gyan vihar school of engineering

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Holographic Data Storage

CSE Dept, GNDEC, Bidar Page 1

CHAPTER-1

INTRODUCTIONWith its omnipresent computers, all connected via the Internet, the Information Age has

led to an explosion of information available to users. The decreasing cost of storing data, and the

increasing storage capacities of the same small device footprint, has been key enablers of this

revolution. While current storage needs are being met, storage technologies must continue to

improve in order to keep pace with the rapidly increasing demand.

Devices that use light to store and read data have been the backbone of data storage for

nearly two decades. Compact discs revolutionized data storage in the early 1980s, allowing multi-

megabytes of data to be stored on a disc that has a diameter of a mere 12 centimeters and a

thickness of about 1.2 millimeters. In 1997, an improved version of the CD, called a digital

versatile disc (DVD), was released, which enabled the storage of full-length movies on a single

disc.

CDs and DVDs are the primary data storage methods for music, software, personal

computing and video. A CD can hold 783 megabytes of data. A double-sided, double-layer DVD

can hold 15.9 GB of data, which is about eight hours of movies. These conventional storage

mediums meet today's storage needs, but storage technologies have to evolve to keep pace with

increasing consumer demand. CDs, DVDs and magnetic storage all store bits of information on

the surface of a recording medium. In order to increase storage capabilities, scientists are now

working on a new optical storage method called holographic memory that will go beneath the

surface and use the volume of the recording medium for storage, instead of only the surface area.

Three-dimensional data storage will be able to store more information in a smaller space and offer

faster data transfer times.

Holographic memory is developing technology that has promised to revolutionaries the

storage systems. It can store data up to 1 Tb in a sugar cube sized crystal. Data from more than

1000 CDs can fit into a holographic memory System. Most of the computer hard drives available

today can hold only 10 to 40 GB of data, a small fraction of what holographic memory system can

hold. Conventional memories use only the surface to store the data. But holographic data storage

systems use the volume to store data. It has more advantages than conventional storages.


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HOLOGRAMS

A hologram is a recording of the optical interference pattern that forms at the intersection of two

coherent optical beams. Typically, light from a single laser is split into two paths, the signal path

and the reference path. The beam that propagates along the signal path carries information,

whereas the reference is designed to be simple to reproduce. A common reference beam is aplane

wave: a light beam that propagates without converging or diverging. The two paths are overlapped

on the holographic medium and the interference pattern between the two beams is recorded. A key

property of this interferometric recording is that when it is illuminated by a readout beam, the

signal beam is reproduced. In effect, some of the light is diffracted from the readout beam to

reconstruct a weak copy of the signal beam. If the signal beam was created by reflecting light

off a 3D object, then the reconstructed hologram makes the 3D object appear behind the

holographic medium. When the hologram is recorded in a thin material, the readout beam can

differ from the reference beam used for recording and the scene will still appear.


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CHAPTER 2

HOLOGRAPHIC MEMORY LAYOUT

Fig 2.1


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2.11 Hardware for holographic data storage

Fig2.11

Figure shows the most important hardware components in a holographic storage system: the SLM

used to imprint data on the object beam, two lenses for imaging the data onto a matched detector

array, a storage material for recording volume holograms, and a reference beam intersecting the

object beam in the material. What is not shown in Figure is the laser source, beam-forming optics

for collimating the laser beam, beam splitters for dividing the laser beam into two parts, stages for

aligning the SLM and detector array, shutters for blocking the two beams when needed, and wave

plates for controlling polarization.

Assuming that holograms will be angle-multiplexed (superimposed yet accessed

independently within the same volume by changing the incidence angle of the reference beam), a

beam-steering system directs the reference beam to the storage material. Wavelength multiplexing

has some advantages over angle-multiplexing, but the fast tunable laser sources at visible

wavelengths that would be needed do not yet exist.

The optical system shown in Figure, with two lenses separated by the sum of their focal

lengths, is called the 4-f configuration, since the SLM and detector array turn out to be four

focal lengths apart. Other imaging systems such as the Fresnel configuration (where a single lens

satisfies the imaging condition between SLM and detector array) can also be used, but the 4-f

system allows the high numerical apertures (large ray angles) needed for high density. In addition,

since each lens takes a spatial Fourier transform in two dimensions, the hologram stores the

Fourier transform of the SLM data, which is then Fourier transformed again upon readout by the

second lens. This has several advantages: Point defects on the storage material do not lead to lost


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bits, but result in a slight loss in signal-to-noise ratio at all pixels; and the storage material can be

removed and replaced in an offset position, yet the data can still be reconstructed correctly. In

addition, the Fourier transform properties of the 4-f system lead to the parallel optical search

capabilities offered by holographic associative retrieval. The disadvantages of the Fourier

transform geometry come from the uneven distribution of intensity in the shared focal plane of the

two lenses, which we discuss in the axicon section below.

2.2Holographic digital data storage testers

In order to study the recording physics, materials, and systems issues of holographic digital data

storage in depth, we have built three precision holographic recording testers. Each of these

platforms is built around the basic design shown in above figure, implementing mapping of single

SLM pixels to single detector pixels using precision optics in the object beam, and angle

multiplexing in the reference beam. In addition, care has been taken in the design and assembly of

the components listed above but not shown in figure, in order to allow experimental access to a

wide range of holographic data storage parameters with minimal instrumental contributions to the

raw error rate. The three testers, described in the following sections, are called the PRISM tester,

the DEMON I platform, and the DEMON II platform.


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The PRISM tester, built as part of the DARPA Photo Refractive Information Storage Materials

consortium, was designed to allow the rigorous evaluation of a wide variety of holographic

storage materials. This tester was designed for extremely low-baseline BER performance,

flexibility with regard to sample geometry, and high stability for both long recording exposures

and experimental repeatability. The salient features of the PRISM tester are shown in Figure.

Fig2.2

The SLM is a chrome-on-glass mask, while the detector array is a lowframe- rate, 16-bit-per-pixel

CCD camera. Custom optics of long focal length (89 mm) provide pixel matching over data pages

as large as one million pixels, or one megapel. A pair of precision rotation stages directs the

reference beam, which is originally below the incoming object beam, to the same horizontal plane

as the object beam. By rotating the outer stage twice as far as the inner, the reference-beam angle

can be chosen from the entire 360-degree angle range, with a repeatability and accuracy of

approximately one microradian. (Note, however, that over two 30-degree-wide segments within

this range, the reference-beam optics occlude some part of the objectbeam path.) The storage

material is suspended from a three-legged tower designed for interferometric stability (better than

0.1 mm) over time periods of many seconds. The secondary optics occupy approximately 2 feet by

4 feet of optical table space, and the tower and stages approximately 4 feet by 4 feet. The system

signal processing techniques. The reference/object-beam geometry was restricted to the 90-degree

geometry, and the reference beam deflected with a galvanometrically actuated mirror through a

simple 4-f system, limiting the variation of the angle to 610 degrees. A transmissive liquid crystal

SLM, capable of displaying arbitrary data patterns, was pixel-matched onto a small, 60-Hz CCD

camera in two stages. First, a precision five element zoom lens demagnified the SLM (640 3 480


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pixels with 42-mm pitch) to an intermediate image plane (same pixel count on 18-mm pitch).

Then a set of Fourier lenses identical to those in the PRISM imaged this plane 1:1 onto the

detector array (640 3 480 pixels, but 9-mm pitch). Because of the finer pitch on the CCD, only the

central 320 3 240 field of the SLM was detected. To implement true pixel matching, the detector

was aligned so that light from each SLM pixel fell squarely on a single detector pixel (thus

ignoring three of every four pixels on the CCD). Laser light from the green 514.5-nm line of an

argon-ion laser was delivered to the platform with a single-mode polarization-preserving optical

fiber, which produces a clean Gaussian intensity profile apparatus prior to the object/reference

beamsplitter was as much as 400 mW. Simple linear stages move the SLM in two axes and the

CCD in three axes for alignment. The entire system, not including the laser, occupies 18 3 24

inches of optical table space.

The first experiment performed on the DEMON I tester was the demonstration of multiple

hologram storage at low raw BER (BER without error correction) using modulation codes, which

allow decoding over smaller pixel blocks than the global thresholding described above. Using an

8-mm-thick LiNbO3:Fe crystal storage medium and a strong modulation code (8:12), 1200

holograms were superimposed and read back in rapid succession with extremely low raw BER (,2

3 1028) . In addition, the DEMON I platform has been used to implement both associative

retrieval and phase-conjugate readout, as described below.


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DEMON II

The DEMON II holographic storage platform, shown in Figure 7, was designed to achieve high-

density holographic data storage using short-focal-length optics, while including aspects of the

previous two test platforms. DEMON II combines the large data pages of the PRISM tester with

the dynamic SLM and the 90-degree geometry configuration of the DEMON I platform. Here, the

SLM is a reflective device fabricated by IBM Yorktown, containing 1024 x 1024 pixels and

illuminated via a polarizing beam splitter cube. A novel apodizer, described in the next section,

provides uniform illumination over the entire data page without sacrificing input power. The

magnification from the 12.8-mm pitch of the SLM pixels to the 12-mm pitch of the 41-Hz CCD

camera (1024 x 1024 pixels, 41 frames per second) is built into the Fourier optics (effective focal

length 30 mm). A pair of scan lenses provides an improved relay of the reference beam from the

galvanometrically actuated mirror to the LiNbO3 crystal, providing diffraction-limited

performance over an angular scan range of 615 degrees.

The laser light is provided by a diode-pumped solidstate laser (532 nm, doubled Nd-YAG);

waveplates and polarizing beamsplitters provide control over the power in the reference beam and

object beam. The use of two separate elements in the back Fourier lens (between the storage

material M

and detect more than two brightness levels per pixel, it is possible to have more than one bit of

data per pixel. The histogram of a hologram with six gray-scale levels made possible by the

predistortion technique is shown in Figure 13. To encode and decode these gray-scale data pages,

we also developed several local-thresholding methods and balanced modulation codes.

If pixels take one of g brightness levels, each pixel can convey log2 g bits of data. The total

amount of stored information per page has increased, so gray-scale encoding appears to produce a

straightforward improvement in both capacity and readout rate. However, gray scale also divides

the systems signal-to-noise ratio (SNR) into g - 1 part, one for each transition between

brightness levels. Because total SNR depends on the number of holograms, dividing the SNR for

gray scale (while requiring the same error rate) leads to a reduction in the number of holograms

that can be stored. The gain in bits per pixel must then outweigh this reduction in stored

holograms to increase the total capacity in bits.


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Capacity estimation

To quantify the overall storage capacity of different grayscale encoding options, we developed an

experimental capacity-estimation technique. In this technique, the dependence of raw BER on

readout power is first measured experimentally. A typical curve is shown in Figure 14(a). The

capacity-estimation technique then produces the relationship betweenM, the number of holograms

that can be stored, and raw BER [Figure 14(b)]. Without the capacity-estimation technique,

producing Figure 14(b) would require an exhaustive series of multiple hologram experiments.

In general, as the raw BER of the system increases, the number of holograms,M, increases slowly.

In order to maintain a low user BER (say, 10212) as this raw- BER operating point increases, the

redundancy of the ECC code must increase. Thus, while the number of holograms increases, the

ECC code rate decreases. These two opposing trends create an optimal raw BER, at which the

user capacity is maximized. For the ReedSolomon ECC codes we commonly use, this optimal

raw BER is approximately 1023. By computing these maximum capacities for binary data pages

and grayscale data pages from g = 2 to g = 6, we were able to show that gray-scale holographicdata pages provide an advantage over binary encoding in both capacity and readout rate. The use

of three gray levels offered a 30% increase in both capacity and readout rate over conventional

binary data pages.


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Associative retrieval

As mentioned in the Introduction, volume

holographic data storage conventionally

implies that data imprinted on an object

beam will be stored volumetrically [Figure

2,2a)], to be read out at some later time by

illumination with an addressing reference beam [Figure 2,2a ]. However, the same hologram (the

interference pattern between a reference beam and a data-bearing object beam) can also be

illuminated by the object beam [Figure 2.2c]. This reconstructs all of the angle-multiplexed

reference beams that were used to record data pages into the volume. The amount of power

diffracted into each output beam is proportional to the 2D cross -correlation between the input

data page (being displayed on the SLM) and the

stored data page (previously recorded with that

particular reference beam). Each set of output beams

can be focused onto a detector array, so that each

beam forms its own correlation peak. Because both

the input and output lenses perform a two-dimensional

Fourier transform in spatial coordinates, the optical

system is essentially multiplying the Fourier

transforms of the search page and each data page and

then taking the Fourier transform of this product (thus

implementing the convolution theorem

optically). Because of the volume nature of the

hologram, only a single slice through the 2D

correlation function is produced (the other dimension

has been

used already, providing the ability to correlate

against multiple templates simultaneously).

The center of each correlation peak represents the 2D

inner product (the simple overlap) between the input

page being presented to the system and the associated stored page. If the patterns which compose

Holographic data storage system: (a) two coherent beams,

one carrying a page of information, interfere within a

photosensitive material to record a hologram. (b)

Illuminating the hologram with the reference beam

reconstructs a weak copy of the original information-

bearing beam for capture with a detector array. (c)Illuminating multiple stored holograms with a new page

of search information reconstructs all of the reference

beams, computing in parallel the correlation between the

search data and each of the stored pages.


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these pages correspond to the various data fields of a database, and each stored page represents a

data record, the optical correlation process has just simultaneously compared the entire database

against the search argument . This parallelism gives content-addressable holographic data storage

an inherent speed advantage over a conventional serial search, especially for large databases. For

instance, if an un-indexed conventional retrieve-from-disk-and compare software-based

database is limited only by sustained hard-disk readout rate (25 MB/s), a search over one million 1

KB records would take ~40 s. In comparison, with off-the-shelf, video-rate SLM and CCDtechnology,

Two-color, photon-gated holography provides a promising solution to the long-standing problem

of destructive readout in read/write digital holographic storage. In lithium niobate, optimization of

the sensitivity requires control over stoichiometry (or doping), degree of reduction, temperature,

gating wavelength, and gating intensity. Two-color materials differ fundamentally from one-color

materials in that the dynamic range or M# can be increased by using higher writing intensity, and

the sensitivity can be increased with higher gating intensity. Another route to increasing the M#

would be to find a material which exhibits a two-color erase process. Substantial progress has

been made in recent years in the field of two-color holography, and further progress can be

expected on this complex and challenging problem.


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CHAPTER-3

HOLOGRAPHIC VERSATILE DISC

An HVD (holographic Versatile Disc), a holographic storage media, is an advanced optical disc

thats presently in the development stage. Polaroid scientist J. van Heerden was the first to come

up with the idea for holographic three-dimensional storage media in 1960. An HVD would be a

successor to todays Blu-ray and HD-DVD technologies. It can transfer data at the rate of 1

Gigabit per second. The technology permits over 10 kilobits of data to be written and read in

parallel with a single flash. The disc will store upto 3.9 terabyte (TB) of data on a single optical

disk. Holographic data storage, a potential next generation storage technology, offers both high

storage density and fast readout rate. In this article, I discuss the physical origin of these attractive

technology features and the components and engineering required to realize them. I conclude by

describing the current state of holographic storage research and development efforts in the context

of ongoing improvement to established storage technologies.

BRIEF HISTORY

Although holography was conceived in the late 1940s, it was not considered a potential

storage technology until the development of the laser in the 1960s. The resulting rapid

development of holography for displaying 3-D images led researchers to realize that holograms

could also store data at a volumetric density of as much as 1/ where is the wave-length of the light

beam used. Since each data page is retrieved by an array of photo detectors, rather than bi-by-bit,

the holographic scheme promises fast readout rates as well as high density. If a thousand

holograms, each containing a million pixels, could be retrieved every second, for instance, then

the output data rate would reach 1 Gigabit per second.In the early 1990s, interest in volume-holographic data storage was rekindled by the

availability of devices that could display and detect 2-D pages, including charge coupled devices

(CCD), complementary metal-oxide semiconductor (CMOS) detector chips and small liquid-

crystal panels. The wide availability of these devices was made possible by the commercial

success of digital camera and video projectors. With these components in hand, holographic-

storages researchers have begun to demonstrate the potential of their technology in the laboratory.


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CHAPTER-4

FEATURES

Data transfer rate: 1 gbps.The technology permits over 10 kilobits of data to be written and read in parallel with a

single flash.

Most optical storage devices, such as a standard CD saves one bit per pulse.HVDs manage to store 60,000 bits per pulse in the same place.1 HVD5800 CDs830 DVD160 BLU-RAY Discs.


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CHAPTER-5

HVD STRUCTURE

HVD structure is shown in fig 3.1 the following components are used in HVD.

1. Green writing/reading laser (650 nm).

2. Red positioning/addressing laser (650 nm).

3. Hologram (data).

4. Polycarbon layer.

5. Photopolymeric layer (data-containing layer).

6. Distance layers.

7. Dichroic layer (reflecting green light).

8. Aluminum reflective layer (reflecting red light).

9. Transparent base.

10. PIT.

Fig:5.1


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Fig:5.2(Hvd write system)


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RECORDING DATA

To read the data from an HVD, you need to retrieve the light pattern stored in the

hologram.

In the HVD read system, the laser projects a light beam onto the holograma light beam -

- a light beam that is identical to the reference beam.

An advantage of a holographic memory system is that an entire page of data can be

retrieved quickly and at one time. In order to retrieve and reconstruct the holographic page of data

stored in the crystal, the reference beam is shined into the crystal at exactly the same angle at

which it entered to store that page of data. Each page of data is stored in a different area of the

crystal, based on the angle at which the reference beam strikes it.

The key component of any holographic data storage system is the angle at which the

reference beam is fired at the crystal to retrieve a page of data. It must match the original reference

beam angle exactly. A difference of just a thousandth of a millimeter will result in failure to

retrieve that page of data.

During reconstruction, the beam will be diffracted by the crystal to allow the recreation of

the original page that was stored. This reconstructed page is then projected onto the CMOS, whichinterprets and forwards the digital information to a computer.


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Fig:5.6 HVD Read System


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MORE ON HVD

High Storage capacity of 3.9 terabyte (TB) enables user to store large amount of data.

Records one program while watching another on the disc.

Edit or reorder programs recorded on the disc.

Automatically search for an empty space on the disc to avoid recording over a program.

Users will be able to connect to the Internet and instantly download subtitles and other

interactive movie features

Backward compatible: Supports CDs and DVDs also.

The transfer rate of HVD is up to 1 gigabyte (GB) per second which is 40 times faster than

DVD.

An HVD stores and retrieves an entire page of data, approximately 60,000 bitsof

information, in one pulse of light, while a DVD stores and retrieves one bitof data in one

pulse of light.


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CHAPTER-6

ADVANTAGES OF HDSS

With three-dimensional recording and parallel data readout, holographic memories can outperform

existing optical storage techniques. In contrast to the currently available storage strategies,

holographic mass memory simultaneously offers high data capacity and short data access time

(Storage capacity of about 1TB/cc and data transfer rate of 1 billion bits/second).

Holographic data storage has the unique ability to locate similar features stored within a crystal

instantly. A data pattern projected into a crystal from the top searches thousands of stored

holograms in parallel. The holograms diffract the incoming light out of the side of the crystal, with

the brightest outgoing beams identifying the address of the data that most closely resemble the

input pattern. This parallel search capability is an inherent property of holographic data storage and

allows a database to be searched by content.

Because the interference patter ns are spread uniformly throughout the material, it endows

holographic storage with another useful capability: high reliability. While a defect in the medium

for disk or tape storage might garble critical data, a defect in a holographic medium doesn't wipe

out information. Instead, it only makes the hologram dimmer. No rotation of medium is required as

in the case of other storage devices. It can reduce threat of piracy since holograms cant be easily

replicated.


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DISADVANTAGES OF HDSS

Manufacturing cost HDSS is very high and there is a lack of availability of resources which

are needed to produce HDSS. However, all the holograms appear dimmer because their patterns

must share the material's finite dynamic range. In other words, the additional holograms alter a

material that can support only a fixed amount of change. Ultimately, the images become so dim

that noise creeps into the read-out operation, thus limiting the material's storage capacity.

A difficulty with the HDSS technology had been the destructive readout. The re-

illuminated reference beam used to retrieve the recorded information also excites the donor

electrons and disturbs the equilibrium of the space charge field in a manner that produces a gradual

erasure of the recording. In the past, this has limited the number of reads that can be made before

the signal-to -noise ratio becomes too low. Moreover, writes in the same fashion can degrade

previous writes in the same region of the medium. This restricts the ability to use the three-

dimensional capacity of a photorefractive for recording angle-multiplexed holograms. You would

be unable to locate the data if theres an error of even a thousandth of an inch.


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Chapter-7

COMPARISION

Parameters DVD BLU-RAY HVD

Capacity 4.7 GB 25 GB 3.9 TB

Laser wave length

650 nm

(red)

405 nm

(blue)

532 nm

(green)

Disc diameter 120 mm 120 mm 120 mm

Hard coating No yes Yes

Data transfer rate

(rawdata)

11.08 mbps

36 mbps 1 gbps


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INTERESTING FACTS

It has been estimated that the books in the U.S. Library of Congress, the largest library in

the world, could be stored on Six HVDs. The pictures of every landmass on Earth - like the ones

shown in Google Earth - can be stored on two HVDs.

With MPEG4 ASP encoding, a HVD can hold anywhere between 4,600-11,900 hours of

video, which is enough for non-stop playing for a year.


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CHAPTER9

HVD AT A GLANCE

Fig9.1

Media type : Ultra-high density optical disc.

Encoding : MPEG-2, MPEG-4 AVC (H.264), and VC-1.

Capacity : Theoretically up to 3.9 TB.

Usage : Data storage, High-definition video, & he possibility of ultra High-definition

video.


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STANDARDS

On December 9, 2004 at its 88th General Assembly the standards body Ecma International

created Technical committee 44, dedicated to standardizing HVD formats based on Optwares

technology.

On June 11, 2007, TC44 published the first two HVD standards ECMA-377, defining a

200 GB HVD recordable cartridge and ECMA-378,defining a 100 GB HVD-ROM disc.

Its next stated goals are 30 GB HVD cards and submission of these standards to the

International Organization for Standardization for ISO approval.


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POSSIBLE APPLICATION FIELDS

There are many possible applications of holographic memory. Holographic memory

systems can potentially provide the high speed transfers and large volumes of future computer

system. One possible application is data mining.

Data mining is the processes of finding patterns in large amounts of data. Data mining is

used greatly in large databases which hold possible patterns which cant be distinguished by human

eyes due to the vast amount of data. Some current computer system implement data mining, but the

mass amount of storage required is pushing the limits of current data storage systems. The manyadvances in access times and data storage capacity that holographic memory provides could exceed

conventional storage and speedup data mining considerably. This would result in more located

patterns in a shorter amount of time.

Another possible application of holographic memory is inpetaflop computing. A petaflop isa thousand trillion floating point operations per second. The fast access extremely large amounts of

data provided by holographic memory could be utilized in petaflop architecture. Clearly advances

are needed to in more than memory systems, but the theoretical schematics do exist for such a

machine. Optical storage such as holographic memory provides a viable solution to the extreme

amount of data which is required for a petaflop computing.


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CONCLUSION

The information age has led to an explosion of information available to users while

current storage needs are being met,storge technology must continue to improve in order to keep

pace with the rapidly increasing demand.

However, conventional data storage technologies where individual bits are stored as

distinct magnetic or optical changes on the surface of recording medium are approaching physical

limits. Storing information throughout the volume of a medium not just on its surface-offers an

intriguing high capacity alternative. Holographic data storage is a volumetric approach, although

conserved decade ago, has made recent progress towards practicality with the appearance of lower

cost enabling technologies.HVD gives a practical way to exploit the holographic technologies to store data up to

3.9TB on a single disc. it can transfer data at the rate of 1GB per second. The technology permits

bits of data to be written and red in a parallel with a single flash. So an HVD would be a

successor to todays Blu-ray and HD-HVD technologies


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References

E. Chuang, W. Liu, J.J. Drolet, and D. Psaltis, Holographic Random Access Memory(HRAM), Proceedings of the IEEE, vol. 87, no. 11, pp. 1931-1940, 1999.

Literature review, www.entelky.com/holography/letrew.htm, 2000.

P.S. Raman jam, S. Hvilsted, and R.H. Berg, New polymer materials for erasableholographic storage,Risc National Laboratory, Solid State Physics Department, 2000.

E. Chuang, J.J. Drolet, G. Barbasthathis, W. Liu, and D. Psaltis, Compact PhaseConjugate Holographic Memory, website, 2000.


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Web references:

www.holopc.com www.wikeipedia.com www.engeeniringseminars.com www.computer.howstuffworks.com www.tech-faq.com/hvd.shtml www.ibm.com - IBM Research Press Resources Holographic Storagea

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