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VISHVESHWARAIAH TECHNOLOGICAL UNIVERSITY

S.D.M COLLEGE OF ENGINEERING AND TECHNOLOGY

A seminar report on Holographic Data Storage

Submitted by

Sharath H N (2SD06CS092) 8th semester

DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING

2009-10

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VISHVESHWARAIAH TECHNOLOGICAL UNIVERSITY

S.D.M COLLEGE OF ENGINEERING AND TECHNOLOGY

CERTIFICATE Certified that the seminar work entitled “HOLOGRAPHIC DATA STORAGE” is a bonafide work presented by SHARATH H N bearing USN 2SD06CS092 in a partial fulfillment for the award of degree of Bachelor of Engineering in COMPUTER SCIENCE AND ENGINEERING branch, under Visveswaraya Technological University, Belgaum during the year 2009-10. The seminar report has been approved as it satisfies the academic requirements with respect to seminar work presented for the Bachelor of Engineering Degree. Staff in charge H.O.D Name: Sharath H N USN: 2SD06CS092

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HOLOGRAPHIC DATA STORAGE

By,

Sharath H N

Roll no 100

CSE ‘B’ Devision.

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INDEX

1. Abstract……………………………………………………………………………5

2. Introduction………………………………………………………………………..5

3. Different types of data storage systems……………………………………………5

3.1. Magnetic data storage system……………………………………………5

3.1.1. Magnetic disk…………………………………………………..5

3.1.2. Floppy disk……………………………………………………..8

3.2. Optical data storage system……………………………….……………...8

3.2.1 CD technology………………………………………………….8

3.2.2 DVD technology…………...……………………………………9

4. Holographic data storage systems…………………………………………...……10

4.1. Theory behind holographic data storage systems……………………….10

4.2. Advantages of holographic data storage systems……………………….12

4.3. Working…………………………………………………………………12

4.3.1. Spatial Light Modular (SLM)…………………………………12

4.3.2 The components involved……………………………………...13

4.3.3. writing……….………………………………………………...13

4.3.4. Reading………………………………………………………..14

4.3.5. Multiplexing…………………………………………………...14

4.3.6. Errors…………………………………………………………..17

4.4. Holographic Versatile Disk…………………………………………17

4.4.1. Some features………………………………………………….17

4.4.2. Disk structure…………………………….................................18

4.4.3. Limitations of HVD…………………………………………...18

4.4.4. Comparison of HVD…………………………………………..19

5. Future development and challenges……………………………………………..19

6. Conclusion……………………………………………………………………….20

7. References………………………………………………...…………………….14

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1. ABSTRACT: This paper provides a description of different type of data storage systems along with examples. Later it introduces the concept holographic data storage system (HDSS), a three dimensional data storage system which has a fundamental advantage over previously mentioned conventional read/write memory systems. The theory behind HDSS and working is seen, which is followed by some advantages of HDSS with respect to other systems are discussed. Later the working is seen which discusses reading, writing and some multiplexing techniques. Then Holographic Versatile Disk (HVD) is discussed. The future development and challenges of holographic memory is then presented, followed by conclusion.

2. INTRODUCTION:

It is estimated that until now people have produced more than 5 exabytes (5 billion gigabytes) of data, the majority of which is in digital form. Since this figure is always growing and analogue media is constantly being converted to digital, new methods of storing this data are needed. Currently the two main storage methods i.e. magnetic and optical are just about keeping ahead of these needs; unfortunately this is not always going to be the case.

Holographic data storage is a potential replacement technology in the area of high-capacity data storage currently dominated by magnetic and conventional optical data storage.

3. Different types of Data storage systems

They are

• Magnetic Data storage

• Optical data storage

3.1. Magnetic data storage

Magnetic storage and magnetic recording are terms from engineering referring to the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetization in a magnetizable material to store data and is a form of non-volatile memory. [1]

Some of the major storage devices that comes under magnetic data storage systems are

• Magnetic disk

• & Floppy disk

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3.1.1 Magnetic Disk:

Hard disks were invented in the 1950s. They started as large disks up to 20 inches in diameter holding just a few megabytes. Magnetic disk is a plastic disk coated with magnetic material and used for storing computer programs and data as a series of magnetic spots. Most computers contain a hard disk unit for general storage. Hard magnetic disks can store larger amounts of data and come in cartridges that slot into a special drive unit. [2]

Structure and working

On a hard disk (magnetic disk), data is stored in the magnetic coating of the disk’s platters. The platter is a flat disk of either alloy or glass, with a spindle at the centre. Modern platters generally have a diameter of 3.5” in desktops or 2.5” in laptops, although smaller drives are available for devices that require a micro-drive.

The spindle is rotated by an electric motor, and this cause the platter to spin. The speed at which the platter spins is measured in RPM and a higher speed is usually reflective of a higher performance, disk, in terms of data writing and reading.

The magnetic media holds the binary data as with tapes and floppy disks. The data is read from the surface of the platter by a set of ‘heads’ which are fixed so that they can only move between the centre of the platter and the outside edge. The heads are held just above the magnetic media by actuator arms that facilitate this movement across the disk’s platter surface. The heads are not designed to touch the platter surface as physical contact can cause damage to the magnetic media. Each platter has a top side and an underside, and there is usually a head for both. Therefore, a hard disk drive with 5 platters would have 10 heads.

When the disk is not in use, the heads are ‘parked’, usually at the outside edge of the platter.

Data in the magnetic media is organized into cylinders - concentric tracks on the media that are further divided into sectors. A sector is the smallest allocatable logical unit on a drive and usually, but not always, is 512 bytes in size. [3]

Next, the drive moves the heads over the appropriate track on a platter. The time it takes to move the heads is called the seek time. Once over the correct track, the drive waits while the platters rotate the desired sector under the head. The amount of time that takes is called the drive's latency. The shorter the seek time and latency, the faster the drive can do its work.

When the drive electronics determine that a head is over the correct sector to write the data, the drive sends electrical pulses to that head. The pulses produce a magnetic field that alters the magnetic surface of the platter. The variations recorded there now represent the data.

Reading data complements the recording process. The drive positions the read portion of the head over the correct track, and then waits for the correct sector to orbit around. When the particular magnetic specks that represent your data in the right sector and track pass under the read head, the drive's electronics detect the small magnetic changes and convert them back into bits. Once the drive checks the bits for errors and fixes any it sees, it sends the data back to the operating system. [3]

Below is the diagram of the hard disk with platters, spindle and the head.

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3.1.2 Floppy disks

Floppy disks are smaller, simpler, and cheaper disk units that consist of flexible, removable, plastic, diskette coated with the magnetic material. The diskette is enclosed in plastic jacket, which has an opening where the read/write head makes contacts with diskette. A hole in the centre allows a spindle mechanism in the disk drive to position and rotate the diskette. Information recorded on floppy disks by combining the clock and data information along each track using Manchester encoding. Disks encoded in this way are said to have single destiny.

The main features of floppy disks are small physical size and low cost. But this is offset by smaller storage capacity longer acess time and higher failure than hard disk.The floppy disks have been superseded by the emergence of rewritable compact disks having higher capacities. [1]

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3.2. Optical data storage

Today's meaning of optical data storage refers to storage systems that use light for recording and retrieval of information. Optical recording systems potentially have much greater reliability than magnetic recording systems since there is a much larger distance between the read/write element and the moving media. Therefore, there is no wear associated with repeated use of the optical systems. Another advantage of the optical recording systems over the best performing magnetic recording systems - hard drives - is their removability.

The main disadvantage of optical storage when compared to magnetic is slower random data access. This partially comes from the design of the relatively large (and heavy) optical heads. [4]

Optical drives of all kinds operate on the same principle of detecting variations in the optical properties of the media surface. CD and DVD drives detect changes in the light intensity, MO drives - changes in the light polarization. All optical storage systems work with reflected light.[4]

Some of the major optical data storage technologies are

• CD Technology.

• DVD Technology.

• Blue ray disk Technology.

• & Holographic data storage technology.

3.2.1 CD Technology:

The first generation of CD was developed by sony and Philips corporations, for audio systems. To provide high quality audio production and reproduction, 16-bit samples

Of the analog signals are taken at 44100 samples per second. The CDs were required to hold at least an hour of music. [1]

Structure of a CD

A CD is a fairly simple piece of plastic, about four one-hundredths (4/100) of an inch (1.2 mm) thick. During manufacturing, this material is impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. Latera thin, reflective aluminium layer is sputtered onto the disc, covering the bumps. The CD can store up to 600Mbs of data.[5]

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The read laser passes through the polycarbonate underside of the disc to the reflective aluminium layer underneath, reflecting the laser light from the pits and bumps — which represent the binary data on the disc. Note that recordable CDs use a different structure with a 'dye' that can be changed by a laser, thus allowing tracks to be burned on a disc.

3.2.2 The DVD technology

The quest for greater storage capacity resulted in the development of digital versatile disk(DVD) technology in 1996.

The structure of DVD

The size of a DVD disk is same as that of the CD, whereas the storage capacity is increased because of the change made in design process, as mentioned below.

The pits are smaller having minimum length of 0.4 micron.

The tracks are placed more closer, the distance between the tracks are 0.74 micron.

A red light laser with a wavelength of 635nm is used instead of infrared light (780nm) laser used in CDs. The above improvements lead to DVD capacity of 4.74 GBs.[1]

The Multi-Layer Storage of DVD To increase the storage capacity even more, a DVD can have up to four layers, two on each side. The laser that reads the disc can actually focus on the second layer through the first layer. It can be single sided, single layered (capacity-)or single sided, double layered or double sided, double layered.

The below diagram shows double layered, single sided DVD.

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Here is a list of the capacities of different forms of DVDs:

Format Capacity Approx. Movie Time

Single-sided/single-layer 4.38 GB 2 hours

Single-sided/double-layer 7.95 GB 4 hours

Double-sided/single-layer 8.75 GB 4.5 hours

Double-sided/double-layer 15.9 GB Over 8 hours

The capacity of a DVD doesn't double when a whole second layer is added to the disc. This is because when a disc is made with two layers, the pits have to be a little longer, on both layers, than when a single layer is used. This helps to avoid interference between the layers, which would cause errors when the disc is played.[5]

4. Holographic Data storage

Holographic data storage is the methodology that comes under Optical data storage.

When magnetic and optical data storage devices are considered, they rely on individual bits being stored as distinct magnetic or optical changes on the surface of the recording medium.

In order to increase storage capabilities, a new optical storage method is being considered 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.

4.1Theory behind holographic data storage

Holograms are photographic images that are three-dimensional and appear to have depth. Holograms work by creating an image composed of two superimposed 2-dimensional pictures of the same object seen from different reference points. Holography requires the use of light of a single exact wavelength, so lasers must be used. In reflection holograms, the kind of holography that can be viewed in normal light, two laser beams and a photographic plate are used to take an image of the object.

Holography is a method of recording patterns of light to produce a three dimensional object. The patterns of light are called a hologram. The process of creating a hologram begins with a focused beam of light -- a laser beam. This laser beam is split into two separate beams: a reference beam, which remains unchanged throughout much of the process, and an information beam, which passes through an image. When light encounters an image, its composition changes and the image is captured in its waveforms. When these two beams intersect, it creates a pattern of light interference. If this pattern of light interference in a layer of a disc is recorded then the light pattern of the image is recorded.

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To retrieve the information stored, the reference beam is applied directly onto the hologram. When it reflects off the hologram, it holds the light pattern of the image stored there. The holographic memory systems use holograms to store digital instead of analog information. [6]

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4.2 Some of the Advantages of HDSS with respect other data storage methods

• Three-dimensional data storage will be able to store more information in a smaller space and offer faster data transfer times.

• Unlike other technologies that record one data bit at a time, holography records and reads more than a million bits of data with a single flash of light.

• This enables significantly higher transfer rates than current optical storage devices.

• High storage densities and fast transfer rates, combined with durable, reliable, low-cost media, mean that holography is poised to become a compelling choice for next-generation storage.

• In addition, the flexibility of the technology allows a wide variety of holographic storage products to be developed, ranging from handheld devices for consumers to storage products for enterprises.

• IT can be imagined having 50 hours of high-definition video on a single disc, 50 000 songs on a postage stamp, or 500 000 X-rays on a credit card.[7]

4.3 Working of Holographic Data Storage System

4.3.1 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 be turned on and off to create binary 1.s and 0.s. An illustration of this is a window and a window shade. It is possible to pull the shade down over a window to block incoming sunlight. If sunlight is desired again, the shade can be raised. A spatial light modulator contains a two-dimensional array of windows which are only microns wide. 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, exactly the same as what was represented in the SLM. After the laser beam is manipulated, it is sent into the hologram to be recorded. This data is written into the hologram as page form. It is called this due to its representation as a two dimensional plane, or page, of data[10].

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4.3.2 The components involved :

• The holographic memory system is made up of the following basic components:

• a charge-coupled device

• lenses to focus the laser beams

• an LCD panel

• a photopolymer or lithium niobate crystal

• mirrors to direct the laser light

• beam splitters

• and an argon laser.

4.3.3 Writing

The light from the argon laser is split in two by the beam splitter. The signal or object beam will bounce off a mirror and pass through a spatial light modulator or SLM (and LCD showing raw binary data as dark and clear boxes). The signal or object beam will then carry the information from the SLM to the crystal. The second beam or the reference beam, on the other hand, takes another course towards the crystal and upon hitting it along with the object beam, creates an interference pattern that will be used to store the information relayed by the object beam in a certain location in the crystal.

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4.3.4 Retrieving:

The interference pattern induces modulations in the refractive index of the recording material yielding diffractive volume gratings. The reference beam is used during readout to diffract off of the recorded gratings, reconstructing the stored array of bits. The reconstructed array is projected onto a pixelated detector that reads the data in parallel. This parallel readout of data provides holography with its fast data transfer rates (10's to 100's of MBytes/second). The readout of data depends sensitively upon the characteristics of the reference beam. By varying the reference beam, for example by changing its angle of incidence or wavelength, many different data pages can be recorded in the same volume of material and read out by applying a reference beam identical to that used during writing. This process of multiplexing data yields the enormous storage capacity of holography. [5][7]

4.3.5 Multiplexing

Once one can store a page of bits in a hologram, an interface to a computer can be made. The problem arises, however, that storing only one page of bits is not beneficial. Fortunately, the properties of holograms provide a unique solution to this dilemma. Unlike magnetic storage mechanisms which store data on their surface, holographic memories store information throughout their whole volume. After a page of data is recorded in the hologram, a small modification to the source beam before it reenters the hologram will record another page of data in the same volume. This method of storing multiple pages of data in the hologram is called multiplexing. The thicker the volume becomes, the smaller the modifications to the source beam can be.

There are five types of multiplexing

• Angular Multiplexing

• Wavelength Multiplexing

• Spatial Multiplexing

• Peristrophic Multiplexing

• Shift Multiplexing

• Phase-Encoded Multiplexing

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Angular Multiplexing

When a reference beam recreates the source beam, it needs to be at the same angle it was during recording. A very small alteration in this angle will make the regenerated source beam disappear. Harnessing this property, angular multiplexing changes the angle of the source beam by very minuscule amounts after each page of data is recorded (see figure 2). Depending on the sensitivity of the recording material, thousands of pages of data can be stored in the same hologram, at the same point of laser beam entry[10]. Staying away from conventional data access systems which move mechanical matter to obtain data, the angle of entry on the source beam can be deflected by high-frequency sound waves in solids[10]. The elimination of mechanical access methods reduces access times from milliseconds to microseconds.

Wavelength Multiplexing

Used mainly in conjunction with other multiplexing methods, wavelength multiplexing alters the wavelength of source and reference beams between recordings. Sending beams to the same point of origin in the recording medium at different wavelengths allows multiple pages of data to be recorded. Due to the small tuning range of lasers, however, this form of multiplexing is limited on its own.

Spatial Multiplexing

Spatial multiplexing is the method of changing the point of entry of source and reference beams into the recording medium. This form tends to break away from the non-mechanical paradigm because either the medium or recording beams must be physically moved. Like wavelength multiplexing, this is combined with other forms of multiplexing to maximize the amount of data stored in the holographic volume. Two commonly used forms of spatial multiplexing are peristrophic multiplexing and shift multiplexing.

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Peristrophic Multiplexing

This form of spatial multiplexing rotates the recording medium as the light source beams remain in fixed positions[10]. For instance, a holographic cube could be rotated so each of its six sides could take in a source beam. This would provide six times the number of pages which could be stored in the volume. Certain problems arise when implementing this method of multiplexing. The rotational axes needs to be positioned in a way which does not interfere with the laser beams. As with all spatial multiplexing, bringing the recording media back to its original position for data retrieval would need to be very precise. This is much easier to maintain when the media remains static.

Shift Multiplexing

Shift multiplexing alters the point of entry on one surface of the recording media. The recording optics or media could be repositioned to allow the source beam to enter multiple positions on a surface. Depending on the size of the laser beam and thesensitivity of the recording media, the points of entry the source beam takes into it can be immense[10]. This form of multiplexing combined with peristrophic multiplexing could cover a very large percentage of the hologram.

Phase-Encoded Multiplexing

The form of multiplexing farthest away from using mechanical means to record many pages in the same volume of a holograph is called phase-encoded multiplexing. Rather than manipulate the angle of entry of a laser beam or rotate/translate the recording medium, phase-encoded multiplexing changes the phase of individual parts of a reference beam. The main reference beam is split up into many smaller partial beams which cover the same area as the original reference beam. These smaller beamlets vary by phase which changes the state of the reference beam as a whole. The reference beams intersects the source beam and records the diffraction relative to the different phases of the beamlets. The phase of the beamlets can be changed by nonmechanical means, therefore speeding up access times.[10]

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Combining Multiplexing Methods

No single multiplexing method by itself is the best way to pack a hologram full of information. The true power of multiplexing is brought out in the combination of one or more methods. Hybrid wavelength and angular multiplexing systems have been tested and the results are promising. Recent tests have also been formed on spatial multiplexing methods which create a hologram the size of a compact disc, but which hold 500 times more data[10].

4.3.6 Errors

When data is recorded in a holographic medium, certain factors can lead to erroneously recorded data. One major factor is the electronic noise generated by laser beams. When a laser beam is split up (for example, through a SLM ), the generated light bleeds into places where light was meant to be blocked out. Areas where zero light is desired might have minuscule amounts of laser light present which mutates its bit representation. For example, if too much light gets recorded into this zero area representing a binary 0, an erroneous change to a binary 1 might occur.

4.4 A holographic data storage device: Holographic versatile disk (HVD)

(Holographic Versatile Disc) A high-capacity optical disc is the one that combines single beam holographic storage and DVD technologies to provide cartridge capacities reaching 1TB and beyond.

Hence the holographic Versatile Disk offer far more storage and transfer capacity than CDs and DVDs and even "next-generation" DVDs like Blu-ray.

[8]

4.4.1 Some of the features of HVD

• Unlike current CD and the DVD drive technology the HVDs have the capacity to hold up to 300 gigaabytes of information.

• The holographic versatile disc has a transfer rate of 1 Gbit/s.

• Working of HVD is almost similar to the holographic data storage mechanism.

[9]

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4.4.2 Holographic Versatile Disc – Disc Structure The figure below the right shows the cross section of an HVD disc.

As seen in this diagram, holographic recording layer is formed on top of a reflective layer. The simple optical setup of Collinea Technology has allowed the HVD disc to have a reflective layer on the substrate and address pits formed on this layer. This configuration is the key to apply Collinear™ Technology to commercial HVD products. These address pits and the servo technology to read them guarantee the interchangeability of HVD discs, ruggedness against vibration in the real environment, wide system margin against variety of HVD discs from different manufacturers. Of course the servo information also make random access easy.

The servo technology and the address pits are, in fact, not different from those used in the current CD and DVD players and disks. The laser which is used to read address pits is 650nm red laser, also common with DVD players in the market. Another layer called “Dichroic Mirror Layer�Eis placed between the holographic recording layer and the substrate to block the green or blue laser, which are used to read/write holographic information, to reach address pits, thus eliminates noise. In short, HVD is a disc the holographic recording layer of which is formed on top of a conventional optical disk.[9]

4.4.3. Some of the limitations of HVD

• Holographic consumer products need to be very cheap in order to compete with DVD drives, which currently cost much higher, because of the much higher complexities in working and development. [9 ]

• Holographic memory discs have been notably thicker than CDs and DVDs. [5]

• Since HVDs are composed of very complex mechanisms and an investment of over $100 m (778 m) is typically required to develop a new platform. [5]

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4.4.4 Comparison

While HVD is attempting to revolutionize data storage, other discs are trying to improve upon current systems. Two such discs are Blu-ray and HD-DVD, deemed the next-generation of digital storage. Both build upon current DVD technology to increase storage capacity. All three of these technologies are aiming for the high-definition video market, where speed and capacity count.

Blu-ray HD-DVD HVD

Initial cost for recordable disc Approx. $18 Approx. $10 Approx.

$120

Initial cost for recorder/player

Approx. $2,000

Approx. $2,000

Approx. $3,000

Initial storage capacity 54 GB 30 GB 300 GB

Read/write speed 36.5 Mbps 36.5 Mbps 1 Gbps

HVD is still in the late stages of development and it has probably noticed that the projected introductory price for an HVD is a bit steep. An initial price of about $120 per disc will probably be a big obstacle to consumers. However, this price might not be so insurmountable to businesses, which are HVD developers' initial target audience. Optware and its competitors will market HVD's storage capacity and transfer speed as ideal for archival applications, with commercial systems available as soon as late 2006. Consumer devices could hit the market around 2010.[5]

5. Future development and challenges:

In the past, the realization of holographic data storage has been frustrated by the lack of availability of suitable system components, the complexity of holographic multiplexing strategies, and perhaps most importantly, the absence of recording materials that satisfied the stringent requirements of holographic data storage. Recently the development of practical components for holographic systems has rekindled interest in this technology. While the development of the needed components has been accomplished for non holographic markets, the volume of these markets is expected to lead to low-cost, reliable components for holographic data storage. DVD-R (red 680nm) and DVD-B (blue 405-407nm) have been developed for the optical storage market place. These recording sources have the desired characteristics for holographic storage and are attractive due to their small size, ruggedness, and low cost.

The InPhase Technology team has invented several multiplexing techniques that yielded a simple, easily implementable architecture for holographic storage systems.[5]

Changes in both the quality of the laser beam and recording material are being researched, but these improvements must take into consideration the cost-

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effectiveness of a holographic memory system. These limitations to current laser beam and photosensitive technology are some of the main factors for the delay of practical holographic memory systems.[10]

6. Conclusion

May be one day one of these scenarios with data stored in holograms materializes and become reality in the foreseeable future. In collaboration and competition with a large number of scientists from around the globe, the study on the technical feasibility of holographic storage and memory devices with parameters that are relevant for real-world applications should be continued. Whether this research will one day lead to products depends on the insights that leads to gain into these technical issues and how well holography can compete with established techniques in the marketplace.

References :

1. Computer organisation by Carl hamacher.

2. www.softpedia.com.

3. http://www.riedelit.com/Data_Recovery_Technical_Guide.html

4. http://www.usbyte.com/common/optical_data_storage_systems.html

5. www.howstuffworks.com/

6. www.mediastoragedevices.com/holographic-versatile-disc.html

7. Kevin Kurtis from Iphase technologies.

8. http://www.answers.com/topic/holographic-versatile-disc

9. http://www.hvd-forum.org/abouthvd/technology.html

10. Holographic memory, by John Sand.

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