Holographic data storage is a potential next generation storage technology that offers both high storage density and fast readout rate. Holographic memory is a technique that can store information at high density inside cubic crystals. Holographic memory is an emerging technology that is very promising to revolutionalise the storage systems. It can store data up to 1TB in a sugar cube sized crystal. Data from more than 1000 CDs and more than 200 DVDs can fit into a holographic memory System. A DVD in current scenario can only store 4.7GB of data but Holographic storage has the potential to become the next generation of storage media as 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. This paper provides a description of Holographic data storage devices, a three dimensional data storage system which has a fundamental advantage over conventional read/write memory systems. A brief overview of holograms will be presented first. Applications to computer systems are then covered, with the future of holographic memory presented as a conclusion.
KEYWORDS
Holography, photopolymers, diffraction, optical interference, associative retrieval, time-reversed beam, data mining, peta flops, tele-immersing.
INTRODUCTION
Holography breaks through the density limits of the conventional storage by going beyond recording only on the surface, to recording to the full depth of the medium. Unlike other technologies that record one data bit at a time , holography allows a million bits of data to be written and read in parallel simultaneously.
This paper describes holographic data storage system, a three dimensional storage system which stores the data volumetrically.
The fundamental step to understand the holographic storage technology is to know what" holography" actually means. Holography is the study of patterns of light to produce a object in three dimension space. These recorded patterns of lights are the constituents of hologram memory systems.
we need to understand that, the hologram is not a picture and holography is not a picture-making technique. 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).
The need of these systems arises because the magnetic disk is being used to store the information in large repositories. In the near future of IT industry, there will be a flood of data to be handled efficiently so we require bulk storage technology. 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.
HOLOGRAM CREATION AND STORAGE
A thick photosensitive optical material is used to store the data as a result of optical interference. Two coherent beams originated from one laser source are used to create holograms. One beam being the reference beam and the other the signal beam. These beams when interfere, a resulting interference pattern is formed which includes both the amplitude and phase information of both the beams. When a photosensitive material is placed at the point of interference, the interference patterns are recorded on the surface. By adjusting the reference beam angle, wavelength, or media position, a multitude of holograms (theoretically, several thousand) can be stored on a single volume. The beam angle is very important, as different angle could be used to store different data. A key property of this interferometric recording is that when it is illuminated by a readout beam, the signal beam is reproduced. In effect, some of the light is diffracted from the readout beam to "reconstruct" a weak copy of the signal beam. If the signal beam was created by reflecting light off a 3D object, then the reconstructed hologram makes the 3D object appear behind the holographic medium. When the hologram is recorded in a thin material, the readout beam can differ from the reference beam used for recording and the scene will still appear. To make the hologram, the reference and object beams are overlapped in a photosensitive medium, such as a photopolymer or inorganic crystal.
Binary data is stored 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.
(Image ©Geoffrey W. Burr)
Figure 1: How to record and read data using holograms: (a) Holographic storage of a single data bit.
The spherical wave from a single pixel interferes with a coherent plane wave in the reference beam. The resulting
interference pattern changes the refractive properties of the photosensitive medium.
(b) The hologram is read out using the original reference beam, which is diffracted by the stored interference pattern to reconstruct the original spherical wavefront. An image of this beam can be formed on a single detector pixel, resulting in the retrieval of a single bit.
Data can be retrieved quickly as associative retrieval technique has been proved to be successful and very efficient in holographic data storage devices. Since different angle stores different data, multiple records can be searched in parallel. High transfer speed of upto 960 Mbps have been reported by "inphase technologies" during their research period.
ADVANTAGES
In present age, we can see holograms, or 3D images, on credit cards, books, magazine covers, art galleries etc. 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.
High capacity: holographic storage technology allows storage of enormous amount of information in small area.
Throughput: high throughput since data is read from device at a very high speed.
Parallelism: multiple read and write capability of the systems simultaneously.
Longevity: manufacturers belive that using write once read many(worm) approach could provide safe storage of content without degradation for more than 50 years.
DISADVANTAGE
Manufacturing cost of holographic data storage devices is very high and there is a lack of resources needed to build holographic data storage devices.
POSSIBLE FUTURE APPLICATIONS
Holographic storage systems could possibly be used in data mining. Data mining is the processes of finding patterns in large amounts of data. Data mining is used greatly in
large databases which hold possible patterns. Some current computer system implement data mining, but the mass amount of storage required is pushing the limits of current data storage systems. Access time and data storage capabilities of the holographic storage devices have improved significantly and thus speedup 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 extremely large amounts of data provided by holographic memory could be utilized in petaflop architecture. Clearly advances are needed to in more than memory systems, but the theoretical schematics do exist for such a machine. Optical storage such as holographic memory provides a viable solution to the extreme amount of data which is required for a petaflop
computing.
"Hybrid" holographic media is another field where this technology can be implemented. Just as magnetic hard drives are starting to incorporate significant quantities of flash or RAM within the disk, near-term holographic storage media may add some amount of flash memory in the cartridge to provide a degree of rewritability until a suitable rewritable media is developed and productized.
CONCLUSION
Although no one has yet mass-commercialized this technology, many vendors are working on it. InPhase Technologies, which was founded by Lucent, started its research by working on a product capable of storing 200 gigabytes of data, written four times faster than the speed of current DVD drives. Although early versions were not rewritable, the company expected to make holographic storage that could have had been able to rewrite the data.
Figure 2: holographic roadmap (image ©inphase technologies and maxell professional)
Table : holographic storage device advancement by inphase technologies
The first products are likely to be expensive, and only feasible for large organizations with unusual needs for storage. However, vendors expect to make holographic storage available and affordable for the average consumer within the next few years.
Two years ago InPhase Technologies, who had labored for a decade to bring holographic storage to market, bit the dust as VCs at Signal Lake , bought InPhase's remains for holographic storage systems. Holography is a promising technique with which to provide natural-looking 3 dimensional images because of the similarity between the light waves of a hologram and those reflected from a real object. It has a very well using vista in battlefield visualization and firepower control system of weapon. It based on the volume holographic storage system and combined with computer-generated holography.
In 2012 SEGA debuted world's first holographic videogame "The Time Traveller". Holography can also be used to create 3-D environment for various scenarios. Efforts are being made in order to implement the concept of tele-immersing. It would take years to make holograms available commercially at large scale and for that research has to made consistently in this field. Many advances in optical technology and photosensitive materials need to be made before we find holograms in our computer systems.
