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Claims  |
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What is claimed is:
1. An information storage system, comprising:
a storage medium having (n) vertically disposed storage locations and (m)
horizontally disposed storage locations for storing at least (n.times.m)
interference patterns;
an object beam optical channel including means for directing an object beam
to a specified one of the n.times.m locations within said storage medium,
said object beam optical channel further including means for modulating
the directed object beam with information to be stored; and
a reference beam optical channel including means for directing a reference
beam to a specified one of the n.times.m locations within said storage
medium, said directing means of said reference beam optical channel
including,
means for varying an angle of incidence of the reference beam upon the
storage medium,
a plurality of reflectors each of which has an angular orientation, with
respect to said storage medium, that differs from the angular orientation
of others of said plurality of reflectors, said plurality of reflectors
each being disposed for reflecting the reference beam to said storage
means, said plurality of reflectors including an array of (n times m)
discrete reflector elements each having a linear shape and being disposed
in a parallel orientation with one another, said array being organized as
(m) sub-arrays of discrete reflector elements, each of said sub-arrays
including (n) discrete reflector elements, wherein each of said discrete
reflector elements within a sub-array has an angle of inclination that
differs by (.DELTA..alpha.) degrees from an angle of inclination of an
adjacent discrete reflector element, and means for pointing the reference
beam, having the specified angle of incidence, to a selected one of said
plurality of reflectors.
2. An information storage system as set forth in claim 1 wherein said
object beam optical channel directing means is comprised of a first
acoustooptic device and a second acoustooptic device, said first and
second acoustooptic devices being disposed orthogonally with respect to
one another for controllably directing the object beam in two dimensions
to a specified one of said (n.times.m) storage locations.
3. An information storage system as set forth in claim 1 and further
including a radiation detector array having an input that is optically
coupled to an output of said storage medium for detecting radiation output
thereby in response to an application of the reference beam to said
storage medium.
4. An information storage system as set forth in claim 1 wherein each of
said sub-arrays of discrete reflector elements has an angle of inclination
that differs by (.DELTA..alpha.) degrees from an angle of inclination of
an adjacent sub-array.
5. An information storage system as set forth in claim 4 wherein
(.DELTA..alpha.) is related to a separation between adjacent reflector
elements,.DELTA.x, by
.DELTA..alpha.=.DELTA.x/2f.sub.b,
where f.sub.b is a focal length of a lens element that is interposed
between said array of reflectors and said storage medium.
6. An information storage system as set forth in claim 1 wherein said
reference beam optical channel pointing means is comprised of a first
acoustooptic device, and wherein said varying means is comprised of a
second acoustooptic device, said first and second acoustooptic devices of
said reference beam optical channel being disposed orthogonally with
respect to one another for controllably directing the reference beam in
two dimensions.
7. An information storage system as set forth in claim 6 wherein said
reference beam optical channel further includes;
a first lens means interposed between said first and said second
acoustooptic devices and said plurality of reflectors, said first lens
means having a focal length (f.sub.a) and being disposed a distance
(f.sub.a) from said first and said second acoustooptic devices and a
distance (f.sub.a) from said plurality of reflectors; and
a second lens means interposed between said plurality of reflectors and
said storage medium, said second lens means having a focal length
(f.sub.b) and being disposed a distance (f.sub.b) from said storage medium
and a distance (f.sub.b) from said plurality of reflectors.
8. An information storage system as set forth in claim 1 wherein said
object beam optical channel further includes:
a first lens means and a second lens means that are interposed between said
directing means and said modulating means, said first lens means being
disposed at a first focal length (f.sub.1) from said directing means, said
second lens means being disposed at a second focal length (f.sub.2) from
said modulating means, said first lens means and said second lens means
being disposed a distance (f.sub.1 +f.sub.2) from one another; and
a third lens means interposed between said modulating means and said
storage medium, said third lens means having a third focal length
(f.sub.3) and being disposed a distance (f.sub.3) from said modulating
means and a distance (f.sub.3) from said storage medium.
9. An information storage system as set forth in claim 8 and further
including:
a radiation detector array having an input that is optically coupled to an
output of said storage medium for detecting radiation output thereby in
response to an application of said reference beam to said storage medium;
and
a fourth lens means interposed between said radiation detector array and
said storage medium, said fourth lens means having a fourth focal length
(f.sub.4) and being disposed a distance (f.sub.4) from said radiation
detector array and a distance (f.sub.4) from said storage medium.
10. A spatial multiplexer for use with a holographic storage medium for
directing a beam of optical radiation onto the storage medium, the storage
medium having (n) rows and (m) columns of storage locations, said spatial
multiplexer comprising an array of (n times m) discrete reflector elements
each having a linear shape and being disposed in a parallel arrangement
with one another upon a substrate, and wherein each of said discrete
reflector elements has a different angular orientation with respect to one
another.
11. A spatial multiplexer as set forth in claim 10 wherein said array is
organized as (m) sub-arrays of discrete reflector elements, each of said
sub-arrays including (n) discrete reflector elements, wherein each of said
discrete reflector elements within a sub-array has an angle of inclination
that differs by (.DELTA..alpha.) degrees from an angle of inclination of
an adjacent discrete reflector element, wherein each of said sub-arrays of
discrete reflector elements has an angle of inclination that differs by
(.DELTA..alpha.) degrees from an angle of inclination of an adjacent
sub-array, and wherein (.DELTA..alpha.) is related to a separation between
adjacent discrete reflector elements, .DELTA.x, by
.DELTA..alpha.=.DELTA.x/2f.sub.b,
where f.sub.b is a focal length of a lens element that is interposed
between said array and the storage medium.
12. A spatial multiplexer as set forth in claim 10 wherein a separation (d)
between adjacent discrete reflector elements is given by
d=f.sub.a .DELTA..theta.,
where f.sub.a is a focal length of a lens element that is interposed
between said array and a means for directing the optical beam to a
specified one of said discrete reflector elements, and where
.DELTA..theta. is an angular change of the optical beam emergent from the
directing means.
13. A spatial multiplexer as set forth in claim 12 wherein a width of each
of said discrete reflector elements is equal to (d), and wherein
d.gtoreq..lambda.f.sub.a /A,
where A is a diameter of the optical beam emergent from the directing
means.
14. An information storage system, comprising:
a storage medium having (n) storage locations disposed along a first axis
and (m) storage locations disposed along a second axis that is orthogonal
to the first axis;
an object beam optical channel including means for directing an object beam
to a specified one of said storage locations within said storage medium,
said object beam optical channel further including means for modulating
the directed object beam with information to be stored; and
a reference beam optical channel including,
an angle multiplexer means for varying an angle of the reference beam with
respect to a point on the second axis,
a spatial multiplexer means for varying an angle of the reference beam with
respect to a point on the first axis, and
a plurality of reflectors each of which has an angular orientation, with
respect to said storage medium, that differs from the angular orientation
of all others of said plurality of reflectors, said plurality of
reflectors each being disposed for receiving the reference beam from said
angle multiplexer means and from said spatial multiplexer means and for
reflecting the reference beam to one of said storage locations, wherein
said plurality of reflectors includes an array of (n times m) discrete
reflector elements each having a linear shape, each of said reflector
elements being disposed in a parallel orientation with one another, said
array being organized as (m) sub-arrays of discrete reflector elements,
each of said sub-arrays including (n) discrete reflector elements, wherein
each of said discrete reflector elements within a sub-array has an angle
of inclination that differs by (.DELTA..alpha.) degrees from an angle of
inclination of an adjacent discrete reflector element, and wherein each of
said sub-arrays of discrete reflector elements has an angle of inclination
that differs by (.DELTA..alpha.) degrees from an angle of inclination of
an adjacent sub-array.
15. An information storage system as set forth in claim 14 wherein
(.DELTA..alpha.) is related to a separation between adjacent reflector
elements, .DELTA.x, by
.DELTA..alpha.=.DELTA.x/2f.sub.b,
where f.sub.b is a focal length of a lens element that is interposed
between said array of reflectors and said storage medium.
16. An information storage system as set forth in claim 14 wherein said
object beam optical channel directing means is comprised of a first
acoustooptic device and a second acoustooptic device, said first and
second acoustooptic devices being disposed orthogonally with respect to
one another for controllably directing the object beam along the first
axis and along the second axis to a specified one of said storage
locations.
17. An information storage system as set forth in claim 14 wherein said
angle multiplexing means is comprised of a first acoustooptic device,
wherein said spatial multiplexing means is comprised of a second
acoustooptic device, and wherein said first and second acoustooptic
devices of said reference beam optical channel are disposed orthogonally
with respect to one another.
18. An information storage system as set forth in claim 14 and further
including a radiation detector array having an input that is optically
coupled to an output of said storage medium for detecting radiation output
thereby in response to an application of the reference beam to said
storage medium. |
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Claims |
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Description |
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FIELD OF THE INVENTION
This invention relates generally to, optical storage systems and, in
particular, to random access optical storage systems that store volume
holograms.
BACKGROUND OF THE INVENTION
The optical storage and retrieval of information in holographic form has
the potential to provide very high storage densities. Furthermore, by
storing a given unit of information in a redundant fashion within a
storage medium, there is provided a tolerance against a loss of
information in any one given location within the storage medium. Also, a
potential exists to store a large number of units of information at a
given storage location, thereby further increasing storage density.
As can be appreciated, the realization of these various aspects of an
optical information storage and retrieval system relies to a great degree
upon the optical components and, in particular, upon the mechanisms for
accurately scanning object and reference optical beams onto the storage
medium. Typically, both beams are used simultaneously in order to store an
information-encoded interference pattern into the storage medium. For
information retrieval, only the reference beam is employed, in conjunction
with a radiation detector array.
One conventional approach employs a rotating crystal to vary an angle of
incidence of the reference optical beam upon the storage medium. This
technique, known as angle multiplexing, enables a plurality of
interference patterns to be stored within a region of the storage medium.
The use of a rotating crystal implies that a mechanical assembly be
employed. However, for a number of reasons the use of mechanical
components is undesirable. For example, mechanical components generally
require a significant amount of power to operate, occupy a significant
amount of space, and may present both a repeatability and a reliability
problem.
In an article entitled "Storage of 500 high-resolution holograms in a
LiNbO.sub.3 crystal", Optics Letters, Vol 16, No. 8 (Apr. 15, 1991), F. H.
Mok, M. C. Tackitt, and H. M. Stoll describe the recording (at room
temperature) of as many as 500 high-resolution, uniformly diffracting
volume holograms in a single Fe-doped LiNbO.sub.3 crystal. The holograms
were stored by angularly multiplexing a plane-wave reference beam. The
incidence angle of the reference beam was changed by using an optics
assembly having a mirror mounted on a rotation stepper motor and a 1X
telescope focussed at infinity.
It is an object of this invention to provide an optical information storage
system that employs a fixed array of reflectors to spatially scan an angle
multiplexed reference beam onto a holographic storage medium.
A further object of the invention is to provide a beam steering apparatus
for positioning a reference optical beam at a prescribed location and with
a prescribed angle of incidence upon a holographic storage medium.
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome and the objects of the
invention are realized by a spatial multiplexer for use with a holographic
storage medium, and by an information storage system employing the spatial
multiplexer. The spatial multiplexer directs a beam of optical radiation
onto the storage medium, the storage medium having (n) rows and (m)
columns of storage locations. The spatial multiplexer includes an array of
(n times m) discrete reflector elements each having a linear shape. The
discrete reflector elements are disposed in a parallel arrangement with
one another upon a substrate, and each has a different, unique angular
orientation with respect to the storage medium.
The array is organized as (m) sub-arrays of discrete reflector elements,
each of the sub-arrays including (n) discrete reflector elements. Each of
the discrete reflector elements within a sub-array has an angle of
inclination that differs by (.DELTA..alpha.) degrees from an angle of
inclination of an adjacent discrete reflector element. Furthermore, each
of the sub-arrays of discrete reflector elements has an angle of
inclination that differs by (.DELTA..alpha.) degrees from an angle of
inclination of an adjacent sub-array. The term (.DELTA..alpha.) is related
to a separation between adjacent discrete reflector elements, .DELTA.x, by
.DELTA..alpha.=.DELTA.x/2f.sub.b,
where f.sub.b is a focal length of a lens element that is interposed
between the array and the storage medium.
A separation (d) between adjacent discrete reflector elements is given by
d=f.sub.a .rarw..theta.,
where f.sub.a is a focal length of a lens element that is interposed
between the array and a plurality of acoustooptic devices that
angle-multiplex and steer the optical beam to a specified one of the
discrete reflector elements, and where .DELTA..theta. is an angular change
of the optical beam emergent from the acoustooptic modulators.
Further in accordance with the invention an information storage system
includes a storage medium having (n) storage locations disposed along a
first axis (x-axis) and (m) storage locations disposed along a second axis
(y-axis) that is orthogonal to the first axis. An object beam optical
channel includes acoustooptic devices for directing an object beam to a
specified one of the storage locations within the storage medium. The
object beam optical channel further includes a spatial light modulator for
modulating the directed object beam with information to be stored. The
system further includes a reference beam optical channel having an angle
multiplexer and a spatial multiplexer for storing a plurality of
interference patterns within a single one of the storage locations. The
angle multiplexer includes a first acoustooptic device for varying an
angle of the reference beam with respect to a point on the second axis.
The spatial multiplexer includes a second acoustooptic device for varying
an angle of the reference beam with respect to a point on the first axis.
The reference beam optical channel further includes a plurality of
reflectors each of which has an angular orientation, with respect to the
storage medium, that differs from the angular orientation of all others of
the plurality of reflectors. Each of the plurality of reflectors are
disposed for receiving the reference beam from the first and second
acoustooptic devices and for reflecting the reference beam to one of the
storage locations.
A deflection of the reference beam by the angle multiplexer changes the
angle of incidence of the reference beam on the storage medium, which
provides the multiple angles required for angle multiplexing a plurality
of superimposed holograms within a single storage location. A deflection
of the reference beam by the spatial multiplexer specifies one of the
plurality of reflectors, and thus selects one of the storage locations
upon which the angle multiplexed reference beam is incident.
BRIEF DESCRIPTION OF THE DRAWING
The above set forth and other features of the invention are made more
apparent in the ensuing Detailed Description of the Invention when read in
conjunction with the attached Drawing, wherein:
FIG. 1 is a schematic diagram of a holographic random access memory;
FIG. 2 illustrates a two dimensional rectangular grid, wherein each small
square represents a distinct storage location within a storage medium, and
wherein a large number of angle-multiplexed holograms are stored within
each location;
FIG. 3 is a schematic diagram showing an acoustooptic device (AOD)
directing an object beam to different locations within the storage medium;
FIG. 4 illustrates two elements of a mirror array, operating in conjunction
with a vertically-oriented AOD and two lenses, for redirecting light to
two distinct locations within the storage medium, wherein separations
between mirror strips of the mirror array and the storage locations are
not drawn to scale;
FIG. 5 is a schematic diagram showing three collimated beams having
different angles of incidence on the storage medium;
FIG. 6 is a simplified diagram showing a method for fabricating the mirror
strips of the mirror array;
FIG. 7a is a cross-sectional profile of a sub-array of mirror strips of the
mirror array, wherein each sub-array has (n) mirror strips; and
FIG. 7b is an elevational view of the mirror array showing (m) sub-arrays
of the (n) mirror strips of FIG. 7a, with each of the (n times m) mirror
strips having a unique orientation.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates an optical storage system 10 wherein information is
stored as volume holograms and is accessed by acoustooptic devices. The
system 10 includes an optical recording or storage medium 12 comprised of,
by example, iron doped Lithium Niobate (LiNbO.sub.3 :Fe). The storage
medium 12 may be comprised of any suitable three-dimensional holographic
storage medium, with holograms being stored at multiple locations
(spatial-multiplexing) within the storage medium 12. Individual storage
locations 12a are arranged in an (n.times.m) two-dimensional rectangular
grid, as depicted in FIG. 2, with a plurality of angle-multiplexed
holograms being stored within each distinct storage location 12a.
At present, the storage of 10.sup.3 volume holograms at a single location
of a LiNbO.sub.3 crystal has been achieved, with each hologram having a
space-bandwidth-product, SBP, of 320.times.220. This corresponds to a
total of 0.7.times.10.sup.8 bits of stored information. The system of the
invention is not limited to these storage densities however, and is
readily scalable to significantly greater storage densities.
The system 10 includes an Object Beam (OB) optical channel and a Reference
Beam (RB) optical channel. The storage medium 12 is common to both the OB
and RB channels. Acoustooptic devices (AODs) 14 and 16 provide for
scanning the OB that passes through an Acoustooptic Tunable Filter (AOTF)
24. AODs 26 and 28 provide for scanning the RB that passes through a
second AOTF 30. In a presently preferred embodiment of the invention, the
AODs 14, 16, 26 and 28 are comprised of TeO.sub.2 and are responsive to an
applied electrical signal to shift the beams passing therethrough. A
spatial light modulator (SLM) 18 functions as an input device for encoding
the OB with information to be stored, and a multi-channel read-out
detector array 20 is used as an output interface.
In accordance with an aspect of the invention the system 10 further
includes, within the RB optical channel, a mirror array 22 comprised of a
plurality of sub-arrays 22a of individual mirror elements or strips 22b.
As will be described below, the mirror array 22 may be fabricated using a
technique known for use in fabricating blazed gratings.
A plurality of lens elements (L) are provided at various positions within
the reference beam and object beam paths, and are described below where
appropriate. Although shown as simple lenses, it should be realized that
each of the lens elements may include a plurality of constituent lenses.
Spatial and angle multiplexing of the holographic storage medium 12
requires three distinct scanning mechanisms.
A first scanning mechanism directs the information-containing optical beam
from the SLM 18 to different storage locations 12a.
A second scanning mechanism directs the reference optical beam to different
ones of the storage locations 12a, and thus accomplishes spatial
multiplexing of the RB.
A third scanning mechanism controllably varies an angle of incidence of the
RB at each storage location 12a, and thus accomplishes angular (angle)
multiplexing of the RB.
These three scanning mechanisms are now described in detail.
With reference to information beam scanning; in order to store the
holograms within the storage medium 12, each "page" of input information
is first loaded onto the SLM 18. A liquid crystal television device is one
suitable embodiment for the SLM 18. The SLM 18 is illuminated by a plane
wave OB having an angle of incidence that is determined by electrical
signals applied to the pair of crossed AODs 14 and 16. AODs 14 and 16 are
disposed perpendicularly to one another, with the OB being directed
through the area of intersection. The resulting light distributions at the
SLM 18, and at a plane within the storage medium 12, constitute Fourier
transform pairs. The location 12a within the storage medium 12 where light
is focused is varied by changing the angle of illumination of the SLM 18.
That is, the crossed AODs 14 and 16 determine which location 12a within
the storage medium 12 is illuminated with the input image from the SLM 18.
FIG. 3 illustrates information beam scanning within the storage medium 12.
Let .DELTA.x be the separation between successive ones of location 12a on
the recording medium. Then,
.DELTA.x=f.sub.3 .DELTA..theta..sub.3, (1)
where .DELTA..theta..sub.3 is a change in angle of illumination of the SLM
18 required to produce .DELTA.x, and f.sub.3 is the focal length of lens
L.sub.3. The plane wave incident on the SLM 18 is the image of the plane
wave emerging from the AODs 14 and 16. As a result, .DELTA..theta..sub.3
and .DELTA..theta..sub.1, that is the corresponding change in angle of the
emerging beam at the AODs 14 and 16, are related by
.DELTA..theta..sub.3 /.DELTA..theta..sub.1 =f.sub.1 /f.sub.2,(2)
where f.sub.1 and f.sub.2 are the focal lengths of the lenses L.sub.1 and
L.sub.2, respectively. Similarly, A.sub.SLM and A.sub.AOD, the aperture of
the SLM 18 and the aperture of the AODs 14 and 16, respectively, are
related by
A.sub.AOD /A.sub.SLM =f.sub.1 /f.sub.2. (3)
The required useful apertures for the lenses are determined by the number
of distinct locations 12a on the storage medium 12. By example, if
spatially-multiplexed holograms are recorded at L locations, then A.sub.3,
the minimum useful aperture of L.sub.3, is given by
A.sub.3 .gtoreq.A.sub.SLM +(L+1).DELTA.x.
Given that
A.sub.2 /f.sub.2 =A.sub.3 /f.sub.3, and A.sub.1 /f.sub.1 =A.sub.2
/f.sub.2,(4)
then
A.sub.2 =f.sub.2 /f.sub.3 (A.sub.SLM +(L+1).DELTA.x), (5)
and
A.sub.1 =f.sub.1 /f.sub.3 (A.sub.SLM +(L+1).DELTA.x), (6)
where A.sub.2 and A.sub.1 are the minimum useful apertures of L.sub.2 and
L.sub.1, respectively.
As an example of the foregoing, for exemplary desired system 10
characteristics of:
.DELTA..theta..sub.1 =7.times.10.sup.-3 rad, (TeO.sub.2 AOD)
.lambda.=488 nm,
A.sub.SLM =20 mm,
L=16,
and
.DELTA.x=5 mm,
then
the requirements of Equations (1) through (6) may be satisfied by a set of
lenses specified as follows:
L.sub.1 :f.sub.1 =100 mm, A.sub.1 =50 mm
L.sub.2 :f.sub.2 =50 mm, A.sub.2 =30 mm
L.sub.3 :f.sub.3 =400 mm, A.sub.3 =95 mm.
Having discussed information beam scanning, a discussion of the spatial
multiplexing of the RB is now provided. The vertical AOD 26 in the RB
optical channel (FIG. 1) steers the RB to a location within the storage
medium 12 at which the input image has been directed. The vertical AOD 26
accomplishes this by deflecting the collimated RB vertically to one of the
discrete reflector elements, or mirror strips 22b, of the array 22 of (n
times m) vertically stacked mirror strips. Each of the mirror strips 22b
has a unique angular orientation with respect to the storage medium 12.
The orientation of each mirror strip 22b is prescribed so as to redirect
the incident light (both horizontally and vertically) to one of the
(n.times.m) locations within the storage medium 12. In this manner, the
deflection angle of the vertical AOD 26 specifies which mirror strip 22b
is illuminated which, in turn, selects the location of the RB on the
storage medium 12.
As can be realized, the mirror array 22 is an important component of the
system 10. The mirror array 22 is constructed such that the number of
discrete mirror strips 22b is equal to the number of distinct locations
within the storage medium 12. Thus, for (n.times.m) storage locations 12a,
there are (n times m) discrete mirror strips 22b.
As seen in FIG. 6, a presently preferred technique for fabricating the
mirror array 22 employs a technique used for fabricating blazed
diffraction gratings. This technique involves using a diamond tip 40 to
cut grooves into a coating 42 that is deposited upon a suitable substrate
44. By example, the substrate 44 may be glass and the coating 42 may be a
layer of gold. The angles of the grooves are accurately controlled by the
tilt of the diamond tip 40 with respect to the plane of the substrate 44.
The width of each groove is controlled by the number of cuts in the same
groove.
By example only, each groove is approximately 80 micrometers wide, and an
(n row times m column) element mirror array 22 includes (m) sub-arrays
22a. As seen in FIG. 7a, each sub-array 22a includes (n) mirror strips
22b. The change in angle from one mirror strip 22b to the next adjacent
one is, by example, 0.5.degree. such that a maximum mirror tilt is
(m.times.0.5.degree.). The (m) sub-arrays 22a are identical to one another
except that each sub-array 22a is formed within an associated "ramp". The
difference is ramp angles between neighboring sub-arrays 22a is also, by
example, 0.5.degree.. As a result, each mirror strip 22b of the mirror
array 22 has a unique angular orientation with respect to all others of
the mirror strips 22b, and with respect to the storage medium 12.
In general, the angular inclination of the mirror strips 22b within a
sub-array 22a is orthogonal to the angular inclination of the sub-array
22a. In FIG. 7b the sub-arrays 22a are inclined along the x-axis while the
mirror strips 22b are inclined along the y-axis. In general, for an n by m
array of storage locations, there are m sub-arrays 22a of n mirror strips
22b.
It should be realized that the angular inclination between adjacent mirror
strips 22b, and between adjacent mirror sub-arrays 22a, is a function of
the optical characteristics and orientation of the lens elements L.sub.a
and L.sub.b, and may be other than 0.5.degree..
FIG. 4 depicts two reflector elements of the mirror array 22, constructed
as described above, operating in conjunction with the VAOD 26 to redirect
light to two distinct locations. The angle of the collimated beam emerging
from the VAOD 26, in conjunction with lens L.sub.a, determines on which
mirror strip 22b the light will be focused. Each of the mirror strips 22b
is tilted, as described above, and each thus adds a different angle to the
direction of the principal ray of the reflected RB. Lens L.sub.b
collimates the diverging light to illuminate different locations on the
storage medium 12. In order for the mirror array 22 to redirect the RB to
a predetermined location, the change in angle between successive mirror
strips 22b, .DELTA..alpha., is related to the separation between
neighboring mirror strips 22b, .DELTA..alpha., by
.DELTA..alpha.=.DELTA.x/2f.sub.b, (7)
where f.sub.b is the focal length of L.sub.b. It should be noted that
.DELTA..alpha. is equal to half of the angle between principal rays
redirected by successive mirror strips 22b. The separation between mirror
strips 22b, d in FIG. 7a, is given by
d=f.sub.a .DELTA..theta., (8)
where f.sub.a is the focal length of lens L.sub.a and .DELTA..theta. is the
corresponding angular change of the emergent beam from the VAOD 26. The
width of each mirror strip 22b, which is nominally set equal to d, is
preferably at least large enough to accommodate the spot size of the
focused light. Therefore,
d.gtoreq..lambda.f.sub.a /A, (9)
where A is the beam diameter at the VAOD 26.
As an example, when using a 200 MHz TeO.sub.2 VAOD 26 having a 0.5 cm
aperture, .DELTA..theta.=2.8=10.sup.-4. Selecting f.sub.a =300 mm and
f.sub.b =300 mm satisfies the requirements expressed in Equations (7)
through (9). For this case, d=84 micrometers and
.DELTA..alpha.=0.5.degree.. For this example, it is assumed that
.DELTA.x=5 mm.
Having described the spatial multiplexing of the RB a discussion is now
made of angle multiplexing of the RB. The horizontal AOD (HAOD) 28 causes
the angle of the RB to change on the storage medium 12, without also
changing the location of the RB. This is accomplished, as depicted in FIG.
5, as follows. The light deflected by the HAOD 28 remains on the same
mirror strip 22b so long as the vertical deflection remains constant. It
should be noted that since the mirror array 22 is in the Fourier plane of
the storage medium 12, the position of the RB on the storage medium 12
remains unchanged. The deflection by the HAOD 28 changes the angle of
incidence of the RB on the storage medium 12, which provides the multiple
angles required for angle multiplexing superimposed holograms. In this
manner, the VAOD 26, via the mirror strip array 22, selects a location of
the RB on the storage medium 12, whereas the HAOD 28 selects an angle of
incidence of the RB.
Recording of information within the storage medium 12 is accomplished by
illuminating a certain location on the storage medium 12 simultaneously
with both the RB and the OB. The angular and spatial address of the
hologram to be recorded is selected by the AODs 14, 16, 26, and 28 in the
OB and RB optical channels, as described above. In that the AODs introduce
Doppler frequency shifts which, in turn, may cause drift in the
interference pattern to be recorded, the AOTFs 24 and 30 are employed to
compensate for undesired frequency shifts.
Each stored SLM 18 page of information is associated with a unique RB,
characterized by an angle of incidence and a spatial location. Any one of
the stored pages may be accessed by illuminating the storage medium 12
with the appropriate RB. This readout RB is generated by the same
components that generate the record RB. The Fourier transform of the page
associated with the RB is thus reconstructed. The output detector array
20, which is positioned at the Fourier plane of the storage medium 12,
registers the image of the reconstructed page of information. A pulsed
laser is preferably used as the readout RB light source. So long as
sufficient photons are delivered in a compressed time slot, the access
time of the recoding medium 12 is minimized. Firing of the RB laser is
synchronized with the launching of the acoustic waves within the AODs 26
and 28, and the readout laser is charged to full firing potential while
data is read from the detector array 20.
The page access time of the system 10 is equal to the time required for the
AODs 26 and 28 to redirect the RB. As was noted above, the AODs are, in a
presently preferred embodiment of the invention, comprised of TeO.sub.2.
This material is both efficient and provides large deflection angles. The
acoustic transit time, the limiting factor for access time, for a 5 mm
aperture is typically less than 10 microseconds.
The data readout rate of the system 10 is determined by the data transfer
rate of the detector array 20. By example, a single readout channel CCD
can output data at up to 20 MHz, and CCDs with multiple (64) readout
channels can support a readout rate of more than one gigabit per second.
The number of photo-generated electrons registered by the detector 20,
N.sub.e, is governed by: the optical energy per pulse available in the RB,
E.sub.r ; the quantum efficiency of the detector 20, .eta..sub.D ; the
diffraction efficiencies of the holograms, .eta..sub.h ; and the overall
diffraction efficiency of the TeO.sub.2 AODs 26 and 28 in the RB optical
path, .eta..sub.A. The number of photo-generated electrons can be
determined from the following relationship:
N.sub.e =(.eta..sub.h .eta..sub.A .eta..sub.D E.sub.r)/3ev,(10)
where the energy per photon at 488 nm is approximately 3 electron volts
(ev). A typical value of .eta..sub.A is 10%; .eta..sub.D may be as high as
80%; and, for 10.sup.3 holograms stored within one location, .eta..sub.h
is 10.sup.-4 . If each hologram contains 10.sup.6 bits of data, and a
readout laser pulse with 1 mJ (milli-Joule) of energy is employed, then
the number of photo-generated electrons within each pixel of the detector
array 30 is greater than 10.sup.3. This number of electrons is well above
the noise floor of many available CCD detectors arrays.
Iron doped Lithium Niobate (LiNbO.sub.3 :Fe) is a presently preferred
material for the storage medium 12, in that LiNbO.sub.3 :Fe is well
characterized, exhibits a non-volatile nature (after fixing), and also
exhibits a long, dark-storage time (10.sup.5 years). These characteristics
make LiNbO.sub.3 :Fe a desirable material for archival storage.
Applications that require occasional or no updating, e.g. map storage and
templates used for high-resolution and high-speed graphics, are well
suited for use with a recording medium 12 comprised of LiNbO.sub.3 :Fe.
Using a 1 Watt laser, the recording speed of the system 10, when using
LiNbO.sub.3 :Fe as the storage medium 12, is approximately 10.sup.6
bits/sec. Using a 10 W laser increases the recording speed to 10.sup.7
bits/sec. Proper selection of the oxidation/reduction ratio of the iron
concentration in LiNbO.sub.3 :Fe is expected to increase the
photo-sensitivity, and to provide a recording speed approaching 10.sup.8
bits/sec.
Although described in the context of a presently preferred embodiment of
the invention, it will be understood by those skilled in the art that
changes in form and details may be made therein without departing from the
scope and spirit of the invention. For example, any suitable
three-dimensional recording medium (photorefractive or other) can be
employed with little or no modification. Also by example, the SLM 18 may
be other than the liquid crystal television described above.
As such, the teaching of the invention is not to be construed to be limited
to only the presently preferred embodiment described above, but is instead
intended to be given a scope commensurate with the scope of the claims
that follow.
* * * * *
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