JP2005234181A - Hologram recording method and hologram reproducing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hologram reproducing method by which, when data are rewritten, crosstalk by the original data is prevented and reproduction light with little noise and a high S/N ratio can be obtained as well as to provide a hologram recording method by which rewriting of data can be performed at a high speed without passing through an erasing step. <P>SOLUTION: In a prescribed recording area of an optical recording medium in which a hologram concerned with first data has been recorded, a hologram concerned with second data is overwritten and recorded under conditions which generate reproduction light in a polarized state different from the polarized state of reproduction light obtained from the hologram concerned with the first data. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a hologram recording method and a hologram reproducing method, and more particularly to a hologram recording method capable of rewriting data recorded as a hologram and a hologram reproducing method capable of reproducing the rewritten data.

  As a next-generation computer file memory, a holographic memory that has both a large capacity derived from a three-dimensional recording area and a high speed derived from a two-dimensional batch recording / reproducing system has attracted attention. In the holographic memory, 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. Digital data can be recorded / reproduced by digitizing binary digital data “0, 1” instead of an analog image as “bright, dark” and recording / reproducing the hologram as a hologram. Recently, S / N and bit error rate evaluation based on the specific optical system and volume multiplex recording system of this digital holographic memory system, or a proposal for two-dimensional encoding has been made, such as the influence of aberration of the optical system, etc. Research from a more optical viewpoint is also progressing.

  6A and 6B 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. 6B). A new hologram can be recorded. That is, holograms can be multiplex-recorded in substantially the same volume by recording while slightly moving 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 media materials for holographic memory, photopolymer materials, photorefractive materials, photochromic materials represented by azopolymers, and the like have been actively studied. Among these, a recording medium using a photorefractive material or a photochromic material can erase recorded data and record new data. Since these rewritable recording media can be repeatedly recorded, in addition to storing large amounts of information, they are also expected to be used as backup memories such as hard disks.

Conventionally, when rewriting data recorded as a hologram, it has been common to record a new hologram after erasing the recorded hologram all at once by uniformly irradiating the entire recording area with light. . Further, in a recording medium using a photorefractive material or a photochromic material, holograms can be erased collectively by raising the temperature of the recording medium and holding it at a high temperature (Non-patent Document 1).
Appl.Phys.Lett.60,4-5 (1992)

  However, the conventional data rewriting method has a problem that processing time becomes long because a new hologram is recorded after erasing a hologram related to unnecessary data. On the other hand, the hologram can be overwritten without erasing, but depending on the recording / reproducing conditions, crosstalk may occur due to the hologram that has not been erased, and the quality of the reproduced light may deteriorate.

  The present invention has been made to solve the above problem, and an object of the present invention is to provide a hologram recording method capable of rewriting data at high speed without going through an erasing process. Another object of the present invention is to provide a hologram reproduction method capable of preventing reproduction of crosstalk due to original data and obtaining reproduction light with low noise and large S / N when data is rewritten. There is.

  In order to achieve the above object, the hologram recording method of the present invention provides a reproduction light obtained from a hologram related to the first data in a predetermined recording area of an optical recording medium on which the hologram related to the first data is recorded. The hologram relating to the second data is overwritten and recorded on the condition that the reproduction light having a polarization state different from the polarization state is generated.

  In the hologram recording method of the present invention, when the hologram according to the second data is overwritten and recorded in the predetermined recording area of the optical recording medium in which the hologram according to the first data is recorded, the first data is recorded. The hologram according to the second data is overwritten and recorded on the condition that the reproduction light having a polarization state different from the polarization state of the reproduction light obtained from the hologram is generated. When recorded by this method, the first data is not erased, but the polarization state of the reproduction light obtained from the hologram related to the second data is the polarization of the reproduction light obtained from the hologram related to the first data. Since it is different from the state, it is possible to reproduce the second data without crosstalk. That is, data can be rewritten at high speed without going through an erasing process.

  In the above hologram recording method, the recording condition of the second data is appropriately determined according to the recording condition when the first data is recorded. For example, when the reproduction light is linearly polarized light, the hologram related to the second data is overwritten and recorded on the condition that the reproduction light having a polarization direction different from the polarization direction of the reproduction light obtained from the hologram related to the first data is generated. can do. When the reproduction light is circularly polarized, the hologram related to the second data is overwritten and recorded under the condition that the reproduction light is circularly polarized in the reverse direction to the reproduction light obtained from the hologram related to the first data. Can do.

  Further, the reproduction light is applied to the predetermined area of the optical recording medium in which the hologram related to the first data is recorded with the readout light having a polarization state different from the polarization state of the readout light for reproducing the hologram related to the first data. The hologram related to the second data may be overwritten and recorded on the condition that the readout light is not generated from the hologram related to the first data by the readout light.

  Further, the hologram according to the second data can be recorded by overwriting based on the recorded reproduction condition by recording the reproduction condition of the hologram according to the first data on the optical recording medium. The condition for overwriting and recording the hologram according to the second data can be set in advance according to the reproduction condition of the hologram according to the first data.

  Further, the polarization state of the reproduction light is determined by a combination of the polarization state of the signal light and the reference light on which the hologram is recorded and the polarization state of the readout light for reproducing the hologram. Therefore, the hologram recording method of the present invention is a hologram recording method for overwriting and recording the hologram according to the second data in a predetermined recording area of the optical recording medium in which the hologram according to the first data is recorded, A combination of the polarization state of the signal light and the reference light recording the hologram related to the first data and the polarization state of the readout light reproducing the hologram related to the first data, and the signal recording the hologram related to the second data It can also be referred to as a method of overwriting recording the hologram related to the second data so that the combination of the polarization state of the light and the reference light and the combination of the polarization state of the readout light for reproducing the hologram related to the second data is different.

  In order to achieve the above object, the hologram reproduction method of the present invention provides reproduction light obtained from the hologram related to the first data in a predetermined recording area of the optical recording medium in which the hologram related to the first data is recorded. When the hologram related to the second data is overwritten and recorded under the condition that the reproduction light having a polarization state different from the polarization state of the optical recording medium is recorded, the conditions for recording and reproducing the hologram related to the second data are set in the optical recording medium. Recording in advance, determining the polarization state of the reproduction light obtained from the hologram according to the second data based on the recorded conditions, selectively detecting the reproduction light of the determined polarization state, The hologram related to the second data is reproduced.

  Since the first data is not completely erased, the obtained reproduction light includes reproduction light obtained from the hologram related to the first data and reproduction light obtained from the hologram related to the second data There is. According to this hologram reproducing method, the polarization state of the reproduction light obtained from the hologram is determined based on the condition on which the hologram relating to the second data is recorded, and the reproduction light of the determined polarization state is selectively selected. Since it is detected, it is possible to detect the reproduction light obtained from the hologram related to the second data separately from the reproduction light obtained from the hologram related to the first data. Therefore, when reproducing the second data, crosstalk due to the first data can be prevented, and reproduced light with less noise and a large S / N can be obtained.

  As described above, according to the hologram recording method of the present invention, there is an effect that data can be rewritten at high speed without going through an erasing process. In addition, according to the hologram reproducing method of the present invention, when data is rewritten, it is possible to prevent crosstalk due to the original data and obtain reproduced light with less noise and large S / N. can get.

Hereinafter, embodiments of the present invention will be described in detail.
(Hologram recording medium)
FIG. 1 shows the configuration of a hologram recording medium (optical recording medium). The hologram recording medium 24 used in the present embodiment is formed in a disk shape as will be described later, but in the figure, a part cut out in a rectangular shape is shown. As shown in FIG. 1A, the hologram recording medium 24 is composed of an optical recording layer 23 molded into a thick film having a thickness of, for example, 100 μm or more. If the optical recording layer carrier has insufficient strength, as shown in FIG. 1 (B) or (C), a substrate 25 made of a plate-like transparent medium such as quartz or plastic is provided on one or both sides. Can be provided.

  The optical recording layer 23 is made of a material exhibiting light-induced birefringence (light-induced anisotropy and light-induced dichroism). A material exhibiting light-induced birefringence can record the polarization direction of incident light in response to the polarization state of light incident thereon. For example, a polymer or polymer liquid crystal having a photoisomerizable group in the side chain, or a polymer in which a photoisomerizable molecule is dispersed is irradiated with linearly polarized light, and photoisomerization is induced. Depending on the direction, anisotropy of the refractive index occurs, and the polarization direction can be recorded and stored. At this time, if the reference light is irradiated at the same time, the polarization direction of the signal light can be recorded as a hologram.

  A normal hologram is recorded with the polarization directions of signal light (object light) and reference light being the same (parallel). The hologram recorded or recorded in this way is referred to as an intensity modulation hologram in this specification. On the other hand, the above-mentioned material exhibiting light-induced birefringence can record the signal light as a hologram by making the polarization directions of the signal light and the reference light orthogonal. The hologram recorded or recorded in this way is referred to as a polarization modulation hologram in this specification. However, similarly to the intensity modulation hologram, the polarization modulation hologram can be spatially intensity-modulated in binary according to the two-dimensional data.

  For example, S-polarized signal light can be recorded as an intensity-modulated hologram by S-polarized reference light, and can be recorded as a polarization-modulated hologram by P-polarized reference light. The S-polarized signal light recorded as the intensity-modulated hologram can be reproduced as P-polarized diffracted light by the P-polarized readout light, and the S-polarized signal light recorded as the polarization-modulated hologram is It can be reproduced as s-polarized diffracted light by the polarized readout light. At the time of recording, instead of changing the polarization angle of the reference light, the polarization angle of the signal light may be changed.

  As the polymer having a photoisomerizable group in the side chain, a material having at least one polymer selected from the polyester group and having an azobenzene skeleton in the side chain is preferable. In particular, a polyester having cyanoazobenzene in the side chain represented by the following chemical formula (see JP-A-10-340479) is suitable.

(Recording / playback conditions)
In the present invention, the first data is recorded so that the polarization state of the reproduction light from the hologram recording the first data is different from the polarization state of the reproduction light from the hologram recording the second data. The second data is overwritten and recorded in the area, and the second data is selectively reproduced by utilizing the fact that the polarization state of the reproduction light is different. Any recording / reproducing condition may be applied as long as the recording / reproducing method having this feature can be implemented.

  Tables 1 to 4 below show examples of combinations of signal light, reference light, readout light, and reproduction light (hereinafter referred to as “recording / reproduction conditions”). Table 1 shows recording / reproduction conditions (A group) in which the polarization state of the reproduction light is S-polarized light, and Table 2 shows recording / reproduction conditions (B group) in which the polarization state of the reproduction light is P-polarization. Table 3 shows recording / reproduction conditions (C group) in which the polarization state of the reproduction light is RC polarization (clockwise circular polarization), and Table 4 shows the polarization state of the reproduction light is LC polarization (counterclockwise circular polarization). The recording / reproducing conditions (D group) to become are shown. The recording / playback conditions belonging to each group are represented by a combination of a group name and a number assigned outside the table frame, for example, “A-1”.

  One recording / reproducing condition is selected from each of two or more arbitrary groups of the A to D groups to form a set. For example, the recording / reproducing condition (A-1) in Table 1 and the recording / reproducing condition (B-3) in Table 2 are set as a set. When rewritable recording is performed, the second data is overwritten and recorded on the recording / reproducing condition (B-3) in the area where the first data is recorded on the recording / reproducing condition (A-1). Further, when rewriting the second data, overwriting is again performed under the recording / reproducing condition (A-1). In this way, a recording image having a good S / N can be obtained by sequentially using the recording / reproduction conditions as a set and changing the recording / reproduction conditions for each rewrite.

(Hologram recording / reproduction method)
Hereinafter, with reference to the drawings, three embodiments in which the hologram recording method / reproduction method of the present invention is applied to data rewriting in a digital holographic storage will be described in detail. In these embodiments, two arbitrary groups of the A to D groups are selected, and recording / reproduction conditions are selected from each of the selected groups to form a set. In the following embodiment, an example in which two groups are selected from the above four groups will be described. However, three groups or four groups are selected, and recording / playback conditions are selected from each of the selected groups. It can also be a pair.

[First Embodiment]
In the first embodiment, the recording / reproducing condition (A-1) in Table 1 and the recording / reproducing condition (B-3) in Table 2 are used as a set. When rewritable recording is performed, the second data is overwritten and recorded on the recording / reproducing condition (B-3) in the area where the first data is recorded on the recording / reproducing condition (A-1). Further, when rewriting the second data, overwriting is again performed under the recording / reproducing condition (A-1). In this way, the recording / reproducing conditions as a set are alternately used repeatedly, and the recording / reproducing conditions are changed for each rewrite.

  Next, the hologram recording / reproducing apparatus used in this embodiment will be described.

As shown in FIG. 2, 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 polarizing beam splitter 16, there are a half-wave plate 52 that rotates the polarization plane of the light reflected by the polarizing beam splitter 16 by 90 degrees and a quarter-wave plate 54 that converts linearly polarized light into circularly polarized light. However, they are separately arranged in the optical path so that they can be inserted into and retracted from the optical path. On the light transmission side of the quarter-wave plate 54, 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 spherical surface by condensing the laser light for reference light. Objective lenses 20 that generate reference light composed of waves 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 a spherical reference wave as reference light.

  On the light transmitting side of the polarizing beam splitter 16, a shutter 12 for blocking the light transmitted through the polarizing beam splitter 16 and an optical rotator 26 that rotates the polarization plane by 90 degrees are respectively inserted into the optical path and the optical path. It is arranged so that it can be evacuated. On the light transmission side of the optical rotator 26, a reflection mirror 28 and lenses 30, 32, and 34 are configured to reflect the laser light for signal light at a reflection angle of 45 ° and change the optical path toward the hologram recording medium. 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. Between the spatial light modulator 36 and the lens 34, a half-wave plate 59 and a quarter-wave plate 56 that converts linearly polarized light into circularly polarized light are disposed so as to be inserted into and retracted from the optical path. The spatial light modulator 36 has a polarization modulation function. For example, when the incident light to the spatial light modulator 36 is S-polarized light, the emitted light is P-polarized light.

  The lenses 30 and 32 collimate the laser light into a large-diameter beam and irradiate the spatial light modulation element 36. The lens 34 uses the light modulated and transmitted by the spatial light modulation element 36 as signal light on the hologram recording medium 24. To collect light. 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 simultaneously irradiated with the signal light and the reference light.

  On the reproduction light transmitting side of the hologram recording medium 24, a detector 40 is arranged which is composed of a lens 38 and an imaging device such as a CCD, and converts the received reproduction light into an electric signal and outputs it. The detector 40 is connected to a personal computer 42. Between the lens 38 and the detector 40, a quarter-wave plate 58 that converts circularly polarized light into circularly polarized light and light in a predetermined polarization direction from the reproduction light (for example, 0 ° polarization component, 45 ° polarization component, or 90 ° Each of the analyzers 44 that selectively transmit (polarized components) is disposed so as to be separately inserted into and retracted from the optical path.

  The personal computer 42 includes a CPU, a ROM, a RAM, a memory, etc., and is connected to an input / output device (not shown) such as a display and a keyboard, and generates a pattern according to a recording signal supplied from the personal computer at a predetermined timing. It is connected to the spatial light modulation element 36 through the pattern generator 46. In the memory, recording / reproducing conditions when performing the overwrite recording process are stored in a table according to the recording / reproducing conditions when performing the first recording process. In the present embodiment, the recording / reproducing condition (A-1) in Table 1 is used as the recording / reproducing condition when performing the first recording process, and Table 2 is the recording / reproducing condition when performing the first overwrite recording process. Recording / reproducing conditions (B-3) are stored in a table. As described above, when rewriting is performed a plurality of times, these recording / reproducing conditions are used alternately.

  Further, in the personal computer 42, the shutter 12, the optical rotator 26, the half-wave plates 52 and 59, the quarter-wave plates 54, 56, and 58, and the analyzer 44 are separately inserted into the optical path. A driving device 48 is connected to drive and simultaneously retract the shutter 12 and the like inserted in the optical path from the optical path. The personal computer 42 is connected to a driving device 50 that drives the xz stage 22.

  Next, a recording / reproducing process 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. It is determined whether recording is in a non-recorded area (first recording process) or recording in an area where a hologram has already been recorded (overwrite recording process). Here, the first recording information to be recorded is the first data, and the recording information to be overwritten is the second data.

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

  In the present embodiment, the first data is recorded using both the signal light and the reference light as S-polarized light. Therefore, in the case of the first recording process, the driving device 48 is driven in step 104, and the shutter 12, the half-wave plate 52, and the quarter-wave plates 54 and 56 are retracted from the optical path, and the laser beam passes. The optical rotator 26 and the half-wave plate 59 are inserted into the optical path. Further, 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 106, the laser light is irradiated and the recording signal of the first data is output from the personal computer 42 at a predetermined timing from the recording start position, and the shift multiple recording process of the hologram to the hologram recording medium 24 is executed. To do.

  The P-polarized light that has passed through the polarization beam splitter 16 is converted to S-polarized light by the optical rotator 26, then modulated by the spatial light modulator 36 to become P-polarized light, and further converted to S-polarized light by the half-wave plate 59 and signal light. Is applied to the hologram recording medium 24. On the other hand, the S-polarized light reflected by the polarization beam splitter 16 is applied to the hologram recording medium 24 as reference light. The signal light and the reference light are S-polarized light, and the polarization direction of the signal light and the polarization direction of the reference light are parallel. Therefore, the first data is recorded as an intensity modulation hologram.

  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. If 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 in order 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.

In shift multiplex recording using a spherical reference wave, a distance that determines the shift amount of the disc-shaped hologram recording medium, that is, a distance δ _spherical that allows the holograms to be separated independently is given by the following equation (1).

Here, 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 θ _s is the signal light And the reference light. From the above equation (1), the greater the thickness L of the hologram recording medium, the smaller the shift amount δ determined according to the distance at which the holograms can be separated independently from each other. Can be increased.

  The personal computer 42 spatially modulates the recording signal of each page 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 while the hologram recording medium 24 is rotated. This is supplied to the element 36.

  In the next step 108, the recording condition of the first recording process, that is, information on the polarization state of the signal light and the reference light when the first data is recorded is recorded in a specific area on the hologram recording medium 24, and processed. Exit the routine. As the specific area, for example, a so-called FAT area in which a file allocation table (FAT) that associates a file to be recorded with a recording area in which the file is recorded can be used.

  If it is determined in step 102 that the overwrite recording process is performed, in step 110, the quarter-wave plates 54, 56, 58, and the half-wave plate 52 are retracted from the optical path so that the laser beam can pass. At the same time, the shutter 12 and the analyzer 44 are inserted into the optical path, and the hologram recording medium 24 is irradiated with reading light in step 112 to read the recording / reproducing conditions when the first recording process is performed. The laser light transmitted through the polarization beam splitter 16 is shielded by the shutter 12, and only the laser light reflected by the polarization beam splitter 16 is irradiated onto the hologram recording medium 24. The reproduction light diffracted by the hologram recording medium 24 passes through the lens 38, and only the reproduction light having a predetermined polarization component is selectively transmitted by the analyzer 44, and the reproduction light received by the detector 40 is detected by the detector 40. It is converted into an electrical signal and input to the personal computer 42.

  In the next step 114, the table stored in the memory is read out, and in step 116, the second data is overwritten and recorded from the read / write conditions of the first recording process with reference to the read table. Determine the recording conditions.

  In the present embodiment, the condition that the signal light is P-polarized light and the reference light is S-polarized light is set as the overwrite recording condition. In this case, when S-polarized light is irradiated as readout light, P-polarized reproduction light is obtained. When the overwriting recording condition is determined, in step 118, the driving device 48 is driven to retract the shutter 12 and the analyzer 44 from the optical path so that the laser beam can pass, and the optical rotator 26 is moved in the optical path. Insert into.

  In the next step 120, the laser light is irradiated, and the recording signal of the second data is output from the personal computer 42 at a predetermined timing from the recording start position, and the shift multiple recording process of the hologram to the hologram recording medium 24 is executed. To do.

  The P-polarized light that has passed through the polarization beam splitter 16 is converted into S-polarized light by the optical rotator 26, then modulated by the spatial light modulator 36, and irradiated to the hologram recording medium 24 as P-polarized signal light. On the other hand, the S-polarized light reflected by the polarization beam splitter 16 is applied to the hologram recording medium 24 as reference light. The signal light is P-polarized light and the reference light is S-polarized light, and the polarization direction of the signal light is orthogonal to the polarization direction of the reference light. Therefore, the second data is recorded as a polarization modulation hologram.

  In the next step 122, the recording condition of the overwrite recording process, that is, information on the polarization state of the signal light and the reference light when the second data is recorded is recorded in a specific area on the hologram recording medium 24, and the processing routine is performed. Exit.

  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 and the analyzer 44 are inserted into the optical path in step 124, and the hologram recording medium 24 is irradiated with readout light in step 126. Then, the recording / reproducing conditions at the time of the last recording process are read. That is, the recording / reproduction condition of the latest data that has been overwritten is read. In the present embodiment, since the polarization state of the readout light is common (both S polarization) in the initial recording process and the overwrite recording process, the recording / reproducing conditions can be read by irradiating the same readout light.

  Whether or not the recording / reproducing condition is the last when the recording process is performed can be determined based on the intensity of the obtained reproducing light. For example, when data that has been overwritten and data that has been overwritten are read by irradiating read light in the same polarization state, a plurality of reproduction lights can be obtained corresponding to each of the data before and after overwriting. The intensity of the reproduction light corresponding to the latest data is greater than the intensity of the reproduction light corresponding to the other data. Therefore, the recording / reproducing condition read from the reproducing light having the highest intensity is the recording / reproducing condition when the recording process is performed last. In this embodiment, when S-polarized readout light is irradiated, two reproduction lights of S-polarized light and P-polarized light are obtained. In this case, the intensity difference between the two is detected by rotating the transmission axis of the analyzer 44, and the recording / reproducing conditions are read from the reproducing light having a higher intensity.

  Hereinafter, a case where the second data overwritten and recorded under the condition (B-3) is reproduced will be described. In the next step 128, reproduction processing of the second data is executed. The laser beam that has passed through the polarization beam splitter 16 is shielded by the shutter 12, and the S-polarized light reflected by the polarization beam splitter 16 is applied to the hologram recording medium 24 as readout light. In this example, the reading light for reading the recording / reproducing condition and the reading light for reproducing the second data are lights having the same polarization state. Therefore, the second data reproduction process can be performed following the reading of the recording / reproduction conditions.

  The reproduction light diffracted by the hologram recording medium 24 includes an S-polarized component corresponding to the first data and a P-polarized component corresponding to the second data. This reproduced light is transmitted through the lens 38, and only the P-polarized component corresponding to the second data is selectively transmitted by the analyzer 44 and received by the detector 40. The received reproduction light is converted into an electrical signal by the detector 40 and input to the personal computer 42. As described above, since the S polarization component corresponding to the first data is blocked by the analyzer 44, the second data is reproduced without crosstalk. Further, noise due to the first data is prevented, and reproduction light having a large S / N can be obtained.

  As described above, in this embodiment, since the second data is overwritten and recorded without erasing the first data, the data can be rewritten at a high speed. In addition, when overwriting the second data, the recording / reproduction is performed so that reproduction light having a polarization direction different from that of the first data can be obtained by referring to the recording / reproduction conditions when the first data is recorded. Since the conditions are determined, the analyzer can be used to easily separate and detect the reproduction light of the second data from the reproduction light of the first data. That is, the second data can be reproduced without crosstalk.

  In the present embodiment, when it is determined that the hologram reproduction process is selected, the recording / reproduction condition at the time of the last recording process is read, and the reproduction process is performed based on the obtained recording / reproduction condition. Although the example has been described, the recording / reproducing process may be performed directly without reading the recording / reproducing conditions. For example, when the reproduction light diffracted by the hologram recording medium includes an S-polarized component corresponding to the first data and a P-polarized component corresponding to the second data, the transmission axis of the analyzer is rotated. Thus, by comparing the intensities of the two components contained in the reproduction light with a photodetector, it is possible to know which component is the reproduction light from the latest data. Further, the data can be reproduced from the reproduction light component of the latest data.

  In addition, when a plurality of pages of holograms are recorded in the same area, only some pages may be rewritten. Even in this case, the recording / reproduction conditions in which the latest data is recorded can be known by comparing the intensity of these two reproduction lights with a photodetector by rotating the transmission axis of the analyzer.

  For example, when holograms of all 2 pages are overwritten and recorded under the condition (B-3) and only the first page is rewritten under the condition (A-1), the first light beam is irradiated with the S-polarized reference light. From this page, mainly S-polarized reproduction light is emitted, and from the second page, mainly P-polarized reproduction light is emitted. Since a multi-page hologram is recorded from the first page, in this example, the intensity of the reproduction light for the first page is compared, and the intensity of the S-polarized reproduction light is greater than the intensity of the P-polarized reproduction light. In this case, it is understood that the condition (A-1) is a recording / reproducing condition in which the latest data is recorded. Note that the number of hologram pages to be read for comparison may be any number, but a small number of pages is preferable for speeding up.

  In the hologram recording / reproducing apparatus shown in FIG. 2, among the elements that can be inserted into or retracted from the optical path, the symbols of the elements that are inserted according to the recording / reproducing conditions of the present embodiment are shown together in Table 5 below. The elements other than the element to be inserted are retracted from the optical path.

[Second Embodiment]
In the second embodiment, the recording / reproducing condition (C-5) in Table 3 and the recording / reproducing condition (D-3) in Table 4 are used as a set. When rewritable recording is performed, the second data is overwritten and recorded on the recording / reproducing condition (D-3) in the area where the first data is recorded on the recording / reproducing condition (C-5). Further, when the second data is rewritten, overwriting is again performed under the recording / reproducing conditions (C-5). In this way, the recording / reproducing conditions as a set are alternately used repeatedly, and the recording / reproducing conditions are changed for each rewrite.

  In the second embodiment, hologram recording and reproduction are performed using the same hologram recording and reproduction apparatus as in the first embodiment. With reference to FIG. 4, the recording / reproducing processing routine of the second embodiment will be described. In addition, the same code | symbol is attached | subjected to the same part as the processing routine of 1st Embodiment, and description is abbreviate | omitted.

  In the present embodiment, the first data is recorded with the signal light as clockwise circularly polarized light (RC polarized light) and the reference light as counterclockwise circularly polarized light (LC polarized light). Therefore, in the case of the first recording process, the driving device 48 is driven in step 200, and the shutter 12, the half-wave plates 52 and 59, the quarter-wave plate 58, and the analyzer 44 are retracted from the optical path. While allowing the laser beam to pass, the optical rotator 26 and the quarter-wave plates 54 and 56 are inserted into the optical path.

  In the next step 202, the laser light is radiated and a recording signal of the first data 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 24. To do. The P-polarized light transmitted through the polarization beam splitter 16 is converted to S-polarized light by the optical rotator 26, then modulated by the spatial light modulator 36 to become P-polarized light, and further converted to RC-polarized light by the quarter-wave plate 56 to be signal light. Is applied to the hologram recording medium 24. On the other hand, the S-polarized light reflected by the polarizing beam splitter 16 is converted to LC-polarized light by the quarter-wave plate 54 and irradiated to the hologram recording medium 24 as reference light.

  In the next step 108, the recording conditions of the first recording process are recorded in a specific area on the hologram recording medium 24, and the processing routine is ended.

  If it is determined in step 102 that the overwrite recording process is performed, the driving device 48 is driven in step 204, and the optical rotator 26, the half-wave plates 52 and 59, and the quarter-wave plates 54 and 56 are removed from the optical path. The laser beam is retracted so that the laser beam can pass through, and the shutter 12, the quarter wavelength plate 58, and the analyzer 44 are inserted into the optical path. The recording / reproduction conditions when the recording process is performed are read. The hologram recording medium 24 is irradiated with laser light (S-polarized light) reflected by the polarization beam splitter 16. The reproduction light (RC polarized light) diffracted by the hologram recording medium 24 is converted into S polarized light by the quarter wavelength plate 58 and transmitted through the lens 38, and only the S polarized component is selectively transmitted by the analyzer 44. The reproduction light received by the detector 40 is converted into an electrical signal by the detector 40 and input to the personal computer 42.

  In the next step 114, the table stored in the memory is read out, and in step 116, the second data is overwritten and recorded from the read / write conditions of the first recording process with reference to the read table. Determine the recording conditions.

  In the present embodiment, the condition that the signal light is LC polarized light and the reference light is RC polarized light is set as the overwrite recording condition. In this case, when S-polarized light is irradiated as readout light, LC-polarized reproduction light is obtained. When the overwrite recording condition is determined, in step 208, the driving device 48 is driven, and the shutter 12, the quarter wavelength plate 58, and the analyzer 44 are retracted from the optical path so that the laser beam can pass therethrough. The optical rotator 26 and the quarter-wave plates 54 and 56 are inserted into the optical path.

  In the next step 210, the laser light is irradiated and the recording signal of the second data is output from the personal computer 42 at a predetermined timing from the recording start position, and the hologram is subjected to shift multiple recording processing on the hologram recording medium 24. To do. The P-polarized light that has passed through the polarizing beam splitter 16 is converted into S-polarized light by the optical rotator 26, then modulated to P-polarized light by the spatial light modulator 36, converted to LC-polarized light by the quarter wavelength plate 56, and LC The hologram recording medium 24 is irradiated as polarized signal light. On the other hand, the S-polarized light reflected by the polarization beam splitter 16 is converted into RC-polarized light by the quarter-wave plate 54 and applied to the hologram recording medium 24 as RC-polarized reference light.

  In the next step 122, the recording conditions for the overwrite recording process are recorded in a specific area on the hologram recording medium 24, and the processing routine is terminated.

  Next, the hologram reproduction process will be described. If it is determined in step 100 that the hologram reproduction process has been selected, in step 212, the optical rotator 26, the half-wave plates 52 and 59, and the quarter-wave plates 54 and 56 are retracted from the optical path. In addition to allowing the laser beam to pass, the shutter 12, the quarter wavelength plate 58, and the analyzer 44 are inserted into the optical path, and the hologram recording medium 24 is irradiated with readout light in step 214, and finally the recording process is performed. Read the recording / playback conditions when. In the present embodiment, since the polarization state of the readout light is common (both S polarization) in the initial recording process and the overwrite recording process, the recording / reproducing conditions can be read by irradiating the same readout light.

  In this example, the reading light for reading the recording / reproducing conditions and the reading light for reproducing the second data are lights having the same polarization state. Therefore, the second data reproduction process can be performed following the reading of the recording / reproduction conditions. Hereinafter, a case where the second data overwritten and recorded under the condition (D-3) is reproduced will be described.

  In the next step 216, the second data reproduction process is executed. The laser beam that has passed through the polarization beam splitter 16 is shielded by the shutter 12, and the S-polarized light reflected by the polarization beam splitter 16 is applied to the hologram recording medium 24 as readout light.

  The reproduction light diffracted by the hologram recording medium 24 includes an RC polarization component corresponding to the first data and an LC polarization component corresponding to the second data. This reproduction light is transmitted through the lens 38, the RC polarized light is converted into S polarized light by the quarter wavelength plate 58, the LC polarized light is converted into P polarized light, and the analyzer 44 converts the P polarized light corresponding to the second data. Only the component is selectively transmitted and received by the detector 40. The received reproduction light is converted into an electrical signal by the detector 40 and input to the personal computer 42.

  As described above, since the S polarization component corresponding to the first data is blocked by the analyzer 44, the second data is reproduced without crosstalk. Further, noise due to the first data is prevented, and reproduction light having a large S / N can be obtained.

  As described above, in this embodiment, since the second data is overwritten and recorded without erasing the first data, the data can be rewritten at a high speed. Further, when the second data is overwritten, the circularly polarized reproduction light rotating in a direction different from the first data is obtained with reference to the recording / reproduction conditions when the first data is recorded. Since the recording / reproduction conditions are determined as described above, the reproduction light of the second data can be easily separated and detected from the reproduction light of the first data by using a quarter-wave plate and an analyzer. it can. That is, the second data can be reproduced without crosstalk.

  In the hologram recording / reproducing apparatus shown in FIG. 2, among elements that can be inserted into or retracted from the optical path, elements to be inserted according to the recording / reproducing conditions of the present embodiment are shown together in Table 6 below. The elements other than the element to be inserted are retracted from the optical path.

[Third Embodiment]
In the third embodiment, the recording / reproducing condition (C-7) in Table 3 and the recording / reproducing condition (D-5) in Table 4 are used as a set. When rewritable recording is performed, the second data is overwritten and recorded on the recording / reproducing condition (D-5) in the area where the first data is recorded on the recording / reproducing condition (C-7). Furthermore, when rewriting the second data, overwriting is again performed under the recording / reproducing conditions (C-7). In this way, the recording / reproducing conditions as a set are alternately used repeatedly, and the recording / reproducing conditions are changed for each rewrite.

  In the combination of the above recording / reproducing conditions, when the hologram recorded under the condition (C-7) is irradiated with RC polarized light as the readout light under the condition (D-5), the recording was performed under the condition (C-7). No reproduction light is emitted from the hologram. When the hologram recorded under the condition (D-5) is irradiated with LC polarized light, which is the readout light under the condition (C-7), the reproduction light is emitted from the hologram recorded under the condition (D-5). do not do. Therefore, it is possible to obtain reproduction light with a good S / N.

  In the third embodiment, hologram recording and reproduction are performed using the same hologram recording and reproduction apparatus as in the first embodiment. With reference to FIG. 5, the recording / playback processing routine of the third embodiment will be described. In addition, the same code | symbol is attached | subjected to the same part as the processing routine of 1st Embodiment, and description is abbreviate | omitted.

  In the present embodiment, the first data is recorded with the signal light as RC polarized light and the reference light as LC polarized light. Therefore, in the case of the first recording process, the driving device 48 is driven in step 300, and the shutter 12, the half-wave plates 52 and 59, the quarter-wave plate 58, and the analyzer 44 are retracted from the optical path. While allowing the laser beam to pass, the optical rotator 26 and the quarter-wave plates 54 and 56 are inserted into the optical path.

In the next step 302, the laser light is irradiated and the recording signal of the first data is output from the personal computer 42 at a predetermined timing from the recording start position, and the shift multiple recording process of the hologram to the hologram recording medium 24 is executed. To do. The P-polarized light transmitted through the polarization beam splitter 16 is converted to S-polarized light by the optical rotator 26, then modulated by the spatial light modulator 36 to become P-polarized light, and further converted to RC-polarized light by the quarter-wave plate 56, The hologram recording medium 24 is irradiated as light. On the other hand, the S-polarized light reflected by the polarization beam splitter 16 is converted to LC-polarized light by the quarter-wave plate 54 and irradiated to the hologram recording medium 24 as reference light. Recording conditions for the recording process are recorded in a specific area, and the processing routine is terminated.

  If it is determined in step 102 that the overwrite recording process is performed, the driving device 48 is driven in step 304, and the optical rotator 26, the half-wave plates 52 and 59, the quarter-wave plates 56 and 58, and the analyzer. 44 is retracted from the optical path so that the laser beam can pass through, and the shutter 12 and the quarter-wave plate 54 are inserted into the optical path. In step 306, the hologram recording medium 24 is irradiated with the readout light. The recording / reproduction conditions when the recording process is performed are read.

  The laser beam (S-polarized light) reflected by the polarization beam splitter 16 is converted into LC-polarized light by the quarter wavelength plate 54 and irradiated onto the hologram recording medium 24 as readout light. The reproduction light (RC polarized light) diffracted by the hologram recording medium 24 passes through the lens 38, and the reproduction light received by the detector 40 is converted into an electric signal by the detector 40 and input to the personal computer 42. .

  Note that, as described above, even if the RC polarized light that is the readout light under the condition (D-5) is irradiated, the reproduction light is not emitted from the hologram recorded under the condition (C-7), and the condition (C-7) The reproduction light is not emitted from the hologram recorded under the condition (D-5) even when the LC polarized light that is the readout light is irradiated. Therefore, when the number of times of rewriting is unknown, in order to read out the recording / reproducing condition, the RC-polarized reading light and the LC-polarized reading light are respectively irradiated and the intensity of the obtained reproducing light is compared. Thereby, it is possible to know the recording / reproducing conditions for recording the latest data.

  In the next step 114, the table stored in the memory is read out, and in step 116, the second data is overwritten and recorded from the read / write conditions of the first recording process with reference to the read table. Determine the recording conditions.

  In the present embodiment, the condition that the signal light is LC polarized light and the reference light is RC polarized light is set as the overwrite recording condition. In this case, when the polarized light is irradiated with RC polarized light, LC polarized reproduced light is obtained. When the overwrite recording condition is determined, in step 308, the driving device 48 is driven to retract the shutter 12 from the optical path so that the laser beam can pass, and the optical rotator 26 and the quarter wavelength plate 54, 56 are inserted into the optical path.

  In the next step 310, the laser light is radiated and a recording signal of the second data 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 24. To do. The P-polarized light that has passed through the polarizing beam splitter 16 is converted into S-polarized light by the optical rotator 26, then modulated to P-polarized light by the spatial light modulator 36, converted to LC-polarized light by the quarter wavelength plate 56, and LC The hologram recording medium 24 is irradiated as polarized signal light. On the other hand, the S-polarized light reflected by the polarization beam splitter 16 is converted into RC-polarized light by the quarter-wave plate 54 and applied to the hologram recording medium 24 as RC-polarized reference light.

  In the next step 122, the recording conditions for the overwrite recording process are recorded in a specific area on the hologram recording medium 24, and the processing routine is terminated.

  Next, the hologram reproduction process will be described. If it is determined in step 100 that the hologram reproduction process has been selected, in step 312, the optical rotator 26, the half-wave plates 52 and 59, and the quarter-wave plates 56 and 58 are retracted from the optical path. In addition to allowing the laser beam to pass, the shutter 12 and the quarter-wave plate 54 are inserted into the optical path, and in step 314, the hologram recording medium 24 is irradiated with reading light, and finally the recording process is performed. Read the recording / playback conditions. As described above, in this embodiment, since the polarization state of the readout light is different between the initial recording process and the overwrite recording process, the recording / reproducing conditions are read by irradiating each of the two types of readout light.

  In the next step 316, the second data reproduction process is executed based on the read recording / reproduction condition. Hereinafter, a case where the second data overwritten and recorded under the condition (D-7) is reproduced will be described. The laser beam that has passed through the polarization beam splitter 16 is shielded by the shutter 12, and the S-polarized light reflected by the polarization beam splitter 16 is converted into RC polarization by the quarter-wave plate 54, and is read into the hologram recording medium 24 as readout light. Irradiated.

  In the present embodiment, the signal light when the first data is recorded is RC polarized light, and the reference light is LC polarized light. In this case, when LC-polarized light is irradiated as readout light, RC-polarized reproduction light is obtained. On the other hand, the signal light when the second data is recorded is LC polarized light, and the reference light is RC polarized light. In this case, when the polarized light is irradiated with RC polarized light, LC polarized reproduced light is obtained. As described above, since the polarization state of the readout light is different between the first data and the second data, when the second data is reproduced by irradiating the RC polarized light as the readout light, the first data is reproduced. Not.

  The reproduction light diffracted by the hologram recording medium 24 includes only LC polarized light corresponding to the second data. This reproduction light passes through the lens 38 and is received by the detector 40. The received reproduction light is converted into an electrical signal by the detector 40 and input to the personal computer 42. As described above, since only the reproduction light corresponding to the second data is emitted and the reproduction light corresponding to the first data is not emitted, only the second data is reproduced without crosstalk. Further, noise due to the first data is prevented, and reproduction light having a large S / N can be obtained.

  As described above, in this embodiment, since the second data is overwritten and recorded without erasing the first data, the data can be rewritten at a high speed. In addition, when overwriting the second data, the reproduction light of the second data is obtained with the readout light that does not reproduce the first data with reference to the recording / reproduction conditions when the first data is recorded. Since the recording / reproduction conditions are determined as described above, only the reproduction light of the second data can be detected. That is, the second data can be reproduced without crosstalk.

  In the hologram recording / reproducing apparatus shown in FIG. 2, among elements that can be inserted into or withdrawn from the optical path, elements to be inserted according to the recording / reproducing conditions of the present embodiment are shown together in Table 7 below. The elements other than the element to be inserted are retracted from the optical path.

  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, a wavelength multiplex recording method, or a phase multiplex recording method.

(A)-(C) are schematic which shows the structure of a hologram recording medium. It is the schematic of the hologram recording / reproducing apparatus of this Embodiment. It is a flowchart which shows the recording / reproducing process routine of 1st Embodiment. It is a flowchart which shows the recording / reproducing process routine of 2nd Embodiment. It is a flowchart which shows the recording / reproducing process routine of 3rd Embodiment. (A) And (B) is a figure explaining the shift multiple recording method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Laser oscillator 12 Shutter 16 Polarization beam splitter 18 Reflection mirror 20 Objective lens 22 xz stage 23 Optical recording layer 24 Hologram recording medium 26 Optical rotator 28 Reflection mirror 30, 32, 34, 38 Lens 36 Spatial light modulation element 40 Detector 42 Personal Computer 44 Analyzer 46 Pattern Generator 48, 50 Drive Device 52 Half Wave Plate 54, 56, 58 1/4 Wave Plate

Claims (8)

  Under the condition that the reproduction light having a polarization state different from the polarization state of the reproduction light obtained from the hologram according to the first data is generated in a predetermined recording area of the optical recording medium in which the hologram according to the first data is recorded, A hologram recording method for overwriting and recording a hologram according to second data.   When the reproduction light is linearly polarized light, the hologram related to the second data is overwritten and recorded on the condition that the reproduction light having a polarization direction different from the polarization direction of the reproduction light obtained from the hologram related to the first data is generated. Item 2. The hologram recording method according to Item 1.   When the reproduction light is circularly polarized light, the hologram related to the second data is overwritten and recorded on the condition that the reproduction light is circularly polarized in the reverse direction to the reproduction light obtained from the hologram related to the first data. Item 2. The hologram recording method according to Item 1.   Reproduction light is generated in a predetermined area of the optical recording medium in which the hologram related to the first data is recorded, with the readout light having a polarization state different from the polarization state of the readout light for reproducing the hologram related to the first data. A hologram recording method for overwriting and recording the hologram according to the second data under the condition that the readout light is not generated from the hologram according to the first data by the readout light. A hologram recording method for overwriting and recording a hologram according to second data in a predetermined recording area of an optical recording medium in which a hologram according to first data is recorded,
Recording the hologram according to the second data and the combination of the polarization state of the signal light and the reference light recording the hologram according to the first data and the polarization state of the readout light reproducing the hologram according to the first data A hologram recording method for overwriting and recording a hologram related to second data so that a combination of a polarization state of signal light and reference light to be read and a combination of a polarization state of readout light for reproducing the hologram related to the second data is different.   6. The hologram reproduction condition according to the first data is recorded on the optical recording medium, and the hologram according to the second data is overwritten and recorded based on the recorded reproduction condition. 2. The hologram recording method according to 1.   The hologram recording method according to claim 1, wherein a condition for overwriting and recording the hologram according to the second data is set in advance according to a reproduction condition of the hologram according to the first data. . On the condition that the reproduction light having a polarization state different from the polarization state of the reproduction light obtained from the hologram according to the first data is generated in a predetermined recording area of the optical recording medium in which the hologram according to the first data is recorded, When the hologram related to the second data is overwritten and recorded,
The recording and reproducing conditions of the hologram related to the second data are recorded in advance on the optical recording medium, and the polarization state of the reproducing light obtained from the hologram related to the second data is recorded based on the recorded conditions. A hologram reproducing method for determining and reproducing the hologram according to the second data by selectively detecting reproduction light in the determined polarization state.

Images