U.S. patent application number 09/056798 was filed with the patent office on 2001-06-07 for optical storage medium, optical storage method, optical storage apparatus, optical reading method, optical reading apparatus, optical retrieving method and optical retrieving apparatus.
Invention is credited to ISHII, TSUTOMU, KAWANO, KATSUNORI, NISHIKATA, YASUNARI.
Application Number | 20010002895 09/056798 |
Document ID | / |
Family ID | 26371418 |
Filed Date | 2001-06-07 |
United States Patent
Application |
20010002895 |
Kind Code |
A1 |
KAWANO, KATSUNORI ; et
al. |
June 7, 2001 |
OPTICAL STORAGE MEDIUM, OPTICAL STORAGE METHOD, OPTICAL STORAGE
APPARATUS, OPTICAL READING METHOD, OPTICAL READING APPARATUS,
OPTICAL RETRIEVING METHOD AND OPTICAL RETRIEVING APPARATUS
Abstract
An optical storage medium of the present invention enables
storage of data with high precision at high speed, and rewriting of
data at high speed without an erasing process. Optical storage,
reading and retrieving methods and optical storage, reading, and
retrieving apparatuses using the medium are also provided. The
optical storage medium has at least a polarization-sensitive member
having the photo-induced birefringence property, such as a member
made of polyester polymer having cyanoazobenzene as a side chain.
The above apparatuses have spatial light modulator capable of
modulating polarization. The modulator provides information of bit
of two-dimensional data to each corresponding pixel by application
or non-application of a voltage, and modulates the polarization of
the beam incident on each pixel. Thereby, a signal beam transmitted
through the spatial light modulator having a spatial polarization
modulation corresponding to the two-dimensional data is obtained.
The signal beam illuminates the optical storage medium, and at the
same time, a reference beam illuminates the same region in the
medium where the signal beam illuminates. Thus a hologram of the
polarization modulation of the signal beam corresponding to the
two-dimensional data is stored in the optical storage medium.
Inventors: |
KAWANO, KATSUNORI;
(NAKAI-MACHI, JP) ; NISHIKATA, YASUNARI;
(NAKAI-MACHI, JP) ; ISHII, TSUTOMU; (NAKAI-MACHI,
JP) |
Correspondence
Address: |
OLIFF & BERRIDGE
PO BOX 19928
ALEXANDRIA
VA
22320
|
Family ID: |
26371418 |
Appl. No.: |
09/056798 |
Filed: |
April 8, 1998 |
Current U.S.
Class: |
369/103 ;
G9B/7.026; G9B/7.027; G9B/7.105; G9B/7.139; G9B/7.147;
G9B/7.169 |
Current CPC
Class: |
G11B 7/006 20130101;
G11B 7/25 20130101; G11B 7/245 20130101; G11B 7/24 20130101; G11B
7/0065 20130101; G11B 7/128 20130101 |
Class at
Publication: |
369/103 |
International
Class: |
G11B 007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 16, 1998 |
JP |
10-32834 |
Apr 11, 1997 |
JP |
9-94194 |
Claims
What is claimed is:
1. An optical storage medium comprising a polarization-sensitive
member having a photo-induced birefringence property.
2. The optical storage medium as set forth in claim 1, further
comprising a light transmitting substrate, wherein the
polarization-sensitive member is formed on at least one side of the
substrate as a layer.
3. The optical storage medium as set forth in claim 1, wherein the
polarization-sensitive member comprising at least one element
selected from the group consisting of a polymer having a
photoisomarizable moiety as a side chain and a liquid crystal
polymer.
4. The optical storage medium as set forth in claim 1, wherein the
polarization-sensitive member comprising a polymer and
photoisomarizable molecules dispersed therein.
5. The optical storage medium as set forth in claim 3, wherein the
photoisomarizable moiety includes azobenzene structure.
6. The optical storage medium as set forth in claim 3, wherein one
of the polymer and the liquid crystal polymer is a polyester-type
polymer.
7. The optical storage medium as set forth claim 1, wherein the
substrate is in a form of a disk.
8. An optical storage method comprising the steps of: providing a
signal beam retaining spatial polarization modulated data modulated
with a spatial light modulator capable of modulating polarization;
and illuminating an optical storage medium with the signal beam and
a reference beam simultaneously for storing a hologram of the
polarization modulated data retained by the signal beam in the
optical storage medium.
9. The optical storage method as set forth in claim 8, wherein a
polarization angle of the signal beam is modulated in accordance
with the data information.
10. The optical storage method as set forth in claim 8, further
comprising the steps of: changing a polarizing direction of at
least one of the signal beam and the reference beam; and storing
and multiplexing a hologram of light intensity modulated data and a
phase modulated data in a same region of one of stored hologram of
the optical storage medium.
11. The optical storage method as set forth in claim 10, wherein
polarizing directions of the signal beam and reference beam are in
parallel.
12. The optical storage method as set forth in claim 10, wherein
polarizing directions of the signal beam and reference beam are
orthogonal to each other.
13. The optical storage method as set forth in claim 8, wherein the
optical storage medium is in a form of a disk, further comprising
the steps of rotating the disk and moving an optical storage head
containing the spatial light modulator in a direction of a diameter
of the disk.
14. An optical storage method comprising the steps of: providing a
signal beam retaining spatial polarization modulated data modulated
with a spatial light modulator capable of modulating polarization;
and illuminating with the signal beam and a reference beam an
optical storage medium for erasing a stored hologram of a
polarization modulated data retained by a preceding signal beam and
storing a new hologram of the polarization modulated data retained
by the signal beam in the optical storage medium
simultaneously.
15. An optical storage apparatus comprising: a light source that
emits coherent light; a spatial light modulator that modulates
polarization of the coherent light from the light source in
accordance with data and obtains a signal beam retaining spatial
polarization modulated data; an optical system that transmits the
signal beam to an optical storage medium; and a reference beam
generating system that generates a reference beam to illuminate the
optical storage medium.
16. The optical storage apparatus as set forth in claim 15, wherein
the spatial light modulator rotates a polarization angle of the
signal beam in accordance with the data.
17. The optical storage apparatus as set forth in claim 15, wherein
the spatial light modulator comprises an electro-optical element
and transparent electrodes formed thereon.
18. The optical storage apparatus as set forth in claim 17, wherein
the electro-optical element comprises a liquid crystal.
19. The optical storage apparatus as set forth in claim 15, wherein
the optical storage medium is in a form of a disk.
20. The optical storage apparatus as set forth in claim 19, further
comprising: a medium driving mechanism that rotates the optical
storage medium; and a head moving mechanism that moves an optical
head including the light source, the spatial light modulator, the
optical system and the reference beam generating system in a
direction of a diameter of the optical storage medium.
21. The optical storage apparatus as set forth in claim 15, further
comprising the optical storage medium.
22. An optical storage medium storing a hologram of spatial
polarization modulated data retained by a signal beam cooperatively
with a reference beam.
23. The optical storage medium as set forth in claim 22, wherein a
new hologram of a light intensity modulated data or phase modulated
data is multiplexed on the hologram by modulating a polarizing
direction of the signal beam or the reference beam in a same region
of the optical storage medium.
24. The optical storage medium as set forth in claim 22, wherein
the optical storage medium is in a form of a disk.
25. An optical reading method comprising the steps of: illuminating
with a read beam an optical storage medium storing a hologram
generated cooperatively by a reference beam and a signal beam
retaining spatial polarization modulated data; and reading the
polarization modulated data retained by a diffracted light from the
hologram.
26. The optical reading method as set forth in claim 25, wherein a
polarizing direction of the read beam is the same as that of the
reference beam.
27. The optical reading method as set forth in claim 26, wherein an
incident direction of the read beam to the optical storage medium
is opposite to the same of the reference beam.
28. The optical reading method as set forth in claim 25, wherein
the diffracted light has the same polarizing direction as the
signal beam by modulating the polarizing direction of the
diffracted light by a polarizer or half-wave plate.
29. The optical reading method as set forth in claim 25, wherein
the diffracted light is separated into two polarization components
orthogonal to each other and a comparative operation on their light
intensities is performed to obtain a result as a reading
output.
30. An optical reading method comprising the steps of: illuminating
with a read beam an optical storage medium storing a hologram
generated cooperatively by a reference beam and a signal beam
retaining spatial polarization modulated data; separating a
diffracted light from the hologram into two polarization components
orthogonal to each other; performing a comparative operation on
light intensities of the two polarization components; and reading
the data based on a result of the comparative operation.
31. An optical reading method comprising the steps of: illuminating
with a read beam having linear polarization an optical storage
medium that stores a first and a second holograms multiplexingly in
a same region thereof, the first hologram of spatial polarization
modulated data that is created with a first beam having a first
polarizing direction and the second hologram of light intensity
modulated or phase modulated data that is created with a second
beam having a second polarizing direction; and extracting one of
the holograms from the region of the optical storage medium by
separating a polarization component of a diffracted light
therefrom.
32. The optical reading method as set forth in claim 31, wherein
the first beam and the second beam includes a reference beam and a
signal beam having alternatively a parallel polarizing direction
and an orthogonal direction to the reference beam, the read beam
having the same polarizing direction with the reference beam and
the polarization component of the diffracted light having the same
polarizing direction with the signal beam.
33. The optical reading method as set forth in claim 25, wherein
the optical storage medium is in a form of a disk, further
comprising the steps of rotating the disk and moving an optical
reading head including an optical system for the read beam in a
direction of a diameter of the disk.
34. An optical reading apparatus comprising: an optical system that
emits a read beam on an optical storage medium storing a hologram
of spatial polarization modulated data retained by a signal beam
generated cooperatively by the signal beam and a reference beam
each of which has a polarizing direction; a polarizing beamsplitter
that separates a diffracted light from the hologram into
polarization components; and a photodetector that detects a
distribution of a polarization modulation of the diffracted light
based on the polarization components.
35. The optical reading apparatus as set forth in claim 34, wherein
the polarizing direction of the read beam is the same as that of
the reference beam.
36. The optical reading apparatus as set forth in claim 35, wherein
the optical system emits the read beam on the optical storage
medium from an opposite direction to the reference beam.
37. The optical reading apparatus as set forth in claim 34, wherein
the polarizing beamsplitter separates the diffracted light into two
polarization components which are orthogonal to each other and the
photodetector comprises two detectors each of which independently
detects corresponding one of the two polarization components.
38. The optical reading apparatus as set forth in claim 37, further
comprising: an comparative operation element that performs a
comparative operation on outputs from the two detectors.
39. The optical reading apparatus as set forth in claim 34, further
comprising: a driving mechanism that rotates the optical storage
medium; an optical reading head that includes the optical system,
the polarizing beamsplitter and the photodetector; and a head
moving mechanism that moves the optical reading head in a direction
of a diameter of the optical storage medium.
40. The optical reading apparatus as set forth in claim 34, firther
comprising the optical storage medium.
41. An optical retrieving method comprising the steps of:
illuminating with a read beam an optical storage medium storing a
hologram generated cooperatively by a reference beam and a signal
beam retaining spatial polarization modulated data as retrieving
object data; transmitting a diffracted light from the hologram
through a spatial light modulator that modulates polarization of
the diffracted light in accordance with retrieving data; and
detecting matching between the retrieving data and retrieving
object data based on a polarization modulation of the transmitted
diffracted light.
42. An optical retrieving method comprising the steps of:
illuminating with a read beam an optical storage medium storing a
hologram generated cooperatively by a reference beam and a signal
beam retaining spatial polarization modulated data as retrieving
object data; transmitting a diffracted light from the hologram
through a spatial light modulator that modulates polarization of
the diffracted light in accordance with retrieving data; and
detecting correlation between the retrieving data and retrieving
object data based on a polarization modulation of the transmitted
diffracted light.
43. The optical retrieving method as set forth in claim 41, wherein
the optical storage medium is in a form of a disk, further
comprising the steps of rotating the disk, and moving an optical
retrieving head containing the spatial light modulator in a
direction of a diameter of the disk.
44. An optical retrieving apparatus comprising: an optical system
that emits a read beam on an optical storage medium storing a
hologram generated cooperatively by a reference beam and a signal
beam retaining spatial polarization modulated data as retrieving
object data; a spatial light modulator that modulates polarization
of a diffracted light from the hologram in accordance with
retrieving data; a polarizing beamsplitter that separates the
diffracted light from the spatial light modulator into two
polarization components; and a photodetector that detects a
polarization modulation of the diffracted light based on the
polarization components.
45. The optical retrieving apparatus as set forth in claim 44,
wherein the polarizing beamsplitter separates the diffracted light
into two polarization components which are orthogonal to each other
and the photodetector comprises two detectors each of which
independently detects corresponding one of the two polarization
components.
46. The optical retrieving apparatus as set forth in claim 45,
further comprising: an comparative operation element that performs
a comparative operation on outputs from the two detectors.
47. The optical retrieving apparatus as set forth in claim 44,
wherein the spatial light modulator comprises an electro-optical
element and transparent electrodes formed thereon.
48. The optical retrieving apparatus as set forth in claim 47,
wherein the electro-optical element comprising a liquid
crystal.
49. The optical retrieving apparatus as set forth in claim 44,
further comprising: a driving mechanism that rotates the optical
storage medium; an optical retrieving head that includes the
optical system, the polarizing beamsplitter and the photodetector;
and a head moving mechanism that moves the optical retrieving head
in a direction of a diameter of the optical storage medium.
50. The optical retrieving apparatus as set forth in claim 44,
further comprising the optical storage medium.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method and apparatus for
storing data in an optical storage medium, reading the data from
the optical storage medium, retrieving the data from the optical
storage medium, and also relates to the optical storage medium used
for such data storing, reading and retrieving.
[0003] 2. Discussion of the Related Art
[0004] A rewritable optical disk such as a phase change disk or a
magneto-optical disk has already used widely. The storing density
of the optical disk is larger than that of a general magnetic disk
by at least one digit. However, it is still insufficient for
digital storage of image information. To enhance the storing
density, it is necessary to reduce the beam spot diameter to
shorten the distance to the adjacent track or bit.
[0005] A DVD-ROM is put into practice by such development of
technique. The DVD-ROM with 12 cm diameter can store 4.7 GByte data
on one side. A writable/erasable DVD-RAM with 12 cm diameter can
realize high-density storage of 5.2 GByte data on both sides, which
is more than 7 times as large as the capacity of a CD-ROM and
corresponds to the capacity of more than 3,600 floppy disks.
[0006] The optical disk has been improved to obtain higher density
and larger capacity from year to year. However, since the optical
disk stores the data in the two-dimensional surface, the storing
density is restricted by the light diffraction limit and is nearing
5 Gbit/cm.sup.2. To obtain larger capacity, three-dimensional
storage (volume holographic storage) further utilizing a depth
direction is required.
[0007] Materials for the three-dimensional (volume holographic)
optical storage medium are, for example, a photopolymer material,
photorefractive material and the like. Since these materials change
their refractive indexes by absorbing relatively week light beam,
it is possible to use the change of the photo-induced refractive
index for storing information. Therefore, these materials can be
used for multiplexed holographic storage that realizes the larger
capacity.
[0008] An example of high-density storage utilizing the
photopolymer material is discussed in "SPIE Vol. 2514,355".
Shift-multiplexed holograms are stored in a disk, that is made from
DuPont's 150-100 photopolymer and rotated, using a spherical wave
as a reference beam. As a result, the storing density of 10 times
as large as that of a CD currently used, 10 bit/.mu. m.sup.2, is
obtained.
[0009] An example of high-density storage using the photorefractive
material is described in "OPTICAL ENGINEERING Vol. 34, 2193
(1995)". It is reported that 20,000-page holograms are
multiple-stored in Fe-doped LiNbO.sub.3 crystal of the size of
10.times.10.times.22 mm, and thereby about 1-GByte data storage is
achieved.
[0010] The holographic memory can store the large capacity of data
as described above, and in addition, it can write and read the
pieces of data disposed two-dimensionally. Accordingly, it is
possible to perform high-speed data storing, reading, retrieving,
correlation detection and transfer by using the holographic memory.
Specifically, the following data retrieving method is disclosed by
Japanese Patent Application Laid-Open No. 3-149660 (1991).
[0011] FIG. 26 shows a device for retrieval. A laser 101 emits a
laser beam to read pieces of two-dimensional retrieving object data
holographically stored from an optical memory 102. A data pattern
image is written to a spatial light modulator 103 of the optical
address type. Two-dimensional retrieving data is written to a
spatial light modulator 104 of the electric address type that is a
liquid-crystal display (LCD) panel.
[0012] The spatial light modulator 104 is illuminated with a laser
beam as a read beam from a laser 105 through an analyzer 106. The
polarization state of the beam is changed in accordance with the
retrieving data, and the light transmitted through the spatial
light modulator 104 is reflected off a half-mirror prism 107. Then
the light forms an image on a readout surface of the spatial light
modulator 103 of the optical address type.
[0013] Thus, the polarization state of the read beam is modulated
by the spatial light modulator 103 for each pixel in accordance
with the retrieving object data. The read beam illuminates a
photodetecting array 109 through an analyzer 108, and the
photodetecting array 109 performs batch detection as to whether
there is any read beam transmitted through the pixels. Thus the
batch detection of matching between bits of the retrieving object
data and those of the retrieving data is possible.
[0014] "Conjugate Image Plane Correlator with Holographic Disk
Memory", A. Kutanov and Y. Ichioka, OPTICAL REVIEW Vol. 1.3, No.
4,1996, pp. 258-263 describes a data storage method and data
correlation detecting method as follows.
[0015] FIG. 27 shows a device used in the storage method and
correlation detecting method. In storing the data, two-dimansional
data to be stored is displayed on a spatial light modulator 111 of
the electric address type that is an LCD panel. A signal beam 112
having a two-dimensional amplitude modulation transmitted through
the spatial light modulator 111 is Fourier transformed on a Fourier
plane P1 by a lens 113 and illuminates an optical memory 114. At
the same time, a reference beam 115 illuminates the optical memory
114 and the two-dimensional data is stored as a Fourier-transform
hologram in the optical memory 114.
[0016] In detecting correlation, the two-dimensional retrieving
data is displayed on the spatial light modulator 111 of the
electric address type, and in addition, a read beam 116 having
conjugate relation with the reference beam 115 used in storing
illuminates the optical memory 114. The diffracted beam of the
two-dimensional retrieving object data is read out from the
hologram stored in the optical memory 114, and the diffracted beam
is transformed on a Fourier plane P2 by the lens 113. Then the beam
illuminates the spatial light modulator 111.
[0017] Accordingly, the transmitted beam from the spatial light
modulator 111 is an optical product of the retrieving data and the
retrieving object data. If the retrieving data and the retrieving
object data match with each other, a strong correlation peak
appears on a Fourier plane P3 through a lens 117. By detecting the
peak, correlation between two-dimensional images can be found.
[0018] As an optical storage medium in which the hologram can be
rewritten, an optical storage medium made of liquid crystal polymer
is disclosed by Japanese Patent Application Laid-Open No. 2-280116
(1990), and an optical storage medium made of a phase change
material is disclosed by Japanese Patent Application Laid-Open No.
4-30192 (1991).
[0019] As described so far, attentions have recently been paid to
the holographic memory to improve the memory capacity and
processing speed, and the retrieving method discussed with
reference to FIG. 26 and the storage method and the correlation
detecting method discussed with reference to FIG. 27 have been
proposed. Furthermore, enhancement of the signal-to-noise ratio
(S/N) has been researched to realize high-density storage.
[0020] However, the conventional retrieving method, storage method
and correlation detecting method explained with reference to FIGS.
26 and 27 have adopted a spatial light modulator of an amplitude
(intensity) modulation type that is an LCD panel 104 or 111.
Therefore, the following problems have been caused.
[0021] As shown in FIG. 28, like the spatial light modulators 104
and 111, an LCD for displaying data is constructed by forming a
liquid crystal cell 124 containing a liquid crystal 121 and
electrodes 122 and 123 on both sides of the liquid crystal 121 and
disposing polarizers 126 and 127 on the outside of the liquid
crystal cell. Dichromatic polarizers are used as the polarizers 126
and 127 because they can be downsized easily. However, since the
transmittnace of the dichromatic polarizer in the direction of
transmission axis is as low as 70-80%, if two dichromatic
polarizers are used together, 50% transmission loss is caused.
[0022] Therefore, in the case where data storing and reading are
performed by utilizing the spatial light modulator that is an LCD
panel, the light intensity is reduced in both of the storing and
reading processes, and thereby S/N is also reduced. As a result,
deterioration of hologram storing density or retrieving precision
occurs. If the laser power is raised for increasing the signal
intensity, a life span of the laser is shortened.
[0023] In storing and reading the data utilizing the holographic
memory, there are the following noise factors which determine a bit
error rate (BER):
[0024] (1) noises irrelevant to the quality of the hologram caused
by a photodetecting array such as a CCD or the like;
[0025] (2) diffracted light from the adjacent hologram (crosstalk
between pages);
[0026] (3) crosstalk between pixels in a reconstructed image;
and
[0027] (4) fluctuation of diffraction efficiency in a single page
or between pages caused by the defect of a crystal or optical
system.
[0028] Information storage utilizing the amplitude (intensity)
modulation is apt to be affected by various noises, and the storing
density of the storage medium depends on the signal to the noise
ratio (S/N). Therefore, several coding attempts have been made to
restrict BER in the same way as other filing systems.
[0029] In the case where pieces of two-dimensional data having
correspondence of [clear, dark] to [0, 1] are multiple-stored in
holograms, a crosstalk resulting from the fluctuation of the
diffraction efficiency occurs because the whole light intensity of
the signal beam used in storing cannot be constant according to the
data. To avoid the problem, a differential encoding method is
adopted, in which [dark-clear] corresponds to [0], and [clear-dark]
corresponds to [1]. However, in this case, the coding ratio is 0.5,
in other words, using efficiency of pixels is decreased.
[0030] As described above, if the spatial light modulator of the
amplitude modulation type is employed to input data or retrieve
data, some problems arise such as a low light using efficiency,
deterioration of S/N, need for a special encoding, and so forth.
Consequently, high-density storage, which is one of the
characteristics of the holographic memory, cannot be sufficiently
achieved in fact.
[0031] Moreover, the conventional data retrieving method explained
with reference to FIG. 26 has problems in that:
[0032] (1) an expensive spatial light modulator 103 of the optical
address type is required;
[0033] (2) a highly precise alignment of the spatial light
modulator 103 of the optical address type and the spatial light
modulator 104 of the electric address type is required;
[0034] (3) storage of a hologram in the optical memory 102 requires
another spatial light modulator, and so forth.
[0035] The conventional storage method and correlation detecting
method described with reference to FIG. 27 can avoid the above
problems (1) to (3). However, since the detection of correlation
between the data depends upon whether a correlation peak exists, a
serious problem occurs. That is, it is impossible to detect
matching between a bit of data and that of other data that are
complex and of high-density, though a correlation value between
pieces of data can be obtained. Therefore, these methods are not
suitable for a computer filing system capable of retrieving.
[0036] It is possible to rewrite the hologram by using
photo-refractive materials such as Ba.sub.2TiO.sub.3, LiNbO.sub.3,
SBN (Sr.sub.xB.sub.1-xNb.sub.2O.sub.6), or the liquid crystal
polymeric material disclosed in Japanese Patent Application
Laid-Open No. 2-280116 or the phase change material disclosed in
Japanese Patent Application Laid-Open No. 4-30192.
[0037] However, the conventional optical storage medium and optical
storage method using thereof, in principle, cause some changes in
the materials in a portion of high light intensity, and in
contrast, do not cause any change in the materials in a portion of
low light intensity. Therefore, if it is desired to rewrite the
data without an erasing process, a problem occurs. Suppose that, in
a region, the content of the previously stored data causes changes
in materials by high light intensity and the content of the new
data does not cause changes in materials because of low light
intensity. In the region, the content of the data that has caused
changes in materials remains, and as a result, it is impossible to
rewrite the data.
[0038] Consequently, when the data is to be rewritten, it is
necessary to erase the previously stored data by an erasing process
such as illuminating the whole surface of the optical storage
medium with a laser beam and to write the new data. It takes so
much time to rewrite the data, and accordingly, high processing
speed, that is one of advantages of the holographic memory, is
lost.
SUMMARY OF THE INVENTION
[0039] The present invention has been made in view of the above
circumstances and has an object to provide an optical storage
apparatus and optical storage method using thereof which can
perform high-density storing at high speed and can rewrite data at
high speed without an erasing process.
[0040] Another object of the present invention is to provide an
optical reading apparatus and optical reading method using thereof
which can read out data stored in an optical storage medium with
very high precision at high speed.
[0041] Further object of the present invention is to provide an
optical retrieving apparatus and optical retrieving method using
thereof which can retrieve a necessary piece of data with high
precision at high speed from an optical storage medium in which a
large amount of data is stored.
[0042] Further object of the present invention is to provide an
optical storage medium suitable to high-speed storing, reading,
retrieving data with high precision, and high speed rewriting of
data without requiring an erasing process.
[0043] To achieve the objects and in accordance with the purpose of
the invention, as embodied and broadly described herein, an optical
storage medium of the present invention is a polarization-sensitive
member that have a photo-induced birefringence property. The
optical storage medium of the present invention may also be a
sheet-like light-transmitting material which has the
polarization-sensitive member with the photo-induced birefringence
property at least on one side as a layer.
[0044] The optical storage method of the present invention provides
a signal beam that retains spatial polarization modulated data
modulated by a spatial light modulator capable of modulating
polarization of a light beam. The signal beam and a reference beam
simultaneously illuminate an optical storage medium for storing a
hologram of the polarization modulated data retained by the signal
beam in the optical storage medium.
[0045] In the optical reading method of the present invention, a
read beam illuminates an optical storage medium storing a hologram
generated cooperatively by a reference beam and a signal beam
retaining spatial polarization modulated data. The data is then
read out based on a polarization modulation of a diffracted beam
from the hologram.
[0046] In the optical retrieving method of the present invention, a
read beam illuminates an optical storage medium storing a hologram
generated cooperatively by a reference beam and a signal beam
retaining spatial polarization modulated data as retrieving object
data. A diffracted beam from the hologram is transmitted through a
spatial light modulator that modulates polarization of a light beam
in accordance with retrieving data. Matching between the retrieving
object data and the retrieving data can be detected based on the
polarization modulation of the transmitted diffracted beam.
[0047] In a conventional holography, a light intensity modulation
based on an interference pattern of a signal beam and a reference
beam is recorded as a change of a refractive index or an absorption
in an optical storage medium. Accordingly, it is necessary that the
polarizing directions of the signal beam and reference beam are in
parallel. Amplitude and phase of the signal beam can be stored, but
storage of polarizing direction is limited to only one direction.
Therefore, in conventional holographic storage or data retrieval, a
spatial light modulator of an amplitude modulation type has been
used as described above.
[0048] In contrast, a material showing photo-induced birefringence
(also referred to as photo-induced dichroism or photo-induced
anisotropy) senses a polarization state of a light beam incident
thereon, and is able to store a polarizing direction of the
incident beam. As described later, inventors of the present
invention have found materials having particularly excellent
storage characteristics as a result of their researches and
experiments.
[0049] Paying attention to the point, the present invention
constructs an optical storage medium by making a sheet from a
light-transmitting material and forming a polarization-sensitive
layer having the photo-induced birefringence at least on one side
of the sheet. Hereinafter, such optical storage medium according to
the present invention is referred to as polarization-sensitive
optical storage medium.
[0050] The polarization-sensitive optical storage medium can store
a hologram generated by the photo-induced birefringence
corresponding to the polarization modulation by an interference
pattern of two light waves when the polarizing directions of the
signal beam and reference beam are orthogonal to each other. In
this specification, such hologram is referred to as polarization
hologram in contrast with the hologram recorded by usual light
intensity modulation. By illuminating the polarization hologram
with a read beam having the same polarizing direction as that of
the reference beam used in recording, a diffracted beam retaining
the polarizing direction of the signal beam is available.
[0051] Paying attention to this point, the optical storage method
is devised to obtain the signal beam retaining data information
based on a spatial polarization modulation by a spatial light
modulator capable of modulating polarization, illuminate the
optical storage medium with the signal beam and reference beam at
the same time, and thereby store the polarization modulation of the
signal beam as a hologram in the optical storage medium. The
optical reading method of the present invention illuminates the
optical storage medium with a read beam, in which the signal beam
retaining the data information based on the spatial polarization
modulation is stored as the hologram by the reference beam, and
thereby reads out the data information in accordance with the
polarization modulation of the diffracted beam from the
hologram.
[0052] Also, paying attention to the above point, the optical
retrieving method according to the present invention illuminates
the optical storage medium with the read beam, in which the signal
beam retaining pieces of retrieving object data owing to the
spatial polarization modulation is stored as the hologram by the
reference beam. The diffracted beam from the hologram illuminates
the spatial light modulator that modulates polarization in
accordance with the retrieving data. Based on the polarization
modulation of the light transmitted through the spatial light
modulator, matching between the retrieving data and the retrieving
object data is detected.
[0053] Since the spatial light modulator capable of modulating
polarization can be constructed without a polarizer, there is no
light transmission loss. Moreover, the signal beam retains the data
information in the form of the spatial polarization modulation;
therefore the light intensity modulation of the signal beam is
constant. Consequently, the optical storage method according to the
present invention can prevent the deterioration of S/N of the
signal beam, and thereby can store the data with high precision at
high speed.
[0054] Therefore, according to the optical storage method of the
present invention, the data stored in the optical storage medium
can be read out with high precision at high speed. The optical
retrieving method of the present invention makes it possible to
easily retrieve the required data from the optical storage medium
that stores a large amount of data with high precision at high
speed.
[0055] Furthermore, as described later, as a result of the research
and experiments, the inventors of the present invention have found
that, if data is stored as a polarization hologram in an optical
storage medium by the optical storage method of the present
invention, new data can be overwritten as another polarization
hologram on the optical storage medium according to the optical
storage method of the present invention without erasing the
previously stored data by an erasing process such as radiation of
laser beam over the whole surface of the optical storage
medium.
[0056] Accordingly, the above-described optical storage method of
the present invention enables high speed rewriting of data without
an erasing process.
[0057] Additional objects and advantages of the invention will be
set forth in part in the description which follows and in part will
be obvious from the description, or may be learned by practice of
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0058] The accompanying drawings, which are incorporated in and
constitute a part of this specification illustrate embodiments of
the invention and, together with the description, serve to explain
the objects, advantages and principles of the invention. In the
drawings:
[0059] FIGS. 1(a) and 1(b) are sectional views showing examples of
an embodiment of an optical storage medium according to the present
invention;
[0060] FIGS. 2(a), 2(b) and 2(c) are chemical formulas representing
a trans-structure of azobenzene, cis-structure of azobenzene, and a
chemical structure of polyester polymer having cyanoazobenzene as a
side chain, respectively;
[0061] FIGS. 3(a) and 3(b) illustrate holograms based on a light
intensity modulation and a polarization modulation,
respectively;
[0062] FIG. 4 is an optical system employing degenerate four-wave
mixing used in an experiment;
[0063] FIGS. 5(a), 5(b) and 5(c) show diffracted beam intensities
corresponding to the time for recording a hologram in the cases
where polarizing directions of a signal beam and reference beam are
in parallel, orthogonal to each other and at an angle of
45.degree., respectively;
[0064] FIG. 6 shows a relationship between a polarizing direction
of a diffracted beam and the light intensity in the case where the
polarizing directions of the signal beam and reference beam are in
parallel;
[0065] FIG. 7 shows a relationship between a polarizing direction
of a diffracted beam and the light intensity in the case where the
polarizing directions of the signal beam and reference beam are
orthogonal to each other;
[0066] FIG. 8 shows a relationship between a polarizing direction
of a diffracted beam and the light intensity in the case where the
polarizing directions of the signal beam and reference beam are at
a degree of 45.degree.;
[0067] FIG. 9 illustrates polarization multiplexing based on
rotation of the polarization angle of the reference beam;
[0068] FIG. 10 shows an optical system for recording and
reproducing the hologram used in an experiment;
[0069] FIG. 11 shows a relationship between the light intensity and
the polarization angle of the diffracted beam from a previously
stored hologram in the case of rewriting;
[0070] FIG. 12 shows a relationship between the light intensity and
the polarization angle of the diffracted beam from a hologram
stored later in the case of rewriting;
[0071] FIG. 13 shows an embodiment of an optical storage apparatus
according to the present invention;
[0072] FIG. 14 illustrates formation of a recording track by using
the optical storage apparatus shown in FIG. 13;
[0073] FIG. 15 shows an example of a spatial light modulator
capable of modulating polarization used in the optical storage
apparatus shown in FIG. 13;
[0074] FIG. 16 illustrates a polarization modulation obtained by
the optical storage apparatus shown in FIG. 13;
[0075] FIG. 17 shows an embodiment of an optical reading apparatus
according to the present invention;
[0076] FIG. 18 illustrates a polarization modulation of a
diffracted beam obtained by the optical reading apparatus shown in
FIG. 17;
[0077] FIG. 19 shows another example of the optical storage
apparatus and optical reading apparatus according to the present
invention;
[0078] FIG. 20 illustrates a comparative operating method for
increasing S/N of reading output;
[0079] FIGS. 21(a), 21(b) and 21(c) illustrate examples of
multiplex hologram storage with rotation of polarization direction
of the signal beam;
[0080] FIG. 22 illustrates data storage with rotation of the
polarization angle of the signal beam;
[0081] FIG. 23 illustrates the comparative operating method in
reading in the case of data storage with rotation of the
polarization angle of the signal beam;
[0082] FIG. 24 shows an embodiment of an optical retrieving
apparatus according to the present invention;
[0083] FIG. 25 illustrates retrieval of data by utilizing the
optical retrieving apparatus shown in FIG. 24;
[0084] FIG. 26 shows a conventional retrieving apparatus;
[0085] FIG. 27 shows a conventional storage apparatus and
correlation detecting apparatus; and
[0086] FIG. 28 shows a conventional spatial light modulator having
the same configuration as an LCD.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0087] Preferred embodiments are now described in detail based on
the drawings.
[0088] First Embodiment
[0089] FIG. 1(a) shows an embodiment of an optical storage medium
according to the present invention. On one side of a transparent
substrate 11 such as a glass substrate, a polarization-sensitive
layer 12 having photo-induced birefringence is formed to construct
the polarization-sensitive optical storage medium 10. In this case,
when data is stored, the polarization-sensitive layer 12 is
illuminated with a signal beam 1 and reference beam 2 as shown in
the figure.
[0090] To realize volume holographic storage (three-dimensional
storage), the polarization-sensitive layer 12 is required to be at
least about 10 .mu.m in thickness and it is preferable to be
thicker. If the layer 12 attains a thickness of 1 mm, a storing
amount as much as about 100 CD-ROMs is available.
[0091] FIG. 1(b) shows another example of the optical storage
medium according to the present invention. Here, the optical
storage medium 10 as a whole is the polarization-sensitive layer
12. The thickness of the polarization-sensitive layer 12, namely,
the thickness of the optical storage medium 10 is the same as that
of the polarization-sensitive layer 12 shown in FIG. 1(a).
[0092] In both cases shown in FIGS. 1(a) and 1(b), the optical
storage medium 10 is formed like a sheet, that is, the medium 10 is
formed to have a sufficiently large area for its thickness. The
optical storage medium 10 may be in the form of a disk.
[0093] Any material can be employed as the polarization-sensitive
layer 12 as long as it has the photo-induced birefringence and able
to record the polarization hologram. Preferred examples are,
polymer or liquid crystal polymer having a photoisomarizable moiety
as a side chain and polymer in which photoisomerizable molecules
are dispersed. As the photoisomarizable moiety or molecule, those
containing azobenzene structure are suitable.
[0094] Azobenzene shows trans-cis photoisomerization by
illumination of light. In the trans-form, the molecular structure
is shown as the chemical formula in FIG. 2(a). In the cis-form, the
molecular structure is shown as the chemical formula in FIG.
2(b).
[0095] Due to such photoisomerization, there are relatively many
trans-form azobenzene molecules before photo-excitation, and after
the photo-excitation, there exist relatively many cis-form
azobenzene molecules. Moreover, illumination of azobenzene with
linearly polarized light gives a direction in photoisomerization,
and thereby the direction appears in an absorption and refractive
index. In general, these properties are referred to as
photo-induced birefringence and photo-induced dichroism or
photo-induced anisotropy. By illumination of the azobenzene with
circularly-polarized light or non-polarized light, the excited
anisotropy can be erased.
[0096] If a hologram is recorded in the optical storage medium
whose polarization-sensitive layer 12 is made from polymer or
liquid crystal polymer having azobenzene as a side chain, or
polymer in which azobenzene molecules are dispersed, a signal beam
1 and reference beam 2, which are both coherent, illuminate the
same region of the optical storage medium 10 simultaneously.
[0097] In this case, if the polarizing directions of the signal
beam 1 and reference beam 2 are in parallel, for example, if both
signal beam 1 and reference beam 2 are s-polarized as shown in FIG.
3(a), the light intensity modulation is generated in the optical
storage medium 10 by the interference pattern of the signal beam 1
and reference beam 2. In a portion of high light intensity,
azobenzene is strongly photo-excited; therefore a lot of cis-form
azobenzene molecules are generated. In contrast, in a portion of
low light intensity, there are small number of cis-form azobenzene
molecules. Accordingly, a grating of the absorption or refractive
index corresponding to the light intensity modulation is formed as
a hologram.
[0098] In contrast, suppose that the polarizing directions of the
signal beam 1 and reference beam 2 are orthogonal to each other. If
the signal beam 1 is p-polarized and the reference beam 2 is
s-polarized as shown in FIG. 3(b), the same light intensity
modulation does not occur as that in the case of parallel
polarizing directions of the signal beam 1 and the reference beam
2. Instead, the polarizing directions are modulated spatially and
periodically. Therefore, a linearly polarized portion 8 and
elliptically polarized portion 9 periodically appear in turn.
[0099] In this case, the light intensity modulation is uniform.
However, azobenzene molecules having the same direction with the
modulated polarizing direction is strongly photo-excited than those
having other directions. As a result, in the linearly polarized
portion 8, many cis-form azobenzene molecules exist and a grating
of the absorption or refractive index is formed as a hologram.
[0100] Hereinafter, the hologram based on the light intensity
modulation in the case where the polarizing directions of the
signal beam 1 and reference beam 2 are in parallel as shown in FIG.
3(a) is referred to as light intensity hologram, and the hologram
based on the polarization modulation in the case where the
polarizing directions of the signal beam 1 and reference beam 2 are
orthogonal to each other as shown in FIG. 3(b) is referred to as
polarization hologram.
[0101] By the use of polymer or liquid crystal polymer having
azobenzene as a side chain or polymer in which azobenzene molecules
are dispersed as the polarization-sensitive layer 12 of the optical
storage medium 10, a hologram can be recorded since anisotropy of
azobenzene is induced irrespective of whether the polarizing
directions of the signal beam 1 and reference beam 2 are in
parallel or orthogonal to each other.
[0102] As shown in FIGS. 3(a) or 3(b), the optical storage medium
10 wherein the hologram has thus been recorded is illuminated with
a phase conjugate beam of the reference beam 2 used in recording,
namely, a light beam having the same wavefront but an opposite
direction as the reference beam 2. Then a phase conjugate beam of
the signal beam 1 used in recording, namely, a light beam with the
same wavefront but an opposite direction as the signal beam 1, is
generated as a diffracted beam 4 from the hologram.
[0103] In the case shown in FIG. 3(a), if both signal beam 1 and
reference beam 2 are s-polarized, the recorded hologram is a light
intensity hologram, namely, the hologram formed based on the light
intensity modulation, and the diffracted beam 4 is also
s-polarized. In the case where both signal beam 1 and reference
beam 2 are p-polarized, the diffracted beam 4 is also
p-polarized.
[0104] In contrast, if the signal beam 1 is p-polarized and the
reference beam 2 is s-polarized as shown in FIG. 3(b), the recorded
hologram is a polarization hologram, namely, the hologram formed
based on the polarization modulation, and the diffracted beam 4 is
p-polarized which is the same as the signal beam 1. If the signal
beam 1 is s-polarized and the reference beam 2 is p-polarized, the
diffracted beam 4 is s-polarized which is the same as the signal
beam 1.
[0105] A case where the signal beam 1 has s-polarization component
and p-polarization component is discussed as follows. It is assumed
that the signal beam 1 is linearly polarized having equal
s-polarization component and p-polarization component (polarizing
directions is 45.degree. from each of s-polarizing direction and
p-polarizing direction). If the diffraction efficiencies of the
light intensity hologram and the polarization hologram is equal,
the read diffracted beam 4 has the same polarizing direction as the
signal beam 1.
[0106] In contrast, if the diffraction efficiencies of the light
intensity hologram and the polarization hologram is not equal, the
polarizing direction of the read diffracted beam is different from
that of the signal beam 1 because the diffraction efficiencies of
the s-polarization and p-polarization are different from each
other. However, a diffracted beam 4 having the same polarizing
direction as the signal beam 1 is available by disposing a
polarizer or a half-wave plate in an optical path of the diffracted
beam 4.
[0107] As one of the preferred examples of the material of the
polarization-sensitive layer 12, polyester polymer having
azobenzene as a side chain represented by the chemical formula
shown in FIG. 2(c) can be adopted. It is confirmed that the
polarization hologram can be stored in the material by an optical
system employing degenerate four-wave mixing shown in FIG. 4.
[0108] An argon ion laser radiating a laser beam suitable for
polyester polymer having cyanoazobenzene as a side chain is used as
a light source 91. The laser beam with 515 nm wavelength from the
argon ion laser 91 is s-polarized perpendicular to the face of FIG.
4. Part of the laser beam is reflected off a half mirror 92a,
transmitted through a shutter 93a, reflected off a mirror 94a, and
transmitted through a half-wave plate 95 to obtain the signal beam
1. The polarizing direction of the signal beam 1 can be arbitrarily
changed by the half-wave plate 95.
[0109] The signal beam 1 is transmitted through the half mirror 92b
and illuminates a sample optical storage medium 10 made from
polyester polymer having cyanoazobenzene as a side chain. At the
same time, part of the laser beam transmitted through the half
mirror 92a is reflected off a half mirror 92c, transmitted through
a shutter 93b, reflected off a mirror 94b, and illuminates the
optical storage medium 10 as the reference beam 2 of s-polarized.
Here, a shutter 93c is closed.
[0110] When readout is performed, the shutters 93a and 93b are
closed and the shutter 93c is open. The laser beam transmitted
through the shutter 93c is reflected off the mirror 94c and
illuminates the optical storage medium 10 as the s-polarized read
beam 3. The diffracted beam 4 is reflected off the half mirror 92b
and transmitted through an analyzer 96 to be fetched. The
polarizing direction of the diffracted beam 4 can be analyzed by
rotating the analyzer 96.
[0111] A hologram is recorded by the signal beam 1 of 4 mW-light
power and approximately 100 .mu.m-diameter and the reference beam 2
of 100 mW-light power and 2 mm-diameter. The time for recording is
varied with five-second units. The hologram is read out after the
storage has been completed. The light power of the read beam 3 is
200 mW. The illumination time of the read beam 3 for a single
reading process is limited to 0.5 second in the fear of destruction
of the hologram if the read beam 3 illuminates it for a long
time.
[0112] FIGS. 5(a), 5(b) and 5(c) show dependencies of the light
intensity of the diffracted beam 4 in accordance with the time for
recording the hologram in the cases where the signal beam 1 is
s-polarized, p-polarized and 45.degree.--polarized (the medium
between the s-polarization and p-polarization). As described above,
the reference beam 2 and the read beam 3 are s-polarized.
[0113] As it is clear from FIGS. 5(a), 5(b) and 5(c), the hologram
can be recorded irrespective of the polarization state of the
signal beam 1. It is also found from FIGS. 5(a), 5(b) and 5(c) that
the light intensity of the diffracted beam 4 reaches a stationary
state after the storage for approximately 80 seconds. Moreover, it
is confirmed that the stored hologram can be maintained more than
several months in a room temperature.
[0114] FIG. 6 shows the polarizing direction of the diffracted beam
4 if the signal beam 1 is s-polarized as shown in FIG. 5(a). The
horizontal axis represents polarization rotating angle of the
analyzer 96, in which points of 90.degree. and 270.degree.
correspond to s-polarization. The vertical axis represents the
transmission light intensity of the analyzer 96. From the figure,
it can be found that the transmission light intensity takes the
highest value at the polarization rotating angles of 90.degree. and
270.degree.. Accordingly, if the hologram recording is performed
with the s-polarized signal beam 1, the diffracted beam 4 is also
s-polarized.
[0115] FIG. 7 shows the polarizing direction of the diffracted beam
4 if the signal beam 1 is p-polarized as shown in FIG. 5(b). The
horizontal axis represents polarization rotating angle of the
analyzer 96, in which points of 0.degree. and 180.degree.
correspond to p-polarization. The vertical axis represents the
transmission light intensity of the analyzer 96. From the figure,
it can be found that the transmission light intensity takes the
highest value at the polarization rotating angles of 0.degree. and
180.degree.. Accordingly, if the storage is performed with the
p-polarized signal beam 1, the diffracted beam 4 is also
p-polarized.
[0116] FIG. 8 shows the polarizing direction of the diffracted beam
4 if the signal beam 1 is 45.degree.-polarized as shown in FIG.
5(c). The horizontal axis represents polarization rotating angle
and the vertical axis represents the transmission light intensity
of the analyzer 96. From the figure, it can be found that the
transmission light intensity takes the highest value at the
polarization rotating angles of 140.degree. and 320.degree.. Since
the transmission light intensity of the analyzer 96 takes the
highest value at the points of polarization rotating angles of
135.degree. and 315.degree. if the diffracted beam 4 is the phase
conjugate beam that retains the polarization of the signal beam 1.
Therefore, also in this case, it can be recognized that the
diffracted beam 4 retains the polarization of the signal beam 1 for
the most part.
[0117] The difference of 5.degree.-polarization angle seems to be
caused by the optical system, and in particular, by the
polarization property of the half mirror 92b. The difference can be
easily modified by providing the polarizer or half-wave plate in
the optical path of the diffracted beam 4.
[0118] As a result, the optical storage medium made from polyester
polymer having cyanoazobenzene as a side chain can store
polarization of the signal beam as a hologram. It is also possible
to read out the polarization of the diffracted beam from the
optical storage medium. Accordingly, it is also possible to
multiple-record the holograms each of which retains data
information based on the intensity modulation or phase modulation
on the same region of the optical storage medium made from
polyester polymer having cyanoazobenzene as a side chain by
changing the polarizing direction of the signal beam.
[0119] Moreover, to examine whether it is possible to
multiple-store holograms in the same region of the optical storage
medium made from polyester polymer having cyanoazobenzene as a side
chain by changing the polarization angle of the reference beam,
holograms are recorded with various polarization angles of the
reference beam 2 in the optical system shown in FIG. 4, and the
polarization state of the diffracted beam 4 is examined. The
polarization angle of the reference beam 2 is changed by the
half-wave plate (not shown in FIG. 4) provided in the optical path
of the reference beam 2.
[0120] FIG. 9 shows a relationship between the polarization angle
of the diffracted beam 4 and the light intensity through analyzer
in the case where the signal beam 1 is p-polarized, reference beam
2 is p-polarized, and the read beam 3 is s-polarized. The light
intensity of the diffracted beam 4 marks a peak at the points of
polarization angle of the diffracted beam 4 of approximate
90.degree. or 270.degree.. Therefore, it is found that the
diffracted beam 4 is approximately s-polarized.
[0121] In contrast, as shown in FIG. 7 corresponding to FIG. 5(b),
if the signal beam 1 is p-polarized, the reference beam 2 is
s-polarized and the read beam is s-polarized, the diffracted beam 4
is p-polarized. As it is clear from the comparison of FIG. 7 with
FIG. 9, if the hologram is recorded with rotation of the
polarization angle of the reference beam 2, the polarization angle
of the diffracted beam 4 rotates corresponding to the rotation of
the polarization angle of the reference beam 4.
[0122] Therefore, it is possible to record multiple holograms, each
of which retains data information based on the intensity modulation
or phase modulation, on the same region of the optical storage
medium made from polyester polymer having cyanoazobenzene as a side
chain by rotating the polarization angle of the reference beam. The
desirable diffracted beam from the polarization-multiplexed
holograms recorded by the reference beams with the different
polarizing directions can be separated by its polarization
state.
[0123] The rewriting property of the optical storage medium 10 made
from a polyester polymer having cyanobenzene as a side chain is
measured by the optical system for storing and reproducing the
hologram shown in FIG. 10.
[0124] As the light source 81, an argon ion laser that is the same
as the light source 91 of the optical system of degenerate
four-wave mixing in FIG. 4 is used. The laser beam with the
wavelength of 515 nm from the argon ion laser 81 is s-polarized
perpendicular to the face of FIG. 10. Part of the laser beam is
transmitted through a beamsplitter 82, a shutter 83, and a
half-wave plate 84 to obtain the signal beam 1. The polarizing
direction of the signal beam 1 can be arbitrarily changed by the
half-wave plate 84.
[0125] The signal beam 1 is incident on the sample optical storage
medium 10 made from polyester polymer having cyanoazobenzene as a
side chain. At the same time, the laser beam reflected off the
beamsplitter 82 is further reflected off the mirrors 85 and 86 and
illuminates the optical storage medium 10 as the s-polarized
reference beam 2. Thus a hologram is recorded in the optical
storage medium 10.
[0126] In readout process, the shutter 83 is closed and the laser
beam reflected off the beamsplitter 82 is further reflected off the
mirrors 85 and 86 and illuminates the optical storage medium 10 as
the s-polarized read beam 3. The diffracted beam 4 is read from the
optical storage medium 10 and transmitted through an analyzer 87 to
be fetched. The polarizing direction of the diffracted beam 4 can
be examined by rotating the analyzer 87.
[0127] At first, a hologram is stored for approximate 2 seconds by
the signal beam 1 and the reference beam 2 with 1 W/cm.sup.2 light
intensities and about 2 mm diameters. Then the hologram is read by
the read beam 3 with 0.1 W/cm.sup.2 light intensity. After that the
same recording and reading processes are repeated. The illumination
time of the read beam 3 in a single reading process is limited to
0.5 second in the fear of destroying the stored hologram if the
read beam 3 illuminates it for a long time.
[0128] FIG. 11 shows a result of examination of polarizing
direction of the diffracted beam which has been read out. The
horizontal axis represents the polarization rotation angle of the
analyzer 87, where 0.degree. and 180.degree. correspond to
s-polarization. The vertical axis represents the transmission light
intensity of the analyzer 87. As described above, the signal beam
1, reference beam 2 and read beam 3 are s-polarized.
[0129] From FIG. 11, it is found that the transmission light
intensity of the analyzer 87 becomes high at the points of the
polarization rotation degree of the analyzer 87 of 0.degree. and
170.degree.. Accordingly, if a hologram is recorded by the
s-polarized signal beam 1, the diffracted beam 4 is also
s-polarized.
[0130] Next, in the region where s-polarized signal beam has been
recorded as a hologram, p-polarized signal beam is overwritten
without erasing the previously recorded hologram of s-polarized
signal beam. In the optical storage system shown in FIG. 10, the
signal beam 1 is changed to be p-polarized by rotating the
half-wave plate 84 though the reference beam 2 remains to be
s-polarized. Then the hologram is stored for approximate 4 seconds
by the signal beam 1 and reference beam 2 with the same light
intensities and beam diameters as the preceding experiment. The
hologram is read by the read beam 3 with the same light intensity
as the preceding experiment.
[0131] FIG. 12 shows the polarizing direction of the diffracted
beam 4. The horizontal axis represents the polarization rotation
angle of the analyzer 87 and the point of 90.degree. corresponds to
p-polarization. The vertical axis represents the transmission light
intensity in the analyzer 87.
[0132] From the figure, the transmission light intensity through
the analyzer 87 takes the highest value at the point of the
polarization angle 80.degree. of the analyzer 87. Therefore, if the
p-polarized signal beam is stored in the region where the
s-polarized signal beam has been stored without erasing the
previously recorded hologram, the diffracted beam read from the
region has p-polarized.
[0133] In contrast, if an s-polarized signal beam is recorded in a
region where a p-polarized signal beam has been recorded without
erasing the previously recorded hologram, it is confirmed that the
diffracted beam read from the region has s-polarized (though not
shown in the figure).
[0134] As a result of the above experiments, it is found that an
s-polarized or p-polarized signal beam can be recorded in a region
where an s-polarized or p-polarized signal beam has been recorded
in advance without erasing the previously stored hologram.
Moreover, there is no problem in overwriting the hologram again.
Therefore any data can be rewritten at high speed without an
erasing process.
[0135] To store the polarization of the signal beam as a hologram,
to read it and to rewrite data without requiring an erasing process
as described above are also available for an optical storage medium
with a layer of polyester polymer having cyanoazobenzene as a side
chain on only one side thereof. It is not limited to the optical
storage medium using polyester polymer having cyanoazobenzene as a
side chain, and these advantages are available for an optical
storage medium equipped with a layer of polymer or liquid crystal
polymer having a photoisomerizable moiety such as azobenzene as a
side chain or of polymer in which photoisomerizable molecules such
as azobenzene molecules are dispersed at least on one side
thereof.
[0136] Second Embodiment
[0137] FIGS. 13 and 14 show an example of an optical storage method
and optical storage apparatus. The optical storage medium 10 is of
the polarization-sensitive type in the form of a disk described in
the first embodiment.
[0138] As a light source 21 of an optical storage head 20, anything
can be employed as long as it emits a coherent light beam that is
suitable for the polarization-sensitive optical storage medium 10.
In the case where the optical storage medium 10 is made from
polyester polymer having cyanobenzene as a side chain, an argon ion
laser can be used. The argon ion laser generates a laser beam of
515 nm wavelength as described above belonging to the wavelength
with which cyanobenzene is photoisomerized. Part of the laser beam
5 from the light source 21 transmitted through a beamsplitter 25
and is adjusted to be a collimated beam by lenses 22 and 23 and
illuminates a spatial light modulator 30.
[0139] The spatial light modulator 30 is capable of modulating
polarization. As the spatial light modulator 30, a liquid crystal
panel of the electric address type or an electro-optical crystal
equipped with a matrix electrode can be used. Unlike the
above-described spatial light modulator that is the LCD panel shown
in FIG. 28, the spatial light modulator 30 in this embodiment does
not have the polarizer.
[0140] FIG. 15 shows an example of the spatial light modulator 30
capable of modulating polarization. It is a spatial light modulator
of light-valve configuration obtained by forming transparent
electrodes 32 and 33 on inner surfaces of the transparent
substrates 34 and 35, respectively, and then putting an
electro-optical transforming material 31, such as liquid crystal,
between the transparent electrodes 32 and 33. Multiple pixels are
assigned functions as half-wave plates. Bit information of the
two-dimensional data corresponding to each pixel is assigned as
application or non-application of a voltage. Thereby the polarizing
direction of a light beam incident on each pixel is modulated.
[0141] As shown in FIG. 16, the collimated beam 6 is incident on
the spatial light modulator 30 as an s-polarized beam. The axis of
the half-wave plate of a pixel 37a of the spatial light modulator
30, to which the voltage is not applied, is in parallel with the
polarizing direction of the incident beam 6. Therefore, the signal
beam 1a transmitted through the pixel 37a is s-polarized. In
contrast, the axis of the half-wave plate of a pixel 37b of the
spatial light modulator 30, to which the voltage is applied,
rotates at 45.degree. and thereby the polarizing direction of the
incident beam 6 is rotated at 90.degree.. Therefore, the signal
beam 1b transmitted through the pixel 37b is p-polarized.
Accordingly, the signal beam 1 transmitted through the spatial
light modulator 30 has a spatial polarization modulation
corresponding to the two-dimensional data.
[0142] As shown in FIGS. 13 and 14, the signal beam 1 transmitted
through the spatial light modulator 30 is Fourier transformed on a
Fourier plane P1 by a lens 24 and illuminates the optical storage
medium 10. At the same time, the other part of the laser beam 5
from the light source 21 is reflected off the beamsplitter 25 and
further reflected off mirrors 26 and 27 to obtain an s-polarized
reference beam 2. The reference beam 2 illuminates the region of
the optical storage medium 10 where the signal beam 1 illuminates.
Thereby the polarization modulation of the signal beam 1
corresponding to the two-dimensional data can be stored as a
polarization hologram in the optical storage medium 10.
[0143] Multiple polarization holograms can be recorded in different
regions in the circumferential direction of the optical storage
medium 10 by rotating the optical storage medium 10 by a motor 29.
At this time, shift-multiplexing storage is possible if a spherical
wave is used as the reference beam 2. Moreover, by moving the
optical storage head 20 to the direction of diameter of the optical
storage medium 10, the polarization hologram can be stored in the
optical storage medium 10 as if concentric circular recording
tracks are formed as shown in FIG. 14.
[0144] According to the optical storage method and optical storage
apparatus described above, there is no loss of quantity of light in
the spatial light modulator 30 because the spatial light modulator
30 does not have the polarizing plate. Moreover, since the signal
beam 1 retains data information based on the spatial polarization
modulation, the light intensity of the signal beam 1 is spatially
uniform. Therefore, it is possible to prevent reduction of quantity
of light in the spatial light modulator 30 or deterioration of the
S/N of the signal beam 1 caused by fluctuation of the light
intensity of the signal beam 1, and as a result, data can be stored
with high density at high speed. Moreover, a special encoding is
unnecessary.
[0145] Further, according to the optical storage method and optical
storage apparatus as described above, the signal beam 1 transmitted
through the spatial light modulator 30 has the spatial polarization
modulation corresponding to the two-dimensional data. Either a beam
of s-polarization or p-polarization always illuminates the storage
region of the optical storage medium 10. Therefore, unlike the
storage of the light intensity hologram, there is no portion where
the light does not illuminate corresponding to the content of the
two-dimensional data. In addition, a new polarizing direction can
be overwritten without erasing the preceding polarizing direction
as described above.
[0146] As described so far, according to the optical storage method
and optical storage apparatus of this embodiment, data can be
securely rewritten at high speed without an erasing process.
[0147] Third Embodiment
[0148] FIG. 17 shows an embodiment of an optical reading method and
optical reading apparatus according to the present invention. The
optical storage medium 10 is of the polarization-sensitive type in
the form of a disk. In the optical storage medium 10, the signal
beam 1 retaining the two-dimensional data based on the spatial
polarization modulation is stored as a hologram as shown in FIG. 16
by the method and apparatus shown in FIGS. 13 through 16.
[0149] A phase conjugate beam of the reference beam used in storing
is used as a read beam 3 by the read beam optical system 41
including a light source of an optical reading head 40. The beam
illuminates the region of the optical storage medium 10 where the
hologram has been recorded. Then a phase conjugate beam retaining
polarizing direction of the signal beam used in storing can be
obtained as a diffracted beam 4 from the hologram as shown in FIG.
18.
[0150] However, in this case, if the diffraction efficiencies of
the light intensity hologram and the polarization hologram is not
equal, the polarizing direction of the diffracted beam 4 is
different from that of the signal beam 1. The diffracted beam 4
having the same polarizing direction as the signal beam 1 can be
obtained by providing the polarizer or half-wave plate in the
optical path of the diffracted beam 4.
[0151] The diffracted beam 4 is adjusted by a lens 42 to be a
collimated beam and is then incident on the polarizing beamsplitter
43. Then the beam is separated into the s-polarization component 7S
and p-polarization component 7P. The s-polarization component 7S is
detected by a photodetecting array 44S or the p-polarization
component 7P is detected by a photodetecting array 44P. A CCD or
the like can be used as the photodetecting arrays 44S and 44P.
[0152] As shown in FIG. 18, the s-polarization component 7S and
p-polarization component 7P are in the relation of a negative image
and positive image. Therefore, by detecting one of them by the
corresponding photodetecting array, two-dimensional data retained
by the spatial polarization modulation of the diffracted beam 4,
namely, two-dimensional data stored in the optical storage medium
10 can be read.
[0153] It is possible to read multiple holograms stored in
different portions in a circumferential direction of the optical
storage medium 10 by rotating the optical storage medium 10 by a
motor 49. If the optical storage head 40 is moved to the diameter
direction of the optical storage medium 10, the hologram can be
read from the recording tracks formed as the concentric circles on
the optical storage medium 10.
[0154] According to the optical reading method and optical reading
apparatus of this embodiment, data stored in the optical storage
medium 10 can be read with high precision at high speed. The
diffracted beam 4, that is the phase conjugate beam of the signal
beam, automatically cancels aberration or the like of the lens 42
in the optical path, and an image is automatically formed at a
position of the focal distance point of the lens 42. Accordingly,
there is no limitation to alignment.
[0155] Third Embodiment
[0156] FIG. 19 shows another example of the optical storage method,
optical storage apparatus, optical reading method and optical
reading apparatus of the present invention.
[0157] The optical storage method and optical storage apparatus are
substantially the same as those shown in FIGS. 13 through 16 except
that a shutter 28 is disposed to the optical path of the laser beam
transmitted through the beamsplitter 25. The shutter 28 is opened
in storing to obtain a parallel incident beam 6 and the signal beam
1 retaining the spatial polarization modulation.
[0158] In this example of the optical storage method and optical
storage apparatus, data can be rewritten at high speed without
requiring an erasing process as described above.
[0159] In this example of the optical reading method and optical
reading apparatus, completely the same light beam as the reference
beam 2 used in storing is used as the read beam 3, not the phase
conjugate beam of the reference beam 2.
[0160] In reading, the shutter 28 is closed. The laser beam
reflected off the beamsplitter 25 is further reflected off the
mirrors 26 and 27 and illuminates the region of the optical storage
medium 10 where the hologram has been recorded. Then a light beam
retaining the polarizing direction of the signal beam used in
storing can be obtained as the diffracted beam 4 from the hologram
as shown in FIG. 18. The diffracted beam 4 is adjusted by the lens
42 to be the collimated beam and detected by the photodetecting
array 44.
[0161] As shown in FIG. 17, the polarizing beamsplitter or
wavelength plate can be provided between the lens 42 and
photodetecting array 44, though not shown in FIG. 19. With this
configuration, an s-polarization component and a p-polarization
component in the diffracted beam 4 are separately detected.
[0162] Also in this example, data stored in the optical storage
medium 10 can be read out with high precision at high speed.
[0163] Fourth Embodiment
[0164] In the optical reading method and optical reading apparatus
shown in FIG. 17 (or. FIG. 19), the s-polarization component 7S and
p-polarization component 7P of the diffracted beam are separated by
the polarizing beamsplitter 43 and their light intensities are
detected by the photodetecting arrays 44S and 44P. In this
embodiment, the comparative operation is performed on the light
intensities of the s-polarization component 7S and the
p-polarization component 7P to cancel out noises caused by
fluctuation of the diffracted beam 4, influence of the outer light,
defects of the optical storage medium 10 or optical system, or the
like. Thereby, a reading output with higher S/N is available.
[0165] FIG. 20 shows the method of comparative operation. In a
subtraction circuit 45, the detecting output of the photodetecting
array 44S is subtracted from the detecting output of the
photodetecting array 44P for each corresponding pixel (bit).
[0166] It is assumed that the diffracted beam of the i-th order
pixel is p-polarized, the signal component is Ipi, and the noise
component is Ni. For the i-th order pixel, the output of the
photodetecting array 44P is a sum of the signal component Ipi and
the noise component Ni (Ipi+Ni), and the output of the
photodetecting array 44S is only the noise component Ni. The output
of the subtraction circuit 45 is only the signal component Ipi, as
a result of cancellation of the noise component Ni.
[0167] It is assumed that the diffracted beam of the j-th order
pixel is s-polarized, the signal component is Isj, and the noise
component is Nj. For the j-th order pixel, the output of the
photodetecting array 44P is only the noise component Nj, and the
output of the photodetecting array 44S is a sum of the signal
component Isj and the noise component Nj (Isj+Nj). The output of
the subtraction circuit 45 is only the signal component--Isj as a
result of cancellation of the noise component Nj.
[0168] In the case of readout of binary digital data, the positive
output value of the subtraction circuit 45 may be determined as
[1], and the negative output value may be determined as [0], for
example.
[0169] Thus the noise can be canceled for each pixel according to
the above-described optical reading method and optical reading
apparatus. In addition, data value can be determined based on
whether the output value is positive or negative compared to zero
output value as the threshold without depending on the light
intensity of the diffracted beam 4.
[0170] Fifth Embodiment
[0171] As described above, the polarization-sensitive optical
storage medium of the present invention can record the polarization
hologram, and further can record the hologram retaining data
information based on the light intensity modulation or phase
modulation on the same region as the polarization hologram has been
stored by changing the polarizing direction of the signal beam.
[0172] An example of the optical storage method and optical reading
method in this case is discussed as follows. In the storing
process, at first, a hologram is recorded in a region 15 of the
polarization-sensitive optical storage medium 10 with the signal
beam 1 and reference beam 2. Both of the beams are s-polarized as
shown in FIG. 21(a). The hologram at this time is based on the
light intensity modulation as described above with reference to
FIG. 3(a).
[0173] Next, as shown in FIG. 21(b), another hologram is recorded
in the region 15 of the optical storage medium 10 with the signal
beam 1 of p-polarization and reference beam 2 of s-polarization.
The hologram is based on the polarization modulation as shown in
FIG. 3(b). The order of storing the light intensity modulation
hologram and the polarization hologram can be arbitrarily
determined.
[0174] In the storing process, as shown in FIG. 21(c), the region
15 of the optical storage medium 10 where the light intensity
hologram and polarization hologram are multiple-stored is
illuminated with the read beam 3 which is the phase conjugate beam
of the reference beam 2 used in storing. Thereby, the diffracted
beam from the region 15 having s-polarization component diffracted
from the light intensity hologram recorded by the signal beam of
s-polarization and p-polarization component diffracted from the
polarization hologram recorded by the signal beam of
p-polarization.
[0175] As shown in FIGS. 17 and 18, the diffracted beam 4 is
separated into the s-polarization component 7S and the
p-polarization component 7P by the polarizing beamsplitter 43. The
s-polarization component 7S is detected by the photodetecting array
44S, and the p-polarization component 7P is detected by the
photodetecting array 44P. Accordingly, the light intensity hologram
and polarization hologram, namely, data of the signal beam of
s-polarization and data of the signal beam of p-polarization can be
separated.
[0176] According to the above-described methods, holograms can be
multiplexed in the same region and the multiplexed holograms can be
separately read out from the same region. Thus storage of data with
higher precision is available.
[0177] Sixth Embodiment
[0178] The polarization hologram diffracts a beam retaining the
polarizing direction of the signal beam. Therefore, storage of a
large amount of data information based on the different
polarization angles is possible by rotating the polarization angle
of the signal beam. Moreover, at the same time, it is possible to
store a large amount of data information based on the different
light intensities by changing the light intensity of the signal
beam. Thus the high-density storage is realized.
[0179] For example, as shown in FIG. 22, six polarization angles
represented by vectors D1-D6 are set within the range of 90.degree.
from s-polarizing direction (0.degree.) to p-polarizing direction
(90.degree.). The six polarization angles can be coded to represent
six kinds of bit. Therefore they can be represented as numbers
corresponding to base 6 or numbers coded in the form of binary
digit corresponding to 6th power. The lengths of vectors D1-D6
represent light intensities of the signal beam for each of the
polarization angles. Multiple levels can be set to the lengths of
vectors, and they are coded to represent plural bits.
[0180] The signal beam whose polarizing angle has been rotated is
obtained by the spatial light modulator 30 shown in FIGS. 13, 14 or
15. It can also be obtained by wave synthesis of the spatial
intensity modulation of s-polarization and p-polarization by the
beamsplitter.
[0181] In reading, as shown in FIGS. 17 and 18, the diffracted beam
4 from the hologram is separated into the s-polarization component
7S and p-polarization component 7P by the polarizing beamsplitter
43. The s-polarization component 7S is detected by the
photodetecting array 44S and the p-polarization component 7P is
detected by the photodetecting array 44P. Further, as shown in FIG.
23, the detection output of the photodetecting arrays 44S and 44P
are provided to the comparative operation circuit including a
division circuit 46, a square root detection circuit 47 and an
arctangent calculating circuit 48 for performing comparative
operation on each corresponding pixel (bit).
[0182] If it is assumed that the light intensity of the diffracted
beam of a pixel is 1, the polarization angle (s-polarizing
direction is 0.degree.) is .theta., the intensity of the
s-polarization component Is and the intensity of the p-polarization
component Ip are represented as follows:
Is=I cos.sup.2 .theta. (1);
Ip=I sin.sup.2 .theta. (2).
[0183] Therefore, tan.sup.2 .theta. is obtained by dividing the
p-polarization intensity Ip by the s-polarization intensity Is by
the division circuit 46. By the square root calculating circuit 47,
tan .theta. is obtained and polarization angle .theta. is obtained
by the arctangent calculating circuit 48. Thus the data information
based on the difference between the polarization angles of the
signal beams can be read.
[0184] Seventh Embodiment
[0185] FIG. 24 shows an embodiment of the optical retrieving method
and optical retrieving apparatus of the present invention. The
optical storage medium 10 is of the polarization-sensitive type in
the form of the disk. The signal beam 1 retaining the
two-dimensional retrieving object data based on the spatial
polarization modulation as shown in FIG. 16 is stored as the
hologram by the optical storage method and optical storage
apparatus shown in FIGS. 16 to 19.
[0186] The spatial light modulator 30 as shown in FIG. 13-15 or 19
is provided to the optical retrieving head (optical reading head)
60, and two-dimensional retrieving data is written thereto as shown
in FIG. 25. In other words, bit information of the retrieving data
corresponding to each pixel in the spatial light modulator 30 is
provided to the pixel as application or non-application of the
voltage. Each pixel functions as the half-wave plate to rotate the
polarizing direction of a light beam incident thereon in accordance
with the corresponding information bit of the retrieving data.
[0187] Similar to the optical reading method and optical reading
apparatus shown in FIG. 17, the phase conjugate beam of the
reference beam used in recording is obtained as the read beam 3
from the read beam optical system 61 of the optical retrieving head
60 including the light source, and the read beam 3 illuminates the
region of the optical storage medium 10 where the hologram has been
recorded. As a result, the phase conjugate beam retaining the
polarizing direction of the signal beam used in storing is obtained
as the diffracted beam 4 from the hologram as shown in FIG. 25.
[0188] The diffracted beam 4 is converted into the collimated beam
by a lens 62 and forms an image on the spatial light modulator 30.
The converted diffracted beam transmitted through the spatial light
modulator 30 is further transmitted through lenses 63 and 64 which
constitute an imaging optical system. The beam is then incident on
the polarizing beamsplitter 43 and is separated into the
s-polarization component 7S and p-polarization component 7P. The
s-polarization component 7S is detected by the photodetecting array
44S and the p-polarization component 7P is detected by the
photodetecting array 44P.
[0189] In this case, plural holograms stored in different places in
circumferential direction of the optical storage medium 10 are read
by rotating the optical storage medium 10 by the motor 69. The
holograms recorded in the recording track in the form of the
concentric circles on the optical storage medium 10 are read by
moving the optical retrieving head 60 to the direction of diameter
of the optical storage medium 10.
[0190] The diffracted beam 4 retaining the polarization information
of the retrieving object data is the phase conjugate beam retaining
the polarizing direction of the signal beam used in storing.
Consequently, if the retrieving data and the retrieving object data
completely match with each other, the diffracted beam transmitted
through the spatial light modulator 30 is s-polarized for all
pixels owing to a phase modification operation by the phase
conjugate beam in that a distortion of a wavefront is canceled by
passing through a phase distortion medium twice. Therefore,
s-polarization component 7S separated by the polarizing
beamsplitter 43 is [clear] for all pixels, and p-polarization
component 7P is [dark] for all pixels.
[0191] In contrast, if the retrieving data and the retrieving
object data do not match with each other, the phase modification
operation of the phase conjugate beam does not work. Accordingly,
the diffracted beam transmitted through the spatial light modulator
30 is p-polarized in the pixel.
[0192] It is possible to detect whether the retrieving data and
retrieving object data completely match with each other, or
correlation value between the retrieving data and retrieving object
data by monitoring all intensities of the s-polarization component
7S or p-polarization component 7P of the detection output of the
photodetecting array 44S or the photodetecting array 44P. The
comparative operation on all intensities of the s-polarization
component 7S and p-polarization component 7P cancels the noises.
Accordingly, the correlation value between the data or matching
between the data can be detected with higher precision.
[0193] It is assumed that the retrieving data and retrieving object
data do not match with each other. For example, as shown in FIG.
25, the address {m, n} of the retrieving data is s-polarized
(0.degree.-rotation of the axis of the half-wave plate of the
address {m, n} 30c of the spatial light modulator 30), and the
address {m, n} of the retrieving object data is p-polarized. In
this case, the diffracted beam 4 in the address {m, n} becomes
p-polarized by passing through the address {m, n} 30c of the
spatial light modulator 30.
[0194] If the address {k, l} of the retrieving data is p-polarized
(45.degree.-rotation of the axis of the half-wave plate of the
address {k, l} 30d of the spatial light modulator 30) and the
address {k, l} of the retrieving object data is s-polarized, the
diffracted beam 4 in the address {k, l} becomes p-polarized by
passing through the address {k, l} 30d of the spatial light
modulator 30.
[0195] In other words, the diffracted beam transmitted through the
spatial light modulator 30 is s-polarized where the retrieving data
and the retrieving object data match with each other, and
p-polarized where the retrieving data and the retrieving object
data do not match with each other. Accordingly, the s-polarization
component 73S separated by the polarizing beamsplitter 43 is [dark]
in an address where the retrieving data and the retrieving object
data do not match with each other. In contrast, the p-polarization
component 73P is [clear] in an address where the retrieving data
and the retrieving object data do not match with each other.
[0196] Therefore, clearness or darkness in each address of the
s-polarization component 7S or p-polarization component 7P is
detected from the detection output of the photodetecting array 44S
or photodetecting array 44P; accordingly, whether the retrieving
data and the retrieving object data match with each other or not
can be detected for each address (bit). In the same way as the
optical reading method and optical reading apparatus shown in FIG.
17, the comparative operation on the light intensities of the
s-polarization component 7S and p-polarization component 7P is
performed as shown in FIG. 20, thereby the noise is canceled and it
becomes possible to detect matching between the data with higher
precision.
[0197] According to the optical retrieving method and optical
retrieving apparatus, in the state where the retrieving data is
written in the spatial light modulator 30 in the optical retrieving
head 60, the optical retrieving head 60 can be moved to the
recording track formed as the concentric circles on the optical
storage medium 10 and the optical storage medium is rotated by the
motor 69. Thereby only two-dimensional data that matches with the
retrieving data can be retrieved from the optical storage medium 10
accumulating a large amount of data with high precision at high
speed. Moreover, the retrieving data can be set easily and
arbitrarily. Therefore, desired data can be easily retrieved.
* * * * *