III. LITERATURE REVIEW

1. Introduction

Mass storage is the cabinet in which electronic information is kept. The more generous the storage area, the richer, more detailed and more comprehensive is the database. Mass storage is usually defined as a means of preserving computer generated information for subsequent use or retrieval.

Information is said to have been written in a chosen medium if its physical or chemical state is altered under the influence of the signal (or writing beam) energy. The altered state in a storage medium is brought about by making proper use of one or more of its characteristic properties (thermal, chemical, electrical, magnetic, elastic, optical, etc.). Reading of the stored information is accomplished by sensing the altered state using an appropriate technique, and erasing of information is achieved using such means as electrical, magnetic or optical , either alone or in some suitable combination (Tapscott, 1993).

The ability to store and process information in new ways has been essential to humankind's progress. From the early Egyptian papyrus through the Gutenberg press, the Dewey decimal system and eventually, holographic technology, information storage has been the catalyst for increasingly complex technological, scientific and societal systems. Modern science is inextricably bound to information processing with which it exists in a form of symbiosis. Scientific advances have enabled the storage, retrieval and processing of ever more information, which in turn, helped generate the insights needed for further advances.

Recent years have witnessed a significant increase in electronic management of information. In particular many types of information that traditionally were considered to be analog, such as images and sound, are now processed and utilized in digital form. This advancement comes with tremendous opportunities and challenges for information systems to better meet information users needs to manipulate multimedia information in a natural and effective manner. Many industries, among which multimedia, are eagerly looking for new alternatives to their current storage systems, as their databases, and storage requirements are growing larger and larger.

2. Evolution Of Computer Storage

Even before the first commercial electronic computers appeared in 1951 (with the UNIVAC), mass storage, although minuscule by today's standards. was a necessity. As early as the mid-1800'', punch cards were used to provide input to early calculators and other machines. The 1940's ushered in the decade when vacuum tubes were used for storage until finally, tapes drives (serial access) started to replace punch cards in the early 1950's. A couple of years later, magnetic drums appeared on the scene, and in 1957, the first hard drive was introduced as a component of IBM's Ramac 350 (Tapscott, 1993 ).

The first hard drives (random access) were bulky and expensive and were confined to mainframes and minicomputer installations. The personal computer revolution in the early 1980's changed all that , ushering in the introduction of smaller hard drives, and also floppy disks and magnetic tapes used in some computer applications and cartridges used in some video games systems. With the development of personal computing and the emergence of multimedia, larger information storage was needed. The CD-ROM was introduced gaining vast popularity for its capacity (up to 650 MB), durability, efficiency and compact size (Lorentz, 1996 ).

3. The Multimedia Revolution

It may be objected that buzzwords like Multimedia or New Media are not much more than hype currently. But it doesn't take much to realize that these concepts represent a host of human computer interactions that the most innovative thinkers have been developing for years, and which are only now beginning to enter the mainstream. These concepts are being embodied in the best new computer applications, which will have a dramatic impact on the work of all communication professionals in the 1990's.

At the core of this multimedia revolution is the accelerating shift from material information media-including paper, photographic film, audio cassettes and videotape- to computer based simulations of those media. This is accomplished through what we call "digitization" (Ross, 1994), which consists of the transformation of all kinds of information into a uniform system based on two terms, the binary digits 0 and 1, and the infinite possibilities for combining these. This technique has brought a paradigm shift in the electronics industry, as it made possible the compatibility of different sorts of information. Audio, video, and text are recorded in digital form, and are then sent via digital communication carriers, which recognize such messages as bits of information rather than distinct audio and visual forms (Shandle, 1994). The prospect of brewing this exotic multimedia mix for mass consumption is fueling public excitement as well as dozens of new business ventures that are bringing together computer, communications, media, and entertainment in search of new media and digital convergence.

However, multimedia programs eat up a large amount of storage space: About 10 to 1000 times the storage capacity required by other conventional documents. Each second of full motion, full screen digitized video, for example, requires 30 frames of video information, each frame requiring one megabyte of computer storage. This translates into about 30 megabytes of information per second, or 1.8 gigabyte per minute (good compression techniques may bring it down to 12 Mbytes per minute of video). In fact, multimedia and the consequent flood of data information has risen some concerns about computer storage, and the limits that it places on the electronic information products (Arnold, 1993). Currently this storage is provided by two basic types of storage: Magnetic disk storage and optical disk storage.

Magnetic Disk Storage:

The most widely used secondary storage device today is the magnetic disk. This includes both hard disks and removable floppy disks.

Hard Disk Drives: Hard disk drives are the most common type of mass storagedevice thanks to their low cost, high speed and relatively high storage capacity. Hard disks store and retrieve digital data by writing information as a series of magnetic fluctuations on a rigid disk coated with a magnetically coercive material. A magnetic disk rotates at 3.600 to 7.200 rpm (Lorentz, 1996). While a read-write head flies less than 100 nm above the magnetic layer of the disk. The head writes data on the disk by varying a magnetic field that it produces, and reads data by sensing subtle changes in the magnetic field on the disk . Information is stored on the disk in concentric rings or tracks. The performance of a hard drive is measured by two primary criteria: 1. access time, which is the major component of the time it takes to randomly access a unit of data, and 2. transfer rate, which is the number of bytes per second that can be read from or written to a disk drive. Access time depends on the head actuator seek speed and disk rotational speed. Typical access times fall between 10 and 20 ms. The data transfer rate depends largely on the aerial density, the number of bits contained per inch of media, and the rotational speed of the disk. Typical data transfer rates are 4 to 16 MB/s (Arnold, 1994). Disk drive prices range from 25 cents to $ 1 per megabyte, depending on the capacity, size and manufacturer. Hard disk storage systems specialized for multimedia applications are available, and they offer high capacities, typically in excess of 1 GB (Lesser, 1993).

Floppy Disks: Floppy disks come in two main formats, the 5.25 and the 3.5 inch disks. The 5.25 was introduced by Shugart Associates in 1976 and quickly dominated the removable media for the personal computer , until 1981 when Sony introduced 3.5 inch format disk which are today the most common removable storage media for personal computers and can hold up to 2.88 MB of data.(Lorentz, 1996).

Advantages: Magnetic disk offer direct access to the data (Random access memory). This makes them must faster than tape drives, for example,which have sequential or serial access memories. This speed allows for the development of on line information systems. Disks also allow for interrelationships among data to be built into the storage file. It allows for a single update to change many files at one time. With the advances in technology in recent years, very large (several gigabytes) and less expensive devices are now available (Lorentz, 1996).

Disadvantages: Disks are susceptible to the environment. A smoke particle or human hair can cause severe damage to the read/write head. They require a pure and stable environment.

Optical Disk Storage:

A laser is used to burn microscopic pits in the disk service.These pits are used to store large quantities of data including pictures, sound and full motion video. The data is retrieved by a low power laser beam. The most commonly known optical disk storage devices are CD- ROMs and magneto-optical disks.

CD-ROM: CD-ROMs are read-only disks that can hold about 300,000 pages of text or about 680 MB of data, nearly 300 times that of a high density floppy disk. Optical disks are now available in Read Only Memory (ROM) which are recorded prior to purchase. The data can only be retrieved but not edited or added to. It is also available in Write Once Read Many (WORM) which allows the used to record data to be read many times in the future.

Magneto-Optical Storage: A magneto-optical system resembles a 3.5 or 5.25 inch floppy drive, but the disks are thicker and referred to as "cartridges" which unlike fixed hard drive, can be removed. Magneto-optical systems combine two technologies: magnetic and optical. Magnetic fields store the information, and are read by a polarized laser that detects magnetism (Arnold, 1991). MOs offer high capacity and reliability, but they are more costly than hard drives or disk array systems. MOs also provide quick access to off-line files and are an advantage to multimedia users who may need to transport files from one system to another or use large files (Meilach, 1994)

Advantages: Optical disk has the ability to store large quantities of data in a small space. It also allows the data to be easily transported to other locations also offer high quality storage of sound and video.

Disadvantages8 The read only feature of CD-ROM may not be suitable for many applications. The cost of rewritable disk may be prohibitive, even though that price is rapidly dropping. This technology is still in development so equipment purchased today may not be compatible with devices in the future. Standards are just now being defined for these devices to eliminate some of these problems. Although optical disk is a rapidly developing technology, and may replace the magnetic disk just as the magnetic disk replaced magnetic tape, particularly with the advent of multimedia, the capacity of a CD-ROM is relatively small, as one minute of digitized video necessitates storage capacity of about 1.8 Gb Ɠ times more than CD-ROMs can normally handle).

Magnetic and optical systems have long dominated the storage market. However an increasing demand for significantly more storage capacity coupled with the development of a wide variety of applications have taxed the ability of these storage systems. And as the requirements on these systems continue to grow, the limits of current technology will eventually be reached. One solution as identified by researchers and industry leaders, is a holographic storage system. This system in contrast to conventional magnetic and optical recording, requires the unique integration of many different technologies as opposed to one dominant technology (Pappu, 1990). By combining these technologies, it is anticipated that the resulting storage system would be both original and competitive in performance, size, and cost.

4. What Is Holography?

After many years of research and development, holographic storage (also known as volume-holographic-storage) is finally on the verge of becoming a reality. Some of the questions about holography which come to mind immediately might serve as a good starting point . They are: what is a hologram? and how does holography work?

The terms holograms and holography were coined by Dennis Gabor (a Hungarian-born physicist and Nobel prize winner, also known as the father of holography) in 1947. " Holos" stands in Greek for " total, complete" ; "gramma" means letter and writing. It has the same roots of " graphein" ( to write). In ancient Greece, however, the letter was also used as a number or system for the measurement of distinguishable unities, like in the word "kilogram", which does not mean writing with weight but the unity formed by one thousand grams. So if " gram" designates the unity and " holos", the total, the word " hologram" means the unity of the whole as well as the wholeness of the unity.

While photography as we know it today was invented by Nitpce as a culmination of centuries of research in that direction, holography, as many other inventions in the twentieth century, was the by-product of a search for something else. Namely, a method for improving the quality of images recorded on an electronic microscope. As opposed to photography, holography did not come as a consequence of centuries of perfectibility. Dennis Gabor, its inventor, needed in 1947 what was to be later called a laser to make three-dimensional holograms , but he invented holography almost fifteen years ahead of the appearance of the first laser which two engineers from the University of Michigan, namely Emmett Leith and Juris Upatnieks discovered in the early sixties. These two developed a new device which produced a three dimensional image of an object (Heckman, 1986). Building on the discoveries of the "father of holography", they produced the diffuse-light hologram. They were eerie, lifelike. three-dimensional pictures. One of the most startling characteristics of the holograms was that if you smashed the holographic plate, or the negative, every piece was capable of reconstructing the whole scene. Although and image from the tiniest fragments can fade and lose detail, the image itself is still whole, whether the piece came from the middle, bottom or top of the hologram (Jeong, 1975). When the first three-dimensional holograms were made, the technique was labeled "a solution in search of a problem" by the press.

For the purpose of examining some characteristics of holography by contrast, let us expose some of the other basic differences between holography and photography.

First of all, we need to understand that, as opposed to the photograph, the hologram is not a picture and holography is not primarily a picture-making technique. A hologram does not bear an image at all. What it actually does is just to perform the function of a lens, and only diffracts light in a particular way. In this sense, holograms are optical elements, not pictures. They perform optical functions rather than bear an image, and they are not extensions of photographs but a new way of recording, storing and retrieving optical information (information carried by light waves).

A photograph is basically the recording of the differing intensities of the light reflected by the object and imaged by a lens. The light is incoherent, therefore, there are many different wavelengths of light reflecting form the object and even the light of the same wavelength is out of phase (Outwater, 1995). Your emulsion will react to the light image focused by the lens and the chemical change of the silver halide molecules will result form the photon bombardment. There is a point to point correspondence between the object and the emulsion. Any object to be recorded can be thought of as the sum of billions of points on the object which are reflecting more or less light. The lens of the camera focuses each object point to a corresponding point on the film and there it exposes a proportional amount of silver halide. Thus, your record is of the intensity differences on the object which from a pattern that one may ultimately recognize as the object photographed.

In holography, light waves and silver halide emulsion are used, but beyond that , the comparison ceases, as holography uses a vastly different light source: Laser light .

Laser light differs drastically from all other light sources, man-made or natural, in one basic way which leads to several startling characteristics. Laser light can be coherent light. Ideally this means that the light being emitted by the laser is of the same wavelength and is in phase.

Gabor's theory was originally intended to increase the resolving power of electron microscopes (Jeong, 1975). The Nobel Prize physicist proved his theory not with an electron beam but with a light (non-coherent) light beam. The result was the first hologram ever made. The early holograms were legible but were plagued with many imperfections because Gabor didn't have the correct light source to make crisp, clear holograms as we can today. This is one of the reasons his discovery wasn't fully appreciated until the 1960's when the first laser was produced, since holograms could be realized only using coherent light. Since then, holography has become one of the newest and most intriguing of the many techniques with which technology has been concerned, and in many peoples' opinion now seems to likely become extremely helpful in many human endeavors, and will open a broad new horizon for the future (Glantz, 1994). The simplest description of holography is three dimensional recording with lasers. In other words, holography is the technology of recording wave-front (light wave) information and producing reconstructed wave-fronts from those recordings. The record of the wave-front information is called a hologram. Any propagating wave-front phenomenon such as microwaves or acoustic waves is a candidate for application of the principles of holography (Outwater, 1995).

To record a hologram, a laser beam is split into two beams, each sent along a separate path by mirrors. One beam, the object beam, illuminates the object/scene being recorded. The second beam, the reference beam, is directed at an angle to cross paths and interfere with the light reflected off the object from the object beam. If we were to simply illuminate our object with laser light and take a photograph, we would still only be recording the different light intensities of the object; we would not have captured any information about the phase of the light waves after bouncing off the object. That is when the reference beam comes into the scene. The reference source will allow us to record the phase difference of the light wave and thus capture the information which supplies the vital dimensions and depth of the object . Some form of recording medium, such as film, a photo refractive polymer or crystal, a video camera is positioned at the point of interference. The interference pattern is then recorded. As mentioned before, illumination must be accomplished with light of just one wavelength, such as laser, to create a precise interference pattern. To give an illustration, think of pebbles of equal size dropped into a still pond, each sending out concentric waves. Where the waves overlap, there is interference. Some of the waves build on each other constructively, others cancel each other out destructively. This dark light fringe pattern of interference characteristic of the object is recorded. With white light, containing a mixture of wave lengths (different-size pebbles), there is no precise fringe pattern created when the beams interfere. When it is developed, in the case of film, the interference pattern is played back, by illuminating it with a laser beam, recreating the scene or object (Stix, 1993).

To create holograms that appear to move, like those used in advertising, the subject is placed on a turntable that rotates as the subject goes thorough its motions. Images are captured as the table turns, and, on the finished product, narrow bands of hologram are presented to the viewer, each with the subject in a slightly different position (Halas, 1984).

Today, we can see holograms, or 3D images, on credit cards, magazine covers, art galleries. Yet this unique method of capturing information with lasers - the science of holography- has many more applications in the industrial world and is on the verge of revolutionizing data storage technology as we know it.

In traditional photography, light reflecting from the subject is focused by a lens onto a recording medium, typically film. As a result a photograph presents only one perspective of the scene, the perspective from the lens. Only the intensity of light is recorded, not the direction it came from. When recording a hologram, no focusing lens is between the recording medium and the laser light reflected off the subject, so the light falls on the medium from all angles. The image is correct from all perspectives, just like real life. The added dimension of the direction of the light is recorded, as well as the intensity (Hariharan, 1984). It is this combination of sensitivity, high fidelity, and unique optical properties that is exploited in the many current and up-and-coming applications of this recording technique, including data storage.

5. Holography In Data Storage

In this new technology, data is recorded in photo refractive crystals as 3D holograms. Because all holograms provide the perception of 3D images, it is probably necessary to explain exactly what is meant by 3D in data storage terms.

Binary data is written as dark or light "dots" in two dimensional pages, with the pages stacked one on top of the other within a photosensitive crystal. the stacking of pages creates the third dimension . Storing data throughout the whole volume of the recording medium instead of only on the surface is one of the main characteristics and advantages of holographic data storage technology. Crystals of a chemical compound called Strontium-barium-niobate are used most often as the recording media because they combine high sensitivity with high speed. The electronic charge patterns created by the interference of two laser beams is used to create the holograms.

In the most basic systems, light from a laser source passes through a beam splitter that divides the beam into a data beam and an interference beam. The reference beam will eventually be used to create the interference pattern. It is directed into a path that includes a polarization rotor and a page-addressing deflection system (Li, 1994). The data beam, on the other hand, passes into an optical system that expands into the surface of what is called a "page composer", which is implemented as a spatial-light-modulator. Digital data is superimposed on the expanded beam using the spatial-light modulator. The images appear as dark or light spots depending on the value of the digital data. From the page composer, the data beam is converted using Fourier-transform optics. From there, it is focused on the crystallite structure that will hold the hologram. At this point the data beam and reference beam come together again, with the resulting interference pattern on photo refractive material. This in turn, modifies the optical properties of the crystallite material with an electronic charge pattern. Multiple holograms are stored in a single crystal by altering the angle at which the beams enter the crystal (Ajluni, 1994). In the read cycle, instead, the data beam is turned off, allowing only the reference beam to focus on the crystal. The reference beam's location is determined by the particular pages to read. The beam illuminates the interference grating or patterns stored at this location, resulting in the reconstruction of original light-and-dark- spots pattern. The pattern is read by a charge-coupled device that converts the dark and light spots back to digital electronic data (Shandle 1993).

The holographic technology described above, is thought by many in the industry as being ideal for computer storage application, as for use in digital high Definition TV, video, and audio. (Barr,1993), in a article in PC Magazine spoke on holography as the technique that one day may compete with CD-ROM as a storage medium for masses of communication. He added that the medium is able to store a gigabyte of data about double the capacity of a CD-ROM disk, and that data access with holographic devices is ten times as fast as hard disks, and hundreds of times as fast as CD-ROMs. Barr goes on to explain how with this revolutionary medium, high densities are possible because of data being stored as a three-dimensional hologram, and how data is stacked in two dimensional pages, with up to 50 pages in one 3D volume. Some companies (Sony, IBM, Bellcore, and Tamarack), according to Barr, are working on a even higher density storage medium that will store the pages in crystallite material. Because of its vast storage capability and fast access times and data transfer rates, Barr sees holography as the ideal technique for multimedia files that contain video, sound and text.

Unlike traditional storage technologies that capture data only at the media surface, such as tape and disk, holographic media store data throughout their volume. In other words, because holographic images have depth of field, information that is digitized into the computer language of 1s and 0s can be layer deep inside a hologram. Multiple holograms can be recorded in exactly the same spot by changing the angle of the laser beams doing the recording (Wullert, 1994).

According to Stockton of Tamarack (in Deagon, 1994) " The three dimensional aspect of this technology allows storage densities that are at least 10 times greater than existing magnetic or optical recording or optical recording technologies " .

In addition, data are stored and accessed in large blocks, rather than bit by bit, as with magnetic and other optical storage techniques.

Bjorklund ( in Ajluni ,1994 ), manager of the optoelectronic materials department at IBM Almaden Research Center, in San Jose, California, pointed that the compelling reason to go to holographic storage is the rate at which information can be read in and out, up to one million bits per millisecond, some 100 times faster that with current magnetic or laser-disk optical storage .

Jamberdino (Lesser,1994 ) in an article in Defense Electronics spoke of holographic storage as being the best type of data storage technology that can meet the requirements of today's growing databases. According to Jamberdino, holographic technology provide three key elements that other electronic techniques don't: Huge capacity, speed, parallelism, and bandwidth :

a. Huge capacity: Holographic technology allows the storage of enormous amounts of information in a small area.
b. Speed, or throughput is the amount of time that it takes to get to the information once stored.
c. Parallelism is the ability to put information in any form, as for example a thousand by a thousand streams of information and take it out at that same rate (you have the same input and output rates). In other electronic technologies, you get one bit stream at a time.
d. Bandwidth is the amount of information that can be contained in Individual channels

Tazellar ( 1990 ), in his article "Magnetic Vs Optical " mentioned holography as being one of the wildest ideas in mass storage technology :

"Holography...is storing data in crystals with light...Holographic data storage is no hocus pocus... it is a technology whose capacity dwarfs even that of optical disks, while giving you faster access to your data."

Another technology expert, Trout (in Deagon, 1994 ) , who is the development manager for the optical venture of Dupont Imaging Systems said about the new technology: " While holograms used to be more of a curiosity, now we are starting to see potential for large volume applications in graphic arts, automotive, military and aerospace electronics, and information storage."

These are just some of the many testimonies of individuals that foresee the huge potential of this new technology. Many experts in the industry believe that recent breakthroughs in holographic storage technology (like the discovery of new and better types of crystals and the vertiginous advances in laser technology) will have a profound impact in the personal computer industry, including the multimedia&345based applications. Holographic technology truly offers low cost, physically small, portable and easily-filed storage media, consistent with demands for greater-capacity removable storage media as personal computing and consumer electronics merge in the trend toward multimedia.

6. Holographic Data Storage In The Marketplace

According to business experts, the immediate total available markets for memory products exceed $ 100 Billion worldwide, , of which the hard disk segment was $ 47 billion, the magnetic tape segment $ 42 billion, and the optical disk segment $ 6 billion The growth rates are estimated to be greater than 40 % per year.( Henue, 1994 $

Since its launch over 40 years ago, the mass storage industry has become one of the most challenging of high technology marketing arenas. Industry leaders distinguish themselves by consistently delivering high-quality products that meet the needs of manufacturers and end users.

As with many sectors of the electronics industry, the mass storage business and its technologies and products- are evolving continuously. Each new development creates another competitive thrust among manufacturers to develop products that are smaller, faster, and smarter at a reduced cost, and current entrenched technologies are, in fact, moving targets to any new comer. The question that poses it self considering the current milieu of memory technologies and the fierce competitive environment is -what it would take for holographic data storage, to fulfill its promise of being the storage technology of the future?

Various analytic techniques for such forecasting have been developed. The most useful for our purpose, in my opinion, is the triple-gateway methodology. This methodology was applied by Benson, Sage and Cook to evaluate microelectro- mechanical systems or M.e.m.s. as emerging technologies in their very early stages of development ( Benson,. 1993 )

This methodology was chosen primarily because it incorporates both systems-engineering and systems-management concepts. It is based on the proposition that a technology, to reach a mature stage in which it yields useful products or services, must pass through three gateways: the market gateway , the technology gateway, and the systems-management gateway. In other words, an emerging technology must have a market, technical feasibility, and a means of development and delivery, if its to achieve useful deployment. Passing through the technology gateway requires research ability, innovation, technical merit, and a technical champion.. The management gateway includes technology management. finance, enterprise management, and standards. The market gateway, sometimes referred to as the "demand pull" gateway, includes societal and consumer needs and receptiveness and general economic conditions.

In the market gateway analysis we look at the following elements of market uncertainty: new uses, user skepticism about improved performance, and competitive technologies:

a. New uses: There will always be uncertainty where a new use or function is being offered, even if there are only relatively minor changes in the technology.
b. User skepticism: about improved performance characteristics. Many technologies are developed with the notion that they will substitute for existing technologies by providing higher performance at modest or at least acceptable increase in price. Yet the consumer may not be particularly impressed with the performance improvement.
c. Competitive technologies : competitive technologies are often highly dynamic, adding enormous uncertainty to markets.
The management gateway can best be approached through analysis of the characteristics of the organization developing the new emerging technology.

In the systems-management gateway analysis we look at the key area of structure of the firm-in particular, as technological innovation is usually carried out through one of four organizational modes 8
a. An individual entrepreneur, a small hig-technology-oriented firm.
b. A large corporation with multiple products and multiple markets.
c. Conglomerates with multiple organizations and involved in multiple sectors.
These different modes can have dramatically different implications for the risk taking associated with a new technology; the flexibility to exploit new opportunities and the financing, marketing skills, and legal talents available to back up the technology.

In the technology gateway analysis we look at three elements of technology uncertainty: a. Innovativeness of technology: The more innovative a technology the more uncertainty it presents.
b. Number of constituent technologies: Uncertainty may well increase geometrically rather than arithmetically, with the number of technologies involved in an innovation.
c. Manufacturing difficulties: Manufacturing problems can frustrate new technologies.

The Market Gateway:

The data storage market includes drives and media for secondary and tertiary storage applications. In the past, the majority of desktop computing users did not need very high storage capacity. However, during the 1993 - 95 period, the advent of image computing and processing of multimedia documents with still images has quickly raised the floor of the minimum useful desktop storage capacity closer to 1 GB. Holographic storage products are well suited to fill the mass storage requirements of a multimedia storage hierarchy with high density, low cost per megabyte and fast access and data transfer rates. However, factors, such as the lack of a suitable storage material, and lack of consumer awareness, have impeded introduction of holographic storage products (Lorentz, 1996). Deterrents have also included the high initial cost of holographic components and the existence of other competitive products for backup applications. Furthermore, US firms, driven by short-term product strategies, have concentrated on marketing the well-established magnetic storage products .

The Management Gateway:

Holographic storage, as a radical innovation will demand competencies that incumbent , entrenched firms lack. Entrenched firms normally lack 1.technical capability. 2. they have no incentive to abandon their existing product line and markets 3. they fail to see the market potential of the new technology. In this way, new firms will gain a foothold in the industry (Henue, 1996). Tamarack storage Devices of Austin, Texas, is an example. This small Texas company with less that 10 staff members, has attempted to play a leadership role in promoting Holographic storage technology, seeing a huge potential market for storage products, However, small firms can encounter difficulties bringing new technologies to market at any of several points in the commercialization process. Often the most difficult stage is that of converting a prototype into a salable product. Initial manufacturing costs can be prohibitive for small firms, which face significant financial constraint in these stages of commercialization. Venture capital, contribution from wealthy individuals, and government funding can meet such costs. If Tamarack's researchers could develop a concept for a device that could compete with CD-ROMs and Magneto-Optical drives directly, it is possible that a Tamarack-led consortium might support continued development.

The Technological Gateway:

Magnetic and optical systems have long dominated the storage market. However an increasing demand for significantly more storage capacity coupled with the development of a wide variety of applications have taxed the ability of these storage systems. And as the requirements on these systems continue to grow, the limits of current technology will eventually be reached (Gibbs, 1994 ).

Holographic storage uses a combination of technologies:

Holographic storage systems, in contrast to conventional magnetic and optical recording, requires the unique integration of many different technologies as opposed to one dominant technology. By combining these technologies, it is anticipated that the resulting storage system would be both original and competitive in performance, size, and cost. Researchers believe that such a system could even potentially have access times and data rates of 100 to 1000 times faster than today's magnetic hard disks .

Difference between holographic storage and conventional storage techniques:

Information recorded using conventional technologies occupies a discrete location in or on the recording layer. In contrast. a bit recorded by holographic means is stored in the form of an interference pattern that spans the entire area or volume of the hologram. The storage process starts by forming a two dimensional page of digital information on a spatial light modulator (SLM). The SLM is illuminated by a laser beam, resulting in a transmitted light or signal beam. Both the signal beam and a reference beam of light are then directed at the same spot on the recording medium. The intersection of the beams produces an interference pattern that's recorded as a hologram. Numerous holograms may be recorded in the same spot either changing the angle with respect to the recording medium of the sample or the laser beams for each individual hologram.

Some of the key features of holographic storage are: redundancy, parallelism, and multiplexing.
- 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 (Pappu, 1990). Rather, only the signal to noise ratio is affected. To retrieve the stored information , the recorded hologram is illuminated with a replica of the reference beam. This effectively creates a replica of the original signal beam. That beam is then used to form an image of the original SLM (Spatial Light Modulator) data pattern on an array of photodetectors.
- 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 (Ajluni, 1994)
- Multiplexing: Holographic storage also lends itself well to multiplexing. In this process, the holographic structure of one page is intermixed with the holographic structure of other pages. Hence, multiple, independent pages of data can be recorded at the same spot on a relatively thick recording layer. In effect, if multiple holograms were stored in increments of 0.9 nm on a recording medium approximately 1.0 mm thick, then a total of 500 holograms could be stored in one location. Thick recording layers also help minimize the number of defects tolerated. Consequently, multiple pages of data may be retrieved with minimum cross-talk (Pappu, 1990).

Current holographic storage systems require the use of high-performance grade components. Optimum performance necessitates that many of these components be pushed beyond their current performance capabilities. Nevertheless, although it could be up to 5 years before completed holographic storage system is ready to ship, great progress has been made in many of the key system components.

Recent development in the area of integrated CCD (Charged-Coupled Device) arrays have replaced discretely mounted photodiodes (Pappu, 1990). For instance, liquid crystal SLMs have supplanted photographic masks and film transports. Also efficient laser diodes, which can be used directly, or as pumping sources to provide blue-green light, have been developed. with these devices, holographic technology is able to gain more attention as the potential storage technology of the future.

In fact, 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 (Gibbs, 1994), 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.

Despite all the advances made over the past few years, several issues have impeded the successful implementation of holographic storage. Most important has been the lack of a suitable recording material. Yet, with the recently renewed interest in holographic technology researchers are more hopeful that not only will they identify a material system, but integrate it into a workable holographic storage system (Henue, 1996). It is hoped that such a system will one day surpass today's technologies in terms of cost and performance. By some estimates, real industry impacts from this technology could be as close as five years away. Holographic components, will then, by most estimate, be available at highly competitive, mass production prices.


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