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Claims  |
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I claim:
1. A holographic digital data processing system in which data may be stored
and retrieved in a sequential manner comprising
a source of a beam of coherent energy,
a beam divider for dividing the beam of coherent energy into a reference
beam and a data beam,
a modulator for impressing a bit of digital data on at least one of the
beams of coherent energy,
a first beam deflector which deflects the data beam by a first preselected
angle selected from a first matrix array of angles,
means for by-passing the first beam deflector with the reference beam to
establish a unique spatial relationship between the data beam and the
reference beam,
a second beam deflector which deflects both the data beam and the reference
beam by a second preselected angle selected from a second matrix array of
angles,
a recording medium in which a hologram may be formed,
means for focussing the data beam and the reference beam into the recording
medium so that the data beam and the reference beam interfere and form a
hologram of the bit of digital data in the recording medium, the second
preselected angle determining the position of the hologram in the
recording medium and the first preselected angle determining the angle of
intersection of the data beam and the reference beam at the recording
medium whereby the hologram of the bit of digital data has a unique
address within the recording medium and may be stored in the recording
medium in a sequential manner,
means for selectively occluding the reference beam before reaching the
recording medium so that only the data beam is directed onto the recording
medium at the hologram of the bit of digital data thereby forming an image
of the single bit of digital data beyond the recording medium at a fixed
location defined by the reference beam when the hologram is recorded, and
a photodetector located at the image of the bit of digital data so that the
image bit of digital data located at a particular address in the recording
medium may be retrieved individually in a sequential manner.
2. A holographic digital data processing system according to claim 1,
wherein the means for selectively occluding the reference beam is located
in the reference beam before the second beam deflector so that during
retrieval both beam deflectors affect only the data beam.
3. A holographic digital data processing system according to claim 1,
wherein the means for selectively occluding the reference beam is a second
electro-optic modulator.
4. A holographic digital data processing system according to claim 3,
wherein the second electro-optic modulator interrupts the reference beam
before the reference beam by-passes the first beam deflector.
5. A holographic digital data processing system according to claim 4,
wherein the recording medium is a permanent recording medium thereby
forming a read-only memory.
6. A holographic digital data processing system according to claim 4,
wherein the recording medium is erasable and wherein there is further
included means for erasing digital data stored in the recording medium.
7. A holographic digital data processing system according to claim 4,
wherein the first and second beam deflectors are acousto-optic beam
deflectors.
8. A holographic digital data processing system according to claim 7,
wherein there is further included means for collimating the data beam and
the reference beam prior to entrance into the second beam deflector.
9. A holographic digital data processing system according to claim 8,
wherein the means for by-passing is a plurality of reflective elements for
diverting the reference beam around the first beam deflector.
10. A holographic digital data processing system according to claim 4,
wherein the source of a beam of coherent energy is a laser.
11. A holographic digital data processing system according to claim 1,
wherein the beam of coherent energy is modulated prior to division thereby
having both the data beam and the reference beam modulated.
12. A method for holographically processing digital data comprising the
steps of
producing a beam of coherent energy,
dividing the beam of coherent energy into a reference beam and a data beam,
modulating at least one of the beams of coherent energy with a bit of
digital data,
deflecting the data beam by a first preselected angle selected from a first
matrix array of angles,
maintaining the reference beam at a second preselected angle to establish a
unique spatial relationship between the data beam and the reference beam,
deflecting both the data beam and the reference beam by a third preselected
angle selected from a second matrix array of angles,
focussing the data beam and the reference beam onto a recording medium so
that the beams interfere and form a hologram of the bit of digital data in
the recording medium, the third preselected angle determining the position
of the hologram in the recording medium and the first and second
preselected angles determining the angle of intersection of the data beam
and the reference beam at the recording medium whereby the hologram of the
bit of digital data has a unique address within the recording medium and
may be stored in a sequential manner,
selectively occluding the reference beam before reaching the recording
medium so that only the data beam is directed onto the recording medium at
the hologram of the bit of digital data thereby forming an image of the
single bit of digital data beyond the recording medium at a fixed location
defined by the reference beam when the hologram is recorded, and
detecting the bit of digital data at the image so that the unique bit of
digital data located at a particular address in the recording medium may
be retrieved individually in a sequential manner.
13. A method for holographically processing digital data according to claim
12, wherein the modulation step occurs prior to the beam division step so
that the bit of digital data is impressed on both the data beam and the
reference beam.
14. A method holographically processing digital data according to claim 13,
wherein there is further included the step of
collimating the data and reference beam prior to the deflection by the
third preselected angle.
15. A holographic digital data read-out system for sequentially retrieving
bits of digital data stored in a holographic memory, the holographic
memory having a plurality of pages of digital data stored in separate
areas thereof and having the individual data bits comprising each page
superimposed in an area through variance of the direction of intersection
of a first interfering beam for each data bit at formation, the direction
of intersection of the other interfering beam being constant for each page
and intersecting an image plane at a constant location, the system
comprising:
a source of a beam of coherent energy,
a first beam deflector which deflects the beam of coherent energy by a
first preselected angle selected from a first matrix array of angles,
a second beam deflector which deflects the beam of coherent energy by a
second preselected angle selected from a second matrix array of angles,
means for focussing the beam of coherent energy into the holographic
memory, the second preselected angle causing the beam of coherent energy
to intersect the holographic memory at the location of the page of digital
data to be interrogated and the first preselected angle causing the beam
of coherent energy to intersect the holographic memory at the direction of
intersection of the first interfering beam at formation for an individual
digital data bit in the page which is desired to be retrieved, and
a photodetector located in the image plane at the location of intersection
of the other interfering beam during formation so that the imaged bit of
digital data located at a particular address in the holographic memory may
be retrieved individually in a sequential manner. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention is related generally to an improved method and apparatus for
storing and retrieving digital data utilizing holograms and more
specifically, to a novel and improved method and apparatus for
sequentially recording and retrieving individual bits of digital data in
and from such holographic memories.
The use of holograms as a medium for storing digital data is well known in
the art. A conventional holographic data memory is formed by initially
arranging the data to be stored in a planar array. This array is composed
initially as a "page" of data and is placed in an intermediate temporary
storage device commonly referred to as a "page composer". The page
composer is usually an array of cells, each of which may be made either
opaque or transparent according to whether a binary 0 or 1 is to be stored
at that particular cell address. After the entire page of data has been
formed, it is illuminated with laser light and holographically recorded on
a holographic storage medium.
Several formats for recording a large number of such pages in a single
hologram memory have been reported. In one such form, a page array of
small normally transparent cells is composed. This page is then recorded
holographically on a photographic medium as a single hologram by
illuminating the transparent page composer with laser light and focussing
the transmitted laser light onto a small area of the recording medium
where it is caused to interfere with a reference beam. Following the
recording of the first page, another page of digital data is composed in
the page composer, and likewise recorded by interfering with the reference
beam but in another area of the recording medium. This process is
continued until either all of the data is recorded or space in the
recording medium is exhausted.
Another format, which is less commonly employed, involves the use of the
angular selectivity inherent in the recording of so-called Lippmann-Bragg
volume holograms. Such holograms are formed throughout the volume of a
thick recording medium instead of on the surface of a thin planar
recording medium. In a volume holographic recording, each page of data is
superimposed upon a number of others in the same volume of the recording
medium; however, with each exposure, the reference beam is incident on the
medium from a different recording angle.
During the playback or read-out of data from holograms formed according to
the former methods, a reference laser beam is employed. In the previously
described first format, the reference beam is directed onto a selected
small area including the desired hologram with its page array of data. An
image of the original page composer with its array of digital data in the
form of light and dark spots for that page is reconstructed. The
reconstructed page image is formed in a detection plane where an array of
photodetectors is placed to interrogate each individual data bit.
The projected data array is read-out electronically with the photodetectors
which sense the presence or absence of light at each bit position in the
imaged array. The read-out of a page from a Lippmann-Bragg volume
holographic memory is performed in a similar manner. A reference beam is
directed into the volume hologram from the specific angle used to record
that particular page and the resulting data array is imaged and
reconstructed for electronic detection. Other pages in the volume hologram
memory are accessed by orienting the reference beam at different angles
associated with those pages during recording.
These prior art methods for storing and retrieving data present certain
disadvantages. For example, to be practical, a page of data must include a
large number of individual bits of digital data. The parallel recording of
an entire such page of digital data involves the simultaneous illumination
of all of the data bits in the page composer with a common data beam. Such
simultaneous illumination creates intermodulation noise which is caused by
the interference of rays from individual data bits at the recording
medium. During read-out, such noise results in a flare of diffracted light
for which special care must be taken to keep the flare from creating
detection interference on the detector plane. Failure to take such
precautions causes serious signal-to-noise problems. In order to avoid
flare problems, the readout involves using reference beams at relatively
large angles from the data beams to prevent the flare from falling
directly or being scattered onto the detector plane. Since no data bits
can be recorded within a minimum solid angle around the reference beam
where the flare is located, that minimum part of the storage capacity of
the holographic recording medium around the reference beam is essentially
wasted.
Another disadvantage of the prior art systems involves the requirement that
a full page of data must be temporarily stored to enable an optical
version to be composed followed by subsequent storage on the holographic
recording medium.
Another disadvantage arises because a substantial amount of laser beam
power is wasted through losses encountered with the page composer which is
opaque in many places such as the areas where zeroes are stored, the areas
between the data, unused addresses and border areas. Consequently, much of
the laser light used to illuminate the page composer is not transmitted
and recorded, but is wasted by absorption. Hence, for any given laser beam
power that may be available, the time to store data will be significantly
longer than in comparison with a storage method with which all of the
laser power could be used to record the data. The only manner in which
storage time can be reduced is to increase the laser illumination power.
However, given the present state of the art in lasers, such an increase
can only be accomplished at significant increases in cost, both for
operation and equipment.
The page composer required in prior art holographic data memory devices
involves a complex structure utilizing many individually addressable
electro-optic light valve cells, commonly of the order of 10,000 or more,
to achieve practical densities in the storage medium. Such a page composer
is extremely complex to design and to produce; it thus represents a
generally undesirable component in a hologram memory storage system.
A further disadvantage occurs during read-out and has two aspects. This
disadvantage is that the entire page of digital data is presented
simultaneously to the read-out. If, for example, a page of data is a
matrix of 10,000 bits, a like number of photodetectors and their
associated circuitry are required. Obviously, such a device is complex and
costly in design and construction and low in reliability. Furthermore,
unless the read-out is directly integrated into a computer main frame, the
data cannot be utilized at a speed even close to that at which it is
available. Normally, the data is utilized in a sequential manner. Also, in
many instances, only one or a few bits of data from a page are actually
required at a specific time. At such times, generation of all data stored
in that page creates much waste.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a novel
method and apparatus for storing and retrieving digital data in a
holographic digital data processing system.
It is a second object of the invention to provide such a novel method and
apparatus which do not require composition of pages of digital data or
read-out from an array of images constituting a page of digital data.
It is another object of the present invention to provide such an apparatus
which is a significant reduction in complexity from prior art systems.
Briefly, the invention in its broadest aspect is a holographic digital data
processing system in which data may be stored and retrieved in a
sequential manner. The system includes a source of a beam of coherent
energy. A beam divider splits the beam of coherent energy into a reference
beam and a data beam. A modulator then impresses a bit of digital data on
at least one of the beams of coherent energy. A first beam deflector
deflects the data beam by a first preselected angle selected from a first
matrix array of angles. Means are provided for by-passing the first beam
deflector with the reference beam to establish a unique spatial
relationship between the data beam and the reference beam. A second beam
deflector deflects both the data beam and the reference beam by a second
preselected angle selected from a second matrix array of angles. A
recording medium is included in which a hologram may be formed. A means
focusses the data beam and the reference beam into the recording medium so
that the data beam and the reference beam interfere and form a hologram of
the bit of digital data in the recording medium. The second preselected
angle determines the position of the hologram in the recording medium and
the first preselected angle determines the angle of intersection of the
data beam and the reference beam at the recording medium whereby the
hologram of the bit of digital data has a unique address within the
recording medium and may be stored in the recording medium in a sequential
manner. A means selectively occludes the reference beam before reaching
the recording medium so that the data beam is directed onto the recording
medium at the hologram of a stored bit of digital data thereby forming an
image of the single bit of digital data beyond the recording medium at a
fixed location which is defined by the reference beam when the hologram is
recorded. Finally, a photodetector is located at the image of the bit of
digital data so that the image of the bit of digital data stored at a
particular address in the recording medium may be retrieved individually
in a sequential manner.
In a holographic memory apparatus in accordance with the invention, a
sequential storage technique is used to directly input one data bit at a
time into the hologram memory at an appropriate address in the recording
medium as the bit arrives at an input to the system. In this manner, a
page composer is eliminated and the full laser beam power is available to
record the data. A holographic memory system in accordance with the
invention provides high speed recording with practically high data storage
densities for an efficient operation.
Each individual data bit in a page is recorded holographically with a
preselected recording angle between the data and reference beams. Hence,
since the reference beam orientation is constant for a page, there exists
for each data bit in a page a particular intersection angle of the data
beam at the recording medium. This intersection angle serves as the
address information for the bits in a page. The plurality of data bits
within a page are recorded on the medium by superimposing the bits upon
each other to form a localized multiple exposure synthetic page hologram.
An entire array of such pages of holograms may thus be conveniently
recorded on the medium at respectively different locations.
To retrieve any selected bit of digital data from the holographic memory,
the reference beam is first occluded. The data beam is then directed by
the two beam deflectors to the address of the bit in the memory. An image
of the bit is thereby reconstructed at the normal intersection of the
reference beam and an image plane. A photodetector located at that
intersection senses the bit of digital data.
These and other objects, advantages and features of the invention will be
apparent from the following detailed description of the preferred
embodiments taken together with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
In the Drawing:
FIG. 1 is a perspective, partially schematic illustration of a holographic
digital data processing system according to the present invention having
sequential data storage and retrieval;
FIG. 2 is a side schematic illustration of the holographic memory shown in
FIG. 1; the system being shown in the WRITE mode of operation;
FIG. 3 is a similar schematic illustration of the holographic memory as
shown in FIGS. 1 and 2, but being shown in the READ mode of operation; and
FIG. 4 is a schematic view of an alternative form for the beam deflection
portion of the holographic memory shown in FIGS. 1 through 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In referring to the various figures of the drawing hereinbelow, like
reference numerals will be used to refer to identical parts of the
apparatus.
Referring initially to the embodiment shown schematically in FIGS. 1
through 3, a holographic digital data processing system in accordance with
the invention is indicated generally by the reference numeral 10. The
holographic digital data processing system 10 includes a laser 16 or other
source of coherent light. The laser source 16 may operate in the visible,
infrared or ultraviolet portions of the electromagnetic spectrum,
depending upon the spectral sensitivities of a storage medium 18 located
in a recording plane 20 and of a photodetector 108 disposed in a detection
plane 24.
The output of the laser 16 is a laser beam 26 which is aligned along an
optical axis 27. The laser beam 26 encounters an electro-optic data
modulator 28 which intensity modulates the beam 26 in accordance with a
bit of digital data to be stored in the storage medium 18. A data input
signal source 30 provides modulation signals to the modulator 28 on lines
32 and 33 to intensity modulate the laser beam 26. The modulation signals
from data input signal source 30 result in the formation of zeroes and
ones by, for example, respectively inhibiting and allowing the passage of
the laser beam 26 through the modulator 28.
A modulated laser beam 29 from the modulator 28 is incident on a beam
splitter 54 where it is divided into a reference beam 14 and a data beam
12 with both beams being modulated with the bit of digital data. The data
beam 12 is applied to a data beam deflector 37 which deflects the data
beam 12 according to "x" deflection signals on lines 38 and 39 and "y"
deflection signals on lines 40 and 41 which are generated by a data
address control source 36. It should be noted here that the optical axis
27 of the system 10 is aligned nominally parallel with the "z" axis 42 of
an orthogonal coordinate system as shown in FIG. 1. The recording plane 20
and the detection plane 24 are each normal to the optical axis 27 and
parallel to the "x" and "y" axes 44 and 46.
The first or data beam deflector 37 is capable of deflecting the data beam
12 through a multitude of discrete angles selected from a matrix array of
angles. The matrix array of angles is defined by a pair of orthogonally
imposed beam deflection signals, these signals being applied in the "x"
and "y" directions. Each of the respective deflection signals can be
applied in a plurality of discrete levels, each level corresponding to a
discrete angle of data beam deflection in a direction parallel to the "x"
and "y" axes. Two stages of beam deflectors 48 and 50 are combined in data
beam deflector 37 to deflect the data beam 12 in the "y" and "x"
directions respectively. Therefore, by combining the effects of the beam
deflector stages 48 and 50, data beam deflection to a discrete angular
position within a defined solid angle is achieved.
Preferably, the data beam deflector 37 is an electro-optic device; however,
other means by which the deflection may be accomplished including tilting
mirrors are intended to be included within the purview of the invention.
The discrete angular deflection of the data beam 12 is selected to
establish a unique spatial relationship between the data beam and the
reference beam which corresponds with a desired angular relationship
between the reference beam 14 and the data beam 12 during their
intersection in the recording plane 20. The data deflection control
signals on lines 38, 39, 40 and 41 are synchronized to the modulation of
the laser beam 26, as suggested by a sync line 52, so that each bit is
stored with a distinct angular relationship between the data and reference
beams 12 and 14.
The angular deflection of the data beam 12 may be controlled, for example,
to any of 500 discrete angular positions in each of the "x" and "y"
directions. Thus, a total angular array of 250,000 possible bit positions
can be achieved for each recorded page of digital data. The specific
deflector employed is not critical so long as each discrete deflection
angle is the same for all of the rays within the data beam 12. The angle
of intersection .alpha., between the data and reference beams 12 and 14 is
thus an angle whose magnitude and rotational orientation are dependent
upon the relative magnitudes of the "x" and "y" deflections of the data
beam 12 imposed by the data beam deflector 37.
The reference beam 14, in this embodiment, is directed by the beam splitter
54 and a pair of reflectors 56 and 58 past the data deflector 37 such that
after the last reflection, the reference beam 14 appears to emerge or
travel away from the intersection 60 of the optical axis 27 with the
center of the scan of the data beam deflector 37. In this manner, both the
reference and data beams 12 and 14 appear to diverge from a common fixed
origin 60 (see FIG. 2) located within the data beam deflector 37 and on
the optical axis 27. Although reflectors 56 and 58 are used in the
preferred embodiment, any other means for by-passing the first beam
deflector 37 with the reference beam 14 is also included in the purview of
the invention.
In addition, the reference beam 14 passes through a reference beam switch
61. The switch 61 is located optically in that section of the apparatus
between the beam divider 54 and the intersection of the data and reference
beams 12 and 14 at the recording plane 20. The function of the reference
beam switch 61 will be discussed more fully hereinbelow.
The deflected data beam 12 and the reference beam 14 are directed onto a
first lens 62, which is aligned on the optical axis 27. The lens 62 is so
placed as to intercept these two beams at a distance of one focal length
f.sub.1 from the center of scan 60 of the data beam deflector 37. The lens
62 serves to collimate both of the beams 12 and 14 with their relative
unique spatial relationship previously defined as the angle therebetween
prior to the lens 62, and now expressed by the dimension "d", their
relative position being determinative of the discrete recording or
intersection angle .alpha. of the data and reference beams 12 and 14 at
the recording plane 20.
The parallel data and reference beams 12 and 14 are applied to a second
beam or page deflector 64. The page deflector 64, which functions in an
identical manner as the data beam deflector 37, however, which may be of
differing construction, such as acousto-optic devices, also provides
identical two dimensional deflection of both of the beams 12 and 14 in the
"x" and "y" directions to correspond with the desired location of the page
of digital data in the array of holograms to be recorded in the recording
plane 20. The deflection signals for the page deflector 64 are obtained
from a page address control network 66 which determines, such as under
control from a data processor, where the page of digital data is to be
recorded in the recording medium 18. Here again, the deflection angle
imposed by the page deflector 64 is selectable from a second matrix array
of angles. The second matrix array of angles is defined in the same manner
as the first matrix of angles described above.
The parallel entry of the data and reference beams 12 and 14 into the page
deflector 64 accommodates the requirement for some deflectors such as
acousto-optic deflectors that the light beams to be deflected enter
parallel to each other. For other types of deflectors, the beam position
parameters may be changed over an extensive range as determined by the
optical designer who may wish to optimize his design for a specific
application and with particular components in mind.
The lens 62 directs the reference and data beams 12 and 14 parallel to each
other and the optical axis 27 of the lens 62. The rays which constitute
the individual beams 12 and 14 converge to a focussed spot at a distance
of one focal length beyond the lens 62. However, since diffraction effects
render the rays of a laser beam parallel at the focal point, the focal
length f.sub.1 for the lens 62 is made sufficiently long to provide a
region of essentially parallel rays. If the particular type of deflector
used as the page deflector 64 requires parallelism of the rays passing
therethrough, the page deflector 64 should be located at the focal point
of the lens 62. FIG. 1 illustrates the distribution or spacing of the data
and reference beams 12 and 14 at the entrance plane 68 to the page
deflector 64. In this entrance plane 68, which is perpendicular to the
optical axis 27 and preferably located one focal length beyond the first
lens 62, the reference beam 14 arrives at a fixed location, such as 70,
while the position or location 72 where the focussed data beam 12 arrives
is determined by the specific deflection imposed by the data beam
deflector 37.
For example, with a data beam deflector 37 as described above, there is an
array of 500 by 500 possible discrete data positions in the entrance plane
68 through which the data beam 12 may pass. The area 74 through which the
data beam 12 may pass is outlined in the form of a square with dashed
lines 76.
The array of positions enclosed by the lines 76 corresponds to the data
"page" which may be stored in the holographic memory, i.e., the recording
medium 18. The area 74 differs from the conventional previously described
"page composer" in that there is no physical object necessary for
temporary storage of the digital data. Rather, the complete set of
possible data light beam positions in the area 74 effectively constitutes
the page of digital data bits.
In order to direct the light from the virtual page plane 68 onto a specific
small area in the memory recording plane 20, the page deflector 64 is
placed with its center of deflection 78 on the optical axis 27 and at the
entrance plane 68. A page address signal from the page address control
network 66 causes the page deflector 64 to deflect equally both the data
and reference beams 12 and 14 into a direction where a second lens 80 can
cause the beams to intersect and interfere at a desired data storage area,
such as 82, in the recording medium 18.
The size of the illuminated area 82 is a function of the focal length
f.sub.2 of the second lens 80 and its position relative to the page
deflector 64 and the storage plane 20 where the recording medium 18 is
situated. Since the data and reference beams 12 and 14 are parallel to
each other in this embodiment, they are brought together by the second
lens 80 to intersect in the lens' back focal plane, i.e., at one focal
length f.sub.2 behind plane of the lens 80 so that, independent of the
deflection direction imposed by the page deflector 64, the beams 12 and 14
intersect somewhere on the storage plane which is centrally disposed in
the recording medium 18. The location of the intersection of the beams 12
and 14 in the storage medium 18 is dependent upon the deflections
introduced by the page deflector 64, while the angle of intersection is
determined by the deflection imposed by the data beam deflector 37.
The storage medium 18 may be formed of any material which responds to
exposure to light by yielding a change in its optical density or optical
path length, e.g., by changing its index of refraction, and in which the
pattern created due to one exposure is added to, but not erased by
succeeding exposures since thousands of individual bit hologram exposures
may be superimposed to synthesize a single page of digital data. These
qualities are possessed by the majority of storage media used
conventionally for holograms. For nonerasible, read-only memories, these
qualities are possessed by ordinary photographic film and other permanent
recording media.
While the foregoing description has included an electro-optic modulator 28
before the beam divider 54, the modulation may be imposed on either or
both of the data or reference beams 12 and 14 after division. For example,
the modulator 28 can be eliminated and the reference beam switch 61 used
to modulate the reference beam 14.
Referring now to FIG. 3, there is shown the holographic digital data
processing system 10 operating in the READ mode. The reference beam switch
61 is operated to turn off the reference beam 14 leaving only the data
beam 12 passing through the apparatus. The reference beam switch 61 may be
any means for occluding the reference beam 14 and preferably is an
electro-optic modulator which is synchronized with the data address
control network 36 and the page address control network 66 so that the
reference beam is blocked whenever retrieval of digital data stored in the
recording medium 18 is desired. Simultaneously, the modulator 28 is
commanded to pass the laser beam 26 completely. Therefore, only an
unmodulated data beam 12 remains traversing the apparatus.
Since when an individual data bit hologram is formed a unique path is
established for the data beam, that path may be utilized for illuminating
that individual data bit hologram during retrieval of that data bit. In
other words, if the data beam deflector 37 is given a specific bit address
by the data address control network 36 and the page address control
network 66 supplies a specific page address to the page deflector 64, the
data beam is directed along the identical path that was utilized for the
data beam 12 during formation of the individual data bit hologram.
While an individual data bit hologram address is illuminated with the data
beam, the reference beam 14 is reconstructed if a bit of digital data is
stored at that address, i.e., if one of the component interference
patterns of the multiple exposure page hologram was formed with the data
beam at that particular angle. If a single photodetector 108 is placed in
the image plane 24 at the intersection of that reconstructed reference
beam 14, that photodetector 108 may be utilized to sense the presence or
absence of a bit of digital data at that particular address. If no bit was
stored in the address corresponding to the interrogating data read-out
beam, then no component of the hologram pattern is capable of
reconstructing the reference beam 14, and the detector 108 placed at the
reference beam position in the detector plane 24 reads "zero". If a bit
has been recorded there, the detector signal indicates a "one", thereby
completing the requisite binary code. Of course, the code could be
reversed at will.
Because of the construction of the holographic digital data processing
system 10, in particular the fact that the reference beam 14 always
intersects the entrance plane 68 to the page deflector 64 in precisely the
same location, and regardless of the deflection of the reference beam 14
by the page deflector 64, the intersection of the reference beam 14 with
the image plane 24 is constant. Therefore, regardless of which data bit
hologram address is illuminated by the data beam 12, if a bit is present
the reference beam 14 is reconstructed and can impinge on the single
photodetector 108. By interrogating in rapid succession any sequence of
page bit address combinations, a sequence of electronic signals is derived
from the single detector 108 corresponding to the data that was recorded
at each interrogated address.
Although this random access, direct read-out technique is specifically
advantageous in a system where the holograms were recorded by the random
access, direct input technique described above, it can also be used to
read-out data from a hologram memory data storage plane recorded by
conventional methods using the two-step, parallel input, page composer
techniques. The reason is that the essential data-storing components of a
hologram are the same in form, whether the data is stored in parallel or
sequentially.
In the foregoing description of the holographic digital data processing
system 10, the beam deflectors 37 and 64 are each represented by a pair of
boxes, one assigned to each dimension of scanning. The rays of the beams
are shown at the same positions at both the input and output planes of the
deflectors while differing only by the angle of propagation. This is a
schematic representation which does not take into account any one
particular type of deflector. Most beam deflectors at least approximately
meet, for small maximum deflection angles, the general requirement that
the page array imaged at the center of deflection reproduces an
essentially stationary image as projected by the second lens 80 onto the
detector plane 24. This requirement assures that the image is always
projected with the detector 108 in the same place relative to the position
of the imaged data bit. When large angles of deflection are to be employed
or required, compensating optical elements may have to be incorporated to
stabilize the image. One may adopt special deflection techniques whereby
the deflection angles for each bit address can be made different for each
page deflection angle, the difference in angles being programmed into the
deflector's electronic circuitry.
The reference and data laser beams 12 and 14 are selected to provide
sufficiently long coherence lengths for mutual coherence at their
intersection at the data storage medium 18. In the event that a laser is
employed which has a relatively short coherence length, conventional
path-length equalization techniques can be used to insure that the beams
are coherent at the recording medium 18.
The lenses 62 and 80 are indicated in FIG. 1 as simple, single element
lenses. In practice, these lenses may be much more complex and may consist
of several elements depending upon the design requirements of the specific
holographic recording and retrieval apparatus 10.
It should be noted that although the angle of deflection or the entrance
areas of the deflectors 37 and 64 must be considerably larger for the page
deflector 64 than for the data beam deflector 37, the angular deflection
repeatability of both of the deflectors 37 and 64 are of the same order of
magnitude.
Several modifications to system 10 as shown in FIG. 1 may be made within
the scope of the invention. For example, FIG. 4 illustrates the
substitution of two smaller lenses 102 and 104 for the first lens 62 of
FIG. 1. The lens 102 is aligned to pass the reference beam 14 while the
other lens 104 is located so as to pass the data beam 12. The use of a
pair of lenses such as 102 and 104 facilitates the lens design and
significantly increases the angular separation between the reference and
data beams when such separation would benefit the design for specific
applications.
During operation of the holographic digital data processing system 10, when
data bits are to be entered into the holographic memory 18, data signals
are applied to the modulator 28 to turn the laser beam 26 on or off
depending upon the code being transmitted. At the same time, an address
for each data bit in a page is selected with the data address signals
being applied to the data beam deflector 37. A page address signal is
applied to the page deflector 64 so that each data bit is recorded with a
distinct recording of intersection angle between the data and reference
beams 12 and 14 and at a page location such as 82.
The power of the laser beam 26, the data input rate and the time for each
exposure of the data storage medium 18 are selected so that the total
energy per data bit exposure is approximately equal to the optimum
exposure level for the selected recording medium 18. The appropriate laser
parameters are further selected on the basis of obtaining a bright
reconstructed image for read-out by the detector 108.
Since the embodiment of the holographic digital data processing system 10
in FIG. 1 directs the reference beam 14 to the same detector location 108,
the photodetector at that location can also be used to monitor the
modulation of the beams. The photodetector 108 may then be used during
recording as a feedback error signal generator to maintain the proper
modulation as suggested by a line 110 coupled to the data input signal
network 30.
The recording medium 18 may require chemical processing steps (such as with
photographic film) to complete the recording process. In such case the
removal and return of the medium 18 to the system 10 requires precise
registration to preserve alignment of the reconstructed data bits with the
photodetector 108 in the detector plane 24. Other materials for the
recording medium 18 such as lithium niobate can be read-out immediately.
Still other materials may also permit the erasure of data such as by
illumination by a high intensity laser, electrical stimulation from a
network 112 or other processes.
Having described a holographic data recording and playback system according
to the invention, the various advantages thereof can be appreciated. The
requirement of an intermediate page composer has b | | |
