Holographic Data Storage Complete

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HOLOGRAPHIC DATA STORAGE

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Contents

1. Abstract5

2. Introduction..5

3. Different types of data storage systems5

3.1. Magnetic data storage system5

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...94. 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. Working12

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 Disk17

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..195. 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 memorysystems.

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 whichdiscusses 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 alwaysgrowing 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 alwaysgoing 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 datastorage.

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 systemsare

Magnetic disk

& Floppy disk

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3.1.2Floppydisks

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 diskshave been superseded by the emergence of rewritable compact disks having higher capacities.

[1]

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3.2.Opticaldatastorage

Today's meaning of optical data storage refers to storage systems that use light for

recording and retrieval of information. Optical recording systems potentially havemuch 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 - harddrives - 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 (andheavy) 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 thelight 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.

3.2.1CDTechnology:

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. Laterathin, 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.

3.2.2TheDVDtechnology

The quest for greater storage capacity resulted in the development of digital versatile

disk(DVD) technology in 1996.

ThestructureofDVD

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 DVDTo 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 layeredor 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

Single-sided/double-layer

Double-sided/single-layer

Double-sided/double-layer

4.38 GB

7.95 GB

8.75 GB

15.9 GB

2 hours

4 hours

4.5 hours

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 interferencebetween 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 ofthe 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 thevolume 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 beused. 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 twoseparate 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 itswaveforms. 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 HDSSwith 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 compellingchoice for next-generation storage.

In addition, the flexibility of the technology allows a wide variety ofholographic storage products to be developed, ranging from handheld devicesfor 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 sunlightis 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 resultingcross 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, ofdata[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 (andLCD 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 uponhitting 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 duringreadout 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 transferrates (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 beamidentical 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 notbeneficial. 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 itreenters 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. Thethicker 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 regeneratedsource 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 ofmechanical 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 MultiplexingSpatial 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 bephysically 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 andshift 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 berotated 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 bepositioned 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 entermultiple 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 peristrophicmultiplexing 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 partsof 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. Thereference 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 bynonmechanical 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 beentested and the results are promising. Recent tests have also been formed on spatialmultiplexing 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 zerolight 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 arearepresenting 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. Thisconfiguration is the key to apply Collinear Technology to commercial HVD

products. These address pits and the servo technology to read them guarantee theinterchangeability 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 holographicrecording 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 aconventional 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 muchhigher 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 ofover $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 toincrease 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 recordableApprox. $18 Approx. $10 Approx.

disc $120

Initial cost forrecorder/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 $120per 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 andtransfer 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 holographicmultiplexing strategies, and perhaps most importantly, the absence of recordingmaterials 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 neededcomponents 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 desiredcharacteristics 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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