JP4543843B2 - Hologram recording method, hologram recording apparatus, hologram reproducing apparatus, and hologram recording medium - Google Patents

Description

  The present invention relates to a hologram recording method, a hologram recording apparatus, a hologram reproducing apparatus, 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 a storage capacity that surpasses that of the conventional optical disc can be achieved by multiple recording at the same location on the hologram recording medium.

  Of the multiplex recording, the method using phase correlation multiplex has a particularly high possibility of increasing the storage capacity and is attracting attention (for example, see Patent Document 1).

In the method using phase correlation multiplexing, a phase mask made of, for example, ground glass is inserted in the optical path of the reference light. For example, if the ground glass has a rough period of about 3 μm, data is not reproduced if the irradiation position is shifted by 3 μm. Therefore, multiple recording with the position shifted at a pitch of 3 μm is possible. This method can adjust the pitch that can be multiplexed by selecting the rough period of the phase mask, and therefore has the possibility of multiplexing at a higher density than other methods.
Japanese Patent Laid-Open No. 11-242424

  However, in the method using phase correlation multiplexing, the reference light is disturbed by a phase mask such as ground glass, so that noise is generated during reproduction. This noise increases as the number of times of multiplex recording increases, and it becomes impossible to reproduce a signal after several hundred times of multiplex recording. For this reason, there is a problem that even if multiple recording is possible in terms of media performance, the upper limit of the number of times of multiplexing is determined due to this noise.

  In view of the circumstances as described above, an object of the present invention is to provide a hologram recording method, a hologram recording apparatus, a hologram reproducing apparatus, and a hologram recording medium capable of performing multiplex recording on a hologram recording medium at a higher density. is there.

A. The hologram recording method according to the present invention records a hologram by phase correlation multiplexing in the first direction of the hologram recording medium, and wavelength-multiplexes in a second direction different from the first direction of the hologram recording medium. And holographic recording.

  In the present invention, hologram recording is performed by phase correlation multiplexing in the first direction of the hologram recording medium. However, hologram recording is performed by wavelength multiplexing in a second direction different from the first direction of the hologram recording medium. Therefore, it is difficult for the multiplex recording in the second direction to be affected by noise associated with phase correlation multiplexing. Accordingly, in the present invention, it is possible to perform multiplex recording with higher density in the second direction, and multiplex recording onto the hologram recording medium can be performed with higher density.

  Here, the phase filter used for the phase correlation multiplexing may have substantially no effect as a phase filter in the second direction.

  Since there is substantially no effect as a phase filter in the second direction, the influence of noise associated with phase correlation multiplexing is eliminated for the multiplex recording in the second direction.

  The hologram recording medium may be a disk type, the first direction may be a circumferential direction of the hologram recording medium, and the second direction may be a radial direction of the hologram recording medium.

By making the hologram recording medium a disk type, the drive system can be made simpler and easy to handle. By performing wavelength multiplex recording that requires time for switching in the radial direction (track direction) where switching at relatively high speed is not required, hologram recording can be performed at high speed as a result.
B. A hologram recording apparatus according to the present invention includes a light source that emits laser light having a plurality of types of wavelengths, an optical system that guides the laser light emitted from the light source to a hologram recording medium as signal light and reference light for hologram recording, and , And means for superimposing a data signal on the signal light, and relatively moving the hologram recording medium, the signal light, and the reference light in a first direction and a second direction different from the first direction. And a phase filter that is disposed on the optical path of the reference light and has substantially no effect as a phase filter in the second direction.

  In the present invention, hologram recording is possible by phase correlation multiplexing for the first direction of the hologram recording medium, and hologram recording is possible by wavelength multiplexing for the second direction. Multiple recording can be performed at higher density.

  Here, the hologram recording medium is a disk type, the first direction is a circumferential direction of the hologram recording medium, the second direction is a radial direction of the hologram recording medium, and the hologram recording medium And a means for changing the wavelength of the light source when the signal light and the reference light are relatively moved in the second direction.

By using a holographic recording medium as a disk type, the drive system of the recording apparatus can be made simpler and easy to handle. By performing wavelength multiplex recording that requires time for switching in the radial direction (track direction) where switching at relatively high speed is not required, hologram recording can be performed at high speed as a result.
C. A hologram reproducing apparatus according to the present invention includes a light source that emits laser light of a plurality of types of wavelengths, an optical system that guides the laser light emitted from the light source to a hologram recording medium, the hologram recording medium, and the laser light. Means for relatively moving in a first direction and a second direction different from the first direction; and disposed on an optical path of the laser beam, and substantially as a phase filter with respect to the second direction. And a means for receiving laser light that has passed through the hologram recording medium.

  According to the present invention, it is possible to reproduce a hologram recording medium in which hologram recording is performed by phase correlation multiplexing in the first direction and hologram recording is performed by wavelength multiplexing in the second direction.

  Here, the hologram recording medium is a disk type, the first direction is a circumferential direction of the hologram recording medium, the second direction is a radial direction of the hologram recording medium, and the hologram recording medium And means for changing the wavelength of the light source when the laser beam is relatively moved in the second direction.

By making the hologram recording medium a disk type, the drive system of the reproducing apparatus can be made simpler and easy to handle. Further, it is possible to reproduce the hologram recording medium on which the hologram is recorded as described above.
D. The hologram recording medium according to the present invention is characterized in that hologram recording is performed by phase correlation multiplexing in the first direction, and hologram recording is performed by wavelength multiplexing in a second direction different from the first direction. To do.

  In the present invention, it is possible to provide a hologram recording medium capable of performing multiple recording at a higher density. Here, the hologram recording medium may be a disk type, the first direction may be a circumferential direction of the hologram recording medium, and the second direction may be a radial direction of the hologram recording medium.

  As described above, according to the present invention, multiple recording on a hologram recording medium can be performed at a higher density.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[First Embodiment]

  FIG. 1 is a schematic diagram showing an optical unit 100 of a hologram recording / reproducing apparatus according to the first embodiment of the present invention. FIG. 2 is a perspective view showing the configuration of the hologram recording medium 101.

  As shown in FIG. 1, the hologram recording / reproducing apparatus records and reproduces information on a hologram recording medium 101 and includes an optical unit 100.

  The optical unit 100 includes a recording / reproducing light source 111, a beam expander 112, a polarization beam splitter 113, a mirror 121, a spatial light modulator 122, a lens 123, a mirror 124, a one-dimensional phase filter 125, a lens 126, a lens 127, and a lens 128. CCD sensor 129 is included.

  The hologram recording medium 101 has a disk shape (disk type), has a protective layer 102, a recording layer 103, and a protective layer 104, and is a recording medium that records interference fringes due to signal light and reference light.

  The protective layers 102 and 104 are layers for protecting the recording layer 103 from the outside world.

  The recording layer 103 records interference fringes as a change in refractive index (or transmittance), and is an organic material that can change the refractive index (or transmittance) according to the intensity of light. It can be used regardless of the material or 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.

  As the organic material, for example, a photopolymerization type photopolymer can be used. In the photopolymerization type photopolymer, in the initial state, monomers are uniformly dispersed in the matrix polymer. When this is irradiated with light, the monomer is polymerized at the exposed portion, and the refractive index changes.

  As described above, when the refractive index (or transmittance) of the recording layer 103 changes in accordance with 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). Recording on the hologram recording medium 101 is possible.

  The hologram recording medium 101 is moved (radial direction of the hologram recording medium 101) and rotated (circumferential direction of the hologram recording medium 101) by a driving unit (not shown) to record the image of the spatial light modulator 122 as a large number of holograms. Can do. The hologram recording medium 101 itself may not move, but the optical unit 100 may move in the radial direction of the hologram recording medium 101.

  The recording / reproducing light source 111 is a laser light source capable of changing the wavelength of the emitted laser light, and for example, a tunable laser is used. This will be described later.

  The beam expander 112 is an optical element that expands the diameter of the laser light emitted from the recording / reproducing light source 111.

  The polarization beam splitter 113 is an optical element that splits the laser light incident from the beam expander 112 into signal light and reference light.

  The mirror 121 is an optical element that reflects the light and changes its direction in order to irradiate the incident signal light on the first surface side of the hologram recording medium 101. The mirror 124 is an optical element that reflects the light and changes its direction in order to irradiate the incident reference light on the second surface side opposite to the first surface of the hologram recording medium 101.

  The spatial light modulator 123 is an optical element that spatially modulates signal light (in this case, two-dimensionally) and superimposes a data signal. As the spatial light modulator 123, a transmissive ferroelectric liquid crystal element which is a transmissive element can be used. In addition, it is possible to use a reflection type element such as a DMD (Digital micro mirror), a reflection type liquid crystal, or a GLV (Grating Light Value) element for the spatial light modulator.

  The lens 123 is an optical element that focuses signal light on the hologram recording medium 101. An image (real image) displayed on the spatial light modulator 122 is converted into a Fraunhofer diffraction image (Fourier transform image) at the focal position of the lens 123 to form an image.

  The one-dimensional phase filter 125 has substantially no effect as a phase filter in the radial direction of the hologram recording medium 101, and has an effect as a phase filter only in the circumferential direction of the hologram recording medium 101. It is. Such a one-dimensional phase filter 125 can be realized by, for example, putting a scratch in the radial direction of the hologram recording medium 101 randomly or at a predetermined period on the surface of ground glass.

  The lens 126 is an optical element that forms an image by converting the reference light reflected by the mirror 124 into a Fraunhofer diffraction image (Fourier transform image) at the focal position of the lens 126.

  The lens 127 converts the reference light that has passed through the lens 126 into parallel light.

  Therefore, as shown in FIG. 3, the signal light is incident from the first surface side of the hologram recording medium 101 via the spatial light modulator 123 and the lens 123, and the reference light is the one-dimensional phase filter 125 and the lens 126, The light enters from the second surface side opposite to the first surface of the hologram recording medium 101 via 127. As a result, the hologram recording medium 101 interferes with the signal light and the reference light to form a diffraction grating (hologram) at the irradiation point, and data is recorded on the hologram recording medium 101.

  The lens 128 collects the diffracted light reproduced from the hologram recording medium 101 on the CCD sensor 129. At this time, the real image displayed on the spatial light modulator 122 is formed on the detection surface of the CCD sensor 129. That is, the lens 128 is an inverse Fourier transform lens that forms an image on the CCD sensor 129 by performing inverse Fourier transform on the Fourier transform image of the spatial light modulator 122 recorded on the hologram recording medium 101.

The CCD sensor 129 is a light receiving element that detects reproduced signal light, and is a two-dimensional light receiving element in which the light receiving elements are arranged in two directions.
(Example of light source 111 for recording and reproduction)

  FIG. 4 is a diagram showing an example of a tunable laser used as the recording / reproducing light source 111 described above.

  The recording / reproducing light source 111 includes a laser diode 130, a collimating lens 131, a diffraction grating 132, a mirror 133, a holding member 134, and a rotation driving unit 135.

  The laser diode 130 emits multimode laser light. For example, it emits blue laser light of about 410 nm.

  The collimating lens 131 converts the laser light emitted from the laser diode 130 into parallel light.

  As shown in FIG. 5, the diffraction grating 132 generates primary light in a different direction for each wavelength, and the primary light of a specific wavelength (for example, 410 nm) is returned to the laser diode 130 among them. The angle is set. As a result, only the wavelength component increases in the laser diode 130 and a single mode is obtained. Most of the laser light emitted by the laser diode 130 is not first-order light but zero-order light, and is reflected by the diffraction grating 132 like a mirror. That is, the recording / reproducing light source 111 is basically a Littrow type external resonator laser.

  The mirror 133 changes the optical path of the laser light reflected by the diffraction grating 132 in a specific direction.

  The holding member 134 holds the diffraction grating 132 and the mirror 133. A predetermined angle is always maintained between the diffraction grating 132 and the mirror 133. Thereby, even if the diffraction grating 132 rotates, the emission direction of the laser beam can be fixed by the mirror 133.

  The holding member 134 is rotatably held by a shaft 136. In the holding member 134, the diffraction grating 132 and the mirror 133 are disposed on one side of the shaft 136, and the rotation shaft 137 is integrally provided on the other side.

  The rotation drive unit 135 rotates the holding member 134 about the shaft 136. The main body 138, the screw 139 for pressing the rotation shaft 137, and the direction in which the rotation shaft 137 is pressed by the screw 139. A leaf spring 140 is provided to apply an elastic force to the rotation shaft 137 from the opposite direction. The screw 139 is rotated by, for example, a rotary drive motor (not shown).

In the recording / reproducing light source 111 configured as described above, the wavelength of the blue laser light of, for example, about 410 nm can be varied by about 5 nm by rotating the diffraction grating 132.
(Function and effect of hologram recording / reproducing device)

  In the hologram recording / reproducing apparatus configured as described above, the one-dimensional phase filter 125 has substantially no effect as a phase filter in the radial direction of the hologram recording medium 101 (direction in FIG. 3B). Therefore, the hologram recording is performed by phase correlation multiplexing in the circumferential direction of the hologram recording medium 101 (direction of FIG. 3A). . On the other hand, hologram recording is performed by changing the wavelength of the recording / reproducing light source 111 in the radial direction of the hologram recording medium 101 (direction in FIG. 3B), that is, by wavelength multiplexing.

  In the present embodiment, by using the one-dimensional phase filter 125 as described above and recording the hologram by wavelength multiplexing in the radial direction, noise caused by the phase filter does not affect the radial direction, Multiple recording can also be performed in the radial direction.

  In the reproducing operation in the hologram recording / reproducing apparatus, the signal light is not applied to the hologram recording medium 101 using a shielding plate (not shown), but only the reference light is applied to the hologram recording medium 101, and the reflected light is applied. This can be done by receiving light with the CCD sensor 129.

  Here, in this embodiment, since it is a reflection type that irradiates the reproduction light and receives the reflected light, the dependency of the reproduction wavelength on the hologram recording medium 101 is particularly steep, and if the wavelength differs by about 0.1 nm. Even if the recording locations overlap, it is possible to reproduce only the recording signal corresponding to the reproduction wavelength. Accordingly, if the recording / reproducing light source 111 can change the wavelength by about 5 nm,

5 nm / 0.1 nm = 50
Thus, multiple recording can be performed about 50 times in the radial direction.

  On the other hand, the spot diameter of the laser beam on the hologram recording medium 101 is 1 mmφ, and phase correlation multiplexing can be performed in the circumferential direction of the hologram recording medium 101 at a pitch of 5 μm.

1 mm / 5 μm = 200
Thus, multiple recording can be performed about 200 times in the circumferential direction.

  With this, considering both the circumferential direction and the radial direction,

200 × 50 = 10000
Thus, multiplexing can be performed about 10,000 times.
[Second Embodiment]

  FIG. 6 is a schematic diagram showing an optical unit 200 of a hologram recording / reproducing apparatus according to the second embodiment of the present invention.

  The hologram recording / reproducing apparatus according to the first embodiment irradiates signal light from the first surface side of the hologram recording medium 101, irradiates reference light from the second surface side of the hologram recording medium 101, and reflects during reproduction. Although configured to receive light, the hologram recording / reproducing apparatus according to the second embodiment irradiates the signal light and the reference light from the first surface side of the hologram recording medium 101 and reproduces the hologram recording medium 101 at the time of reproduction. The difference is that light is received on the second surface side. Here, the same elements as those shown in FIG.

  Here, the spot diameter of the laser beam on the hologram recording medium 101 is 1 mmφ, and phase correlation multiplexing can be performed in the circumferential direction of the hologram recording medium 101 at a pitch of 5 μm.

1 mm / 5 μm = 200
Thus, similar to the first embodiment, multiple recording can be performed about 200 times in the circumferential direction.

  On the other hand, in the radial direction of the hologram recording medium 101, the wavelength is changed by 0.5 nm from 405 nm to 410 nm, the position is sent by 100 μm accordingly, and wavelength multiplex recording is performed.

1 mm / 100 μm = 10
Thus, 10 times of multiplexing were performed.

  FIG. 7 shows experimental data on dependency of reproduction wavelength when hologram recording is performed only once at a reference light incident angle of 45 ° and a wavelength of 410 nm on a hologram recording medium 101 having a thickness of 1 mm. From this graph, it can be seen that reproduction is impossible when the wavelength is smaller than 0.5 nm. Conversely, if the wavelengths are different by 0.5 nm or more, only the recording signal corresponding to the reproduction wavelength is reproduced even if the recording locations overlap.

  With this, considering both the circumferential direction and the radial direction,

200 × 10 = 2000
Thus, multiplexing can be performed about 2000 times.
[Third Embodiment]

  FIG. 8 is a schematic diagram showing an optical unit 300 of a hologram recording / reproducing apparatus according to the third embodiment of the present invention. This optical unit 300 is a type called collinear, and the same optical system is used for signal light and reference light.

  The optical unit 300 includes a recording / reproducing light source 311, a beam expander 312, a spatial light modulator 313, a polarizing beam splitter 314, a Faraday element 315, a lens 316, and a CCD sensor 317.

  The recording / reproducing light source 311 and the beam expander 312 have the same configuration as that shown in FIG.

  The spatial light modulator 313 includes a transmissive ferroelectric liquid crystal element 318 and a one-dimensional phase filter 319. As shown in FIG. 9, the transmissive ferroelectric liquid crystal element 318 in the spatial light modulator 313 includes a substantially circular first region 320 through which signal light passes, and a second region 321 through which reference light passes. Have As shown in FIG. 10, in the first region 320, the signal light is spatially modulated and the data signal is superimposed. A one-dimensional phase filter 319 is overlaid on the second region 321.

  The one-dimensional phase filter 319 has substantially no effect as a phase filter in the radial direction of the hologram recording medium 101, and has an effect as a phase filter only in the circumferential direction of the hologram recording medium 101. It is.

  Here, the polarization directions of the signal light and the reference light emitted from the spatial light modulator 313 are in the radial direction C of the hologram recording medium 101 as shown in FIG. The polarization beam splitter 314 is an optical element that transmits signal light and reference light having a polarization direction in that direction.

  The Faraday element 315 is an optical element that rotates the polarization direction of light by 45 °. The polarization directions of the signal light and the reference light are rotated by 45 ° by the Faraday element 315. The reflected light reflected from the hologram recording medium 101 is further rotated by 45 ° by the Faraday element 315. Therefore, the polarization direction of the reflected light reflected from the hologram recording medium 101 is rotated by 90 ° from the direction of the transmission axis of the polarizing beam splitter 314, and the reflected light reflected from the hologram recording medium 101 is transmitted through the polarizing beam splitter 314. Without being reflected and guided to the CCD sensor 317.

  The lens 316 is an optical element that focuses the signal light and the reference light on the hologram recording medium 101. An image (real image) displayed on the spatial light modulator 313 is converted into a Fraunhofer diffraction image (Fourier transform image) at the focal position of the lens 316 and imaged. The image is converted into a Fraunhofer diffraction image (Fourier transform image) at the focal position. As a result, the hologram recording medium 101 interferes with the signal light and the reference light to form a diffraction grating (hologram) at the irradiation point, and data is recorded on the hologram recording medium 101.

  Further, the diffracted light reproduced from the hologram recording medium 101 is condensed on the CCD sensor 317 via the lens 316, the Faraday element 315 and the polarization beam splitter 314. At this time, a real image displayed on the spatial light modulator 313 is formed on the detection surface of the CCD sensor 317. That is, the lens 316 is also an inverse Fourier transform lens that forms an image on the CCD sensor 317 by performing inverse Fourier transform on the Fourier transform image of the spatial light modulator 313 recorded on the hologram recording medium 101.

The CCD sensor 317 is a light receiving element that detects the reproduced signal light, and is a two-dimensional light receiving element in which the light receiving elements are arranged in two directions.
(Function and effect of hologram recording / reproducing device)

  In the hologram recording / reproducing apparatus configured as described above, the one-dimensional phase filter 319 is substantially effective as a phase filter in the radial direction of the hologram recording medium 101, as in the first and second embodiments. However, since it has an effect as a phase filter only in the circumferential direction of the hologram recording medium 101, hologram recording is performed in the circumferential direction of the hologram recording medium 101 by phase correlation multiplexing. On the other hand, hologram recording is performed in the radial direction of the hologram recording medium 101 while changing the wavelength of the recording / reproducing light source 311, that is, by wavelength multiplexing. Therefore, multiple recording on the hologram recording medium 101 can be performed at a higher density.

  In addition, in this embodiment, since the signal light and the reference light are used as a common optical system, it is possible to reduce the size and the number of components. In particular, in order to implement the present invention, it is necessary to interpose a one-dimensional phase filter in the optical path of the reference light. Since such a one-dimensional phase filter 319 can be integrated with the spatial light modulator 313, the configuration is further improved. It becomes possible to make it simple.

  In the reproducing operation of this hologram recording / reproducing apparatus, only the reference light portion of the spatial modulator is transmitted, so that the signal light is not applied to the hologram recording medium 101 and only the reference light is applied to the hologram recording medium 101. The reflected light can be received by the CCD sensor 317.

It is a schematic diagram showing the optical unit of the hologram recording device which concerns on the 1st Embodiment of this invention. It is a perspective view which shows the structure of a hologram recording medium. FIG. 4 is a schematic plan view showing a recording operation on a hologram recording medium. It is a schematic diagram which shows an example of the light source for recording / reproducing. It is a schematic diagram for demonstrating operation | movement of the light source for recording / reproducing. It is a schematic diagram showing the optical unit of the hologram recording device which concerns on the 2nd Embodiment of this invention. It is a graph which shows the experimental data of the dependence of the reproduction | regeneration wavelength in a hologram recording medium. It is a schematic diagram showing the optical unit of the hologram recording device which concerns on the 3rd Embodiment of this invention. It is explanatory drawing of a spatial light modulator (the 1). It is explanatory drawing of a spatial light modulator (the 2). It is a figure for demonstrating the polarization direction of light.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Optical unit 101 Hologram recording medium 111 Recording / reproducing light source 112 Beam expander 113 Polarizing beam splitter 121 Mirror 122 Spatial light modulator 123 Lens 124 Mirror 125 One-dimensional phase filter 126 Lens 127 Lens 128 Lens 129 CCD sensor

Claims (6)

Hologram recording by phase correlation multiplexing with respect to the first direction of the hologram recording medium;
A hologram recording method for performing hologram recording by wavelength multiplexing in a second direction different from the first direction of the hologram recording medium ,
The hologram recording method according to claim 1, wherein the phase filter used for the phase correlation multiplexing has substantially no effect as a phase filter in the second direction. The hologram recording method according to claim 1,
The hologram recording medium is a disk type,
The first direction is a circumferential direction of the hologram recording medium;
The hologram recording method, wherein the second direction is a radial direction of the hologram recording medium. A light source that emits laser light of multiple types of wavelengths;
An optical system for guiding the laser light emitted from the light source to a hologram recording medium as signal light and reference light for hologram recording;
Means for superimposing a data signal on the signal light;
Means for relatively moving the hologram recording medium, the signal light and the reference light in a first direction and a second direction different from the first direction;
A hologram recording apparatus, comprising: a phase filter disposed on an optical path of the reference light and having substantially no effect as a phase filter in the second direction. The hologram recording apparatus according to claim 3, wherein
The hologram recording medium is a disk type,
The first direction is a circumferential direction of the hologram recording medium;
The second direction is a radial direction of the hologram recording medium;
A hologram recording apparatus comprising: means for changing a wavelength of the light source when the hologram recording medium, the signal light, and the reference light are relatively moved in the second direction. A light source that emits laser light of multiple types of wavelengths;
An optical system for guiding laser light emitted from the light source to a hologram recording medium;
Means for relatively moving the hologram recording medium and the laser beam in a first direction and a second direction different from the first direction;
A phase filter disposed on the optical path of the laser beam and having substantially no effect as a phase filter in the second direction;
And a means for receiving laser light that has passed through the hologram recording medium. A hologram reproducing apparatus according to claim 5, wherein
The hologram recording medium is a disk type,
The first direction is a circumferential direction of the hologram recording medium;
The second direction is a radial direction of the hologram recording medium;
A hologram reproducing apparatus comprising: means for changing a wavelength of the light source when the hologram recording medium and the laser beam are relatively moved in the second direction.

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