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ABSTRACT 2. PROPERTIES OF HOLOGRAMS
This paper provides an overview of holographic memory, a A hologram is a block or sheet of photosensitive material which
developing three-dimensional data storage system for computers. records the diffraction of two light sources. To create a hologram,
In order to relay more in-depth concepts of holographic memory, laser light is first split into two beams, a source beam and a
a brief section on the physics of holography is presented first. reference beam. The source beam is then manipulated and sent
Following the background information, methods of modifying into the photosensitive material. Once inside this material, it
holograms to capture meaningful computer data are covered. intersects the reference beam and the resulting diffraction of laser
Various methods of multiplexing are also presented, each section light is recorded on the photosensitive material, resulting in a
covering the potentials and problems of their implementations. hologram. Once a hologram is recorded, it can be viewed with
Brief discussions of error correction, interfacing with I/O systems, only the reference beam. The reference beam is projected into the
and future applications are also provided. This paper will explain hologram at the exact angle it was projected during recording.
how holographic memory could revolutionize next generation When this light hits the recorded diffraction pattern, the source
computers by providing storage for terabytes of information at beam is regenerated out of the refracted light. An exact copy of
access times of gigabits per second. the source beam is sent out of the hologram and can be read by
optical sensors. For example, a hologram that can be obtained
Keywords from a toy store illustrates this idea. Precise laser equipment is
used at the factory to create the hologram. A recording material
Holographic Memory, Optical Memory, Spatial Multiplexing,
which can recreate recorded images out of natural light is used so
the consumer does not need high-tech equipment to view the
information stored in the hologram. Natural light becomes the
1. INTRODUCTION reference beam and human eyes become the optical sensors.
Although conventional storage methods adapt to the growing
needs of computer systems, they are reaching their fundamental
limits. Often improvements made to these storage methods 3. APPLICATION TO BINARY
decrease access times or reduce the size of stored bits, but the In order for holographic technology to be applied to computer
design of such systems is based on serial access, or reading in a systems, it must store data in a form that a computer can
one dimensional streams of bits. Conventional storage also relies recognize. In current computer systems, this form is binary. In
on mechanical devices to retrieve data, such as the arm which the previous section, it was mentioned that the source beam is
passes over magnetic platters in a hard drive. As computer manipulated. In common holograms, this manipulation is the
systems continue to become faster, they will need a way to access creation of an optical image such as a ball or human face. In
larger amounts of data in shorter periods of time. This paper computer applications, this manipulation is in the form of bits.
provides a description of holographic memory, a three- The next section explains the spatial light modulator, a device that
dimensional data storage system which has fundamental converts laser light into binary data.
advantages over conventional read/write memory systems.
3.1 Spatial Light Modulator (SLM)
A brief overview of the properties of holograms will be presented A spatial light modulator is used for creating binary information
first. This will cover the way in which data can be stored in a out of laser light. The SLM is a 2D plane, consisting of pixels
hologram with the diffraction of laser light. Applying these which can be turned on and off to create binary 1’s and 0’s. An
properties to computer memory systems will follow, including the illustration of this is a window and a window shade. It is possible
description of page data, a method in which another dimension is to pull the shade down over a window to block incoming sunlight.
added to the accessing of stored data. Various methods of If sunlight is desired again, the shade can be raised. A spatial
multiplexing are covered as well. Multiplexing is the method of light modulator contains a two-dimensional array of “windows”
manipulating the position of a hologram to fit multiple pages of which are only microns wide. These windows block some parts
data in the same volume. Error correction and applications to of the incoming laser light and let other parts go through. The
computer systems are then covered, with the future of holographic resulting cross section of the laser beam is a two dimensional
memory presented as a conclusion. array of binary data, exactly the same as what was represented in
the SLM. After the laser beam is manipulated, it is sent into the
Permission is granted to make copies of this document for personal or hologram to be recorded. This data is written into the hologram
classroom use. Copies are not to be made or distributed for profit or commercial as page form. It is called this due to its representation as a two
purposes. To copy otherwise, or in any way publish this material, requires written
dimensional plane, or page, of data.
4.2 Wavelength Multiplexing
Used mainly in conjunction with other multiplexing methods,
wavelength multiplexing alters the wavelength of source and
reference beams between recordings. Sending beams to the same
point of origin in the recording medium at different wavelengths
allows multiple pages of data to be recorded. Due to the small
tuning range of lasers, however, this form of multiplexing is
limited on its own.
4.3 Spatial Multiplexing
Spatial multiplexing is the method of changing the point of entry
Figure 1. A Spatial Light Modulator implemented with a of source and reference beams into the recording medium. This
LCD panel(from ) form tends to break away from the non-mechanical paradigm
because either the medium or recording beams must be physically
moved. Like wavelength multiplexing, this is combined with
3.2 Page Data Access other forms of multiplexing to maximize the amount of data
Because data is stored as page data in a hologram, the retrieval of stored in the holographic volume. Two commonly used forms of
this data must also be in this form. Page data access is the method spatial multiplexing are peristrophic multiplexing and shift
of reading stored data in sheets, not serially as in conventional multiplexing.
storage systems. It was mentioned in the introduction that
conventional storage was reaching its fundamental limits. One
such limit is the way data is read in streams. Holographic
4.3.1 Peristrophic Multiplexing
This form of spatial multiplexing rotates the recording medium as
memory reads data in the form of pages instead. For example, if a
the light source beams remain in fixed positions. For instance,
stream of 32 bits is sent to a processing unit by a conventional
a holographic cube could be rotated so each of its six sides could
read head, a holographic memory system would in turn send 32 x
take in a source beam. This would provide six times the number
32 bits, or 1024 bits due to its added dimension. This provides
of pages which could be stored in the volume.
very fast access times in volumes far greater than serial access
methods. The volume could be one Megabit per page using a
Certain problems arise when implementing this method of
SLM resolution of 1024 x 1024 bits at 15-20 microns per pixel.
multiplexing. The rotational axes needs to be positioned in a way
which does not interfere with the laser beams. As with all spatial
4. MULTIPLEXING multiplexing, bringing the recording media back to its original
Once one can store a page of bits in a hologram, an interface to a position for data retrieval would need to be very precise. This is
computer can be made. The problem arises, however, that storing much easier to maintain when the media remains static.
only one page of bits is not beneficial. Fortunately, the properties
of holograms provide a unique solution to this dilemma. Unlike
magnetic storage mechanisms which store data on their surface,
holographic memories store information throughout their whole
volume. After a page of data is recorded in the hologram, a small
modification to the source beam before it reenters the hologram
will record another page of data in the same volume. This method
of storing multiple pages of data in the hologram is called
multiplexing. The thicker the volume becomes, the smaller the
modifications to the source beam can be.
4.1 Angular Multiplexing
When a reference beam recreates the source beam, it needs to be
at the same angle it was during recording. A very small alteration
in this angle will make the regenerated source beam disappear.
Harnessing this property, angular multiplexing changes the angle
of the source beam by very minuscule amounts after each page of Figure 2. A simple example of angular multiplexing. A beam
data is recorded (see figure 2). Depending on the sensitivity of steering device is used to modulate the angle of incidence of
the recording material, thousands of pages of data can be stored in the reference beam. Notice the three different angles that the
the same hologram, at the same point of laser beam entry. beam enters the recording medium.(from )
Staying away from conventional data access systems which move
mechanical matter to obtain data, the angle of entry on the source 4.3.2 Shift Multiplexing
beam can be deflected by high-frequency sound waves in Shift multiplexing alters the point of entry on one surface of the
solids. The elimination of mechanical access methods reduces recording media. The recording optics or media could be
access times from milliseconds to microseconds. repositioned to allow the source beam to enter multiple positions
on a surface. Depending on the size of the laser beam and the
sensitivity of the recording media, the points of entry the source bleeds into places where light was meant to be blocked out. Areas
beam takes into it can be immense. This form of multiplexing where zero light is desired might have minuscule amounts of laser
combined with peristrophic multiplexing could cover a very large light present which mutates its bit representation. For example, if
percentage of the hologram. too much light gets recorded into this zero area representing a
binary 0, an erroneous change to a binary 1 might occur. Changes
in both the quality of the laser beam and recording material are
being researched, but these improvements must take into
consideration the cost-effectiveness of a holographic memory
system. These limitations to current laser beam and
photosensitive technology are some of the main factors for the
delay of practical holographic memory systems.
5.2 Page-Level Parity Bits
Once error-free data is recorded into a hologram, methods which
read data back out of it need to be error free as well. Data in page
format requires a new way to provide error control. Current error
control methods concentrate on a stream of bits. Because page
data is in the form of a two dimensional array, error correction
needs to take into account the extra dimension of bits. When a
Figure 3. Shift multiplexing illustration. The original point of page of data is written to the holographic media, the page is
entry of the source beam is denoted at A. The holographic separated into smaller two dimensional arrays. These sub sections
cube is shifted along B and the new point of entry is at C. are appended with an additional row and column of bits. The
added bits calculate the parity of each row and column of data.
4.4 Phase-Encoded Multiplexing An odd number of bits in a row or column create a parity bit of 1
The form of multiplexing farthest away from using mechanical and an even number of bits create a 0. A parity bit where the row
means to record many pages in the same volume of a holograph is and column meet is also created which is called an overall parity
called phase-encoded multiplexing. Rather than manipulate the bit. The sub sections are rejoined and sent to the holographic
angle of entry of a laser beam or rotate/translate the recording medium for recording.
medium, phase-encoded multiplexing changes the phase of
individual parts of a reference beam. The main reference beam is
split up into many smaller partial beams which cover the same
area as the original reference beam. These smaller “beamlets”
vary by phase which changes the state of the reference beam as a
whole. The reference beams intersects the source beam and
records the diffraction relative to the different phases of the
beamlets. The phase of the beamlets can be changed by non-
mechanical means, therefore speeding up access times.
4.5 Combining Multiplexing Methods
No single multiplexing method by itself is the best way to pack a
hologram full of information. The true power of multiplexing is
brought out in the combination of one or more methods. Hybrid
wavelength and angular multiplexing systems have been tested
and the results are promising. Recent tests have also been formed
on spatial multiplexing methods which create a hologram the size
of a compact disc, but which hold 500 times more data.
5. ERROR CORRECTION Figure 4. Example of a 3x3 array of input bits(a), appended
It is inevitable that storing massive amounts of data in a small with parity bits(b), returned with an error(c), and located by
volume will be error prone. Factors exist in both the recording the check bits(d).(from )
and retrieval of information which will be covered in the
following subsections, respectively. In order for holographic When data is read back from storage, another row and column are
memory systems to be practical in next generation computer added called parity check bits. Because the row of parity bits
systems, a reliable form of error control needs to be created. evens out the data, the addition or subtraction of a bit of stored
data will cause two of the parity check bits to become a one. The
overall parity check bit becomes a one and the place of error is
5.1 Recording Errors calculated. The calculation occurs by finding where the column
When data is recorded in a holographic medium, certain factors parity check bit and the row parity check bit meet up in the
can lead to erroneously recorded data. One major factor is the original data. This erroneous bit is flipped and the data is read out
electronic “noise” generated by laser beams. When a laser beam error free. If there happens to be two or more errors in the
is split up ( for example, through a SLM ), the generated light
original data, the overall parity check bit becomes a zero and the in more than memory systems, but the theoretical schematics do
page is re-read. exist for such a machine. Optical storage such as holographic
memory provide a viable solution to the extreme amount of data
which is required for petaflop computing.
Like error control, the I/O interface to modern computer systems
needs to be tailored to data retrieval in page format. Bits are no 8. CONCLUSION
longer read from a stream, they are sent to the computer as sheets. The future of holographic memory is very promising. The page
Clearly the I/O interface needs to be changed to accommodate for access of data that holographic memory creates will provide a
this. One of the problems with such large amounts of data window into next generation computing by adding another
being fed to a processor is that the incoming data may exceed the dimension to stored data. Finding holograms in personal
processor’s throughput. This is where interfacing needs to bridge computers might be a bit longer off, however. The large cost of
the data in a coherent fashion between memory and processor. In high-tech optical equipment would make small-scale systems
the following subsections, two kinds of interfacing are covered implemented with holographic memory impractical. Holographic
which vary in a unique way. memory will most likely be used in next generation super
computers where cost is not as much of an issue. Current
magnetic storage devices remain far more cost effective than any
6.1 Smart Interfacing other medium on the market. As computer systems evolve, it is
Smart interfacing is a method of controlling the way data is sent to
not unreasonable to believe that magnetic storage will continue to
the processor from holographic memory by a pre-defined set of
do so. As mentioned earlier, however, these improvements are
logical commands. These logical commands come from
not made on the conceptual level. The current storage in a
outside the stored memory and are provided to control the way
personal computer operates on the same principles used in the first
data is managed before going to the processor. An example of
magnetic data storage devices. The parallel nature of holographic
these pre-defined instructions are the fixed set of rules used by
memory has many potential gains on serial storage methods.
error detection and correction. Because these rules stay the same
However, many advances in optical technology and
throughout memory retrieval, they can be hard coded into the
photosensitive materials need to be made before we find
smart interfacing agent.
holograms in computer systems.
6.2 Intelligent Interfacing 9. REFERENCES
Seemingly the same as smart interfacing by name, intelligent
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to transform incoming data in a non-static manner. These signals
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incoming data in a meaningful way. For example, a data Holographic Storage for Large Relational Databases, Optical
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data which is not a part of the pattern being searched for.
Intelligent interfacing agents can contain the functionality of  P. A. Mitkas and L. J. Irakliotis. Three-Dimensional
smart interfaces such as error control, but have the added feature Optical Storage for Database Processing Optical Memory and
of dynamically changing the way data passes through it. Neural Networks, Volume 3, Number 2, 1994
 When Silicon Hits Its Limits, What’s Next? / Creating
7. POSSIBLE APPLICATIONS Holographic Storage Byte Magazine, April 1996
There are many possible applications of holographic memory.
Holographic memory systems can potentially provide the high-
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speed transfers and large volumes of future computer systems.
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One possible application is data mining. Data mining is the
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process of finding patterns in large amounts of data. Data mining
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is used greatly in large databases which hold possible patterns
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amount of data. Some current computer systems implement data
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mining, but the mass amount of storage required is pushing the
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Retrieval at Rome Laboratory Proceedings of the 5th NASA
access times and data storage capacity that holographic memory
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provides could exceed conventional storage and speed up data
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Holographic Memory Design for Petaflop Computing
Another possible application of holographic memory is in petaflop Proceedings of HTMT meeting, July, 1998, Princeton
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amounts of data provided by holographic memory systems could there yet? [Online]
be utilized in a petaflop architecture. Clearly advances are needed
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OE-Reports Number 187, July 1999