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. 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, but storage technologies have to evolve to keep pace with increasing consumer demand. CDs, DVDs and magnetic storage all store bits of information on the surface of a recording medium. 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. Three-dimensional data storage will be able to store more information in a smaller space and offer faster data transfer times. Holographic memory is developing technology that has promised to revolutionalise the storage systems. It can store data upto 1 Tb in a sugar cube sized crystal. Data from more than 1000 CDs can fit into a holographic memory System. Most of the computer hard drives available today can hold only 10 to 40 GB of data, a small fraction of what holographic memory system can hold. Conventional memories use only the surface to store the data. But holographic data storage systems use the volume to store data. It has more advantages than conventional storage systems. It is based on the principle of holography.
Scientist Pieter J. van Heerden first proposed the idea of holographic (three-dimensional) storage in the early 1960s. A decade later, scientists at RCA Laboratories demonstrated the technology by recording 500 holograms in an iron-doped lithium-niobate crystal and 550 holograms of high-resolution images in a light-sensitive polymer material. The lack of cheap parts and the advancement of magnetic and semiconductor memories placed the development of holographic data storage on hold.

HOLOGRAPHY

A hologram is a block or sheet of photosensitive material which records the interference of two light sources. To create a hologram, laser light is first split into two beams, a source beam and a reference beam. The source beam is then manipulated and sent into the photosensitive material. Once inside this material, it intersects the reference beam and the resulting interference of laser light is recorded on the photosensitive material, resulting in a hologram. Once a hologram is recorded, it can be viewed with only the reference beam. The reference beam is projected into the hologram at the exact angle it was projected during recording. When this light hits the recorded diffraction pattern, the source beam is regenerated out of the refracted light. An exact copy of the source beam is sent out of the hologram and can be read by optical sensors. For example, a hologram that can be obtained from a toy store illustrates this idea. Precise laser equipment is used at the factory to create the hologram. A recording material which can recreate recorded images out of natural light is used so the consumer does not need high-tech equipment to view the information stored in the hologram. Natural light becomes the reference beam and human eyes become the optical sensors. Holography was invented in 1947 by the Hungarian-British physicist Dennis Gabor (1900-1979), who won a 1971 Nobel Prize for his invention.

CREATING HOLOGRAMS [pic]

RETRIEVING HOLOGRAMS

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APPLICATION TO BINARY In order for holographic technology to be applied to computer systems, it must store data in a form that a computer can recognize. In current computer systems, this form is binary. In the previous section, it was mentioned that the source beam is manipulated. In common holograms, this manipulation is the creation of an optical image such as a ball or human face. In computer applications, this manipulation is in the form of bits. The next section explains the spatial light modulator, a device that converts laser light into binary data.

Spatial Light Modulator (SLM)

A spatial light modulator is used for creating binary information out of laser light. The SLM is a 2D plane, consisting of pixels which can be turned on and off to create binary 1.s and 0.s. An illustration of this is a window and a window shade. It is possible to pull the shade down over a window to block incoming sunlight. If sunlight is desired again, the shade can be raised. A spatial light modulator contains a two-dimensional array of windows which are only microns wide. These windows block some parts of the incoming laser light and let other parts go through. The resulting cross section of the laser beam is a two dimensional array of binary data, exactly the same as what was represented in the SLM. After the laser beam is manipulated, it is sent into the hologram to be recorded. This data is written into the hologram as page form. It is called this due to its representation as a two dimensional plane, or page of data. Spatial light modulator is a Liquid Crystal Display panel that consists of clear and dark areas corresponding to the binary information it represent. Spatial light modulator is actually that device which makes holography applicable to computers. So it is one of the important components of Holographic Data Storage System.

Implementation The components of Holographic data storage system is composed of
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 camera

They can be classified into three sections namely recording medium, optical recording system and photodetector array. The laser is used because it provides monochromatic light. Only the interference pattern produced by the monochromatic beam of light is stable in time. Lithium niobate crystal is used as photosensitive material on which hologram is recorded. It has certain optical characteristics that make it behave as photosensitive material. CCD camera detects the information in the light, converts to digital information and forward it to computer.

RECORDING OF DATA IN HOLOGRAPHIC MEMORY SYSTEM 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 information from the page of binary code is carried by the signal beam around to the light-sensitive lithium-niobate crystal. 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.
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RETRIEVAL OF DATA FROM HOLOGRAPHIC MEMORY SYSTEM An advantage of a holographic memory system is that an entire page of data can be retrieved quickly and at one time. In order to retrieve and reconstruct the holographic page of data stored in the crystal, the reference beam is shined into the crystal at exactly the same angle at which it entered to store that page of data. Each page of data is stored in a different area of the crystal, based on the angle at which the reference beam strikes it. During reconstruction, the beam will be diffracted by the crystal to allow the recreation of the original page that was stored. This reconstructed page is then projected onto the charge-coupled device (CCD) camera, which interprets and forwards the digital information to a computer. CCD is a 2-D array of thousands or millions of tiny solar cells, each of which transforms the light from one small portion of the image into electrons. Next step is to read the value (accumulated charge) of each cell in the image. In a CCD device, the charge is actually transported across the chip and read at one corner of the array. An analog-to-digital converter turns each pixel's value into a digital value. CCDs use a special manufacturing process to create the ability to transport charge across the chip without distortion. This process leads to very high-quality sensors in terms of fidelity and light sensitivity. CCD sensors have been mass produced for a longer period of time, so they are more mature. They tend to have higher quality and more pixels. 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.
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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.

MULTIPLEXING

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

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 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. The elimination of mechanical access methods reduces access times from milliseconds to microseconds.

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

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

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 or translate the recording medium, phase-encoded multiplexing changes the phase of individual parts of a reference beam. The main reference beam is split up into many smaller partial beams which cover the same area as the original reference beam. These smaller beamlets vary byphase which changes the state of the reference beam as a whole. The reference beams intersects the source beam and records the diffraction relative to the different phases of the beamlets. The phase of the beamlets can be changed by non-mechanical means, therefore speeding up access times.

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Recording Errors

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

Page-Level Parity Bits
Once error-free data is recorded into a hologram, methods which read data back out of it need to be error free as well. Data in page format requires a new way to provide error control. Current error control methods concentrate on a stream of bits. Because page data is in the form of a two dimensional array, error correction needs to take into account the extra dimension of bits. When a page of data is written to the holographic media, the page is separated into smaller two dimensional arrays. These sub sections are appended with an additional row and column of bits. The added bits calculate the parity of each row and column of data. An odd number of bits in a row or column create a parity bit of 1 and an even number of bits create a 0. A parity bit where the row and column meet is also created which is called an overall parity bit. The sub sections are rejoined and sent to the holographic medium for recording.

MERITS OF HOLOGRAPHIC MEMORY
Holographic memory offers storage capacity of about 1 TB. Speed of retrieval of data in tens of microseconds compared to data access time of almost 10ms offered by the fastest hard disk today. By the time they are available they can transfer an entire DVD movie in 30 seconds. Information search is also faster in holographic memory. Consider the case of large databases that are stored on hard disk today. To retrieve any piece of information you first provide some reference data. The data is then searched by its address, track, sector and so on after which it is compared with the reference data. In holographic storage entire pages can be retrieved where contents of two or more pages can be compared optically without having to retrieve the information contained in them. Also HDSS has no moving parts. So the limitations of mechanical motion such as friction can be removed.

CHALLENGES During the retrieval of data the reference beam has to be focused at exactly the same angle at which it was projected during recording. A slight error can cause a wrong data page to be accessed. It is difficult to obtain that much of accuracy. The crystal used as the photographic filament must have exact optical characteristics such as high diffraction efficiency, storage of data safely without any erasure and fast erasure on application of external stimulus light ultra violet rays. With the repeated number of accesses the holograms will tend to decay.

POSSIBLE 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. 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 can’t 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. 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 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. Holographic memory can be used as extended DRAM with 10ns access time, Hard disk drives ,CD ROMs of large storage capacity and rockmounted (combining numerous DASDs) of petabytes storage capacity.

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RECENT DEVELOPMENTS The research on holographic memory is taking place in well guarded and rich companies like IBM, ROCKWELL and InPhase. InPhase claims to have developed a holographic memory of size slightly larger than a DVD. It has a capacity of about 100GB. They are trying to push it upto 1TB. IBM and ROCKWELL claims to have developed a recording medium less sensitive than lithium niobate crystals.

Holographic memory vs. conventional storage devices

|Storage Medium |Access Time |Data Transfer Rate |Storage Capacity |
|Holographic Memory |2.4 (s |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 |

Comparing the access times holographic memory lies midway between that of main memory and magnetic disk. Data transfer rate is 10GB/s which is higher than that of other storage devices and, 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.

SUMMARY
|STORAGE CAPACITY |TERABYTE CLASS |
|TECHNOLOGY USED |OPTICS AND HOLOGRAPHY |
|DATA TRANSFER SPEED |HIGH |
|TIME FOR IMPLEMENTATION |4 |
|COST OF IMPLEMENTATION |HIGH |
|COMPANIES INVOLVED |IBM,ROCKWELL,LUCENT |

HOLOGRAPHIC MEMORY LAYOUT

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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. As computer systems evolve, it is not unreasonable to believe that magnetic storage will continue to do so. As mentioned earlier, however, these improvements are not made on the conceptual level. 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.

REFERENCES http://www.howstuffworks.com http://www.sandj@cda.mrs.umn.edu http://www.stanford.edu/~svngam/
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