex recording method, a plurality of pages of holograms can be recorded in the same area by rotating the disk. When the wavelength of the laser beam, the film thickness of the recording medium, the NA of the objective lens, etc. are set appropriately, the disk is simply rotated so that the recording position moves several tens of μm to record the hologram of the next page. The hologram of the next page can be recorded and reproduced without crosstalk with the already recorded page in substantially the same area. This utilizes the fact that since the reference light is a spherical wave, the angle of the reference light is changed by the shift (movement of several tens of μm) of the disc-shaped hologram recording medium.
The distance that determines the shift amount of the disc-shaped hologram recording medium for shift multiplex recording using the spherical reference wave, that is, the distance δ _{spherical at} which the respective holograms can be separated independently is given by the following equation (1).

Where λ is the wavelength of the laser beam, z ₀ is the distance between the objective lens that generates the spherical reference wave and the hologram recording medium, L is the film thickness of the hologram recording medium, NA is the numerical aperture of the objective lens, and θs is the signal light It is an angle with reference light. From the above equation (1), the greater the thickness L of the hologram recording medium, the smaller the shift amount δ determined in accordance with the distance at which the holograms can be separated independently from each other. Can be increased.

  In the personal computer, with the hologram recording medium rotated, the recording signal of each page is sent to the spatial light modulator at a timing determined so that each page of the hologram is recorded at an interval of the shift amount δ from the recording start position. Supply.

  In the present embodiment, similarly to normal digital holographic storage, signal light is Fourier transformed by a lens and irradiated to a hologram recording medium that is a recording medium. By using a lens, a relatively strong light intensity can be obtained on the hologram recording medium surface. At this time, if the distance between the Fourier transform lens and the hologram recording medium is equal to the focal length of the lens, a Fourier transform hologram is recorded. In general, hologram recording is performed by slightly shifting the recording medium from the focus position of the lens for the purpose of suppressing the intensity of the 0th-order diffracted light of the signal light on the recording surface.

  FIG. 3 is a diagram schematically showing a state of hologram recording at the defocus position. On the hologram recording medium surface, the signal light is collected to some extent by the lens, and the diffraction pattern of the data page appears. This diffraction pattern corresponds to the pattern (periodicity) of the digital data page, and has the largest spread when the digital data page is a random pattern.

  On the other hand, the reference light is irradiated to a wider area than the irradiation area of the signal light diffraction pattern so as to cover all the diffraction patterns of the signal light. Refractive index or absorption changes occur where the light is strengthened due to interference between the signal light and the reference light, and these changes are small where the light is weakened. With this phenomenon, hologram recording of each page is performed.

  If it is determined in step 102 that the recording process of the hologram is the second time, for example, by reading the number of times of writing, in step 104, the recording start position of the hologram that has already been recorded is set as the recording start position of the first page of the hologram. Further, the optical rotator 26 is retracted from the optical path, and the second recording process of the hologram is executed with the shift amount δ as described above.

  The amount of shifting the recording start position of the first page of the hologram is preferably ½ of the shift amount δ when recording each page. After shifting the recording start position of the first page of the hologram, hologram recording is performed in the same manner as described above with the shift amount δ. Thereby, each hologram page is newly recorded at a recording position different from the recording position of each page of the already recorded hologram.

  FIG. 5 shows the relationship between the diffracted light intensity at the time of reproduction from the hologram data page already recorded on the hologram recording medium and the diffracted light intensity at the time of reproduction from the newly recorded hologram data page for four pages. . In each newly recorded hologram data page, the recording start position of the first page is shifted in advance by a half of the shift amount, so that the diffracted light from the hologram data page already recorded on the hologram recording medium. Recording is performed so that the maximum point of the diffracted light intensity is located at a position where the intensity is minimum. Since the position where the diffracted light intensity from the hologram data page that has already been recorded is minimized is a position that is out of the Bragg condition of this data base, each hologram page is newly recorded at this minimal position. This eliminates crosstalk during recording and playback.

  Further, when the hologram is recorded for the second time, the optical rotator 26 is retracted, so that the polarization direction of the signal light and the polarization direction of the reference light when recording each page of the hologram are made parallel. Therefore, the polarization state of the signal light when recording each page of the hologram already recorded on the hologram recording medium is different from the polarization state of the signal light when newly recording each page of the hologram .

  In this embodiment, the example in which the polarization plane of the signal light is rotated to change the polarization state has been described, but the polarization plane of the reference light may be rotated to change the polarization state. In this case, an optical rotator similar to the above is disposed on the light reflection side of the polarization beam splitter so as to be inserted into the optical path and retractable from the optical path.

  In the above description, an example is described in which each page of the first hologram is recorded with the optical rotator 26 inserted in the optical path, and each page of the second hologram is recorded with the optical rotator 26 retracted from the optical path. However, conversely, each page of the first hologram is recorded with the optical rotator 26 retracted from the optical path, and each page of the second hologram is recorded with the optical rotator 26 inserted in the optical path. May be.

  Next, the hologram reproduction process will be described. If it is determined in step 100 that the hologram reproduction process has been selected, the shutter 12 is inserted into the optical path in step 110. As a result, the laser light transmitted through the polarization beam splitter 16 is shielded by the shutter 12, so that only the reference light is applied to the hologram recording medium on which the hologram is recorded. The reproduction light diffracted by the hologram recording medium is transmitted through the lens 38, only the reproduction light having a predetermined polarization component is selected and transmitted by the analyzer 44, and the reproduction light received by the detector 40 is converted into an electric signal by the detector 40. Is input to the personal computer 42 and displayed on a display provided in the personal computer.

  In the above description, an example in which shift multiplex recording is performed by rotating the hologram recording medium has been described. However, shift multiplex recording may be performed by moving the hologram recording medium in a straight line. Instead of moving to, signal light and reference light may be scanned onto the hologram recording medium.

  In the above description, the embodiment using the shift multiplex recording method has been described. However, the present invention can also be applied to an angle multiplex recording method and a wavelength multiplex recording method.

  When applied to the angle multiplexing recording method, at the time of the first hologram recording, as shown in FIG. 6, the signal recording light and the reference light are simultaneously recorded while changing the angle formed by the reference light with respect to the signal light by a predetermined angle θ. The information of the signal light is multiplexed and recorded on the hologram recording medium as a hologram of a plurality of pages by changing the recording angle by irradiating the light.

  When newly recording each page of a hologram on a recorded hologram recording medium, after the recording start position is shifted by θ / 2, the first recording is performed while changing the angle formed by the reference light with respect to the signal light by a predetermined angle θ. Record each page of the hologram in the same way as time. As a result, each page of the hologram is newly recorded at a recording angle different from the recording angle of each page of the already recorded hologram. The angle of the reference light for newly recording each page of this hologram is equal to the angle at which the light intensity of the reproduction light from each page of the hologram recorded on the hologram recording medium is minimized. Further, as described in the shift multiplex recording method, the angle at which the diffracted light intensity from the hologram data page that has already been recorded is minimized is also an angle that deviates from the Bragg condition of this data page.

  In the above description, an example in which multiple recording is performed by changing the angle of the reference light has been described. However, multiple recording may be performed by changing the angle of the signal light with respect to the reference light.

  When applied to the wavelength multiplexing recording method, at the time of the first hologram recording, the angle formed by the signal light and the reference light is kept constant, and the signal light and the reference light are changed while changing the wavelength of the signal light and the reference light by a predetermined wavelength Δλ. The optical recording medium is simultaneously irradiated with light, and the information of the signal light is multiplexed and recorded on the hologram recording medium as a hologram of a plurality of pages.

  When newly recording each page of a hologram on a recorded hologram recording medium, the wavelength of the signal light and reference light is shifted by Δλ / 2, and then the wavelength of the signal light and reference light is changed by Δλ. However, each page of the hologram is recorded as in the first recording. Thus, each page of the hologram is newly recorded at a wavelength different from the wavelength of the signal light and the reference light when each page of the already recorded hologram is recorded. The wavelengths of the signal light and the reference light for newly recording each page of the hologram are equal to the wavelength at which the light intensity of the reproduction light from each page of the hologram recorded on the hologram recording medium is minimized. Further, as described in the shift multiplex recording method, the wavelength at which the diffracted light intensity from each data page of the already recorded hologram is minimized is also a wavelength that is outside the Bragg condition of this data page.

  Whether applied to the angle multiplex recording method or the wavelength multiplex recording method, the polarization direction of the signal light and the polarization direction of the reference light when each page of the hologram recorded on the recording medium is recorded. When parallel, the polarization direction of the signal light when newly recording each page of the hologram is orthogonal to the polarization direction of the reference light, and the signal light when recording each page of the hologram recorded on the recording medium When the polarization direction and the polarization direction of the reference light are orthogonal, the polarization direction of the signal light and the polarization direction of the reference light when recording each new page of the hologram are made parallel and already recorded on the recording medium. The polarization state of the signal light or reference light when recording each page of the hologram is different from the polarization state of the signal light or reference light when newly recording each page of the hologram Preferred.

It is the schematic of the hologram recording / reproducing apparatus of this Embodiment. It is the schematic which shows the structure of a hologram recording medium. It is the schematic which shows the relationship between a signal light irradiation area | region and a reference light irradiation area | region. It is a flowchart which shows the hologram recording / reproducing process routine of this Embodiment. It is a diagram showing the relationship between the diffracted light intensity from the recorded hologram data page and the diffracted light intensity from the newly recorded hologram data page. It is a figure explaining the angle multiplexing recording method which is one of the hologram multiplexing recording methods. (A), (B) is a figure explaining the shift multiple recording method.

Explanation of symbols

10 Laser Transmitter 36 Spatial Light Modulator

Claims (16)

The recording angle is changed by irradiating the rewritable optical recording medium with the signal light and the reference light simultaneously while changing the angle formed by the signal light and the reference light, and the information of the signal light is converted into a hologram of a plurality of pages. A hologram recording method for newly recording a hologram by overwriting in a recording area in which a plurality of pages of holograms are multiplexed and recorded, using a hologram recording method for performing multiple recording on the optical recording medium ,
Recording when the hologram to be overwritten is recorded so that the intensity of diffracted light from the hologram of a plurality of pages that are multiplexed and recorded in the recording area to be overwritten is minimized at the recording angle when the hologram is newly recorded as different recording angles and angle, a hologram recording method for newly recording a hologram. By irradiating the rewritable optical recording medium with the signal light and the reference light at the same time while keeping the angle between the signal light and the reference light constant and relatively moving at least one of the signal light and the reference light and the optical recording medium. Using a hologram recording method in which the information of the signal light is multiplexed and recorded on the optical recording medium as a hologram of a plurality of pages by changing the recording position, the hologram is overwritten in the recording area where the holograms of a plurality of pages are multiplexed and recorded. A new hologram recording method for recording,
Recording when a hologram to be overwritten is recorded so that the intensity of diffracted light from the holograms of a plurality of pages that are multiplexed and recorded in the recording area to be overwritten is minimized at the recording position when the hologram is newly recorded. A hologram recording method for newly recording a hologram as a recording position different from the position. The angle between the signal light and the reference light is kept constant, the signal light and the reference light are simultaneously rewritable while changing the wavelengths of the reference light and the signal light, and the information of the signal light is displayed on a plurality of pages. A hologram recording method for newly recording a hologram by overwriting in a recording area in which a plurality of holograms are recorded in a multiplexed manner using a hologram recording method of multiplex recording on the optical recording medium as a hologram of
The hologram to be overwritten so that the diffracted light intensity from the holograms of a plurality of pages that are multiplexed and recorded in the recording area to be overwritten is minimized with respect to the wavelengths of the reference light and the signal light when the hologram is newly recorded. A hologram recording method for newly recording a hologram at a wavelength different from the wavelengths of the reference light and signal light at the time of recording. Polarization state of signal light or reference light when multiple holograms are recorded on the recording area of the optical recording medium, and polarization state of signal light or reference light when the hologram is newly recorded by overwriting the recording area The hologram recording method according to any one of claims 1 to 3, wherein: When the polarization direction of the signal light when the hologram is multiplexed and recorded on the recording area of the optical recording medium and the polarization direction of the reference light are parallel, a signal for overwriting the recording area and newly recording the hologram The polarization direction of the light and the polarization direction of the reference light are orthogonalized,
When the polarization direction of the signal light and the polarization direction of the reference light are orthogonal when the hologram is multiplexed and recorded on the recording area of the optical recording medium, when the hologram is newly recorded by overwriting the recording area 5. The hologram recording method according to claim 4 , wherein the polarization direction of the signal light and the polarization direction of the reference light are made parallel to each other. The hologram recording method according to claim 1, wherein the optical recording medium includes a photorefractive material. The hologram recording method according to claim 1, wherein the optical recording medium includes a photochromic material. The hologram recording method according to claim 1, wherein the optical recording medium has a polarization sensitive material. The hologram recording method according to claim 1, wherein the optical recording medium has at least one polymer selected from a polyester group. The hologram recording method according to claim 9, wherein the polymer has an azobenzene skeleton in a side chain. A light source that emits coherent light;
A stage for rotating or moving the optical recording medium;
After separating the coherent light into light for reference light and light for signal light, the reference light and the signal light are simultaneously irradiated to the rewritable optical recording medium , and the angle formed by the reference light and the signal light is Light separation optical path changing means for changing the optical path of the light separated so as to change;
A space that is arranged in the optical path of the signal light and modulates the signal light according to the supplied recording signal for each page and generates signal light for recording each page of the hologram A light modulation element;
When recording the first hologram, the recording angle is changed by irradiating the rewritable optical recording medium simultaneously with the signal light and the reference light while changing the angle formed by the signal light and the reference light. A recording signal for each page is supplied to the spatial light modulator so that signal light information is multiplexed and recorded on the optical recording medium as a hologram of a plurality of pages, and the light source, the stage, and the light separation optical path change While controlling the means,
In the case of newly recording a hologram by overwriting in a recording area in which a plurality of pages of holograms are multiplexed and recorded, the recording angle at which the hologram is newly recorded is recorded in the recording area that is overwritten in the overwritten recording area. intensity of diffracted light from the hologram of the page so that a local minimum, said as different recording angles the recording angle when recording the hologram is overwritten, so as to newly record a hologram, a recording signal for each page Control means for supplying a spatial light modulation element to control the light source, the stage, and the light separation optical path changing means;
Hologram recording apparatus comprising: A light source that emits coherent light;
A stage for rotating or moving the optical recording medium;
After separating the coherent light into light for reference light and light for signal light, the reference light and the signal light are simultaneously irradiated to the rewritable optical recording medium , and the angle formed by the reference light and the signal light is Light separation optical path changing means for changing the optical path of the light separated so as to be constant ;
A space that is arranged in the optical path of the signal light and modulates the signal light according to the supplied recording signal for each page and generates signal light for recording each page of the hologram A light modulation element;
When recording the first hologram , the signal light and the reference light can be rewritten simultaneously while the angle formed by the signal light and the reference light is kept constant and the optical recording medium is rotated or moved with respect to the signal light and the reference light. A recording signal for each page is recorded on the optical recording medium so that the recording position is changed by irradiating the optical recording medium, and the information of the signal light is multiplexed and recorded on the optical recording medium as a hologram of a plurality of pages. And controlling the light source, the stage, and the light separation optical path changing means,
In the case where a new hologram is recorded by overwriting in a recording area in which a plurality of pages of holograms are recorded in a multiplexed manner, the recording positions for newly recording the holograms are recorded in a multiplexed manner in the recording area to be overwritten. The recording signal for each page is recorded so that the hologram is newly recorded as a recording position different from the recording position when the overwritten hologram is recorded so that the diffracted light intensity from the hologram of the page is minimized. Control means for supplying a spatial light modulation element to control the light source, the stage, and the light separation optical path changing means;
Hologram recording apparatus comprising: A light source configured to irradiate by changing the wavelength of the coherent light by a predetermined wavelength;
A stage for rotating or moving the optical recording medium;
After separating the coherent light into light for reference light and light for signal light, the reference light and the signal light are simultaneously irradiated to the rewritable optical recording medium , and the angle formed by the reference light and the signal light is Light separation optical path changing means for changing the optical path of the light separated so as to be constant ;
A space that is arranged in the optical path of the signal light and modulates the signal light according to the supplied recording signal for each page and generates signal light for recording each page of the hologram A light modulation element;
When recording the first hologram, the angle between the signal light and the reference light is kept constant, and the signal recording light and the reference light are simultaneously rewritten while changing the wavelengths of the reference light and the signal light. And supplying a recording signal for each page to the spatial light modulator so that the information of the signal light is multiplexed and recorded on the optical recording medium as a hologram of a plurality of pages , and the light source, the stage, and the light While controlling the separation optical path changing means,
When a new hologram is recorded by overwriting in a recording area in which multiple pages of holograms are recorded in a multiplexed manner, the wavelengths of the reference light and the signal light for newly recording the hologram are multiplexed in the overwritten recording area. as intensity of diffracted light from the hologram of a plurality of pages being recorded is minimized, as the wavelength different from the wavelength of the reference light and the signal light when recording a hologram to be overwritten, so as to newly record a hologram A control means for supplying a recording signal for each page to the spatial light modulator to control the light source, the stage, and the light separation optical path changing means;
Hologram recording apparatus comprising: An analyzer that transmits a component in a predetermined polarization direction of diffracted light from each page of a hologram recorded on the optical recording medium;
A detector for detecting the intensity of transmitted light that has passed through the analyzer;
The hologram recording apparatus according to claim 11 , further comprising: An optical rotator for rotating the polarization plane of the signal light is further arranged in the optical path of the signal light,
Polarization state of signal light or reference light when multiple holograms are recorded on the recording area of the optical recording medium, and polarization state of signal light or reference light when the hologram is newly recorded by overwriting the recording area The hologram recording device according to claim 11 , wherein the hologram recording device is different from the hologram recording device. When the polarization direction of the signal light when the hologram is multiplexed and recorded on the recording area of the optical recording medium and the polarization direction of the reference light are parallel, a signal for overwriting the recording area and newly recording the hologram The polarization direction of the light and the polarization direction of the reference light are orthogonalized,
When the polarization direction of the signal light and the polarization direction of the reference light are orthogonal when the hologram is multiplexed and recorded on the recording area of the optical recording medium, when the hologram is newly recorded by overwriting the recording area The hologram recording apparatus according to claim 15 , wherein the polarization direction of the signal light is parallel to the polarization direction of the reference light.

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