CONTENTS

INTRODUCTION
HOLOGRAPHY
PHOTOGRAPHY AND HOLOGRAPHY
COMPONENTS OF HDSS
STORING OF DATA
RETRIEVAL OF DATA
MULTIPLEXING
PAGEDATA ACCESS
MATERIAL AND MEDIA REQUIREMENTS
PRESENT SCENARIO
MEMORY HIERARCHY
FUTURE CONCIEVABLE PRODUCTS
APPLICATIONS
ADVANTAGES
DISADVANTAGES
CONCLUSION
REFERENCES

ABSTRACT

With its omnipresent computers, all connected via the Internet, the Information Age has led to an explosion of information available to users. The decreasing costs of storing data, and the increasing storage capacities of the same small device footprint, have been key enablers of this revolution. While current storage needs are being met, storage technologies must continue to improve in order to keep pace with the rapidly increasing demand.

In order to increase storage capabilities, scientists are now working on a new optical storage method, called holographic memory that will go beneath the surface and use the volume of the recording medium for storage, instead of only the surface area.

Both magnetic and conventional optical data storage technologies, where individual bits are stored as distinct magnetic or optical changes on the surface of a recording medium, are approaching physical limits beyond which individual bits may be too small or too difficult to store. Storing information throughout the volume of a medium—not just on its surface—offers an intriguing high-capacity alternative. Holographic data storage is a volumetric approach, which although conceived decades ago, has made recent progress toward practicality with the appearance of lower-cost enabling technologies, significant results from longstanding research efforts, and progress in holographic recording materials.

This seminar deals with the recording and retrieval of data in holographic memories and the advantages and disadvantages and the future of holographic memory.

INTRODUCTION

Devices that use light to store and read data have been the backbone of data storage for nearly two decades. Compact discs revolutionized data storage in the early 1980s, allowing multi-megabytes of data to be stored on a disc that has a diameter of a mere 12 centimeters and a thickness of about 1.2 millimeters. In 1997, an improved version of the CD, called a digital versatile disc (DVD), was released, which enabled the storage of full-length movies on a single disc.

CDs and DVDs are the primary data storage methods for music, software, personal computing and video. A CD can hold 783 megabytes of data, which is equivalent to about one hour and 15 minutes of music, but Sony has plans to release a 1.3-gigabyte (GB) high-capacity CD. A double-sided, double-layer DVD can hold 15.9 GB of data, which is about eight hours of movies. These conventional storage mediums meet today's storage needs, With its omnipresent computers, all connected via the Internet, the Information Age has led to an explosion of information available to users. The decreasing cost of storing data, and the increasing storage capacities of the same small device footprint, have been key enablers of this revolution. While current storage needs are being met, storage technologies must continue to improve in order to keep pace with the rapidly increasing demand.

In order to increase storage capabilities, scientists are now working on a new optical storage method, called holographic memory that will go beneath the surface and use the volume of the recording medium for storage, instead of only the surface area.

Both magnetic and conventional optical data storage technologies, where individual bits are stored as distinct magnetic or optical changes on the surface of a recording medium, are approaching physical limits beyond which individual bits may be too small or too difficult to store. Storing information throughout the volume of a medium—not just on its surface—offers an intriguing high-capacity alternative. Holographic data storage is a volumetric approach, which although conceived decades ago, has made recent progress toward practicality with the appearance of lower-cost enabling technologies, significant results from longstanding research efforts, and progress in holographic recording materials.

HOLOGRAPHY

Holography was invented in 1948 by Hungarian physicist Dennis Gabor (1900-1979), for which he received the Nobel Prize in physics in 1971. He received patent GB685286 on the invention. The discovery was an unexpected result of research into improving electron microscopes at the British Thomson-Houston Company in Rugby, England, but the field did not really advance until the invention of the laser in 1960.
First of all the question is What is HOLOGRAPHY?

In holography, one beam (the reference beam) comes directly from the laser, while the other (the object beam) comes from the same laser but impinges on the object, and is distorted by it, before striking the photographic film. What is recorded on the film is the interference pattern produced by the two beams. After development, if the hologram is illuminated by a beam of light from the direction of the reference beam, the object beam is recreated, and the object "appears."

A hologram is a three-dimensional picture .It is a recording of an interference pattern made by the interaction of two beams of light.

Laser light is used in holography because it has coherence. This means that the waves of light coming from the laser are not only of the same wavelength, but that all the waves are "in step", like soldiers marching together. White light, from the sun or a light bulb, is made up of many different wavelengths, so it is like a group of people walking along, some taking short strides, some longer ones, all out of step, and going in various directions.

PHOTOGRAPHY AND HOLOGRAPHY

Another important question that comes to the mind is what is the difference between Holography and photography?

A photograph has an actual physical image; a hologram contains information about the size, shape, brightness and contrast of the object being recorded. This information is stored in a very microscopic and complex pattern of interference. The interference pattern is made possible by the properties of light generated by a LASER.

When you shine a light on the hologram, the information that is stored as an interference pattern takes the incoming light and re-creates the original optical wave front that was reflected off the object. Your eyes and brain now perceives the object as being in front of you once again.

The difference between holography and photography is best understood by considering what a Black & White (B&W) photograph actually is: it is a point-to-point recording of the intensity of light rays that make up an image. Each point on the photograph records just one thing, the intensity (i.e. the square of the amplitude of the electric field) of the light wave that illuminates that particular point.

However, the light which makes up a real scene is not only specified by its amplitude and wavelength, but also by its phase. In a photograph, the phase of the light from the original scene is lost. In a hologram, both the amplitude and the phase of the light (usually at one particular wavelength) are recorded. When reconstructed, the resulting light field is identical to that which emanated from the original scene, giving a perfect three-dimensional image .

COMPONENTS OF HDSS

Over the past decade, the Defense Advanced Research Projects Agency (DARPA) and high-tech giants IBM and Lucent's Bell Labs have led the resurgence of holographic memory development. Prototypes developed by Lucent and IBM differ slightly, but most holographic data storage systems (HDSS) are based on the same concept. Here are the basic components that are needed to construct an HDSS:
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 (CCD) camera

STORAGE OF DATA

In holographic data storage, an entire page of information is stored at once as an optical interference pattern within a thick, photosensitive optical material (Figure 1).

Figure 1

Intersecting two coherent laser beams within the storage material does this. The first, called the object beam, contains the information to be stored; the second, called the reference beam, is designed to be simple to reproduce—for example, a simple collimated beam with a planar wavefront. The resulting optical interference pattern causes chemical

and/or physical changes in the photosensitive medium: A replica of the interference pattern is stored as a change in the absorption, refractive index, or thickness of the photosensitive medium.

A large number of these interference gratings or patterns can be superimposed in the same thick piece of media and can be accessed independently, as long as they are distinguishable by the direction or the spacing of the gratings. Such separation can be accomplished by changing the angle between the object and reference wave or by changing the laser wavelength.


When the blue-green argon laser is fired, a beam splitter creates two beams. One beam, called the 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 raw binary data as clear and dark boxes. The signal beam around to the light-sensitive lithium-niobate crystal carries the information from the page of binary code. Some systems use a photopolymer in place of the crystal. A second beam, called the reference beam, shoots out the side of the beam splitter and takes a separate path to the crystal. When the two beams meet, the interference pattern that is created stores the data carried by the signal beam in a specific area in the crystal -- the data is stored as a hologram.

RETRIEVAL OF DATA

When the stored interference grating is illuminated with one of the two waves that were used during recording [Figure 2(a)], some of this incident light is diffracted by the stored grating in such a fashion that the other wave is reconstructed. Illuminating the stored grating with the reference wave reconstructs the object wave, and vice versa [Figure 2(b)]. Interestingly, a backward-propagating or phase-conjugate reference wave, illuminating the stored grating from the “back” side, reconstructs an object wave that also propagates backward toward its original source [Figure 2(c)].

Any particular data page can then be read out independently by illuminating the stored gratings with the reference wave that was used to store that page. Because of the thickness of the hologram, this reference wave is diffracted by the interference patterns in such a fashion that only the desired object beam is significantly reconstructed and imaged on an electronic camera. The theoretical limits for the storage density of this technique are around tens of terabits per cubic centimeter.

In addition to high storage density, holographic data storage promises fast access times, because the laser beams can be moved rapidly without inertia, unlike the actuators in disk drives. With the inherent parallelism of its page wise storage and retrieval, having a large number of relatively slow, and therefore low-cost, parallel channels can reach a very large compound data rate.



The data to be stored are imprinted onto the object beam with a pixelated input device called a spatial light modulator (SLM); typically, this is a liquid crystal panel similar to those on laptop computers or in

modern camcorder viewfinders. To retrieve data without error, the object beam must contain a high-quality imaging system—one capable of directing this complex optical wavefront through the recording medium, where the wavefront is stored and then later retrieved, and then onto a pixelated camera chip (Figure 3).

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

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 that 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.
1. 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. 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. Staying away from conventional data access systems that move mechanical matter to obtain data, the angle of entry on the source beam can be deflected by high-frequency sound waves in solids. The elimination of mechanical access methods reduces access times from milliseconds to microseconds.

2. 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.

3. 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.

4. 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 that

cover the same area as the original reference beam. These smaller beamlets vary by phase that changes the state of the reference beam as a whole. The reference beam 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 non-mechanical means, therefore speeding up access times.

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.

PAGE DATA ACCESS

Because 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 in conventional storage systems. It was mentioned in the introduction that conventional storage was reaching its fundamental 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 a stream of 32 bits is sent to a processing unit by a conventional read head, a holographic memory system would in turn send 32 x 32 bits, or 1024 bits due to its added dimension. 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 bits at 15-20 microns per pixel.

MATERIAL AND MEDIA REQUIREMENTS

Desirable parameters described so far include storage capacity, data input and output rates, stability of stored data, and device compactness, all of which must be delivered at a specified (very low) user BER. To a large extent, the possibility of delivering such a system is limited by the properties of the materials available as storage media. The connections between materials properties and system performance are complex, and many tradeoffs are possible in adapting a given material to yield the best results. Here we attempt to outline in a general way the desirable properties for a holographic storage medium and give examples of some promising materials.

Properties of foremost importance for holographic storage media can be broadly characterized as “optical quality,” “recording properties,” and “stability.” These directly affect the data density and capacity that can be achieved, the data rates for input and output, and the BER.

As mentioned above, for highest density at low BER, the imaging of the input data from the SLM to the detector must be nearly perfect, so that each data pixel is read cleanly by the detector. The recording medium itself is part of the imaging system and must exhibit the same high degree of perfection. Furthermore, if the medium is moved to access different areas with the readout beam, this motion must not compromise the imaging performance. Thus, very high standards of optical homogeneity and fabrication must be maintained over the full area of the storage medium. With sufficient materials development effort and care in fabrication, the

necessary optical quality has been achieved for both inorganic photorefractive crystals and organic photopolymer media.

Because holography is a volume storage method, the capacity of a holographic storage system tends to increase as the thickness of the medium increases, since greater thickness implies the ability to store more independent diffraction gratings with higher selectivity in reading out individual data pages without crosstalk from other pages stored in the same volume. For the storage densities necessary to make holography a competitive storage technology, a media thickness of at least a few millimeters is highly desirable. In some cases, particularly for organic materials, it has proven difficult to maintain the necessary optical quality while scaling up the thickness, while in other cases thickness is limited by the physics and chemistry of the recording process.

Much basic research in holographic storage has been performed using photorefractive crystals as storage media. Of these crystals, Fe-doped lithium niobate has been the workhorse. Its sensitivity is sufficient for demonstration purposes, but lacks a factor of 100 for practical application. Since photorefractive are reversible materials, they suggest the possibility of a rewritable holographic storage medium. However, because they are linear and reversible, they are subject to erasure during readout.

Materials for writing permanent volume holograms generally involve irreversible photochemical reactions that are triggered by the bright regions of the optical interference pattern. A photopolymer material, for example, polymerizes in response to optical illumination: material diffuses from darker to brighter regions so that short monomer chains can bind together

to form long molecular chains. And in a so-called direct-write or photochromic material, the illuminated molecules undergo a local change in their absorption or index of refraction, which is driven by photochemistry or photo-induced molecular reconfiguration.

Most erasable holographic materials are inorganic photorefractive crystals doped with transition metals or rare-earth ions. These crystals are often available in centimetre-thick samples and include lithium niobate, strontium barium niobate and barium titanate doped with iron, cerium, praseodymium or manganese.

These materials react to the light and dark regions of an interference pattern by transporting and trapping electrons, which subsequently leads to a local change in the index of refraction. The trapped charge can be rearranged by later illumination, so it is possible to erase recorded holograms and replace them with new ones.

PRESENT SCENARIO

After more than 30 years of research and development, a desktop holographic storage system (HDSS) is close at hand. There is still some fine tuning that must be done before such a high-density storage device can be marketed, but IBM researchers have suggested that they will have a small HDSS device ready as early as 2003. These early holographic data storage devices will have capacities of 125 GB and transfer rates of about 40 MB per second. Eventually, these devices could have storage capacities of 1 TB and data rates of more than 1 GB per second -- fast enough to transfer an entire DVD movie in 30 seconds. So why has it taken so long to develop an HDSS, and what is there left to do?

When the idea of an HDSS was first proposed, the components for constructing such a device were much larger and more expensive. For example, a laser for such a system in the 1960s would have been 6 feet long. Now, with the development of consumer electronics, a laser similar to those used in CD players could be used for the HDSS. LCDs weren't even developed until 1968, and the first ones were very expensive. Today, LCDs are much cheaper and more complex than those developed 30 years ago. Additionally, a CCD sensor wasn't available until the last decade. Almost the entire HDSS device can now be made from off-the-shelf components, which means that it could be mass-produced.

Although HDSS components are easier to come by today than they were in the 1960s, there are still some technical problems that need to be worked out. For example, if too many pages are stored in one crystal, the strength of each hologram is diminished. If there are too many holograms

stored on a crystal, and the reference laser used to retrieve a hologram is not shined at the precise angle, a hologram will pick up a lot of background from the other holograms stored around it. It is also a challenge to align all of these components in a low-cost system.

Better multiplexing techniques are certainly welcome, but a fundamental means of increasing capacity will be needed if holographic memories are to make inroads against compact discs. Holographic memories have been shown to be significantly faster at present than are compact-disc systems, but speed alone is rarely enough for a new technology to supplant an entrenched one. What is generally needed is another basic advantage, such as greater storage capacity.

As in any holographic medium, data are stored throughout the volume of the recording layer of the 3-D disk. The head has a detector array for reading out an entire page of data and a beam deflector for angle multiplexing. A spatial-light modulator, which imprints the page of data onto the signal beam (such as the LCD screen used in current demonstrations), could also be incorporated into the head. Even though a 3-D disk stores information in three dimensions, the number of bits that could theoretically be stored per square micron of disk surface can be computed for the purpose of comparing this areal density to that of a conventional CD. Such a comparison is reasonable because a 3-D disk can be as thin as a CD. It turns out that for thicknesses less than two millimeters, the areal density of the holographic disk is approximately proportional to the thickness of the recording medium. We can increase the surface density, moreover, by simply increasing the thickness of the holographic layer. Density of 100 bits per square micron would be possible with a material

one millimeter thick. Such a 3-D disk would be nearly identical in size and weight to a conventional CD, but it would store 100 times more information.

Among the companies pursuing this basic technology is Holoplex, a small start-up that was co-founded in Pasadena, Calif. The company has built a high speed memory system capable of storing up to 1,000 fingerprints, for use as a kind of selective lock to restrict access to buildings or rooms. Although the capacity of this system is approximately half that of a CD, its entire contents can be read out within one second. Holoplex is now working on another product that would be capable of storing up to a trillion bits, or almost 200 times what can be put on a CD.

Researchers are confident that technologies will be developed in the next two or three years to meet these challenges. With such technologies on the market, you will be able to purchase the first holographic memory players by the time "Star Wars: Episode II" is released on home 3-D discs. This DVD-like disc would have a capacity 27 times greater than the 4.7-GB DVDs available today, and the playing device would have data rates 25 times faster than today's fastest DVD players.

MEMORY HIERARCHY

In the memory hierarchy, holographic memory lies somewhere between RAM and magnetic storage in terms of data transfer rates, storage capacity, and data access times. The theoretical limit of the number of pixels that can be stored using volume holography is V2/3/l2 where V is the volume of the recording medium and l is the wavelength of the reference beam. For green light, the maximum theoretical storage capacity is 0.4 Gbits/cm2 for a page size of 1 cm x 1 cm. Also, holographic memory has an access time near 2.4 ms, a recording rate of 31 kB/s, and a readout rate of 10 GB/s. Modern magnetic disks have data transfer rates in the neighborhood of 5 to 20 MB/s. Typical DRAM today has an access time close to 10 – 40 ns, and a recording rate of 10 GB/s.


Storage
Medium

Access Time
Data Transfer
Rate
Storage Capacity
Holographic Memory
2.4 ms
10 GB/s
400 Mbits/cm2
Main Memory (RAM)
10 – 40 ns
5 MB/s
4.0 Mbits/cm2
Magnetic Disk
8.3 ms
5 – 20 MB/s
100 Mbits/cm2


Table 1: This table shows the comparison of access time, data transfer rates (readout), and storage capacity (storage density) for three types of memory; holographic, RAM, and magnetic disk.



Holographic memory has an access time somewhere between main memory and magnetic disk, a data transfer rate that is an order of magnitude better than both main memory and magnetic disk, and a storage capacity that is higher than both main memory and magnetic disk. Certainly if the issues of hologram decay and interference are resolved, then holographic memory could become a part of the memory hierarchy, or take the place of magnetic disk much as magnetic disk has displaced magnetic tape for most applications.

FUTURE CONCIEVABLE PRODUCTS

In principle, laser beams can be moved with no mechanical components, allowing access times of the order of 10 µs, faster than any conventional disk drive will ever be able to randomly access data. As in other optical recording schemes, and in contrast to magnetic recording, the distances between the “head” and the media are very large, and media can be easily removable. In addition, holographic data storage has shown the capability of rapid parallel search through the stored data via associative retrieval.

On the other hand, holographic data storage currently suffers from the relatively high component and integration costs faced by any emerging technology. In contrast, magnetic hard drives, also known as direct access storage devices (DASD), are well established, with a broad knowledge base, infrastructure, and market acceptance

Four conceivable product scenarios are shown in Figure 13. The first two scenarios use read/write media, while the latter two are designed for WORM materials, which are much easier to develop but must support data retention times as long as tens of years. The first scenario [Figure 13(a)] takes advantage of rapid optical access to a stationary block of media, resulting in a random-access time of the order of 10 µs. The capacity is limited to about 25 GB by the size of the block of media that can be addressed by simple, inexpensive optics. Such a device could bridge the gap between conventional semiconductor memory and DASD, providing a nonvolatile holographic cache with an access time that is between DASD and dynamic random-access memory (DRAM).

Using the same optical components but replacing the stationary block of media with a rotating disk results in performance characteristics similar to those of a disk drive, albeit with terabytes (1012 bytes) of capacity per platter [Figure 13(b)]. In the CD-ROM type of embodiment [Figure 13(c)], holographic data storage takes advantage of the fact that single-exposure full-disk replication has been demonstrated. The player for the holographic ROM is conceptually very simple: a CMOS camera chip replaces the photodiode from a conventional ROM player, and the reconstructed data page is then imaged with suitable optics onto that camera.

Figure 13

Combining one of the DASD-type R/W heads and possibly a number of CD-ROM-type readers, a robotic picker, and sufficient tiles of media, a data warehouse with petabyte (1015 bytes) capacity in a standard 19-inch rack is conceivable .While the access time to any of the stored files is determined by the robotic picker and will be of the order of tens of seconds, the aggregate sustained data rate could be enormous. In this scenario, the relatively high component cost of a read/write holographic engine is amortized over a large volume of cheap media to obtain competitive cost per gigabyte.

APPLICATIONS

There are many possible applications of holographic memory. Holographic memory systems can potentially provide the high-speed transfers and large volumes of future computer systems.
PARALLELISM: In conventional storage, data is recorded and retrieved serially. Holographic storage, on the other hand, uses the information capacity of an optical wave-front so that data can be recorded and retrieved in parallel, one page at a time. Due to the page-oriented nature of holographic storage, the potential exists for extremely high data rates, subject only to the limitations imposed by I / O (input/output) devices. Holographic storage systems can have data rates approaching 1.0 Gbytes / sec. In addition, because beam deflection, as opposed to moving parts, is used to access the stored holograms, access times in the 10-ms range could be achieved. Hence searching can be very fast since it uses parallel search.
DATAMINING: One possible application is data mining. Data mining is the process of finding patterns in large amounts of data. Data mining is used greatly in large databases which hold possible patterns which cannot be distinguished by human eyes due to the vast amount of data. Some current computer 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 storage capacity that holographic memory provides could exceed conventional storage and speed up data mining considerably. This would result in more located patterns in a shorter amount of time.

PETAFLOP COMPUTING: Another possible application of holographic memory is in petaflop computing. A petaflop is a thousand trillion floating point operations per second. The fast access in extremely large amounts of data provided by holographic memory systems could be utilized in a petaflop architecture. Clearly advances are needed in more than memory systems, but the theoretical schematics do exist for such a machine. Optical storage such as holographic memory provide a viable solution to the extreme amount of data which is required for petaflop computing.

ADVANTAGES

HIGH CAPACITY: Holographic storage devices having 125GB capacity has already been realized. Eventually these devices could have storage capacities of 1TB or even 1PB.

HIGH TRANFER RATE: Early holographic data storage devices have transfer rates up to 40MB/sec .It is expected that it would have transfer rates up to 1GB/sec

PARALLEL SEARCHING: Holographic data storage enables rapid parallel searching as data is stored and retrieved in parallel. Hence searching is very fast.

REDUNDANCY: Because a single page of bits may be stored at one time, the information content of the page is intermingled. Thus any defect occurring in the recording medium would not destroy the data bits .Rather, only the signal to noise ratio is affected.

BANDWIDTH: Bandwidth is the amount of information that can be contained in Individual channels.

DISADVANTAGES

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

The recording mechanism for photopolymers also leads to some disadvantages, including shrinkage of the material wit polymerization and the possibility of nonlinear response. Both of these distort the reconstructed holograms and thus cause errors in decoding digital data.

On the other hand, holographic data storage currently suffers from the relatively high component and integration costs faced by any emerging technology. In contrast, magnetic hard drives, also known as direct access storage devices (DASD), are well established, with a broad knowledge base, infrastructure, and market acceptance.

Another important problem that arises is that the whole recording medium and apparatus should exhibit high degree of perfection. That is it should be able to even withstand the slightest of jerks.

CONCLUSION

The future of holographic memory is very promising. The page access of data that holographic memory creates will provide a window into next generation computing by adding another dimension 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 systems implemented 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 than any other medium on the market. The current storage in a personal computer operates on the same principles used in the first magnetic data storage devices. The parallel nature of holographic memory has many potential gains on serial storage methods. However, many advances in optical technology and photosensitive materials need to be made before we find holograms in computer systems.

Holographic techniques may provide a long sought ideal: a mass memory with archival permanence and yet electronic accessibility. It also promises to provide a long wished-for mass storage device for data processing that is devoid of any mechanical motion and which integrates in a single unit, permanent recording with high speed electronic random accessibility. Holographic technology is looking toward a wide range of commercial markets, including, multimedia computing, video-on-demand, high-definition television, portable computers, and consumer video.

REFERENCES

E. Chuang, “Methods and architecture for rewritable holographic memories,” Ph.D. dissertation, California Inst. Technol., Pasadena, 1998.silicon/liquid crystal devices. Jean-Jacques is currently with Micro Display Corporation, San Pablo, CA.

Holographic Random Access Memory (HRAM):ERNEST CHUANG, WENHAI LIU, JEAN-JACQUES P. DROLET, ASSOCIATE MEMBER, IEEE,AND DEMETRI PSALTIS, SENIOR MEMBER, IEEE.

IBM Journal of Research and Development ,HOLOGRAPHIC MEMORIES by, J. Ashley

PROCEEDINGS OF THE IEEE, VOL. 87, NO. 11, NOVEMBER 1999

http://www.optics.arizona.edu/Glenn/holograp1.htm

http://www.research.ibm.com/journal/rd/443/ashley.html

http://optics.caltech.edu/Publications/Papers/Chuang%20HRAM.pdf.