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WHATS THE NEW BUZZ

Holographic memory is developing technology that has promised to revolutionalise the storagesystems.It can store data upto 1 Tb in a sugar cube sized crystal. Data from more than 1000 CDs can fitinto a holographic memory System.

Holographic data storage systems use the volume to store data. It is based on the principle ofholography

WHAT IS HOLOGRAPHY?

Holography was invented in 1947 by the Hungarian-British physicist Dennis Gabor (1900-1979),who won a 1971 Nobel Prize for his invention.

Holography is a technique of recording patterns of light to produce three dimensional object.

COMPONENTS OF HOLOGRAPHIC MEMORY

The holographic memory system is made up of the following basic components:Blue-green argon laserBeam splitters to spilt the laser beamMirrors to direct the laser beamsLCD panel (spatial light modulator)Lenses to focus the laser beamsLithium-niobate crystal or photopolymerCharge coupled device camera

RECORDING OF DATA IN HDSS

DETAILS ABOUT CCDCCD is a 2-D array of thousands or millions of tiny solar cells, each of which transforms thelight from one small portion of the image into electrons. Next step is to read the value(accumulated charge) of each cell in the image. In a CCD device, the charge is actuallytransported across the chip and read at one corner of the array. An analog-to-digital converterturns each pixel's value into a digital value. CCDs use a special manufacturing process to createthe ability to transport charge across the chip without distortion.WHAT IS SLM?

A spatial light modulator is used for creating binary information out of laser light. The SLM is a2D plane, consisting of pixels which can be turned on and off to create binary 1.s and 0.s.PAGE LEVEL PARITY BITS

Once error-free data is recorded into a hologram, methods which read data back out of it need tobe error free as well. Because page data is in the form of a two dimensional array, errorcorrection needs to take into account the extra dimension of bits. When a page of data is writtento the holographic media, the page is separated into smaller two dimensional arrays. These subsections are appended with an additional row and column of bits.MULTIPLEXING TECHNIQUE

Unlike magnetic storage mechanisms which store data on their surface, holographic memoriesstore information throughout their whole volume.

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Fortunately, the properties of holograms provide a unique solution to this dilemma.This method of storing multiple pages of data in the hologram is called multiplexing.POSSIBLE APPLICATIONS

One possible application is data mining. Data mining is the process of finding patterns in largeamounts of data.

Another possible application of holographic memory is in petaflop computing. A petaflop is athousand trillion floating point operations per second.

Holographic memory can be used as extended DRAM with 10ns access time,

Holographic memory vs.conventional storage devicesADVANTAGES

In contrast to the currently available storagestrategies, holographic mass memory simultaneously offers high data capacity and short data

access time(Storage capacity of about1TB/ccand data transfer rate of 1 billion bits/second)

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

ADVANTAGES(CONT)

Because the interference patterns are spread uniformly throughout thematerial, it endows holographic storage with another usefulcapability: high reliability.

No rotation of medium is required as in the case of other storagedevices.It can reduce threat of piracy since holograms cant be easily replicated.

DISADVANTAGES

Manufacturing cost HDSS is very high.

And there is a lack of availability of resources which are needed to produce HDSS.

Writes in the same fashion can degradeprevious writes in the same region of the medium.

The re-illuminated reference beam used to retrieve the recorded information, also excites thedonor electrons and disturbsthe equilibrium of the space charge field in a manner that produces a gradual erasure of therecording.In the past, this has limited the number of reads that can be made before the signal-to -noise ratiobecomes too low.

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CONCLUSION

The future of holographic memory is very promising. The page access of data that holographicmemory creates will provide a window into next generation computing by adding anotherdimension to stored data.

Holographic memory will most likely be used in next generation super computers where cost isnot as much of an issue.

Reference: http://www.seminarprojects.com/Thread-holographic-memory-download-full-seminar-report#ixzz1XvEJhHi6

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holographic data storage full report

Seminar-Report-on-Holographic-Data-Storage-System-HDSS.doc(Size:123 KB / Downloads: 743)Seminar Report On Holographic datastorage system (HDSS)

ABSTRACT:

Holographic memory is a technique thatcan store information at high densityinside crystals Holographic memory isdeveloping technology that has promisedto revolutionalise the storage systems. Itcan store data up to 1Tb in a sugar cubesized crystal. Data from more than 1000CDs can fit into a holographic memorySystem. . Holographic storage has the

potential to become the next generation ofstorage media Conventional memories useonly the surface to store the data. Butholographic data storage systems use thevolume to store data. It has moreadvantages than conventional storagesystems. It is based on the principle ofholography This paper provides adescription of Holographic data storagesystem (HDSS), a three dimensional datastorage system which has a fundamentaladvantage over conventional read/writememory systems. A brief overview ofproperties of holograms will be presentedfirst. Applications to computer systemsare then covered, with the future ofholographic memory presented as aconclusion.

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Presented by BYA.Arjun ReddyANDA.Santosh

ADVANTAGES OF HDDS1 Personal computer with a holographictechnology platform (HoloPCs) will beable to access data more rapidly and in aqualitatively different manner thanconventional PCs.2 Two-dimensional data saving systemssuch as CDs and DVDs record andretrieve files in a serial fashion -- one bitat a time. Holographic data systems retainand retrieve data in parallel, a total

(million- bit) page of files at a time.3 The interface of holography androbotics may make "holobots" that learnthrough creative identification of usefulpatterns in big quantities of files. 4Eventually, optical neural networks maybe possible using holographic research

INTRODUCTION

Each time you watch a fast-paced DVDmovie or pull down a piece of information

from the Internet or even access the ATMat the corner of your street, you areactually tapping into large repositories ofdigital information. The hard disk, themainstay of personal and corporatestorage, has faithfully obeyed theexponential law. This has happenedlargely due to increases in aerial density,that is, how many bits are crammed into asquare inch. This paper provides adescription of Holographic data storage

system (HDSS), a three dimensional datastorage system which has a fundamentaladvantage over conventional read/write .The first step in understandingholographic memory is to understandwhat "holographic" means. Holography isa method of recording patterns of light toproduce a three-dimensional object. The

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recorded patterns of light are calledhologram memory systems. Holographicmemory is a technique that can storeinformation at high density inside crystalsor photopolymers. As current storage

techniques such as DVD reach the upperlimit of possible data density (due to thediffraction limited size of the writingbeams), holographic storage has thepotential to become the next generation ofstorage media. The advantage of this typeof data storage is that the volume of therecording media is used instead of just thesurface.

WORKING OF HDSS

RECORDING DATA ON MEDIUM

Creating holograms is achieved by meansof two coherent beams of light split fromone laser source, one being the referencebeam and the other the signal beam. Whenboth these beams interfere with oneanother, a resulting interference pattern isformed which encompasses the patternboth in amplitude and phase informationof the two beams. When an appropriatephotorefractive material is placed at the

point of interference, the interferencepatterns are recorded inside the material.The beam's angle is crucial, and it can'tvary by more than a fraction of a degree.This apparent flaw in the recordingprocess is actually an asset. It's howholographic storage achieves its high datadensities. By changing either the angle ofthe reference beam or its frequency, youcan write additional data pages in to thesame volume of crystal. The dynamicrange of the medium determines howmany pages it can hold reliably.

When the reference beam illuminates thematerial in the absence of the signal beam,the hologram causes the light to bediffracted in the same direction of theinitial signal beam and all the information

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of the original signal beam isreconstructed

MULTIPLEXINGOnce one can store a page of bits in a

hologram, an interface to acomputer can be made. The problemarises, however, that storing onlyone page of bits is not beneficial.Fortunately, the properties ofholograms provide a unique solution tothis dilemma. Unlike magneticstorage mechanisms which store data ontheir surfaces, holographicmemories store information throughouttheir whole volume. After a

page of data is recorded in the hologram, asmall modification to thesource beam before it reenters thehologram will record another pageof data in the same volume. This methodof storing multiple pages ofdata in the hologram is calledmultiplexing. The thicker the volumebecomes smaller the modifications to thesource beam can be.. IMPLEMENTATION

A holographic data storage systemconsists of a recording medium, anoptical recording system, a photo detectorarray. A beam of coherentlight is split into a reference beam and asignal beam which are usedto record a hologram into the recordingmedium. The recording mediumis usually a photo refractive crystalA hologram is simply the three-dimensionalinterference pattern of the intersection ofthe reference and signalbeams are perpendicular to each other.This interference pattern isimprinted into the crystal as regions ofpositive and negativecharges. To retrieve the stored hologram,a beam of light that has

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the same wavelength and angle ofincidence as the reference beam issent into the crystal and the resultingdiffraction pattern is usedto reconstruct the pattern of the signal

beam. Many differentholograms may be stored in the samecrystal volume by changing theangle of incidence of reference beam

FIG INPLEMENTATION OF HDDThe most common holographic recordingsystem uses laser light, a beamsplitter to divide the laser light intoreference beam and signalbeam, various lenses and mirrors to

redirect the light, a photoreactive crystal, and an array of photodetectors around the crystalto receive the holographic data. To recorda hologram, a beam laserlight is split into two beams by a mirror.These two beams thenbecome the reference and signal beams.The signal beam interacts withan object and the light that is reflected bythe object intersects

the reference beam at right angles. Theresulting interferencepattern contains all the informationnecessary to recreate the imageof the object after suitable processing. Theinterference pattern isrecorded on to a photo reactive materialand may be retrieved at alatter time by using a beam that isidentical to the reference beam.This is possible because the hologram hasthe property that if it isilluminated by either of the beams used torecord it, the hologramcauses light to be diffracted in thedirection of the second beamthat was used to record it, there byrecreating the reflected imageof the object if the reference beam was

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used to illuminate thehologram. So, the reflected must betransformed into a real imagewith mirrors and lenses that can be sent tothe laser detector array.

ADVANTAGESWith three-dimensional recording andparallel data readout,holographic memories can outperformexisting optical storagetechniques. In contrast to the currentlyavailable storagestrategies, holographic mass memorysimultaneously offers high datacapacity and short data access time(Storage capacity of about 1TB/cc

and data transfer rate of 1 billionbits/second).Holographic data storage has the uniqueability to locate similarfeatures stored within a crystal instantly.A data pattern projectedinto a crystal from the top searchesthousands of stored holograms inparallel. The holograms diffract theincoming light out of the sideof the crystal, with the brightest outgoing

beams identifying theaddress of the data that most closelyresemble the input pattern.This parallel search capability is aninherent property ofholographic data storage and allows adatabase to be searched bycontent.Because the interference patterns arespread uniformly throughout thematerial, it endows holographic storagewith another usefulcapability: high reliability. While a defectin the medium for diskor tape storage might garble critical data, adefect in a holographicmedium doesn't wipe out information.Instead, it only makes thehologram dimmer.

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No rotation of medium is required as inthe case of other storagedevices. It can reduce threat of piracysince holograms cant beeasily replicated.

DISADVANTAGES OF HDDSManufacturing cost HDSS is very highand there is a lack ofavailability of resources which are neededto produce HDSS. However,all the holograms appear dimmer becausetheir patterns must share thematerial's finite dynamic range. In otherwords, the additionalholograms alter a material that cansupport only a fixed amount of

change. Ultimately, the images become sodim that noise creeps intothe read-out operation, thus limiting thematerial's storagecapacity.A difficulty with the HDSS technologyhad been the destructivereadout. The re-illuminated referencebeam used to retrieve therecorded information, also excites thedonor electrons and disturbs

the equilibrium of the space charge fieldin a manner that produces agradual erasure of the recording. In thepast, this has limited thenumber of reads that can be made beforethe signal-to -noise ratiobecomes too low. Moreover, writes in thesame fashion can degradeprevious writes in the same region of themedium. This restricts theability to use the three-dimensionalcapacity of a photorefractivefor recording angle-multiplexedholograms. You would be unable tolocate the data if theres an error ofeven a thousandth of an inch.-POSSIBLE APPLICATIONSThere are many possible applications ofholographic memory.

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Holographic memory systems canpotentially provide the high speedtransfers and large volumes of futurecomputer system. One possibleapplication is data mining. Data mining is

the processes of findingpatterns in large amounts of data. Datamining is used greatly inlarge databases which hold possiblepatterns which cant bedistinguished by human eyes due to thevast amount of data. Somecurrent computer system implement datamining, but the mass amount ofstorage required is pushing the limits ofcurrent data storage

systems. The many advances in accesstimes and data storage capacitythat holographic memory provides couldexceed conventional storageand speedup data mining considerably.This would result in morelocated patterns in a shorter amount oftime.Another possible application ofholographic memory is in petaflopcomputing. A petaflop is a thousand

trillion floating pointoperations per second. The fast accessextremely large amounts ofdata provided by holographic memorycould be utilized in petafloparchitecture. Clearly advances are neededto in more than memorysystems, but the theoretical schematics doexist for such a machine.Optical storage such as holographicmemory provides a viable solutionto the extreme amount of data which isrequired for a petaflopcomputing.CONCLUSIONThe future of HOLOGRAPHIC DATASTORAGE SYSYEM is very promising.Thepage access of data that HDSS creates will

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provide a window into nextgeneration computing by adding anotherdimension to stored data.Finding holograms in personal computersmight be a bit longer off,

however. The large cost of high-techoptical equipment would makesmall-scale systems implemented withHDSS impractical. It will mostlikely be used in next generationsupercomputers where cost is not asmuch of an issue. Current magneticstorage devices remain far morecost effective than any other medium onthe market. As computersystem evolve, it is, not unreasonable to

believe that magneticstorage will continue to do so. Asmentioned earlier, however, theseimprovements are not made on theconceptual level. The currentstorage in a personal computer operateson the same principles usedin the first magnetic data storage devices.The parallel nature ofHDSS has many potential gains on serialstorage methods. However,

many advances in optical technology andphotosensitive materials needto be made before we find holograms inour computer systems.REFERENCE.......www.holopc.com........www.wikeipedia.com.........www.engeeniringseminars.com..........www.computer.howstuffworks.comThe Herald - the herald is not one of thesteps, but an archetype. Heis usually the one who introduces the nextstep in the hero'sjourney. In Star Wars the Herald is R2D2a little droid with aholographic message on his hard disc.2. The Call to AdventureIn Star Wars, the holographic message byPrincess Leia is the Call to

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Adventure. 'Help me,' she says, 'you're myonly hope.' The Callusually asks the hero to leave his worldand go on a quest to save anobject or person of great value. Lord ofthe Rings seems to

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ABSTRACTHolographic memory is developing technology that has promised to revolutionalise the storagesystems. It can store data upto 1 Tb in a sugar cube sized crystal. Scientist Pieter J. van Heerdenfirst proposed the idea of holographic (three-dimensional) storage in the early 1960s. A decadelater, scientists at RCA Laboratoriesdemonstrated the technology by recording 500 holograms inan iron-doped lithium-niobate crystal and 550 holograms of high-resolution images in a light-sensitive polymer material. The lack of cheap parts and the advancement of magnetic andsemiconductor memories placed the development of holographic data storage on hold.INTRODUCTIONDevices that use light to store and read data have been the backbone of data storage for nearlytwo 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 athickness of about 1.2 millimeters. In 1997, an improved version of the CD, called a digitalversatile disc (DVD), was released, which enabled the storage of full-length movies on a singledisc.CDs and DVDs are the primary data storage methods for music, software, personal computingand video. A CD can hold 783 megabytes of data. A double-sided, double-layer DVD can hold15.9 GB of data, which is about eight hours of movies. These conventional storage mediumsmeet today's storage needs, but storage technologies have to evolve to keep pace with increasingconsumer 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 anew optical storage method called holographic memory that wili go beneath the surface and usethe 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 offerfaster data transfer times.Holographic memory is developing technology that has promised to revolutionalise the storagesystems. It can store data upto 1 Tb in a sugar cube sized crystal. Data from more than 1000 CDscan fit into a holographic memory System. Most of the computer hard drives available today canhold 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 storagesystems use the volume to store data. It has more advantages than conventional storage systems.

It is based on the principle of holography.HOLOGRAPHYA hologram is a block or sheet of photosensitive material which records the interference of twolight sources. To create a hologram, laser light is first split into two beams, a source beam and areference beam. The source beam is then manipulated and sent into the photosensitive material.Once inside this material, it intersects the reference beam and the resulting interference of laserlight is recorded on the photosensitive material, resulting in a hologram. Once a hologram is

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recorded, it can be viewed with only the reference beam. The reference beam is projected intothe hologram at the exact angle it was projected during recording. When this light hits therecorded diffraction pattern, the source beam is regenerated out of the refracted light. An exactcopy of the source beam is sent out of the hologram and can be read by optical sensors. Forexample, a hologram that can be obtained from a toy store illustrates this idea. Precise laser

equipment is used at the factory to create the hologram. A recording material which can recreaterecorded images out of natural light is used so the consumer does not need high-tech equipmentto view the information stored in the hologram. Natural light becomes the reference beam andhuman eyes become the optical sensors.Holography was invented in 1947 by the Hungarian-British physicist Dennis Gabor(1900-1979),who won a 1971 Nobel Prize for his invention.PotM90 QK E r i i i1 ' ' iWart- WBM)CREATING HOLOGRAMS

Ilus photoresistive maditoiu replicates the fttagfli a* a change in* -iibnorptioti thidkne&sft*)APPLICATION TO BINARYIn order for holographic technology to be applied to computer systems, it must store data in aform that a computer can recognize. In current computer systems, this form is binary. In theprevious section, it was mentioned that the source beam is manipulated. In common holograms,this manipulation ;s the creation of an optical image such as a ball or human face. In computerapplications, this manipulation is in the form of bits. The next section explains the soatial lightmodulator, a device that converts laser light into binary data.

SPATIAL LIGHT MODULATOR (SLM)A spatial light modulator is used for creating binary information out of laser light. The SLM is a2D plane, consisting of pixels which can be turned on and off to create binary 1.s and O.s. Anillustration of this is a window and a window shade. It is possible to pull the shade down over awindow to block incoming sunlight. If sunlight is desired again, the shade can be raised. Aspatial light modulator contains a two-dimensional array of windows which are only micronswide. These windows block some parts of the incoming laser light and let other parts go through.The resulting cross section of the laser beam is a two dimensional array of binary data, exactlythe same as what was represented in the SLM. After the laser beam is manipulated, it is sent intothe hologram to be recorded. This data is written into the hologram as page form. It is called thisdue to its representation as a two dimensional plane, or page of data. Spatial light modulator is a

Liquid Crystal Display panel that consists of clear and dark areas corresponding to the binaryinformation it represent.Spatial light modulator is actually that device which makes holography applicable to computers.Sort is one of the important components of Holographic Data Storage System.IMPLEMENTATIONThe components of Holographic data storage system is composed of> Blue-green argon laser> Beam splitters to spilt the laser beam

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> Mirrors to direct the laser beams> LCD panel (spatial light modulator)> Lenses to focus the laser beams> Lithium-niobate crystal or photopolymer> Charge coupled device camera

SPATIAL LIGHT MODULATOR (SLM)A spatial light modulator is used for creating binary information out of laser light. The SLM is a2D plane, consisting of pixels which can be turned on and off to create binary 1.s and O.s. Anillustration of this is a window and a window shade. It is possible to pull the shade down over awindow to block incoming sunlight. If sunlight is desired again, the shade can be raised. Aspatial light modulator contains a two-dimensional array of windows which are only micronswide. These windows block some parts of the incoming laser light and let other parts go through.The resulting cross section of the laser beam is a two dimensional array of binary data, exactlythe same as what was represented in the SLM. After the laser beam is manipulated, it is sent intothe hologram to be recorded. This data Is written into the hologram as page form. It is called thisdue to its representation as a two dimensional plane, or page of data. Spatial light modulator is a

Liquid Crystal Display panel that consists of clear and dark areas corresponding to the binaryinformation it represent.Spatial light modulator is actually that device which makes holography applicable to computers.So it is one of the important components of Holographic Data Storage System.IMPLEMENTATIONThe components of Holographic data storage system is composed of> Blue-green argon laser> Beam splitters to spilt the laser beam> Mirrors to direct the laser beams> LCD panel (spatial light modulator)> Lenses to focus the laser beams

> Lithium-niobate crystal or photopolymer> Charge coupled device cameraThey can be classified into three sections namely recording medium, optical recording systemand photodetector array. The laser is used because it provides monochromatic light. Only theinterference pattern produced by the monochromatic beam of light is stable in time. Lithiumniobate crystal is used as photosensitive material on which hologram is recorded. It has certainoptical characteristics that make it behave as photosensitive material. CCD camera detects theinformation in the light, converts to digital information and forward it to computer.RECORDING OF DATA IN HOLOGRAPHIC MEMORY SYSTEMWhen the blue-green argon laser is fired, a beam splitter creates two beams. One beam, calledthe object or signal beam, will go straight, bounce off one mirror and travel through a spatial-

light modulator (SLM). An SLM is a Liquid crystal display (LCD) that shows pages of rawbinary data as clear and dark boxes. The information from the page of binary code is carried bythe signal beam around to the light-sensitive lithium-niobate crystal. Some systems use aphotopolymer in place of the crystal. A second beam, called the reference beam, shoots out theside of the beam splitter and takes a separate path to the crystal. When the two beams meet, theinterference pattern that is created stores the data carried by the signal beam in a specific area inthe crystal - the data is stored as a hologram.RETRIEVAL OF DATA FROM HOLOGRAPHIC MEMORY SYSTEM

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An advantage of a holographic memory system is that an entire page of data can be retrievedquickly and at one time. In order to retrieve and reconstruct the holographic page of data storedin the crystal, the reference beam !s shined into the crystal at exactly the same angle at which itentered to store that oage 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. During reconstruction, the beam will

be diffracted by the crystal to allow the recreation of the original page that was stored. Thisreconstructed page is then projected onto the charge-coupled device (CCD) camera, whichinterprets and forwards the digital "'ormation to a computer.|CCD is a 2-D array of thousands or millions of tiny solar cells, each of which transforms thelight from one small portion of the image into electrons. Next step is to read the value(accumulated charge) of each cell in the image. In a CCD device, the charge is actuallytransported across the chip and read at one corner of the array. An analog-to-digital converterturns each pixel's value into a digital value. CCDs use a special manufacturing process to createthe ability to transport charge across the chip without distortion. This process leads to very high-quality sensors in terms of fidelity and light sensitivity. CCD sensors have been mass produced

for a longer period of time, so they are more mature. They tend to have higher quality and morepixels.The key component of any holographic data storage system is the angle at which the secondreference beam is fired at the crystal to retrieve a page of data. It must match the originalreference beam angle exactly. A difference of just a thousandth of a millimeter will result infailure to retrieve that page of data.Reading DataDetectorIPAGE DATA ACCESSBecause data is stored as page data in a hologram, the retrieval of this data must also be in this

form. Page data access is the method of reading stored data in sheets, not serially as inconventional storage systems. It was mentioned in the introduction that conventional storage wasreaching itsifundamental limits. One such limit is the way data is read in streams., Holographic memory reads data in the form of pages instead. For example, if ai! stream of 32 bits is sent to a processing unit by a conventional read head,ia holographic memory system would in turn send 32 x 32 bits, or 1024 bits due to its addeddimension. This provides very fast access times in volumes far greater than serial access

methods. The volume could be one Megabit per page using a SLM resolution of 1024 x 1024 bitsat 15-20 microns per pixel.iMULTIPLEXINGOnce one can store a page of bits in a hologram, an interface to a computer can be made. Theproblem arises, however, that storing only one page of bits is not beneficial. Fortunately, theproperties of holograms provide a unique solution to this dilemma. Unlike magnetic storagemechanisms which store data on their surface, holographic memories store information

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throughout their whole volume. After a page of data is recorded in the hologram, a smallmodification to the source beam before it reenters the hologram will record another page of datain the same volume. This method of storing multiple pages of data in the hologram is calledmultiplexing. The thicker the volume becomes, the smaller the modifications to the source beamcan be.

ANGULAR MULTIPLEXINGWhen a reference beam recreates the source beam, it needs to be at the same angle it was duringrecording. 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 aftereach page of data is recorded. Depending on the sensitivity of the recording material, thousandsof pages of data can be stored in the same hologram, at the same point of laser beam entry.Staying away from conventional data access systems which move mechanical matter to obtaindata, the angle of entry on the source beam can be deflected by high-frequency sound waves insolids. The elimination of mechanical access methods reduces access times from milliseconds tomicroseconds.

WAVELENGTH MULTIPLEXINGUsed mainly in conjunction with other multiplexing methods, wavelength multiplexing alters thewavelength of source and reference beams between recordings. Sending beams to the same pointof origin in the recording medium at different wavelengths allows multiple pages of data to berecorded. Due to the small tuning range of lasers, however, this form of multiplexing is limitedon its own.SPATIAL MULTIPLEXINGSpatial multiplexing is the method of changing the point of entry of source and reference beamsinto the recording medium. This form tends to break away from the non-mechanical paradigmbecause either the medium or recording beams must be physically moved. Like wavelengthmultiplexing, this is combined with other forms of multiplexing to maximize the amount of data

stored in the holographic volume. Two commonly used forms of spatial multiplexing areperistrophic multiplexing and shift multiplexing.Shift Multiplexing

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beam as a whole. The reference beams intersects thesource beam and records the diffraction relative to the different phases of the beamlets. Thephase of the beamlets can be changed by non-mechanical means, therefore speeding up accesstimes.RECORDING ERRORS

When data is recorded in a holographic medium, certain factors can lead to erroneously recordeddata. One major factor is the electronic noise generated by laser beams. When a laser beam issplit up ( for example, through a SLM ), the generated light bleeds into places where light wasmeant to be blocked out. Areas where zero light is desired might have minuscule amounts oflaser light present which mutates its bit representation. For example, if too much light getsrecorded into this zero area representing a binary 0, an erroneous change to a binary 1 mightoccur. 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 holographicmemory system. These limitations to current laser beam and photosensitive technology are someof the main factors for the delay of practical holographic memory systems.PAGE-LEVEL PARITY BITS

Once error-free data is recorded into a hologram, methods which read data back out of it need tobe error free as well. Data in page format requires a new way to provide error control. Currenterror control methods concentrate on a stream of bits. Because page data is in the form of a twodimensional array, error correction needs to take into account the extra dimension of bits. Whena page of data is written to the holographic media, the page is separated into smaller twodimensional arrays. These sub sections are appended with an additional row and column of bits.The added bits calculate the parity of each row and column of data. An odd number of bits in arow or column create a parity bit of 1 and an even number of bits create a 0. A parity bit wherethe row and column meet is also created which is called an overall parity bit. The sub sectionsare rejoined and sent to the holographic medium for recording.MERITS OF HOLOGRAPHIC MEMORY

Holographic memory offers storage capacity of about 1 TB. Speed of retrieval of data in tens ofmicroseconds compared to data access time of almost 10ms offered by the fastest hard disktoday. By the time they are available they can transfer an entire DVD movie in 30 seconds.Information search is also faster in holographic memory. Consider the case of large databasesthat are stored on hard disk today. To retrieve any piece of information you first provide somereference data. The data is then searched by its address, track, sector and so on after which it iscompared with the reference data. In holographic storage entire pages can be retrieved wherecontents of two or more pages can be compared optically without having to retrieve theinformation contained in them. Also HDSS has no moving parts. So the limitations ofmechanical motion such as friction can be removed.CHALLENGES

During the retrieval of data the reference beam has to be focused at exactly the same angle atwhich it was projected during recording. A slight error can cause a wrong data page to beaccessed. It is difficult to obtain that much of accuracy. The crystal used as the photographicfilament must have exact optical characteristics such as high diffraction efficiency, storage ofdata safely without any erasure and fast erasure on application of external stimulus light ultraviolet rays. With the repeated number of accesses the holograms will tend to decay.POSSIBLE APPLICATIONSThere are many possible applications of holographic memory. Holographic memory systems can

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potentially provide the high-speed transfers and large volumes of future computer systems. Onepossible application is data mining. Data mining is the process of finding patterns in largeamounts of data. Data mining is used greatly in large databases which hold possible patternswhich can't be distinguished by human eyes due to the vast amount of data. Some currentcomputer systems implement data mining, but the mass amount of storage required is pushing

the limits of current data storage systems. The many advances in access times and data storagecapacity that holographic memory provides could exceed conventional storage and speed up datamining considerably. This would result in more located patterns in a shorter amount of time.Another possible application of holographic memory is in petaflop computing. A petaflop is athousand trillion floating point operations per second. The fast access in extremely large amountsof data provided by holographic memory systems could be utilized in petaflop architecture.Clearly advances are needed in more than memory systems, but the theoretical schematics doexist for such a machine. Optical storage such as holographic memory provide a viable solutionto the extreme amount of datawhich is required for petaflop computing.Holographic memory can be used as extended DRAM with 10ns access time, Hard disk drives

,CD ROMs of large storage capacity and rockmounted (combining numerous DASDs) ofpetabytes storage capacity.RECENT DEVELOPMENTSThe research on holographic memory is taking place in well guarded and rich companies likeIBM, ROCKWELL and InPhase. InPhase claims to have developed a holographic memory ofsize slightly larger than a DVD. It has a capacity of about 100GB. They are trying to push it upto1TB.IBM and ROCKWELL claims to have developed a recording medium less sensitive than lithiumniobate crystals.HOLOGRAPHIC MEMORY VS. CONVENTIONAL STORAGE DEVICESStorage Medium Access Time Data Transfer Rate Storage Capacity

Holographic Memory 2.4 us lOGB/s 400 Mbits/cm2Main Memory (RAM) 10-40 ns 5 MB/s 4.0 Mbits/cm2Magnetic Disk 8.3 ms 5-20 MB/s 100 Mbits/cm2Comparing the access times holographic memory lies midway between that of main memory andmagnetic disk. Data transfer rate is 10GB/s which is higher than that of other storage devicesand, and a storage capacity that is higher than both main memory and magnetic disk. Certainly ifthe issues of hologram decay and interference are resolved, then holographic memory couldbecome a part of the memory hierarchy, or take the place of magnetic disk much as magneticdisk has displaced magnetic tape for most applications.SUMMARYSTORAGE CAPACITY TERABYTE CLASS

TECHNOLOGY USED OPTICS AND HOLOGRAPHYDATA TRANSFER SPEED HIGHTIME FOR IMPLEMENTATION 4COST OF IMPLEMENTATION HIGHCOMPANIES INVOLVED IBM,ROCKWELL,LUCENTHOLOGRAPHIC MEMORY LAYOUTCONCLUSIONThe future of holographic memory is very promising. The page access of data that holographic

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memory creates will provide a window into next generation computing by adding anotherdimension to stored data. Finding holograms in personal computers might be a bit longer off,however. The large cost of high-tech optical equipment would make small-scale systemsimplemented with holographic memory impractical.Holographic 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 thanany other medium on the market. As computer systems evolve, it is not unreasonable to believethat magnetic storage will continue to do so. As mentioned earlier, however, these improvementsare not made on the conceptual level. The current storage in a personal computer operates on thesame principles used in the first magnetic data storage devices. The parallel nature ofholographic memory has many potential gains on serial storage methods. However, manyadvances in optical technology and photosensitive materials need to be made before we findholograms in computer systems.REFERENCEShttp://www.howstuffworks.comhttp://www.sandj[at]cda.mrs.umn.eduhttp ://www. sta nford.edu/~svngam/Chip magazinePC Quest MagazineCONTENTS'NTRODUCTION 1HOLOGRAPHY 202. APPLICATION TO BINARY 4SPATIAL LIGHT MODULATOR 5:Z. IMPLEMENTATION 5RECORDING OF DATA IN HOLOGRAPHIC MEMORY 6RETRIVAL OF DATA FROM HOLOGRAPHIC MEMORY 7:-. PAGE DATA ACCESS 9

15. MULTIPLEXING 9ANGULAR MULTIPLEXING 9WAVELENGTH MULTIPLEXING 10SPATIAL MULTIPLEXING 11PHASE ENCODED MULTIPLEXING 11:5. RECORDING ERRORS 12PAGE-LEVEL PARITY BITS 13:7. MERITS OF HOLOGRAPHIC MEMORY 1308. CHALLENGES 1439. POSSIBLE APPLICATIONS 14"0. RECENT DEVELOPMENTS 1511. HOLOGRAPHIC Vs CONVENTIONAL STORAGE DEVICES 16

12. SUMMARY 16^3. HOLOGRAPHIC MEMORY LAYOUT 17CONCLUSION 18'z. REFERENCES 19

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