JP2006163021A - Hologram recording device, hologram reproducing method and hologram recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hologram recording device that can reduce influences by birefringence of a hologram substrate. <P>SOLUTION: The recording conditions relating to the birefringence of a substrate such as incident planar angles of two light waves, a recording angle range and recording power such as a recording period are recorded as header information in a hologram recording medium. On actual recording, the header information is read out, and multiple recording such as multi-angle recording is carried out by determining recording conditions based on the header information. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a hologram recording apparatus that performs recording using a hologram, a hologram reproducing method, and a hologram recording medium.

  Development of hologram recording devices that record data using holography is in progress.

  In the hologram recording apparatus, two modulated signal lights (data superimposed) and unmodulated reference light are generated from laser light, and these are irradiated to the same place on the hologram recording medium. As a result, the signal light and the reference light interfere on the hologram recording medium, a diffraction grating (hologram) is formed at the irradiation point, and data is recorded on the hologram recording medium.

  By irradiating the recorded hologram recording medium with reference light, diffracted light (reproduced light) is generated from the diffraction grating formed during recording. Since the reproduction light includes data superimposed on the signal light at the time of recording, the recorded signal can be reproduced by receiving this with a light receiving element.

  In order to record a large amount of information on the hologram recording medium, a large number of holograms may be formed on the hologram recording medium. In this case, the hologram is not necessarily formed at different locations on the hologram recording medium, and the hologram can also be formed at the same location (or overlapping region) of the hologram recording medium. This is so-called multiple recording (angle multiplexing), and various methods such as an angle multiplexing method, a wavelength multiplexing method, and a rotation multiplexing method have been proposed.

  For example, in the angle multiplexing method, the hologram is formed by changing the incident angle of the reference light at the same location on the hologram recording medium. By using the same reference light as that used during recording during reproduction, it is possible to obtain reproduction light and data corresponding to each of a plurality of holograms formed at the same location.

In addition, development of a hologram recording apparatus that aims to increase the storage capacity by using phase correlation multiplexing, which is a type of multiplex recording, has been underway (see, for example, Patent Document 1).
Japanese Patent Laid-Open No. 11-242424

  Glass has mainly been used as the substrate for hologram recording media, but hologram recording can be made at a lower cost by using polycarbonate, APO (amorphous polyolefin), etc., which are widely used for optical disks such as CDs and DVDs. It becomes possible to create a medium.

  However, since a plastic substrate such as polycarbonate originally has intrinsic birefringence, the polarization state after transmission through the substrate changes. Unlike conventional optical discs such as CDs and DVDs that simply write pits on a disc and detect light reflection, in the case of hologram recording that uses two-wave interference such as hologram recording, the polarization state of incident light is There has been a problem of having a very large influence on the recording characteristics.

  FIG. 1 shows experimental results of diffraction efficiency at each recording angle when a hologram is recorded on a substrate having birefringence by the angle multiplexing method. As can be seen from the experimental results, the diffraction efficiency at each recording angle varies. Since such a phenomenon is not observed in a substrate having no birefringence, such as glass, as the substrate of the hologram recording medium, this variation is considered to be due to the influence of birefringence on the substrate.

  In view of the circumstances as described above, an object of the present invention is to provide a hologram recording apparatus, a hologram reproducing method, and a hologram recording medium that can reduce the influence of birefringence of a hologram substrate.

  A hologram recording apparatus according to the present invention is a hologram recording apparatus for recording a hologram on a hologram recording medium in which recording conditions relating to birefringence of a substrate are recorded in advance, and recording condition reading means for reading the recording conditions from the hologram recording medium And control means for controlling to record the hologram on the hologram recording medium based on the recording condition read by the recording condition reading means.

  By performing hologram recording based on the recording conditions related to the birefringence of the substrate recorded on the hologram recording medium, the influence of birefringence at the time of hologram recording can be mitigated, and hologram recording can be performed well. Is possible.

  Here, the present invention can be applied to an angle multiplexing type hologram recording apparatus, and in this case, the recording condition is an incident plane angle of signal light and reference light. As a result, the birefringence axis and the polarization direction of the incident light can be matched, and the difference in birefringence between the inner and outer circumferences of the disk and the influence of vertical birefringence can be minimized.

  The recording condition may be a range of recording angles for angle multiplexing. This makes it possible to perform angle multiplexing within a range that is not significantly affected by vertical birefringence.

  Furthermore, the recording condition can be information regarding recording power for each recording angle. Since the effect of birefringence increases as the angle of incident light increases, increasing the recording power as the angle of incident light increases increases the recording angle range while minimizing the effects of variations in diffraction efficiency. Since it is expanded, the multiplicity can be increased.

  The present invention can be applied to a holographic hologram recording apparatus, and in this case, the recording condition can be the incident plane angles of the signal light and the reference light. As a result, the birefringence axis and the polarization direction of the incident light can be matched, and the difference in birefringence between the inner and outer circumferences of the disk and the influence of vertical birefringence can be minimized.

  Further, the recording condition in the case of the rotational multiplexing method may include a range of recording angles for angle multiplexing for each incident plane angle. This makes it possible to perform angle multiplexing within a range that is not significantly affected by vertical birefringence.

  Furthermore, the recording condition can be information regarding recording power for each recording angle.

  The present invention can be applied to a collinear hologram recording apparatus. In this case, the light modulation element is an element that spatially modulates signal light and superimposes data, and the recording condition is the pattern on the spatial light modulator. It can be information on intensity modulation applied in each circumferential direction. The influence of birefringence in the circumferential direction can be reduced by changing the intensity of the reference light in accordance with the influence of the refractive index in one round.

  As another recording condition for the collinear method, information on the size of each pattern of the signal light and the reference light in the circumferential direction applied to the spatial light modulator may be recorded. Thereby, the difference in diffraction efficiency between the inner and outer circumferences of the disk can be reduced.

  Furthermore, by recording the above recording conditions as recording conditions for each position in the radial direction on the disk, hologram recording can be performed under optimum conditions according to the radial position of the disk.

  Further, it further includes a rotatable wave plate that can change the deflection direction of the signal light and / or the reference light, and the control means variably sets the rotation position of the wave plate based on the recording condition read by the recording condition reading means. You may comprise. Thereby, it is possible to set a recording condition that alleviates the influence of birefringence by simply rotating the wave plate.

  As described above, according to the present invention, it is possible to provide a hologram recording apparatus, a hologram reproducing method, and a hologram recording medium that can reduce the influence of birefringence of a hologram substrate.

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

  In the hologram recording medium (media) on which hologram recording is performed by the hologram recording apparatus of the present embodiment, optimal hologram recording conditions for the medium are preliminarily set in order to reduce the influence of the birefringence of the substrate of the medium. It is recorded. For example, in the case of a disc-type medium, the hologram recording condition is recorded as header information in an innermost area of the disc.

  Hologram recording conditions differ depending on the multiplex recording method.

When the media is a disk, the angle multiplex hologram recording conditions include
The incident plane angle of two light waves at each position in the R direction (radial direction) of the disk;
-Recording angle range (multiple number) at each position in the R direction of the disc,
There is a recording power such as a recording time at each recording angle (recording of each hologram) at each position in the R direction of the disk.

Rotation multiplex hologram recording conditions include
The incident plane angle of two light waves to be recorded in multiple positions at each position in the R direction of the disk,
Recording power such as recording time at each recording angle (angle multiplexing) at each incident plane angle (rotation multiplexing) at each position in the R direction of the disc,
There is a recording angle range at each recording angle (angle multiplexing) of each incident plane angle (rotation multiplexing) at each position in the R direction of the disk.

For collinear hologram recording conditions,
Intensity modulation information given in each circumferential direction of the pattern on the spatial light modulator at each position in the R direction of the disk,
Information on the size of each pattern of the signal light and the reference light in the circumferential direction given on the spatial light modulator at each position in the R direction of the disk;
Recording power such as recording time at each position in the R direction of the disk.

  The position in the R direction (radial direction) of the disk means the position of an area obtained by dividing the disk area by the position in the radial direction for each range having a birefringence value. Therefore, the greater the birefringence of the substrate, the greater the number of regions partitioned in one disc.

  The above-mentioned hologram recording conditions include the disc surface with respect to the inner and outer peripheral positions of the disc, refractive index information nx, ny, nz in the vertical direction, birefringence information (Δnxy, Δnxz, Δnyz) in each direction, and each birefringence. You can actually observe the direction (angle, etc.) of the axis and find the result. As a birefringence measuring method, for example, there are a Senalmon method, a prism coupler method, and an ellipsometer.

  Next, each of the angle recording method, the rotation multiplexing method, and the collinear method hologram recording apparatus that performs hologram recording on a medium based on the header information will be described.

1. Angle multiplexing method (2 plane incidence angle of light wave)
As described above, the influence of birefringence is generally classified into a disk surface direction (nx, ny) and a direction perpendicular to the disk surface (nz). Here, the incident directions of the two light waves of the signal wave and the reference wave in consideration of the influence of birefringence will be described for each of the disk type and card type media.

  The case of a disk type medium will be described with reference to FIG. The medium 1 is obtained by bonding plastic substrates 3 and 4 to both sides of the recording layer 2. Therefore, the influence by the birefringence of the plastic substrate 3 on the front side where the light is incident can be considered. As shown by a refractive index ellipsoid indicated by reference numeral 5 in the figure, in the case of a disk-type medium 1, although it depends on the emission conditions, generally, a plurality of plane directions (nx, ny) from the center of the medium 1 to the radiation direction. There is an axis of refraction. Therefore, when the birefringence axis and the polarization direction of the incident light coincide with each other, the influence of birefringence is minimized, so that the incident plane of the two light waves of the signal light and the reference light is horizontal to the radiation direction of the medium 1. 6 or by bringing the incident plane 7 of the two light waves perpendicular to the radiation direction of the medium 1, the birefringence difference between the inner and outer circumferences of the medium 1, and the vertical birefringence Can be minimized.

  Next, the case of a card-type medium will be described with reference to FIG. In the case of the card-type medium 8, although depending on the injection conditions, it may be considered that the axis of the refractive index ellipsoid 9 is generally in the horizontal or vertical direction with respect to the surface of the card-type medium 8. In the case of the card-type medium 8 as well as the disk-type medium 1, since the influence of birefringence is minimized when the birefringence axis and the polarization direction of the incident light coincide with each other, the card-type medium 8 is horizontal to the birefringence axis. The influence of vertical birefringence or the like can be suppressed by either bringing the two light wave incident plane 10 or bringing the two light wave incident plane 11 perpendicular to the birefringence axis.

  However, the orientation of the birefringence axis in the plane direction is not always constant depending on the medium. Therefore, it is effective to record the incident plane angle of the two light waves on the medium as header information and read the header information at the time of recording to set the incident plane angle of the two light waves. Further, since the direction of the birefringence axis may differ depending on the position in the R direction in the disk-type medium 1, information on the incident plane angle of two light waves at each position in the R direction is described in the header information. An optimal incident plane angle may be set for the recording position in the R direction (radial direction) based on the header information.

(Recording angle range (multiple number))
As shown in FIG. 4, assuming a refractive index ellipsoid 12 for a certain medium, the angle range (recording angle range) of incident light is limited due to the influence of vertical birefringence. In the case of the angle multiplexing method, the influence of vertical birefringence increases as the angle of the incident light from the media normal increases, and therefore the incident light 13 whose angle is fixed is not affected by the birefringence. By making the light incident at a marginal angle within 14, the angle region that is not affected by the birefringence of the other incident light 15 can be increased.

  FIGS. 5A and 5B show an angle range 14 that is not greatly affected by birefringence. As shown in FIG. 5A, when the signal light 16 is incident at a marginal angle within the angular range 14 that is not greatly affected by birefringence, the case shown in FIG. It can be seen that the angular width over which the reference beam 17 can be swung is wider than that of.

  In the case of the angle multiplex method, as described above, by having the incident plane in the horizontal or vertical direction with respect to the birefringence axis, the influence of birefringence in the plane direction is avoided as much as possible. However, the angle multiplexing method is affected by vertical birefringence in order to change the angle of the reference light or signal light during recording. For this reason, it is necessary to change the recording angle range in consideration of vertical birefringence.

  Specifically, when a disc-type medium is assumed as the hologram recording medium, it is considered that the birefringence value differs between the inner and outer circumferences of the medium, so it is necessary to change the angle range of incident light between the inner and outer circumferences. is there. In general, the birefringence becomes larger at the outer periphery and affects the diffraction efficiency. Therefore, at the outer periphery, the range for changing the angle of incident light is narrowed to reduce the multiplicity.

  Therefore, in this embodiment, the recording angle range at each position in the R direction is recorded on the disc as header information, and this header information is read at the time of recording, and the optimum recording angle of incident light with respect to the recording position in the R direction. By setting the range (number of multiples), the influence of vertical birefringence is suppressed.

(Recording power)
As described above, in the case of a disk-type medium, the birefringence value may be different at the inner and outer circumferences of the disk (the birefringence index is larger at the outer circumference). It is beneficial to change the recording power according to the recording position in the direction. Also, since the influence of birefringence increases as the angle of incident light increases, increasing the recording power as the angle of incident light increases increases the recording angle while suppressing the influence of variations in diffraction efficiency. Since the range can be expanded, the multiplicity can be increased. Specifically, the recording power can be selected depending on the beam intensity, the recording time, and the number of pulses in the case of pulse recording.

  By the way, when holographic recording of holograms is performed, the amount of recording material (polymer in the case of organic materials) decreases as recording is performed later, so gradually increase the recording power according to the polymer remaining amount of media. There is a scheduling record to go, but considering the effect of the above birefringence in this method, if recording from the outer periphery where the incident angle is large (the effect of birefringence is large), the remaining amount of polymer scheduling recording and cancellation Therefore, there is a possibility of recording with a certain power.

(Angle-multiplexed hologram recording device)
FIG. 6 is a diagram showing a configuration of an angle multiplexing type hologram recording apparatus.

  As shown in the figure, this angle-multiplexed hologram recording apparatus records and reproduces information on a disk-type medium 1 that is a hologram recording medium.

  The optical unit 100 of this hologram recording apparatus includes a recording / reproducing light source 21, a collimating lens 22, an isolator 23, a half-wave plate 24, a mechanical shutter 25, a polarization beam splitter 26, a half-wave plate 27, a polarization beam splitter 28, 1/2. Wave plate 29, polarizing beam splitter 30, quarter wave plate 31, relay lens 32, pinhole 33, dichroic mirror 34, objective lens 35, servo drive unit 36, quarter wave plate 37 for reference light Pinhole 38 for light, galvanometer mirror 39, reflection liquid crystal 40, magnification adjustment lens 41, CCD camera 42, servo light source 43, collimating lens 44, grating 45, beam splitter 46, condensing lens 47, cylindrical lens 48 and a light receiving element 49.

  The disk-type medium 1 is a recording medium that has a protective layer 50, a recording layer 51, a groove 52, and a reflective layer 53, and records interference fringes due to signal light and reference light. Here, the protective layer 50 is a layer for protecting the recording layer 51 from the outside. The recording layer 51 records interference fringes as a change in refractive index (or transmittance), and any material that changes the refractive index (or transmittance) according to the intensity of light can be used. It can be used without questioning the inorganic material. As the inorganic material, for example, a photorefractive material whose refractive index changes according to the exposure amount by an electro-optic effect such as lithium niobate (LiNbO3) can be used. For example, a photopolymerizable photopolymer can be used as the organic material. In the initial state of the photopolymerization type photopolymer, the monomers are uniformly dispersed in the matrix polymer. When this is irradiated with light, the monomer is polymerized at the exposed portion. In addition, as it polymerizes, a monomer moves from the circumference | surroundings and the density | concentration of a monomer changes with places. As described above, when the refractive index (or transmittance) of the recording layer 51 changes according to the exposure amount, interference fringes caused by interference between the reference light and the signal light are regarded as changes in the refractive index (or transmittance). It can be recorded on the disc type medium 1.

  The disk type medium 1 is rotated by a spindle motor (not shown). Since the hologram recording medium 1 rotates, recording / reproduction on the hologram recording medium 1 is performed along a track formed in the moving direction.

  The groove 52 is provided for performing servo control such as tracking and focusing on the disk-type medium 1. That is, the groove 52 is formed along the track of the disk-type medium 1, and the tracking servo and the focus servo are performed by controlling the condensing position and condensing depth of the signal light so as to correspond to the groove 52. .

  Next, the basic operation of this angle multiplexing type hologram recording apparatus will be described.

  In the optical unit 100, the recording / reproducing light source 21 is a laser light source, and for example, a laser diode (LD) having a wavelength of 405 [nm] or an Nd-YAG laser having a wavelength of 532 [nm] can be used.

  The light beam emitted from the recording / reproducing light source 21 is converted into parallel light by the collimator lens 22 and passes through an isolator 23 to prevent return light. Thereafter, the ratio of the p-polarized light and the s-polarized light is adjusted by the half-wave plate 24, and the light is split into the p-wave signal light line 61 and the s-wave reference light line 62 by the polarization beam splitter 26.

  The intensity of the signal light is adjusted by the half-wave plate 27, and only the p-polarized component is incident on the reflective liquid crystal 40 as a spatial light modulator through the polarization beam splitter 28. The spatial light modulator is an optical element that superimposes data by spatially (in this case, two-dimensionally) modulating signal light. As the spatial light modulator, in addition to the reflective liquid crystal, a DMD (Digital Micro Mirror) or a transmissive liquid crystal element which is a transmissive element can be used.

  The light modulated by the reflective liquid crystal 40 is reflected by the polarization beam splitter 28 because the polarization is rotated by 90 °. The light reflected from the polarization beam splitter 28 is returned to p-polarized light again by the half-wave plate 29, passes through the polarization beam splitter 30, and enters the half-wave plate 31. The half-wave plate 31 has a rotation angle that does not change the polarization direction, and transmits the incident p-polarized light as it is. Thereafter, it is propagated through the relay lens 32. At this time, high-order diffracted light from the liquid crystal is cut by the pinhole 33.

  Subsequently, the signal light passes through the dichroic mirror 34. The dichroic mirror 34 is an optical element for making the recording / reproducing light and the light used for servo have the same optical path. The light from the recording / reproducing light source 21 is transmitted to the surface of the dichroic mirror 34 in response to the wavelength of the laser light being different between the recording / reproducing light source 21 and the servo light source 43. Thin film processing that reflects light is performed.

  The light that has passed through the dichroic mirror 34 is collected by the objective lens 35 and enters the disc-type medium 1.

  On the other hand, the reference light separated by the polarization beam splitter 26 is returned to p-polarized light by the half-wave plate 37, the beam diameter is adjusted by the pinhole 38, the angle is modulated by the galvanometer mirror 39, and the signal light and The light is incident on the same part of the disk-type medium 1 so as to interfere. As a result, interference fringes are formed on the disk-type medium 1. At this time, information spatially modulated by the reflective liquid crystal 40 is recorded on the disk-type medium 1 as a hologram.

  Here, in the case of performing angle multiplexing, the angle of incidence on the disc-type medium 1 is changed by changing the angle of the galvanometer mirror 39, so that control of the incident plane angle and angle multiplexing recording can be performed.

  Further, a mechanical shutter 25 that opens and closes a beam during recording is disposed between the half-wave plate 24 and the polarization beam splitter 26, and the recording time can be varied by opening and closing the mechanical shutter 25.

During reproduction, the signal light is blocked and only the reference light is incident on the disc-type medium 1. At this time, by making the incident angle of the reference light the same as that at the time of recording, the signal recorded on the disk type medium 1 is reproduced when the reference light is incident at the same angle.
When the reference light is incident on the disk type medium 1, diffracted light (reproduced light) is generated from the hologram recorded on the disk type medium 1. The reproduction light follows an optical path opposite to that of the signal light, passes through the objective lens 35, passes through the dichroic mirror 34, and passes through the relay lens 32. Noise is cut by the pinhole 33 on the way through the relay lens 32. Thereafter, the reproduction light enters the half-wave plate 31. Unlike the recording, the half-wave plate 31 rotates the polarization direction by 90 degrees to change the p-polarized light to the s-polarized light. The s-polarized light is reflected by the polarization beam splitter 30, the magnification is adjusted by the magnification adjusting lens 41, and converted by the CCD camera 42 into an electrical signal corresponding to the spatial two-dimensional data in the reflective liquid crystal 40. . The output from the CCD camera 42 is binarized by a signal processing unit (not shown) and converted into time-series binarized data.

  The servo light source 43 is a light source for performing servo control such as tracking servo and focus servo, and emits laser light having a wavelength different from that of the recording / reproducing light source 21. The servo light source 43 is, for example, a laser diode, and has a low sensitivity with respect to the disk-type medium 1 as the oscillation wavelength, for example, 650 nm.

  The servo light emitted from the servo light source 43 is converted into parallel light by the collimator lens 44 and divided into three beams by the grating 45. The laser light emitted from the grating 45 passes through the beam splitter 46 and reaches the dichroic mirror 34, and is placed on the same optical path as the recording / reproducing light by the dichroic mirror 34. Incident.

  The servo light reflected from the disk-type medium 1 returns to the dichroic mirror 34 through the objective lens 35, is reflected here, and returns to the beam splitter 46. The return servo light is reflected by the beam splitter 46 and enters the condensing lens 47, where it is condensed, and then the beam shape is converted from a circular shape to an elliptic shape by the cylindrical lens 48 and received by the light receiving element 49. Is done. From the light receiving element 49, a tracking error signal for tracking servo control, a focus error signal for focus servo control, and the like are output for the return light.

  The servo drive unit 36 is a drive mechanism for performing tracking control and focus control by driving the objective lens 35 in two axial directions by a tracking error signal and a focus error signal from the light receiving element 49. The coils 36A and 36B for driving the objective lens 35 are provided.

  Next, the flow of processing based on the header information in the hologram recording apparatus will be described with reference to the flowchart of FIG.

  The hologram recording apparatus includes a system controller having a CPU and a memory. The system controller is control means for controlling a spindle motor (not shown) and the optical unit 100 that rotate the disk-type medium 1.

  First, the system controller rotates the disk type medium 1, reads the header information recorded on the innermost periphery of the disk type medium 1, and stores the contents in the memory (step S1). Here, in addition to the information for managing the recording position on the disk type medium 1, the header information stored in the memory includes two light waves at each position in the R direction (radial direction) of the disk as a hologram recording condition. Recording power such as recording time at each recording angle (recording of each hologram) at each position in the R direction of the disk.

  In order to remove the insensitive area of the disk-type medium 1, the disk surface is irradiated with incoherent light from an LED or the like (step S2).

  The recording position is determined based on the information for managing the recording position included in the header information, and the spindle motor is rotated so that the irradiation position of the two light waves comes to the recording position (step S3). Next, an incident plane angle is determined based on the header information (step S4).

  Subsequently, the two-dimensional information for recording (SLM data) is displayed on the reflective liquid crystal 40 to transfer the SLM data (step S5). The recording angle range at the recording position in the header information stored in the memory is read (step S6), and the incident angle of the reference light is set by moving the galvanometer mirror 39 so as to be within this recording angle range (step S7). .

  Next, recording time information in the header information stored in the memory is read (step S8), and hologram recording is performed by opening the mechanical shutter 25 only during the recording time (step S9).

  Thereafter, when angle multiplex recording at one position is completed (YES in step S10), the process returns to step S2 to perform angle multiplex recording to the next position, and when not completed (step S10). NO), returning to step S5, recording is performed at the same position at different angles.

  When the angle multiplex recording at all positions is completed (YES in step S11), the entire medium is irradiated with incoherent light as post-processing to change the monomer into a polymer (step S12). This is because in hologram media using a photopolymer, if there is an unreacted monomer, additional recording occurs due to irradiation of reference light during reproduction, which causes a decrease in SNR.

  As described above, in order to alleviate the influence of the birefringence of the medium, the optimum hologram in the angle multiplexing system for the medium, such as the incident plane angle of two light waves, the recording angle range (multiplexing number), the recording power such as the recording time, etc. By recording the recording conditions on the medium and performing actual hologram recording according to the hologram recording conditions, it becomes possible to perform good hologram recording on the medium having birefringence by the angle multiplexing method.

2. Rotation Multiplex Method Next, a rotation multiplex method hologram recording apparatus will be described.

  FIG. 8 is a schematic diagram showing a configuration of an optical unit of a rotation multiplexing type hologram recording apparatus.

  In the optical unit 200 of the rotary multiplex type hologram recording apparatus, the light emitted from the recording / reproducing light source 21 is converted into parallel light by a collimator lens (not shown), and further p-polarized light and s-polarized light by a half-wave plate (not shown). The polarization beam splitter 26 divides the signal into a p-wave signal light line and an s-wave reference light line.

  Only the p-polarized light component of the signal light enters the reflective liquid crystal 40 as a spatial light modulator through the polarization beam splitter 28. The light modulated by the reflective liquid crystal 40 is reflected by the polarization beam splitter 28. The light reflected from the polarization beam splitter 28 is returned to the p-polarized light again by the half-wave plate 29, reflected by the mirror 70 and incident perpendicularly on the medium 1.

  On the other hand, the reference light separated by the polarization beam splitter 26 is returned to p-polarized light by the half-wave plate 37, the angle is modulated by the galvanometer mirror 39, and is incident on the mirror 71 having a three-dimensional curvature surface. The light is reflected by the curvature surface of the mirror 71 and is incident on the same portion of the medium 1 so as to interfere with the signal light. As a result, interference fringes are formed on the disk-type medium 1. At this time, information spatially modulated by the reflective liquid crystal 40 is recorded on the medium 1 as a hologram.

  Here, the angle of the galvanometer mirror 39 is changed by the actuator 72, so that the incident position on the mirror 71 changes, and the incident direction of the signal light on the medium 1 changes accordingly. Angle multiplexing is performed.

  Next, the flow of processing based on the header information in the rotation multiplexing hologram recording apparatus will be described with reference to the flowchart of FIG.

  Rotational multiplex type hologram recording conditions include: the incident plane angle of two light waves to be multiplex-recorded at each position in the R direction of the disk; and each recording angle of each incident plane angle (rotational multiplexing) at each position in the R direction of the disk. There are recording power such as recording time in angle multiplexing), recording angle range at each recording angle (angle multiplexing) of each incident plane angle (rotation multiplexing) at each position in the R direction of the disc.

  This rotary multiplex type hologram recording apparatus includes a system controller having a CPU and a memory. The system controller is control means for controlling a spindle motor (not shown) that rotates the disk-type medium 1 and the optical unit 200.

  First, the system controller rotates the disk-type medium 1, reads the header information recorded on the innermost periphery of the disk-type medium 1, and stores the contents in the memory (step S21). In order to remove the insensitive area of the disc type medium 1, the disc surface is irradiated with incoherent light from an LED or the like (step S22).

  The recording position is determined based on the information for managing the recording position on the medium 1 in the header information, and the spindle motor is rotated so that the laser irradiation position comes to the recording position (step S23). Next, the incident plane angle of the two light waves at the recording position is determined based on the header information (step S24).

  Subsequently, the two-dimensional information for recording (SLM data) is displayed on the reflective liquid crystal 40 to transfer the SLM data (step S25). The recording angle range with respect to the incident plane angle at the current recording position in the header information stored in the memory is read (step S26), and the incident angle of the signal light is set by moving the galvanometer mirror 39 based on this range (step S27). ).

  Next, information on the recording time with respect to the incident plane angle at the current recording position in the header information stored in the memory is read (step S28), and hologram recording is performed by opening the mechanical shutter 25 only during the recording time. (Step S29).

  Thereafter, it is determined whether or not the angle multiplex recording at one recording position is completed (step S30). If completed, the process proceeds to the next step S31, and if not completed, the process returns to step S25. Thus, multiple recording at different angles at the same recording position is performed.

  In step S31, it is determined whether or not rotational multiplex recording at one recording position has been completed. If not completed, the process returns to step S24 to determine the next incident plane angle, and with different rotational angles at the same position. Rotation multiplex recording is performed. When the rotational multiplex recording at one position is completed (YES in step S31), the process proceeds to step S32.

  In step S22, it is determined whether or not rotational multiplex recording has been completed at all positions. If not completed (NO in step S32), the process returns to step S32 to irradiate the disc surface again with incoherent light. Thereafter, the next recording position is determined based on the header information, the spindle motor is rotated so that the irradiation position of the two light waves comes to the recording position, and recording is performed by angle multiplexing and rotation multiplexing at that position.

  When multiple recording at all positions is completed (YES in step S32), the entire medium is irradiated with incoherent light as post-processing to change the monomer into a polymer (step S33).

  As described above, in order to reduce the influence of the birefringence of the medium, by recording the optimum hologram recording conditions in the rotational multiplexing method on the medium, and performing the actual hologram recording according to the hologram recording conditions, Hologram recording can be performed satisfactorily on a medium having birefringence by the rotational multiplexing method.

  By the way, in the rotation multiplexing type optical system, for example, as shown in FIG. In the optical system of the rotation multiplex system, variation in birefringence in the horizontal direction is greatly affected, and if the angle of the reference light 17 is increased with respect to the signal light 16, the influence of vertical birefringence increases accordingly. It is necessary to make the angle between the signal light 16 and the reference light 17 as narrow as possible.

  In the horizontal direction, as shown in FIG. 11, the angle from the y-axis where the incident angle of the signal light 16 coincides with the horizontal birefringence axis is 90 * (n−1) [n = 1, 2, 3 , 4] has the least effect, and gradually increases the influence of birefringence as the angle with the y-axis approaches 45 ° * (2n−1) [n = 1, 2, 3, 4]. To go. Therefore, the rotational recording angle when performing the rotational multiplexing is such that the multiple recording is performed as close to the x axis or the y axis as possible. For example, when performing the multiple recording of N times, the rotational angle selectivity of the rotational multiplexing is 90. * (N-1) ± mα [N, m = 1, 2, 3, 4] It is good to record in order from the part close to the axis. Moreover, since the diffraction efficiency decreases as the hologram is recorded at an angle away from these axes, it is also effective to use a scheduling recording method that takes this point into consideration.

3. Collinear Method As shown in FIG. 12, in the collinear method in which the reference light 17 surrounds the signal light 16, there are two possible effects of birefringence. One is that the angle becomes steep as it goes to the outside of the image area, so that the influence of vertical birefringence increases, and unevenness is likely to occur on the inner and outer circumferences. Second, as shown in FIG. Since all directions of the above contribute to interference, the influence of birefringence becomes large at a position of 45 ° * (2n−1) [n = 1, 2, 3, 4] from the birefringence axis. This is a point where unevenness is likely to occur depending on the angle of the direction.

  Therefore, as the header information, information on the size of the pattern of each of the circumferential signal light and the reference light given on the reflective liquid crystal at each position in the R direction of the disk is recorded, and the inside and outside of the disk are recorded according to this information. By reducing the image area of the signal light and the reference light around the circumference, it becomes possible to eliminate the difference in diffraction efficiency between the inner and outer circumferences of the disk.

  Further, as the header information, intensity modulation information to be given in each circumferential direction of the pattern on the reflective liquid crystal at each position in the R direction of the disk is recorded. Specifically, in a portion where the influence of birefringence increases from the position of 90 * (n−1) to the position of 45 ° * (2n−1) [n = 1, 2, 3, 4] in the horizontal direction. The intensity of the reference light is increased as it goes, and the maximum intensity is obtained at 45 ° * (2n−1). As described above, the influence of birefringence in the circumferential direction can be reduced by changing the intensity of the reference light in accordance with the influence of the refractive index in one round.

  As header information, recording power information such as recording time at each position in the R direction of the disc is recorded.

(Collinear hologram recording device)
FIG. 14 is a diagram showing a configuration of a collinear hologram recording apparatus.

  As shown in the figure, the optical unit 200 of the collinear hologram recording apparatus includes a recording / reproducing light source 21, a collimating lens 22, an isolator 23, a mechanical shutter 25, a polarizing beam splitter 26, a half-wave plate 27, and a polarizing beam splitter. 28, 1/2 wavelength plate 29, polarizing beam splitter 30, 1/4 wavelength plate 31, relay lens 32, pinhole 33, dichroic mirror 34, objective lens 35, servo drive unit 36, pinhole 55, reflective liquid crystal 40, a magnification adjusting lens 41, a CCD camera 42, a servo light source 43, a collimating lens 44, a grating 45, a beam splitter 46, a condensing lens 47, a cylindrical lens 48, and a light receiving element 49.

  The disk type medium 1 is rotated by a spindle motor (not shown). Since the hologram recording medium 1 moves, recording / reproduction on the hologram recording medium 1 is performed along a track formed in the moving direction.

  Next, the basic operation of this hologram recording apparatus will be described.

  In the optical unit 100, the recording / reproducing light source 21 is a laser light source, and for example, a laser diode (LD) having a wavelength of 405 [nm] or an Nd-YAG laser having a wavelength of 532 [nm] can be used.

  The light beam emitted from the recording / reproducing light source 21 is converted into parallel light by the collimator lens 22 and passes through an isolator 23 to prevent return light. Thereafter, the intensity is adjusted by the half-wave plate 24, and only the p-polarized component is incident on the reflective liquid crystal 40 as a spatial light modulator through the polarization beam splitter 28. The spatial light modulator is an optical element that superimposes data by spatially (in this case, two-dimensionally) modulating signal light. As the spatial light modulator, in addition to the reflective liquid crystal, a DMD (Digital Micro Mirror) or a transmissive liquid crystal element which is a transmissive element can be used.

  Here, a pattern in which the data area of the reference light 17 surrounds the data area of the signal light 16 is projected onto the reflective liquid crystal 40 as shown in FIGS. In this embodiment, the signal light 16 and the reference light 17 are propagated through the same optical path. However, the signal light data area may be a pattern surrounding the reference light data area. . Further, a pattern in which the reference light is incident from both sides of the signal light pattern may be used.

  The light modulated by the reflective liquid crystal 40 is reflected by the polarization beam splitter 28 because the polarization is rotated by 90 °. The light reflected from the polarization beam splitter 28 is converted back to p-polarized light by the half-wave plate 29, passes through the polarization beam splitter 30, becomes circularly polarized light by the quarter-wave plate 31, and propagates through the relay lens 32. Is done. At this time, high-order diffracted light from the liquid crystal is cut by the pinhole 33.

  Subsequently, the light passes through the dichroic mirror 34. The dichroic mirror 34 is an optical element for making the recording / reproducing light and the light used for servo have the same optical path. The light from the recording / reproducing light source 21 is transmitted to the surface of the dichroic mirror 34 in response to the wavelength of the laser light being different between the recording / reproducing light source 21 and the servo light source 43. Thin film processing that reflects light is performed.

  The light that has passed through the dichroic mirror 34 is collected by the objective lens 35 and enters the disc-type medium 1. As a result, interference fringes are formed on the disk-type medium 1. At this time, information spatially modulated by the reflective liquid crystal 40 is recorded on the disk-type medium 1 as a hologram.

  Here, the pattern of the reference light is modulated to a certain intensity, and the pattern generates speckles in the medium 1, and reproduction light is generated only from the coincident part of the speckles. Multiple recording with a very fine shift pitch depending on the size becomes possible. Further, a mechanical shutter 25 that opens and closes a beam during recording is disposed between the half-wave plate 24 and the polarization beam splitter 26, and the recording time can be varied by opening and closing the mechanical shutter 25.

  At the time of reproduction, only the pattern corresponding to the reference light is displayed on the reflective liquid crystal 40 and only the reference light component is made incident on the disc-type medium 1. Thereby, diffracted light (reproduced light) is generated from the hologram recorded on the disk-type medium 1. The reproduction light follows an optical path opposite to that of the signal light, passes through the objective lens 35, passes through the dichroic mirror 34, and passes through the relay lens 32. Noise is cut by the pinhole 33 on the way through the relay lens 32. Thereafter, the reproduction light becomes s-polarized light by the quarter-wave plate 31, is reflected by the polarization beam splitter 30, and the magnification is adjusted by the magnification adjustment lens 41, and the space in the reflective liquid crystal 40 is obtained by the CCD camera 42. Converted into electrical signals corresponding to typical two-dimensional data. The output from the CCD camera 42 is binarized by a signal processing unit (not shown) and converted into time-series binarized data.

  The servo light source 43 is a light source for performing servo control such as tracking servo and focus servo, and emits laser light having a wavelength different from that of the recording / reproducing light source 21. The servo light source 43 is, for example, a laser diode, and has a low sensitivity with respect to the disk-type medium 1 as the oscillation wavelength, for example, 650 nm.

  The servo light emitted from the servo light source 43 is converted into parallel light by the collimator lens 44 and divided into three beams by the grating 45. The laser light emitted from the grating 45 passes through the beam splitter 46 and reaches the dichroic mirror 34, and is placed on the same optical path as the recording / reproducing light by the dichroic mirror 34. Incident.

  The servo light reflected from the disk-type medium 1 returns to the dichroic mirror 34 through the objective lens 35, is reflected here, and returns to the beam splitter 46. The return servo light is reflected by the beam splitter 46 and enters the condensing lens 47, where it is condensed, and then the beam shape is converted from a circular shape to an elliptic shape by the cylindrical lens 48 and received by the light receiving element 49. Is done. From the light receiving element 49, a tracking error signal for tracking servo control, a focus error signal for focus servo control, and the like are output for the return light.

  The servo drive unit 36 is a drive mechanism for performing tracking control and focus control by driving the objective lens 35 in two axial directions by a tracking error signal and a focus error signal from the light receiving element 49. The coils 36A and 36B for driving the objective lens 35 are provided.

  Next, the flow of processing based on header information in the collinear hologram recording apparatus will be described with reference to the flowchart of FIG.

  The hologram recording apparatus includes a system controller having a CPU and a memory. The system controller is control means for controlling a spindle motor (not shown) and the optical unit 300 that rotate the disk type medium 1.

  First, the system controller rotates the disk-type medium 1, reads the header information recorded on the innermost periphery of the disk-type medium 1, and stores the contents in the memory (step S41).

  In order to remove the insensitive area of the disk-type medium 1, the disk surface is irradiated with incoherent light from an LED or the like (step S42).

  The recording position is determined based on the information for managing the recording position included in the header information, and the spindle motor is rotated so that the irradiation position of the two light waves comes to the recording position (step S43). Next, the information is displayed on the reflective liquid crystal 40 on the basis of the pattern size information and intensity modulation information of each of the circumferential signal light and reference light given on the liquid crystal stored as header information in the memory. A pattern of SLM data to be performed is determined (step S44). Next, the SLM data pattern is displayed on the reflective liquid crystal 40 to transfer the SLM data (step S45). Subsequently, the recording time information in the header information stored in the memory is read (step S46), and hologram recording is performed by opening the mechanical shutter 25 only during the recording time (step S47).

  If multiple recording at one position is completed (YES in step S48), the process returns to step S42 to perform recording at the next position. If not completed (NO in step S48), step S5 is performed. Returning to the above, multiple recording at the same position is performed.

  When the angle multiplex recording is completed at all positions (YES in step S49), the entire medium is irradiated with incoherent light as post-processing to change the monomer into a polymer (step S50).

  As described above, in order to mitigate the influence of the birefringence of the medium, the optimum hologram recording condition in the collinear method is recorded on the medium, and the actual hologram recording is performed in accordance with the hologram recording condition. It becomes possible to perform holographic recording satisfactorily by a collinear method on a medium having refraction.

  Next, a method for reducing the influence of birefringence using a wave plate will be described.

  FIG. 16 is a diagram showing a method of reducing the influence of birefringence using a wave plate.

  This is based on the header information described above, so that one or both lights pass through the wave plate 59 so that the deflection directions of the signal light 16 incident on the medium and the reference light 17 coincide with each other. It is made to let you. The wave plate 59 is rotatable in a plane substantially horizontal to the medium 1, and the rotation position is variably set based on the header information.

  Further, when angle multiplexing is assumed as the multiplexing method, the polarization direction changes depending on the recording angle, and therefore the wave plate 59 is rotated with respect to the recording angle. Further, since the birefringence value varies depending on the position of the disk 1 in the R direction (radial direction), the wavelength plate 59 is changed to the optimum rotational position depending on the position.

  Similarly, in the case of the rotation multiplexing method, when the polarization direction of the incident light changes due to the influence of the birefringence, it is possible to compensate by rotating the wave plate 59.

  Further, assuming that the hologram is multiplexed and recorded using a shift multiplexing method such as phase correlation multiplexing as the multiplexing method, the angle of the incident light does not change basically, so that the wavelength plate 59 is the R of the disk 1. It only needs to be changed depending on the position in the direction (radial direction).

  Next, as a method of creating a hologram ROM disk, a method in which the present invention is applied to a method of creating a ROM medium using an axicon lens will be described.

  As shown in FIG. 17, when the reference light 17 is incident on the medium 1 through the axicon lens 60, the plurality of reference lights 17 jump to the radial shape of the disk 1 and are arranged in an annular shape as shown in the figure. By using a plurality of SLMs, a hologram of a certain region 64 is recorded at a time. Furthermore, multiplexing at different angles is possible by changing the distance between the axicon lens 60 and the disk 1. In this system, ideally, it is possible to record on the entire surface of the disk at one time, but here, as shown in FIG. 18, the disk 1 is placed in the R direction at a plurality of areas 71, 72, 73, Divided into 74,75. This is because the difference in birefringence between the inner and outer circumferences of the disc 1 seems to be large depending on the molding conditions of the disc-type substrate. Therefore, when using the medium 1 of the substrate such as polycarbonate having a large birefringence as shown in the figure, for example, when the wavelength plate 59 is inserted in the optical path of the signal light 16 and enters the medium, the same as the reference light 17 The wave plate 59 is rotated so as to be in the polarization direction.

  Here, the wavelength plate 59 is included only in the signal light 16, but it may be included only in the reference light 17 or both.

It is a graph which shows the experimental result of the diffraction efficiency of each recording angle at the time of recording a hologram on the board | substrate which has birefringence by an angle multiplexing system. It is a figure for demonstrating the incident direction of two light waves of a signal wave and a reference wave which considered the influence of birefringence about a disk type medium. It is a figure for demonstrating the incident direction of two light waves of a signal wave and a reference wave which considered the influence of birefringence about card type | mold media. It is a figure which shows the relationship between the angle range which is not influenced largely by birefringence, and the incident direction of signal light and reference light. It is a figure which shows the amplitude of the reference light in the angle range which is not influenced largely by birefringence. It is a figure which shows the structure of the hologram recording apparatus of an angle multiplexing system. It is a flowchart which shows the flow of the process based on header information in the hologram recording device of FIG. It is a schematic diagram which shows the structure of the optical unit of the rotation multiplexing type hologram recording apparatus. It is a flowchart which shows the flow of the process based on header information in the hologram recording device of FIG. It is a figure which shows the relationship between the reference light and signal light in a rotation multiplexing system. It is a figure which shows the relationship between the incident angle in the horizontal direction of the signal light of a rotation multiplexing system, and the magnitude | size of the influence of birefringence. It is a figure for demonstrating the influence of the vertical birefringence in a collinear system. It is a figure for demonstrating the influence of horizontal birefringence in a collinear system. 1 is a diagram illustrating a configuration of a collinear hologram recording apparatus. It is a flowchart which shows the flow of the process based on header information in a collinear hologram recording apparatus. It is a figure which shows the method of reducing the influence of birefringence using a wavelength plate. It is a figure for demonstrating the method of producing a hologram ROM disc. It is a figure which shows the division | segmentation method of the area | region of the disc at the time of producing a hologram ROM disc.

Explanation of symbols

1 Disc type media (hologram recording media)
16 Signal Light 17 Reference Light 21 Recording / Reproducing Light Source 22 Collimating Lens 23 Isolator 24 Half Wave Plate 25 Mechanical Shutter 26 Polarizing Beam Splitter 27 Half Wave Plate 28 Polarizing Beam Splitter 29 1/2 Wave Plate 30 Polarizing Beam Splitter 31 1/4 Wavelength Plate 32 Relay lens 33 Pinhole 34 Dichroic mirror 35 Objective lens 37 1/4 wavelength plate 38 Pinhole 39 Galvano mirror 40 Reflective liquid crystal 41 Magnification adjustment lens 42 CCD camera 59 Wavelength plate 60 Axicon lens 61 Signal light line 62 Reference Optical line 70 Mirror 71 Mirror 72 Actuator 100, 200, 300 Optical unit

Claims (33)

A hologram recording apparatus for recording a hologram on a hologram recording medium in which recording conditions related to birefringence of a substrate are recorded in advance,
Recording condition reading means for reading the recording condition from the hologram recording medium;
And a control unit that controls to record a hologram on the hologram recording medium based on the recording condition read by the recording condition reading unit. A laser light source for emitting laser light;
A light branching element for branching the laser light emitted from the laser light source into signal light and reference light;
An optical modulation element that modulates the signal light branched by the optical branching element;
The optical system for condensing the signal light modulated by the light modulation element and the reference light branched by the light branching element at substantially the same location of the hologram recording medium. Hologram recording device. An angle multiplexing type hologram recording apparatus,
The hologram recording apparatus according to claim 2, wherein the recording condition is an incident plane angle of the signal light and the reference light. An angle multiplexing type hologram recording apparatus,
The hologram recording apparatus according to claim 2, wherein the recording condition is a range of recording angles for angle multiplexing.   The hologram recording apparatus according to claim 4, wherein the recording condition includes information on recording power for each recording angle. A rotary multiplex type hologram recording apparatus,
The hologram recording apparatus according to claim 2, wherein the recording condition is an incident plane angle of the signal light and the reference light.   The hologram recording apparatus according to claim 6, wherein the recording condition includes a range of recording angles for angle multiplexing for each incident plane angle.   The hologram recording apparatus according to claim 7, wherein the recording condition includes information relating to a recording power for each recording angle. A collinear hologram recording device,
The light modulation element is an element that spatially modulates the signal light and superimposes data;
3. The hologram recording apparatus according to claim 2, wherein the recording condition is information relating to intensity modulation applied in each circumferential direction of the pattern on the spatial light modulator. A collinear hologram recording device,
The light modulation element is an element that spatially modulates the signal light and superimposes data;
3. The hologram recording apparatus according to claim 2, wherein the recording condition is information on the size of each pattern of the signal light and the reference light in the circumferential direction given to the spatial light modulator. The hologram recording medium is a disc;
The hologram recording apparatus according to claim 1, wherein the recording condition is a recording condition for each radial position on the disk. A rotatable wave plate that changes a deflection direction of the signal light and / or the reference light incident on the hologram recording medium;
3. The hologram recording apparatus according to claim 2, wherein the control unit sets a rotational position of the wavelength plate based on the recording condition read by the recording condition reading unit. A hologram recording method for recording a hologram on a hologram recording medium in which recording conditions related to birefringence of a substrate are recorded in advance,
Reading the recording conditions from the hologram recording medium;
Recording a hologram on the hologram recording medium based on the read recording condition. A hologram recording method comprising: An angle multiplex hologram recording method,
The hologram recording method according to claim 13, wherein the recording condition is an incident plane angle of signal light and reference light. An angle multiplex hologram recording method,
The hologram recording method according to claim 13, wherein the recording condition is a range of recording angles for angle multiplexing.   The hologram recording method according to claim 15, wherein the recording condition includes information relating to a recording power for each recording angle. A rotary multiplex hologram recording method,
The hologram recording method according to claim 13, wherein the recording condition is an incident plane angle of signal light and reference light.   The hologram recording method according to claim 17, wherein the recording condition includes a range of recording angles for angle multiplexing for each incident plane angle.   The hologram recording method according to claim 18, wherein the recording condition includes information related to a recording power for each recording angle. A collinear hologram recording method,
14. The hologram according to claim 13, wherein the recording condition is information relating to intensity modulation applied in each circumferential direction of a pattern on a spatial light modulator that spatially modulates signal light and superimposes data. Recording method. A collinear hologram recording method,
The recording condition is information on the size of each pattern of the signal light and the reference light in the circumferential direction given to a spatial light modulator that spatially modulates signal light and superimposes data The hologram recording method according to claim 13. The hologram recording medium is a disc;
14. The hologram recording method according to claim 13, wherein the recording condition is a recording condition for each radial position on the disk.   14. The rotation position of a rotatable wave plate that changes the deflection direction of signal light and / or reference light incident on the hologram recording medium is set based on the recording conditions. Hologram recording method.   A hologram recording medium on which recording conditions relating to birefringence of a substrate are recorded.   25. The hologram recording medium according to claim 24, wherein the recording condition is an incident plane angle of signal light and reference light for angle multiplexing.   The hologram recording medium according to claim 24, wherein the recording condition is a range of recording angles for angle multiplexing.   27. The hologram recording medium according to claim 26, wherein the recording condition includes information relating to a recording power for each recording angle.   25. The hologram recording medium according to claim 24, wherein the recording condition is an incident plane angle of signal light and reference light for rotational multiplexing.   29. The hologram recording medium according to claim 28, wherein the recording condition includes a range of recording angles for angle multiplexing for each incident plane angle.   30. The hologram recording medium according to claim 29, wherein the recording condition includes information relating to a recording power for each recording angle.   25. The hologram recording medium according to claim 24, wherein the recording condition is information relating to intensity modulation applied in each circumferential direction of a pattern on the spatial light modulator.   25. The hologram recording medium according to claim 24, wherein the recording condition is information on the size of each pattern of the signal light and the reference light in the circumferential direction applied to the spatial light modulator. The hologram recording medium is a disc;
25. The hologram recording medium according to claim 24, wherein the recording condition is a recording condition for each position in the radial direction on the disc.

Images