U.S. patent application number 09/925488 was filed with the patent office on 2002-01-24 for holographic memory and optical information recording and reproducing apparatus using the same.
This patent application is currently assigned to Pioneer Corporation, a Japanese corporation. Invention is credited to Hatano, Hideki, Itoh, Yoshihisa, Kouno, Tomomitsu, Matsushita, Hajime, Tanaka, Satoru, Yamaji, Takashi.
Application Number | 20020008889 09/925488 |
Document ID | / |
Family ID | 13942199 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020008889 |
Kind Code |
A1 |
Tanaka, Satoru ; et
al. |
January 24, 2002 |
Holographic memory and optical information recording and
reproducing apparatus using the same
Abstract
A description is provided of an optical information recording
and reproducing apparatus employing a holographic memory. The
apparatus includes a mask disposed in an optical path of the signal
light beam so as to cover a light intensity distribution of the
signal light beam incident into the holographic memory at nearly a
half portion thereof with respect to the center of the distribution
of the zeroth-order diffracted light of the signal light beam. The
center of a region of the holographic memory where the signal light
beam and the reference light beam intersect with each other, is
shifted by a distance substantially equal to twice the distance
between peaks of the zeroth-order diffracted light or the
first-order diffracted light of the signal light beam.
Inventors: |
Tanaka, Satoru;
(Tsurugashima-shi, JP) ; Kouno, Tomomitsu;
(Tsurugashima-shi, JP) ; Hatano, Hideki;
(Tsurugashima-shi, JP) ; Itoh, Yoshihisa;
(Tsurugashima-shi, JP) ; Matsushita, Hajime;
(Tsurugashima-shi, JP) ; Yamaji, Takashi;
(Tsurugashima-shi, JP) |
Correspondence
Address: |
JOHN F. HAYDEN, ESQ.
Fish & Richardson P.C.
601 Thirteenth Street, NW
Washington
DC
20005
US
|
Assignee: |
Pioneer Corporation, a Japanese
corporation
|
Family ID: |
13942199 |
Appl. No.: |
09/925488 |
Filed: |
August 10, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09925488 |
Aug 10, 2001 |
|
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09533763 |
Mar 23, 2000 |
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6301028 |
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Current U.S.
Class: |
359/30 ; 359/31;
359/32; G9B/7.002; G9B/7.027 |
Current CPC
Class: |
G03H 2001/026 20130101;
G11B 7/0065 20130101; G11B 7/0025 20130101; G03H 2223/12 20130101;
G03H 2001/0268 20130101; G03H 1/26 20130101; G03H 1/265
20130101 |
Class at
Publication: |
359/30 ; 359/31;
359/32 |
International
Class: |
G03H 001/00; G03H
001/22 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 1999 |
JP |
11-88416 |
Claims
What is claimed is:
1. An optical information recording/reproducing apparatus for
recording data on a holographic memory formed of a photorefractive
crystal and reproducing data from the holographic memory, said
apparatus comprising: a support portion for detachably supporting a
holographic memory; a reference light beam supplying portion for
supplying a coherent reference light beam of a first wavelength
incident on the holographic memory; a signal light beam supplying
portion for supplying a coherent signal light beam of the first
wavelength, which is modulated in accordance with image data,
incident on the holographic memory, to intersect the signal light
beam with the reference light beam in the holographic memory
thereby to generate a light interference pattern of the coherent
signal light beam and the reference light beam; and a
photo-detecting portion for detecting a diffracted light caused
from a refractive index grating of the light interference pattern
in the holographic memory caused by irradiation of the reference
light beam; a mask portion disposed in a light path of the signal
light beam so as to cover the light intensity distribution of the
signal light beam incident into the holographic memory
substantially at a half portion thereof with respect to the center
of the zeroth-order diffracted light distribution of the signal
light beam; and a moving portion for shifting the center of a
region of the holographic memory in which the signal light beam and
the reference light beam intersect with each other, by a distance
substantially equal to twice the distance between peaks of the
zeroth-order diffracted light or the first-order diffracted light
of the signal light beam.
2. An apparatus according to claim 1, further comprising a gate
light beam supplying portion for supplying a gate light beam of a
second wavelength into the holographic memory, the gate light beam
enhancing a photo-sensitivity of the holographic memory for one of
activating and deactivating of a refractive index grating in
accordance with the presence or absence of said optical
interference pattern.
3. An apparatus according to claim 2, wherein said gate light beam
supplying portion includes a super-luminescent diode.
4. An apparatus according to claim 2, wherein said gate light beam
supplying portion includes a restricting portion for limiting the
gate light beam irradiated in the region in which the signal light
beam and the reference light beam intersect with each other.
5. An apparatus according to claim 1, wherein the holographic
memory includes a cylindrical body made of a uniaxial crystal
having an optical crystallographic axis in parallel with an axis of
rotational symmetry, and said moving portion further comprises a
transferring portion for moving the cylindrical body in a direction
of the optical crystallographic axis, and for rotating the
cylindrical body about the axis of rotational symmetry.
6. An apparatus according to claim 1, wherein the holographic
memory is a rectangular solid made of a uniaxial crystal having an
optical crystallographic axis in parallel with one surface thereof,
and said moving portion further comprises a moving portion for
moving the reference light beam with respect to the holographic
memory.
7. A holographic memory comprising: a cylindrical body made of a
uniaxial crystal having an optical crystallographic axis in
parallel with an axis of rotational symmetry; a plurality of
refractive index gratings corresponding to three-dimensional
optical interference patterns caused by interference between a
coherent signal light beam of a first wavelength modulated in
accordance with image data and a coherent reference light beam,
wherein the refractive index gratings are arrayed at a regular
interval such that the angular distance between the centers of the
refractive index gratings is substantially equal to twice the
distance between peaks of the zeroth-order diffracted light or the
first-order diffracted light of a light intensity distribution in
the signal light beam.
8. A holographic memory comprising: a rectangular parallelepiped
made of a uniaxial crystal having an optical crystallographic axis
in parallel with one surface thereof; a plurality of refractive
index gratings corresponding to three-dimensional optical
interference patterns caused by interference between a coherent
signal light beam of a first wavelength modulated in accordance
with image data and a coherent reference light beam, wherein the
refractive index gratings are linearly arrayed at a regular
interval such that the distance between the centers of the
refractive index gratings is substantially equal to twice the
distance between peaks of the zeroth-order diffracted light or the
first-order diffracted light of a light intensity distribution in
the signal light beam.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a volume holographic memory
and an optical information recording and reproducing apparatus
using the volume holographic memory.
[0003] 2. Description of the Related Art
[0004] Conventionally, a holographic memory system is known as a
digital recording system using the principle of holography. The
holographic memory system records digital data on a memory medium
made of a photorefractive crystalline such as lithium niobate
(LiNbO.sub.3) or the like, and reproduces the data from the same.
The photorefractive effect is a phenomenon in such that electric
charges generated by photo-excitation move within a crystal thereby
to form a spatial electric field distribution, which combines with
a primary electro-optical effect i.e., Pockels effect to change a
refractive index distribution in the crystal. In a ferroelectric
crystal or the like exhibiting the photo-refractive effect, its
change of the refractive index is responsive even to a fine optical
input pattern of 1,000 lines or more per one millimeter, and this
effective action is generated at a response speed on the order of
microseconds to seconds in real time, though the response speed
varies depending on kinds of materials. Therefore, a variety of
applications for such crystals has been studied as a real time
hologram medium which does not require any developing. The
holographic memory system is capable of recording and reproducing
data on a two-dimensional plane page unit, and also performing a
multiple recording with use of a plurality of the page units. The
volume holographic memory is designed to enable three-dimensional
recording with a crystal medium being of a three-dimensional shape
such as a rectangular parallelepiped or the like. In the volume
holographic memory, which is one type of Fourier transform
holograms, data is recorded at every two-dimensional image page
unit in a dispersed manner within a three-dimensional space of the
volume holographic memory. In the following, the outline of the
holographic memory system will be described with reference to FIG.
1.
[0005] Referring to FIG. 1, an encoder 25 translates digital data
to be recorded in a volume holographic memory 1 into a dot pattern
image consisting of light and dark spots arranged in a plane, and
rearranges the image in a data arrangement, for example, a data
array of 480 pixels in the vertical direction and 640 pixels in the
horizontal direction to generate a unit page sequence data. The
unitary page sequence data is supplied to a spatial light modulator
(SLM) 12 including a panel of a transmission type Thin Film
Transistor (TFT) liquid crystal display (hereinafter also called
simply as "LCD").
[0006] The spatial light modulator 12 has a modulation unit for
performing a modulation processing of 480 pixels in a line and 640
pixels in a row which corresponds to one unit page, and optically
modulates a light beam into an on/off signal of spatial light in
accordance with the unit page sequence data from the encoder 25,
and guides the modulated light beam, i.e., signal light beam to a
lens 13. More specifically, the spatial light modulator 12 passes
therethrough the light beam in response to a logical value "1" of
the unit page sequence data, which is an electric signal, and shuts
off the light beam in response to a logical value "0" thereby to
accomplish the electro-optical conversion in accordance with the
contents of respective bits in the unit page data. Accordingly, the
signal light beam including the unit page sequence is generated by
modulation of the light beam.
[0007] The signal light beam is incident upon the volume
holographic memory 1 through the lens 13. In addition to the signal
light beam, a reference light beam is incident upon the volume
holographic memory 1 at an angle .beta. (hereinafter, referred to
as "incident angle .beta.") relative to a predetermined baseline
perpendicular to an optical path of the signal light beam.
[0008] Both the signal light beam and the reference light beams
interfere with each other within the volume holographic memory 1,
and the resulting interference fringes are stored as a refractive
index grating within the volume holographic memory 1, whereby
recording of data is effected. Also, when the volume holographic
memory 1 is irradiated multiple times with the reference light beam
at different incident angles .beta. to record a plurality of
two-dimensional plane data in an angle multiplexing form, a
recording of three-dimensional data can be accomplished.
[0009] When reproducing the recorded data from the volume
holographic memory 1, only the reference light beam is introduced
into the volume holographic memory 1 at the same incident angle
.beta. as at the time of recording toward the center of a region in
which the signal and reference light beams intersect with each
other. In other words, the reproducing of the recorded data is
different from the recording of the data in that the signal light
beam is not introduced into the volume holographic memory 1.
Therefore, the volume holographic memory 1 diffracts the reference
light beam at the intersection of the refractive index grating
caused by interference fringes. The diffracted light from the
refractive index grating recorded in the volume holographic memory
1 is guided through a lens 21 to a photodetector such as a Charge
Coupled Device (CCD) array 22 on which a light and dark pattern
image i.e., an image of the data arrangement is reproduced. The CCD
22 converts the received image into variations in intensity of an
electric signal to output to a decoder 26 an analog electric signal
having a level corresponding to a distribution of brightness in the
incident image. The decoder 26 compares the analog electric signal
with a predetermined amplitude i.e., a slice level to reproduce
data consisting of the corresponding "1" and "0".
[0010] Since the volume holographic memory records two-dimensional
plane data sequences as described above, angle multiplexing
recording can be performed by changing the incident angle .beta. of
the reference light beam. Specifically, a plurality of
two-dimensional planes, i.e., the recorded units, can be defined
within the volume holographic memory by changing the incident angle
.beta. of the reference light beam. Consequently, three-dimensional
recording can be carried out. Examples of angle multiplexing
recording are described in Japanese Unexamined Patent Publications
Kokai Nos. H2-142979 and H10-97174.
OBJECT AND SUMMARY OF THE INVENTION
[0011] It is therefore an object of the present invention to
provide an optical information recording and reproducing apparatus
which is capable of recording interference patterns in a volume
holographic memory at a high density and capable of being
miniaturized.
[0012] According to the present invention, there is provided an
optical information recording/reproducing apparatus for recording
data on a holographic memory formed of a photorefractive crystal
and reproducing data from the holographic memory, said apparatus
comprising:
[0013] a support portion for detachably supporting a holographic
memory;
[0014] a reference light beam supplying portion for supplying a
coherent reference light beam of a first wavelength incident on the
holographic memory;
[0015] a signal light beam supplying portion for supplying a
coherent signal light beam of the first wavelength, which is
modulated in accordance with image data, incident on the
holographic memory, to intersect the signal light beam with the
reference light beam in the holographic memory thereby to generate
a three-dimensional light interference pattern of the coherent
signal light beam and the reference light beam; and
[0016] a photo-detecting portion for detecting a diffracted light
caused from a refractive index grating of the light interference
pattern in the holographic memory caused by irradiation of the
reference light beam;
[0017] a mask portion disposed in a light path of the signal light
beam so as to cover the light intensity distribution of the signal
light beam incident into the holographic memory substantially at a
half portion thereof with respect to the center of the zeroth-order
diffracted light distribution of the signal light beam; and
[0018] a moving portion for shifting the center of a region of the
holographic memory in which the signal light beam and the reference
light beam intersect with each other, by a distance substantially
equal to twice the distance between peaks of the zeroth-order
diffracted light or the first-order diffracted light of the signal
light beam.
[0019] According to one aspect of the present invention, said
apparatus further comprises a gate light beam supplying portion for
supplying a gate light beam of a second wavelength into the
holographic memory, the gate light beam enhancing a
photo-sensitivity of the holographic memory for one of activating
and deactivating of a refractive index grating in accordance with
the presence or absence of said optical interference pattern.
[0020] According to another aspect of the present invention, said
gate light beam supplying portion includes a super-luminescent
diode.
[0021] According to a further aspect of the present invention, said
gate light beam supplying portion includes a restricting portion
for limiting the gate light beam irradiated in the region in which
the signal light beam and the reference light beam intersect with
each other.
[0022] According to a still further aspect of the present
invention, the holographic memory includes a cylindrical body made
of a uniaxial crystal having an optical crystallographic axis in
parallel with an axis of rotational symmetry, and said moving
portion further comprises a transferring portion for moving the
cylindrical body in a direction of the optical crystallographic
axis, and for rotating the cylindrical body about the axis of
rotational symmetry.
[0023] According to another aspect of the present invention, the
holographic memory is a rectangular solid made of a uniaxial
crystal having an optical crystallographic axis in parallel with
one surface thereof, and said moving portion further comprises a
moving portion for moving the reference light beam with respect to
the holographic memory.
[0024] According to the present invention, there is also provided a
holographic memory comprising:
[0025] a cylindrical body made of a uniaxial crystal having an
optical crystallographic axis in parallel with an axis of
rotational symmetry;
[0026] a plurality of refractive index gratings corresponding to
three-dimensional optical interference patterns caused by
interference between a coherent signal light beam of a first
wavelength modulated in accordance with image data and a coherent
reference light beam,
[0027] wherein the refractive index gratings are arrayed at a
regular interval such that the angular distance between the centers
of the refractive index gratings is substantially equal to twice
the distance between peaks of the zeroth-order diffracted light or
the first-order diffracted light of a light intensity distribution
in the signal light beam.
[0028] In addition, there is provided a holographic memory
according to the present invention comprising:
[0029] a rectangular parallelepiped made of a uniaxial crystal
having an optical crystallographic axis in parallel with one
surface thereof;
[0030] a plurality of refractive index gratings corresponding to
three-dimensional optical interference patterns caused by
interference between a coherent signal light beam of a first
wavelength modulated in accordance with image data and a coherent
reference light beam,
[0031] wherein the refractive index gratings are linearly arrayed
at a regular interval such that the distance between the centers of
the refractive index gratings is substantially equal to twice the
distance between peaks of the zeroth-order diffracted light or the
first-order diffracted light of a light intensity distribution in
the signal light beam.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a diagram showing a constitution of a conventional
volume holographic memory system;
[0033] FIG. 2 is a side view showing a constitution of a volume
holographic memory system according to the present invention;
[0034] FIG. 3 is a perspective view showing an essential part of
the optical information recording and reproducing apparatus
according to the present invention and a light intensity
distribution of a signal light beam resulting from the
apparatus;
[0035] FIG. 4 is a graph showing a light intensity distribution of
a signal light beam resulting from the optical information
recording and reproducing apparatus according to the present
invention;
[0036] FIG. 5 is a perspective view showing an essential part of an
optical information recording and reproducing apparatus having a
cylindrical volume holographic memory mounted thereon according to
the present invention;
[0037] FIG. 6 is a perspective view for explaining, in relation
with a light intensity distribution of a signal light beam, how a
mask is utilized for recording in order a number of holograms
within the cylindrical volume holographic memory;
[0038] FIG. 7 is a perspective view showing an essential part of
the optical information recording and reproducing apparatus of
another embodiment having a rectangular parallelepiped volume
holographic memory mounted thereon according to the present
invention;
[0039] FIG. 8 is a perspective view for explaining, in relation
with a light intensity distribution of a signal light beam, how a
mask is utilized for recording in order a number of holograms
within the rectangular parallelepiped volume holographic memory;
and
[0040] FIG. 9 is a diagram showing a constitution of another volume
holographic memory system according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] Preferred embodiments according to the present invention of
an optical information recording and reproducing apparatus will
hereinafter be described with reference to the accompanying
drawings.
[0042] FIG. 2 illustrates a side view of an optical information
recording and reproducing apparatus as one example.
[0043] At the time of recording, a beam splitter 16 split a light
beam emitted from a laser 15 into two, i.e., a source light beam
which progresses straightly and a reference light beam which is
deflected. The resultant light beams are guided to a signal light
beam optical system and a reference light beam optical system,
respectively.
[0044] In the signal light beam optical system, the source light
beam having passed through the beam splitter 16 is incident onto a
volume holographic memory 10 through a shutter 6a controlled by a
controller, a light beam expander 14, a spatial optical modulator
12 and a Fourier transform lens 13. The automatic shutter 6a limits
a period of time, during which the source beam irradiates the
volume holographic memory. The light beam expander 14 enlarges the
source light beam into a parallel light beam having a predetermined
diameter. The spatial light modulator 12 converts the source light
beam from the beam expander 14 to a signal light beam in accordance
with digital recording data supplied from an encoder 25. The
spatial light modulator 12 is, for example, a two-dimensional plane
LCD having 480 pixels in the vertical direction and 640 pixels in
the horizontal direction (480.times.640). The source light beam is
spatially modulated by the spatial optical modulator 12 in
accordance with recording page data, for example, through a
two-dimensional lattice pattern, such as a diced or checkered like
pattern, representing transmittance/non-transmittance of respective
pixels, and then the signal light beam is subjected to Fourier
transformation by the Fourier transform lens 13. The transformed
signal light beam is converged by the lens 13 to pass toward the
volume holographic memory 10 and provided into the volume
holographic memory 10. In this embodiment, the lens 13 constitutes
a Fourier transform system together with a lens 21 below. The
volume holographic memory 10 having a cylindrical form is arranged
so that a Fourier plane caused by the lens 13 is parallel with a
symmetrical axis of rotation of the volume holographic memory 10.
The volume holographic memory 10 of a photorefractive crystalline
has a cylindrical body made of a uniaxial crystal, such as
LiNbO.sub.3 or the like, and an axis of the optical crystal is
parallel to its rotational symmetry axis.
[0045] In the reference light beam optical system, the reference
light beam is reflected by a mirror 17 and then by a movable mirror
18 to impinge on the volume holographic memory 10. Inside the
volume holographic memory 10, the reference light beam intersects
with the signal light beam supplied from the lens 13 to create
three-dimensional interference fringes. It should be noted that the
optical systems including the mirror 18, the lens 13 and so on are
positioned in such a manner that the reference and signal light
beams do not interfere just on the Fourier plane but in front of
(or behind) the Fourier plane. A controller 20 controls the
reflective movable mirror 18 with respect to the position so as to
move in a direction parallel to the axis of the signal light
beam.
[0046] As shown in FIG. 2, a super-luminescent diode 30 is disposed
near a lower side portion of the volume holographic memory 10 such
that the gate light beam generated from a super-luminescent diode
30 for generating the gate light beam is incident at a
predetermined angle onto the side surface of the memory 10. The
gate light beam includes a light beam of a second wavelength
adapted to increase photosensitivity of the volume holographic
memory 10, the second wavelength being different from those of the
reference light beam and the signal light beam. The gate light beam
activates or deactivates a refractive index grating in accordance
with presence or absence of a light interference pattern in the
volume holographic memory 10. Accordingly, the gate light beam
functions as an erasure light on the refractive index grating which
is produced by the light interference pattern. The
super-luminescent diode 30, which serves as a gate light beam
generating portion, is arranged so as to have the gate light beam
restrictedly irradiating a region in which the reference light beam
and the signal light beam intersect with each other in the volume
holographic memory 10. The super-luminescent diode 30 is on-off
controlled by the controller 20.
[0047] The volume holographic memory 10 has on the vicinity of the
side face thereof a mask 50 disposed in the optical path of the
signal light beam. The mask 50 is formed of an LCD or the like and
controlled by the controller 20 so that an optical intensity
distribution of the signal light beam incident on the volume
holographic memory 10 is partly covered, e.g., the mask covers the
distribution at nearly the half portion thereof with respect to the
center of the zeroth diffracted light.
[0048] When data is to be recorded in the volume holographic memory
10, the signal light beam, the reference light beam and the gate
light beam are irradiated at a time onto a predetermined region of
the volume holographic memory 10, whereby the resultant
interference pattern is recorded as a refractive index grating in
which refractive index varies depending on the interference
pattern. The time duration in which a hologram is formed is
controlled by an automatic shutter 6a of a laser light source
device.
[0049] In the case that a Fourier plane exists within the volume
holographic memory, the signal light beam exhibits its maximum
intensity on the Fourier plane. Therefore, when the reference light
beam interferes with the zeroth light of the signal light beam
having a high light intensity on the Fourier plane, the
photorefractive effect will saturate, so that there occurs a
tendency in that a nonlinear distortion is introduced in a recorded
image. The optical systems constituting the apparatus may be
positioned such that the reference and signal light beams interfere
with each other in front of or behind the Fourier plane to
carefully avoid the problem of nonlinear distortion.
[0050] The cylindrical volume holographic memory 10 of a
cylindrical body is shifted at a predetermined pitch in the
direction of an optical crystallographic axis and rotated at a
predetermined angular pitch on the axis of rotation. That is, the
volume holographic memory 10 is moved by any means, or any
mechanism capable of moving the volume holographic memory 10 in the
vertical direction and rotating the same. The mechanism capable of
moving the volume holographic memory 10 in the vertical direction
and rotating the same, includes a driving unit 19 and a vertical
moving mechanism 19b coupled to the driving unit 19 and having a
turntable 19a. The driving unit 19 is controlled in the rotation
and vertical motion of the table 19a by the controller 20.
[0051] The volume holographic memory 10 is detachably disposed on
the table 19a so that the optical crystallographic axis 9 coincides
with the axis of rotation of the driving unit 19. When the driving
unit 19 is rotated, the volume holographic memory 10 is shifted in
the direction of arrow "A" in FIG. 2 while the volume holographic
memory 10 is rotated in the direction of arrow "B" in FIG. 2. When
the volume holographic memory 10 is shifted vertically in the
direction of arrow "A", recording positions of the interference
fringes created by the reference light beam and the signal light
beam are shifted one by one in the direction of arrow "A", whereby
spatial multiple recording can be realized. Further, the volume
holographic memory 10 is rotated together with the table 19a, so
that the face on which the interference patterns are recorded is
rotated. Thus, angular multiple recording and spatial multiple
recording can be realized.
[0052] While this embodiment shows a moving mechanism for
simultaneously performing the angle multiplexing recording and the
spatial multiplexing recording, it is also possible to use only one
of the mechanism for vertical movements of the volume holographic
memory 10 along the crystal optical axis (in the direction of the
arrow "A") or the mechanism for rotation of the volume holographic
memory 10 (in the direction of the arrow "B") to perform only one
multiplexing recording.
[0053] Also, in place of the vertical movement and rotation
mechanism, it is possible to employ a moving mechanism which can be
separately controlled to move the volume holographic memory 10 in
the direction of the crystal optical axis and to rotate the volume
holographic memory 10. For example, a supersonic motor or the like
may be used for rotating the volume holographic memory 10, while a
separate uniaxial moving stage may be used for moving the volume
holographic memory 10 in the direction of the crystal optical
axis.
[0054] During reproduction, on the other hand, the volume
holographic memory 10, which has been recorded in the
aforementioned manner, is mounted on the rotation mechanism, as it
is during recording. After that, only the reference light beam from
the mirror 18 is allowed to impinge on the volume holographic
memory 10 by closing the shutter 6a and turning off the
super-luminescent diode 30 under the control of the controller 20.
Then, diffraction light diffracted from the interference fringes
recorded in the volume holographic memory 10 is made incident as a
reproduced light beam on the CCD 22 through the inverse Fourier
transform lens 21 to form a reproduced image. The CCD 22 has a
two-dimensional light receiving surface made up of 480.times.640
pixels similarly to the spatial light modulator 12. The CCD 22
transduces the reproduced light received thereby to an electric
signal which is output to a decoder 26. The decoder 26 compares the
input electric signal with a predetermined slice level, and outputs
binary digital data.
[0055] When a signal light beam undergoes Fourier transform in the
spatial light modulator 12 such as an LCD utilized in a Fourier
transform hologram recording, a first-order diffracted light caused
from repetitive dot pattern of the spatial light modulator 12 has
the highest frequency component. In FIG. 3, if z-axis is set in the
optical axis of the signal light beam, y-axis is set in the optical
crystallographic axis 9 of the volume holographic memory 10, and
x-axis is set in the direction perpendicular to both the axes, when
the signal light beam and the reference light beam are brought into
interference to effect recording in the volume holographic memory
10, then a spatial frequency spectrum distribution of light
intensity is generated on a plane parallel with the Fourier plane
so as to be symmetrical with respect to the optical axis of the
signal light beam.
[0056] Hologram recording has the following advantages. That is,
holograms can be stored in a limited space in terms of space
utility, information can be subjected to Fourier transform so that
it can be recorded in a space resulting from inverse-Fourier
transform in a dispersed fashion, and the information can be
recorded with high redundancy.
[0057] By using the spatial frequency on the recording surface
(fsp), the wavelength of light (.lambda.), the focal distance of
the Fourier transform lens 13 (F1), the distance (d) between the
zeroth-order Fourier spectrum and the first-order Fourier spectrum
on the Fourier plane can be given as follows:
.alpha.=rsp.multidot..lambda..multidot.F1
[0058] FIG. 3 shows an intensity distribution of the Fourier
transform image of the signal light beam. Since the pixel pitch,
wavelength and focal distance of the spatial light modulator 12 are
42 .mu.m, 532 nm and 165 mm, respectively, the Fourier spectrum
distance (d) corresponding to the spatial light modulator 12
becomes 2.1 mm according to the above-introduced equation. Thus,
information to be recorded resides in a range of about .+-.2.1 mm
on the optical axis. Accordingly, as shown in FIG. 4,
two-dimensional data appearing in the spatial light modulator 12 is
dispersed in the x-y space or four quadrisected square in
accordance with the intensity distribution of the first-order
diffracted light and zeroth-order diffracted light of the signal
light beam.
[0059] If the spatial light modulator 12 forms the two-dimensional
grid pattern consisting of a great number of pixels each indicative
of presence or absence corresponding to a status of transparent or
not transparent, by means of turning on and off, then the
fundamental frequency components of the Fourier transform image of
the information becomes a frequency half the repeating frequency
component of the pixels.
[0060] For this reason, most part of the fundamental frequency
components indispensable for recording information are concentrated
in the vicinity of the zeroth-order diffracted light spectrum.
Thus, information of harmonic component near the first-order
diffracted light of pixels are relatively not important.
[0061] According to the optical information recording and
reproducing apparatus of the present invention, each of the
two-dimensional data distributions are arrayed at a regular
interval upon recording so that the neighboring first-order
diffracted light patterns of the irradiated signal light beam are
overlaid on one another. According to the constitution, information
can be recorded in a densely packed multiple recording fashion. In
this case, the two-dimensional data distributions are arrayed so
that the first-order diffracted light intensity distribution and
the zeroth-order diffracted light intensity distribution are not
overlaid on each other.
[0062] If the volume holographic memory has a cylindrical shape as
shown in FIGS. 2 and 5, an image made to contain information by the
spatial light modulator 12 is formed on the CCD array 22 as a light
detector. Thus, information is recorded and reproduced. However,
since light passes through the cylindrical curvature surface of the
volume holographic memory, it is necessary for the optical system
to be equipped with a correcting optical system for correcting
distortion of the image formed thereon.
[0063] When the signal light beam passes through the curved
surface, narrower the width of the signal light beam in the curved
direction of the volume holographic memory, more the incident
condition can be approximated to a status that a beam is incident
into a plane surface. Which fact makes it easy to design the
correcting optical system.
[0064] When multiple recording is carried out on the curved
surface, information contained in the curved surface, or the
lateral direction (x-direction), is covered by a mask 50 so that
the recording surface is limited to nearly half the area in the
direction. Thus, the correcting optical system can be arranged with
ease.
[0065] FIG. 6 shows a specific example of constitution in which
when a number of holograms are recorded in the cylindrical volume
holographic memory, the holograms are arrayed so that neighboring
first-order diffracted light patterns of the signal light beam are
overlaid on one another in terms of spatial layout. In the figure,
the recording surface of the cylindrical volume holographic memory
is developed with respect to the axis of rotation of the volume
holographic memory, whereby a light intensity distribution of the
signal light beam is shown in a planar fashion. However, respective
light intensity distributions are actually arrayed in a rotating
fashion on the cylindrical co-ordinates and hence separated from
one another. The terms "arrayed in an rotating fashion and hence
separated from one another" means that if a cylindrical volume
holographic memory having a line at a certain position with respect
to the axis of rotation is rotated on the axis of rotation, the
line will be positioned at a different angular position relative to
the original position in association with the rotation angle of the
volume holographic memory.
[0066] As shown in FIG. 6, in accordance with the control of the
controller 20, when a unit recording operation is carried out to
create a first recording layer, the signal light beam is masked at
the left half part by the mask 50 so that only the right half part
of the signal light beam can pass through the mask 50 (portions
surrounded by dot lines). Conversely, when the unit recording
operation is carried out to create a second recording layer, the
signal light beam is masked in turn at the right half part by the
mask 50 so that only the left half part of the signal light beam
can pass through the mask 50. At this time, the cylindrical volume
holographic memory is shifted in the vertical direction at a
predetermined pitch while rotated on the axis of rotation so that
the recording surface is moved in a spiral fashion. Thus, multiple
recording in terms of angle and multiple recording in terms of
space can be effected at a time. In this case, the distance between
the first layer and the second layer in the height direction
(interval between the zeroth-order diffracted light patterns)
becomes at most a pitch of 2H, and hence information can be
recorded in the most densely packed multiple fashion. In this way,
according to the optical system including the cylindrical volume
holographic memory 10, the volume holographic memory comes to
contain a plurality of refractive index gratings corresponding to a
three-dimensional optical interference pattern caused by
interference between the signal light beam, which is modulated in
accordance with image data, and the reference light beam. Moreover,
according to the optical system including the cylindrical volume
holographic memory 10, the center of respective refractive indices
is rotationally shifted from one another by an angle corresponding
to the distance substantially equal to twice the peak distance
between the zeroth-order diffracted light and the first-order
diffracted light of a light intensity distribution of the signal
light beam.
[0067] When the recording operation is carried out to create a
third and the following recording layers, the right half part and
the left half part of the signal light beam are covered by the mask
alternately in the similar manner, whereby an appropriate multiple
recording can be carried out. In this case, the interval between
the recorded patterns within the same layer becomes at most 2H.
However, since the volume holographic memory is arranged as a
cylindrical shape, the length in which interference is effected in
the recorded portion is sufficiently long. Therefore, if the
optical system provides a sufficient angular resolution, the
distance between the neighboring recording portions can be made
smaller than the pitch of 2H.
[0068] While in the foregoing embodiment, the cylindrical volume
holographic memory 10 is positioned such that its crystal optical
axis is oriented upward, the cylindrical volume holographic memory
10 may be positioned such that the crystal optical axis is oriented
downward as long as it is coaxial with the axis of rotation of the
rotation mechanism. Also, while the foregoing embodiment has been
described for the structure in which the gate light beam generated
from the super-luminescent diode 30 is incident on the side surface
of the cylindrical volume holographic memory 10, the gate light
beam may be incident on the top surface of the volume holographic
memory 10.
[0069] FIG. 7 shows another embodiment in which the volume
holographic memory 10 is made into a rectangular parallelepiped and
the volume holographic memory 10 is arranged to be independently
movable in parallel with the directions of axes of x, y and z. As
shown in FIG. 8, when the unit recording operation is carried out
to create the first recording layer, the mask 50 is controlled by
the controller 20 so that only the upper half part of the signal
light beam can pass through the mask 50. Conversely, when the unit
recording operation is carried out to create the second recording
layer, the mask 50 is controlled so that only the lower half part
of the signal light beam in turn can pass through the mask 50. In
this case, however, when the unit recording operation is carried
out to create the first recording layer and thereafter the same is
carried out on the second one, the volume holographic memory 10
need not be moved in the height direction (y-direction) relative to
each other. If H is taken as the distance between the zeroth-order
diffracted light pattern and the first-order diffracted light
pattern, when the recording is carried out in the same height, or
by scanning in the horizontal direction (x-direction), the distance
between the neighboring recording portions within the same layer is
set to a pitch of 4H. Further, recording is carried out on the
second layer so that the zeroth-order diffracted light pattern is
inserted into the space between the recording portions of the first
layer. In this way, recording is carried out in the most densely
packed fashion. When the recording operation is carried out to
create the third and the following recording layers, the recording
layer is shifted in the downward or upward direction by a pitch of
2H with respect to the first or second layer, and the
above-described recording operation is repeated. Thus, recording is
carried out in the most densely packed fashion. In this way,
according to the optical system including the rectangular
parallelepiped volume holographic memory 10, the volume holographic
memory comes to contain a plurality of refractive index gratings
corresponding to a three-dimensional optical interference pattern
caused by interference between the signal light beam, which is
modulated in accordance with image data, and the reference light
beam. Moreover, according to the optical system including the
rectangular parallelepiped volume holographic memory 10, the center
of respective refractive indexes is directly shifted from one
another by the distance substantially equal to twice the peak
distance between the zeroth-order diffracted light and the
first-order diffracted light of the light intensity distribution of
the signal light beam.
[0070] In the above embodiment description has been made on a case
where the light beam is made incident on the side face of the
volume holographic memory 10, but the constitution of the light
beam incident into the volume holographic memory may be modified to
construct an angular multiple recording system as for example shown
in FIG. 9. That is, the photorefractive crystalline volume
holographic memory may be made of a single crystal axis rectangular
parallelepiped 10 having an optical crystallographic axis parallel
with one plane thereof, a pair of galvanometric mirrors are
employed in order for changing the incident angle of the reference
light beam with respect to the upper face of the volume holographic
memory.
* * * * *