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SREE NARAYANA GURUKULAMKOLENCHERY

DEPARTMENT OF COMPUTER SCIENCEAND ENGINEERING

Seminar ReportnHOLOGR PHIC MEMORY

Submitted by:NIJESH P.CReg No 56 56

2005 2006

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SREE N R Y N GURUKUL MCOLLEGE OF ENGINEERING

K O L E N C H E R Y

DEP RTMENT O COM PUTER SCIENCEND ENGINEERING

CERTIFICATEThis is to cet fib1hat the Sen~inu i eport entitled HOLOGR PHIC MEMORY

Prof. Dr Janah an La1Head of the Department

Place: Kadayiruppu

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CKNOWLEDGEMENT

I express my sincere gratitude to our respected principal K.Rajendran forgranting permission to present the sem inar.

I express my sincere thanks to Dr. Janahan Lal Head of the Com puter ScienceEng ineering department for p roviding me the guidance and facilities for the seminar.

extend my deep sense of gratitude to our Seminar Co-coordinatorProf. Smijesh P.S, Lecturer of Com puter Science Engg. Dept for their valuableguidance as w ell as timely advice which helped us a lot in taking seminar successfully.

also extend m y sincere than ks to all other faculty mem bers of Com puterscience Engineering department and my friends for their support and encouragement.

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BSTR CTHolographic memory is developing technology that

has promised to revolutionalise the storage systems. It can store data upto 1 Tb ina sugar cube sized crystal. Scientist Pieter J van Heerden first proposed the ideaof holographic three-dimensional) storage in the early 1960s. decade later,scientists at RC Laboratories demonstrated the technology by recording 500holograms in an iron-doped lithium-niobate crystal and 550 holograms of high-resolution images in a light-sensitive polymer material. The lack of cheap parts andthe advancement of magnetic and semiconductor memories placed thedevelopment of holographic data storage on hold.

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ONTENTSYTRODUCTION .................................................................... I

HOLOGRAPHY....................................................................... 21. APPLICATION TO BINAR Y .......................................................4

SPATIAL LIGHT MODULATOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 . IMPLEMENTATION . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . 5

RECORDING OF DATA IN HOLOGRAPHIC MEMORY . . . . . . . . . . . . . . . . . . 6RETRIVAL OF DATA FROM HOLOGRAPHIC MEMORY ................ 7

a PAGE DATA ACCESS .............................................................. 9MULTIPLEXING ..................................................................... 9ANGULAR MULTIPLEXING ..................................................... 9WAVELENGTH MULTIPLEXING .............................................. 10SPATIAL MU LTIPLEXING 11PHASE ENCODE D MULTIPLEXING .......................................... 11

= RECORDING ERROR S ........................................................ 12.....................................................AGE -LEVEL PARITY BITS 13

MERITS OF HOLOGRAPHIC MEMORY ....................................... 1338 CHALLENGES........................................................................ 1429 . POS SIBLE APPLICATIONS ........................................................ 4

iI . ........................................................RECENT DEVELOPMENTS 15

............HOLOGRAPHIC VS CONVE NTIONAL STORAG E DEVICES 167 SUMMARY............................................................................. 16

3 HOLOGRAPHIC MEMORY LAYOU T ............................................ 17CONCLUSION....................................................................... 18

z . REFERENCES .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . 19

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I??: f Computer Science Engg. Holographic Memory

INTRODU TIONDevices that use light to store and read data

have been the backbone of data storage for nearly two decades. Compact discsrevolutionized data s torage in the early 1980s, allowing multi-megabytes of data tobe stored on a disc that has a diameter of a m ere 12 centimeters and a thicknessof 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-lengthmovies on a single disc.

CDs and DVDs are the primary data storagemethods for music, software, personal computing and video. A C D can hold 783megabytes 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 m ediums meettoday s storage needs, but storage technologies have to evolve to keep pace withincreasing consumer demand. CDs, DVDs and m agnetic storage all store bits ofinformation on the surface of a recording medium. In order to increase storagecapabilities, scientists are now working on a new optical storage method calledholographic memory that will go beneath the surface and use the volume of therecording medium for storage, instead of only the surface area. Three-dimensiona ldata storage will be able to store more information in a smaller space and offerfaster data transfer times.

Holographic memory is developing technologythat has promised to revolutionalise the storage systems. It can store data upto 1Tb in a sugar cube sized crystal. Data from more than 1000 CDs can fit into aholographic memory System. Most of the computer hard drives available todaycan hold only 10 to 40 GB of data, a small fraction of what holographic memorysystem 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 moreadvantages than conventional storage systems. It is based on the principle of

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391 of Computer Science Engg. Holographic Memory

YOLOGRAPHY

A hologram is a block or sheet of photosensitive material whichrecords the interference of two light sources. To create a hologram, laser light isfirst split into two beams, a source beam and a reference beam. The sourcebeam is then man ipulated and sent into the photosensitive ma terial. Onceinside this ma terial, it intersects the reference beam and the resultinginterference of laser light is recorded on the photosensitive m aterial, resultingin a hologram. Once a hologram is recorded, it can be viewed with only thereference beam. The reference beam is projected into the hologram at the exactangle it was projected during recording. When this light hits the recordeddiffraction pattern, the source beam is regenerated out of the refracted light. Anexact copy of the source beam is sent out of the hologram and can be readby optical sensors. For examp le, a hologram that can be obtained from atoy store illustrates this idea. Precise laser equipment is used at the factory tocreate the hologram. A reco rding material which can recreate recorded imagesout of natural light is used so the consum er does not need high-techequipment to view the information stored in the hologram. Natural lightbecomes the reference beam and human eyes become the optical sensors.

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

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3e pt . of Computer Science Engg. Holographic Memory

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Dept. of Computer Science Engg. Holographic Memory

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APPLICATION TO BINARY

In order for holographic technology to be applied to computersystems it must store data in a form that a computer can recognize. In currentcomputer systems this form is binary. In the previous section it was mentionedt5at the source beam is manipulated. In comm on holograms this manipulation;s the creation of an optical image such as a ball or huma n face. In computer

s ~ a t i a light modulator a device that converts laser light into binary data.5-t-= Xarayana Gurukulam College of Engg. Kolencherry 4

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I??: of Comp uter Science Engg. Holographic Mem ory

SPATIAL LIGHT MODULATOR SLM)A spatial light modulator is used for creating binary information

out of laser light. The SLM is a 2 0 plane, consisting of pixels which can beturned on and off to create binary 1.s and 0.s. An illustration of this is a windowand a window shade. It is possible to pull the shade dow n over a window to blockincoming sunlight. If sunlight is desired again, the shade can be raised. A spatiallight modulator contains a two-dimensional array of windows which are onlymicrons wide. These windows block some pa rts of the incoming laser light and letother parts go through. The resulting cross section of the laser beam is a twodimensional array of binary data, exactly the sam e as what was represented in theSLM. After the laser beam is manipulated, it is sent into the hologram to berecorded. This data is written into the hologram as page form. It is called thisdue to its rep resentation as a two dimensional plane, or page of da ta. Spatial lightmodulator is a Liquid Crystal Display panel that consists of clear and dark areascorresponding to the binary information it represent.

Spatial light modu lator is actually that device which makes holographyapplicable to computers. S o- it s one of the important components of HolographicData Storage System.

IMPLEMENT TIONThe components of Ho lographic data storage system is composed of> Blue-green argon laser> Beam splitters to sp ilt the laser beam

Mirrors to direct the laser beam sLCD panel spatial light modulator)

enses to focus the laser beam sLithium-niobate crystal or pho topolymerCharge coupled device camera

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Dept. of Computer Science Engg. Holographic Mem ory

SPATIAL LIGHT MODULATOR SLM )A spatial light modulator is used for creating binary information

out of laser light. The SLM is a 2D plane, consisting of pixels which can beturned on and off to create binary I .s and 0.s. An illustration of this is a windowand a window shade. It is possible to pull the shade down over a window to blockincoming sunlight. If sunlight is desired again, the shade can be raised. A spatiallight modulator contains a two-dimensional array of windows which are onlymicrons wide. These windows block some parts of the incom ing laser light and letother parts go through. The resulting cross section of the laser beam is a twodimensional array of binary data, exactly the sam e as what was rep resented in theSLM. After the laser beam is manipulated, it is sent into the hologram to berecorded. 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 lightmodulator is a Liquid Crystal Display panel that consists of clear and dark areascorresponding to the binary information it represent.

Spatial light modulator is actually that device which makes holographyapplicable to computers. So4t is one of the important components of HolographicData Storage System.

IMPLEMENT TION

The components of Holographic data storage system is composed ofBlue-green argon laser> Beam splitters to spilt the laser beam

i Mirrors to direct the laser beamsLCD panel spatial light modulator)Lenses to focus the laser beamsLithium-niobate crystal or pho topolymer

> Charge coupled device camera

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Dept. o Com puter Science Engg. Holographic Memory

They can be classified into three sections nam ely recording medium,optical recording system and photodetector array. The laser is used because itprovides monochromatic light. Only the interference pattern produced by themonochromatic beam of light is stable in time. Lithium n iobate crystal is used asphotosensitive material on which hologram is recorded. It has certain opticalcharacteristics that make it behave as photosensitive material. CCD cameradetects the information in the light, conve rts to digital information and forward it to

RECORDING OF D T IN HOLOGR PHIC MEMORY SYSTEM

W hen the blue-green argon laser is fired, a beam splitter creates twobeams. One beam, called the object or signal beam, will go straight, bounce offone mirror and travel through a spa tial- l ight mo du lato r SLM). An SLM is a

around to the light-sensitive lithium-niobate crystal. Some systems use aphotopolymer in place of the crystal. A second beam, called the re feren ce beam ,shoots out the side of the beam sp litter and takes a separate path to the crystal.When the two beams meet, the interference pattern that is created stores the datacarried by the signal beam in a specific area in the crystal -- the data is stored as a

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Dept. of Com puter Science Engg. Holographic M emory

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RETRIEVAL OF DATA FROM HOLOGRAPHIC MEMORY SYSTEMAn 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 andreconstruct the holographic page of data stored in the crys tal, the reference beamis shined into the crystal at exactly the same angle at which it entered to store thatclage of data. Each page of data is stored in a different area of the crystal, based3n the angle at which the reference beam strikes it. During reconstruction theSeam will be diffracted by the crystal to allow the recreation of the original pagethat was stored. This reconstructed page is then projected onto the charge-soupled device (CCD) camera, which interprets and forwards the digitalm ormation to a computer.

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CCD is a 2-D a rray of thousands or millions of tiny solar cells, each ofwhich transforms the light from one sm all portion of the image into electrons. N extstep is to read the value (accumulated charge) of each cell in the image. In a C CDdevice, the charge is actually transported across the chip and read at one cornerof the array. An analog-to-digital converter turns each pixel s value into a digitalvalue. CCD s use a special manufacturing process to create the ability to transportcharge across the chip without distortion. This process leads to very high-qualitysensors in terms of fidelity and light sensitivity. CCD sensors have been massproduced for a longer period of time, so they are m ore mature. They tend to havehigher quality and more pixels.

The key component of any holographic data storage system is theangle at which the second reference beam is fired at the crystal to retrieve a pageof data. It must match the original reference beam angle exactly A difference ofjust a thousandth of a millimeter will result in failure to retrieve that page of data.

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.S z-r. >=Compute rScience Engg. Holographic MemoryP GE D T CCESS

Because data is stored as page data in a hologram, the retrieval of: is data must also be in this form. Page data access is the method of readingstored data in sheets, not serially as in conventional storage systems. Itwas mentioned in the introduction that conventional storage was reaching itsfundamental limits. One such limit is the way data is read in streams.

I Holographic memory reads data in the form o f pages instead. For example, if aII stream of 32 bits is sent to a processing unit by a conven tional read head,

a ho lographic memory system would in turn send 32 x 32 bits, or 1024 bits dueI to its added dimension. This provides very fast access times in volumes far

greater than serial access methods. The volume could be one Megab it per pageusing a SLM resolution of 1024 x 1024 bits at 15-20 m icrons per pixel.

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MULTIPLEXING

Once one can store a page of bits in a hologram, an interface to acomputer can be made. The prob lem arises, however, that storing only one pageof bits is not beneficial. Fortunately, the properties of ho lograms provide a uniquesolution to this dilemm a. Unlike magnetic storage mechanisms which store dataon their surface, holographic memories store information throughout their wholevolume. After a page of data is recorded in the ho logram, a small modification tothe 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 thehologram is called multiplexing. The thicker the volume becom es, the smallerthe m odifications to the source beam can be.

NGUL R MULTIPLEXING

Wh en a reference beam recreates the source beam , it needs to be atthe same angle it was during recording. A very sm all alteration in this angle willmake the regenerated source beam disappear. Harnessing this property,

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Dept. of Computer Science Engg. Holographic Mem ory

Angular multiplexing changes the angle of the source beam by veryminuscule amounts after each page of data is recorded. Depend ing on thesensitivity of the recording material thousands of pages of data can be stored inthe sam e hologram at the same point of laser beam entry. Staying awayfrom conventional data access systems which move mechanical matter to obtaindata the angle of entry on the source beam can be deflected by high-frequencysound waves in solids. The elimination of mechanical access methods reducesaccess times from milliseconds to microseconds.

W VELENGTH MULTIPLEXING

Used mainly in conjunction with other multiplexing methodswavelength multiplexing alters the wavelength of source and reference beamsbetween recordings. Sending beam s to the same point of origin in the recordingmedium at different wavelengths allows multiple pages of data to be recorded.Due to the sm all tuning range of lasers howeve r this form of multiplexingis limited on its own.

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hift Mu iplsxing

SPATIAL MULTIPLEXINGSpatial multiplexing is the method of changing the point of entry of

source and reference beams into the recording medium. This form tends to breakaway from the non-mechanical paradigm because either the medium or recordingbeams mus t be physically moved. Like wavelength mu ltiplexing this iscombined with other forms of m ultiplexing to maximize the am ount of datastored in the holographic volume. Two commonly used forms of spatialmu ltiplexing are peristrophic multiplexing and shift m ultiplexing.

PHASE-ENCODED MULTIPLEXINGThe form of multiplexing farthest away from using mechanical means

to record many pages in the same volume of a holograph is called phase-encodedmu ltiplexing. Rather than manipulate the angle of entry of a laser beam orrotate or translate the recording medium phase-encoded mu ltiplexing changesthe phase of individual parts of a reference beam. The ma in reference beam issplit up into m any sm aller pa rtial beams which cover the same area as the

rence beams intersects the

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Dept. of Computer Science Engg. Holographic M emory

the beamlets. The phase of the beam lets can be changed by non-mechanicalmeans, therefore speeding up access times.

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RE ORDING ERRORS

When data is recorded in a holographic medium, certain factors canlead to erroneously recorded data. One major factor is the electronic noisegenerated by laser beams. W hen a laser beam is split up for example,through a S M ), the generated light bleeds into places where light was meantto be blocked out. Areas where zero light is desired might have minusculeamounts of laser light present which m utates its bit representation. For examp le,if too much light gets recorded into this zero area representing a binary 0, anerroneous change to a binary 1 might occur. Changes in both the quality of thelaser beam and recording material are being researched, but theseimprovem ents must take into consideration the cost-effectiveness of aholographic memory system. These limitations to current laser beam andphotosensitive technology are some of the main factors for the delay ofpractical holographic mem ory systems.

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Dept. of Comp uter Science Engg. Holographic Mem ory

PAGE -LEVEL PARITY BITSOnce 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 anew way to provide error control. Current error control methods concentrate on astream of bits. Because pa ge data is in the form of a two dimen sionalarray error correction needs to take into accou nt the extra dimension ofbits. W hen a page of data is written to the holograp hic media the page isseparated into smaller two dimens ional arrays. These sub sections are appendedwith an additional row and column of bits. The added bits calculate the parity ofeach row and column of data. An odd number of bits in a row or column create aparity bit of 1 and an even number of bits create a 0. A parity bit where the rowand column meet is also created which is called an overall parity bit. The subsections are rejoined and sent to the h olographic medium for recording.

MERITS OF HOLOGR PHIC MEMORY

Holographic mem ory offers storage capacity of about 1 TB. Speed ofretrieval of data in tens of microseconds compared to data access time of almost10ms offered b y the fastest hard disk today. By the time they are available theycan transfer an entire DVD movie in 30 seconds. Information search is also fasterin holographic mem ory. Consider the case of large databases that are stored onhard 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 onafter which it is compared with the reference data. In holographic storage entirepages can be retrieved where contents of two or more pages can be compared

has no mov ing parts. So the limitations of mechanical motion such as friction canbe removed.

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Dept. of Computer Science Engg. Holographic Memory

CH LLENGES

During the retrieval of data the reference beam has to be focused atexactly the same angle at which it was projected during recording. A slight errorcan cause a wrong data page to be accessed. It is difficult to obtain that m uch ofaccuracy. The crystal used as the photographic filament must have exact opticalcharacteristics such as high diffraction efficiency, storage of data safely withoutany erasure and fast erasure on application of external stimulus light ultra violetrays. W ith the repeated number of accesses the holograms will tend to decay.

POS SIBLE PPLIC TIONS

There are many possible applications of holographic memory.Holographic mem ory systems can potentially provide the high-speed transfers andlarge volumes of future computer systems. One possible application is datamining. Data mining is the process of finding patterns in large amounts of data.Data mining is used greatly in large databases which hold possible pa tternswhich can t be distinguished by human eyes due to the vast amount of data.Som e current computer systems implemen t data mining, but the mass amountof storage required is pushing the limits of current data storage systems. Themany advances in access times and data storage capacity that holographicmemory provides could exceed conventional storage and speed up data miningconsiderably. This would result in more located patterns in a shorter amount of

Another possible application of holographic memory is in petaflopcomputing. A petaflop is a thousand trillion floating point operations per second.The fast access in extremely large amounts of data provided by holographicmem ory systems could be utilized in petaflop architecture. Clearly advances areneeded in more than mem ory systems, but the theoretical schem atics do existfor such a machine. Optical storage such as holographic mem ory provide aviable solution to the extreme amoun t of data

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Dept. of Com puter Science Engg. Holographic M emory

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HOLOGRAPHIC MEMORY VS CONVENTIONAL STORAGE DEVICES

Data TransferStorage Medium ccess Time Storage Capacity

5 20 MBIs 1 MbitsJcm2

Comparing the access times holographic memory lies midwaybetween that of main memory and magnetic disk. Data transfer rate is 10GBIswhich is higher than that of other storage devices and, an d a storage capacity thatis higher than both main memory and magnetic disk. Certainly if the issues ofhologram decay and interference are resolved, then holographic memory couldbecome a part of the memory hierarchy, or take the place of magnetic disk muchas magnetic disk has displaced m agnetic tape for most applications.

SUMM RY

COMPAN IES INVOLVED IBM,ROCKW ELL,LUCENT

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ON LUSION

The future of holographic memory is very promising. The pageaccess of data that holographic mem ory creates will provide a window intonext generation computing by adding another dimension to stored data.Finding holograms in personal computers might be a bit longer off however. Thelarge cost of high-tech optical equipment would make small-scale systemsimplemented with holograph ic memory impractical.

Holographic memory will m ost likely be used in next generationsuper computers where cost is not as much of an issue. Current magneticstorage devices remain far more cost effective than any other medium on themarket. As computer systems evolve it is not unreasonable to believe thatmagnetic storage will continue to do so. As mentioned earlier however theseimprovem ents are not made on the concep tual level. The current storage in apersona l computer operates on the same principles used in the first magnetic datastorage devices. The parallel nature of holographic memory has many potentialgains on serial storage methods. However many advances in opticaltechnology and photosensitive materials need to be made before we findholograms in com puter systems.

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I

REFEREN ES

http://www.howstuffworks.com

http:/lwww.sandi@ cda.mrs.umn.edu

http://www.stanford.edu/ svnqam/

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