15-09-2014, 02:45 PM
Holographic Data Storage System
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TECHNICAL REPORT
Holography dates from 1947, when British/Hungarian scientist Dennis Gabor developed the theory of holography while working to improve the resolution of an electron microscope. He was awarded the Nobel Prize in physics in 1971 for his invention and development of the holographic method.
Holographic data storageis a potential technology in the area of high-capacity data storage currently dominated by magnetic and conventional optical data storage. Magnetic and optical data storage devices rely on individual bits being stored as distinct magnetic or optical changes on the surface of the recording medium. Holographic data storage records information throughout the volume of the medium and is capable of recording multiple images in the same area utilizing light at different angles.
Holography:Holography is technique that enablesthree dimensional images to be made. It involves the use of a laser, interference,and light intensity recording and suitable illumination of the recording. The image changes as the position and orientation of the viewing system changes in exactly the same way as if the object were still present, thus making the image appear three dimensional.
Hologram: Hologram is a device which stores the information in it for the projection of stored data in three dimensional manner. It is also referred as holographic projector. The size of hologram is very less.
Holographic data storage systems are far better than conventional data storage systems both in storage capacity and display pattern.
ABSTRACT
The Information age has led to an explosion of information available to users. While current storage needs are being met, storage technologies must continue to improve in order to keep pace with the rapidly increasing demand. However, conventional 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.
Storing information throughout the volume of a medium and 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.
Holographic versatile disc (HVD) is a holographic storage format capable of storing far more data than DVD. Prototype HVD devices have been created with a capacity of 3.9 terabytes (TB) and a transfer rate of 1 gigabit per second. At that capacity, an HVD could store as much information as 830 DVDs or 160 Blu-ray discs.
This seminar report presents an introduction to holographic versatile disc and its technology. It also analyses the advantages that holographic data storage provide over other conventional data storage technologies .The challenges faced by this emerging technology and its future opportunities are also evaluated.
INTRODUCTION
Each time you watch a fast-paced DVD movie or pull down a piece of information from the Internet or even access the ATM at the corner of your street, you are actually tapping into large repositories of digital information. The hard disk, the mainstay of personal and corporate storage, has faithfully obeyed the exponential law. This has happened largely due to increases in aerial density, that is, how many bits are crammed into a square inch. This paper provides a description of Holographic data storage system (HDSS), a three dimensional data storage system which has a fundamental advantage over conventional read/write.
The first step in understanding holographic memory is to understand what "holographic" means.
Holography is a method of recording patterns of light to produce a three-dimensional object. The recorded patterns of light are called hologram memory systems. Holographic memory is a technique that can store information at high density inside crystals or photopolymers. As current storage techniques such as DVD reach the upper limit of possible data density (due to the diffraction limited size of the writing beams), holographic storage has the potential to become the next generation of storage media. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface.
WORKING OF HDSS
Creating holograms is achieved by means of two coherent beams of light split from one laser source, one being the reference beam and the other the signal beam. When both these beams interfere with one another, a resulting interference pattern is formed which encompasses the pattern both in amplitude and phase information of the two beams.Whenan appropriate photorefractive material is placed at the point of interference, The beam's angle is crucial, and it can't vary by more than a fraction of a degree. This apparent flaw in the recording process is actually an asset. . By changing either the angle of the reference beam or its frequency, you can write additional data pages in to the same volume of crystal. The dynamic range of the medium determines how many pages it can hold reliably
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. 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 renters the hologram will record another page of data in the same volume. This method is known as multiplexing. This property enables storing of multiple pages in holographic data storage system.
IMPLEMENTATION
A holographic data storage system consists of a recording medium, an optical recording system, a photo detector array. A beam of coherent light is split into a reference beam and a signal beam which are used to record a hologram into the recording medium. The recording medium is usually a photo refractive crystala simple three-dimensional interference pattern of the intersection of the reference and signal beams are perpendicular to each other. This interference pattern is imprinted into the crystal as regions of positive and negative charges. To retrieve the stored hologram, a beam of light that has the same wavelength and angle of incidence as the reference beam is sent into the crystal and the resulting diffraction pattern is used to reconstruct the pattern of the signal beam. Many different holograms may be stored in the same crystal volume by changing the angle of incidence of reference beam
HOLOGRAPHIC PROJECTION
The stored data in holographic data storage system is projected in a three dimensional pattern. These three dimensional images are known as holograms. This process is known as holographic projection
ADVANTAGES
In contrast to the currently available storage strategies, holographic mass memory simultaneously offers high data capacity and short access time.
Holographic data storage has the unique ability to locate similar features stored within a crystal instantly. A data pattern projected into a crystal from the top searches thousands of stored holograms in parallel.
The holograms diffract the incoming light out of the side of the crystal, with the brightest outgoing beams identifying the address of the data that most closely resemble the input pattern.
This parallel search capability is an inherent property of holographic data storage and allows a database to be searched by content. Because the interference patterns are spread uniformly throughout the material, it endows holographic storage with another useful capability: high reliability.
While a defect in the medium for disk or tape storage might garble critical data, a defect in a holographic medium doesn't wipe out information. Instead, it only makes the hologram dimmer. No rotation of medium is required as in the case of other storage devices.
DISADVANTAGES OF HDDS
1. Manufacturing cost HDSS is very high and there is a lack of availability of resources which are needed to produce HDSS.
2. However, all the holograms appear dimmer because their patterns must share the material's finite dynamic range. In other words, the additional holograms alter a material that can support only fixed amount of change. Ultimately, the images become dim that noise creeps into the read-out operation, thus limiting storage.
3. A difficulty with the HDSS technology had been the destructive readout. The re-illuminated reference beam used to retrieve the recorded information, also excites the donor electrons and disturbs the equilibrium of the space charge field in a manner that produces a gradual erasure of the recording.
4. In the past, this has limited the number of reads that can be made before the signal-to -noise ratiobecomes too low. Moreover, writes in the same fashion can degrade previous writes in the same region of the medium.
5. This restricts the ability to use the three-dimensional capacity of a photorefractive for recording angle-multiplexed holograms. You would be unable to
locate the data if there is an error of even a thousandth of an inch.
CONCLUSION
The future of holographic data storage systemis very promising. The
page access of data that HDSS 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 HDSS impractical. It will most likely be used in next generation supercomputers 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 system 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 HDSS 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 our computer systems