INTERNATIONAL JOURNAL OF COMPUTER ENGINEERING & TECHNOLOGY (IJCET) International Journal of Computer Engineering and Technology (IJCET), ISSN 0976-6367(Print), ISSN 0976 6367(Print) ISSN 0976 6375(Online) Volume 4, Issue 6, November - December (2013), pp. 232-239 IAEME: www.iaeme.com/ijcet.asp Journal Impact Factor (2013): 6.1302 (Calculated by GISI) www.jifactor.com IJCET I A E M E 3-D HOLOGRAPHIC DATA STORAGE D.Sai Anuhya, Smriti Agrawal Department of Information Technology, JB Institute of Engineering and Technology, Hyderabad, India ABSTRACT In response to the rapidly changing face of computing and demand for physically smaller, greater capacity and high bandwidth storage devices, the holographic memory came into existence. Holographic data storage using volume holograms offers high density and fast readout. It can store 1Tb-4Tb of information on a sugar cube sized crystal. That means a single holographic device can replace more than thousands of CD s. This paper provides description about the principle of holography along with the comparisons observed between the conventional optical disks and the holographic disks. Further, it investigates on what is making holographic devices still unfamiliar and summarizes applications of computer systems wherein holography can be used. I. INTRODUCTION Computing technology is ever fast emerging. Ever since the IC (Integrated Circuits) was developed, the number of transistors that engineers can pack on a chip has increased at a phenomenal rate. ICs are made by the Photolithography process in which the patterns of metal or chemically treated silicon are layered one on top of another, on to a die of silicon. Building even smaller chip features requires using light sources with even shorter wavelength. That means designers had to move from visible light, to ultraviolet light, and finally to the X-Rays territory. But using the X rays for the photolithographic introduces a new set of problems. For example the issue of having a reliable X ray source, the X rays cannot be focused with optical lenses and therefore the mask, which produces the required pattern on the silicon, must be the size of the features themselves and furthermore the materials opaque to light are not necessarily opaque to X rays. In the case of optical disks, the wavelength of the light used limits the distance between the bits. It is estimated that in the next five or ten years we will reach the limiting density for storing data on magnetic disks [1, 2, 3]. There is currently much research into other methods of memory and storage. 232
The capacities of today's mass storage devices cannot satisfy the demands of new processes which will be developed in near future. Storage capacities are very competitive with magnetic or optical disk, and the media volumetric storage density is significantly greater since it is a 3-D storage media. II. METHODS OF OPTICAL STORAGE Optical data storage techniques are categorized in three basic groups. The symbol that precedes a technique indicates a method that is already in use to produce commercial products. 1. Surface or 2D recording [3, 4] a. CD/DVD Data are stored in reflective pits and scanned with a focused laser. Disks are easily replicated from a master. b. CD-Recordable Reflective pits are thermally recorded by focused laser. This type is usually lower density than read-only versions. Researchers have proposed blue lasers and electron-trapping materials to achieve density improvements. c. Magneto-optic disks Spots are recorded with a combination of magnetic field and focused laser. d. Near-field optical recording Higher 2D density than with conventional surface recording is achieved by placing a small light source close to the disk. Light throughput and readout speed are issues. e. Optical tape Parallel optical I/O has the advantages of magnetic tape without the long-term interaction between tape layers wound on the spool. Flexible photosensitive media is an issue. 2. Volumetric recording [5, 6, 7] a. Holographic Data are stored in interference fringes with massively parallel I/O. Suitable recording material is still needed. b. Spectral hole burning This technique addresses a small subset of molecules throughout the media by using a tunable narrowband laser. Alternatively, all subsets are addressed with ultra short laser pulses. It may add a fourth storage dimension to holography but requires cryogenic temperatures and materials development. 3. Bit-by-bit 3D recording [7, 8] a. Sparsely layered disks The focus of the CD laser is changed to hit interior layers. DVD standard already includes two layers per side. b. Densely layered disks A tightly focused beam is used to write small marks in a continuous or layered material; read with con-focal (depth-ranging) microscope. c. 2-photon Two beams of different wavelengths mark writes, then read in parallel using fluorescence. Material sensitivity is an issue. 233
III. WHY HOLOGRAPHIC MEMORY? Holographic memory is a three-dimensional data storage system that can store information at high density inside the crystal or photopolymer. It stores information in the form of holographic images. Holographic memory serves as a high-capacity data storage challenging the current domination of the 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 overcomes this limitation by recording information throughout the volume of the medium and is capable of recording multiple images in the same area utilizing light at different angles. Holographic media like other media are available in both the forms, namely, write once where the storage medium undergoes irresistible change, and rewritable media where change is reversible [5, 16, 17, 19]. Traditionally, magnetic and optical data storage records information a bit at a time in a linear fashion, which leads to poor bit transfer rate. However, holographic storage is capable of recording and reading millions of bits in parallel, enabling data transfer rates greater than those attained by traditional optical storage. The holographic storage has additional advantage of being compact. It has the potential of storing up to 1 terabyte or one thousand gigabytes of data in a crystal of the size of a sugar cube. It is being researched and slated as the storage device that will replace hard drives and DVDs in the future [8, 9, 10]. IV. HOW DOES IT WORK? Holographic memory is very promising because it not only offers greater capacity, but the access speeds are very fast because there are few moving parts and no contact is required [1]. Photographic holograms are made by recording interference patterns of a reference beam of light and a signal beam of light reflected off an object. Photosensitive material holds this interference pattern, and the image can be reproduced by applying an identical beam of light to the reference beam onto the photosensitive material. Many variations of the object can be recorded on a single plate of material by changing the angle or the wavelength of the incident light. This is illustrated in the figure 1. Each frame of an animation is stored by varying the angle of the incident light. Prototypes developed by Lucent and IBM differ slightly, but most holographic data storage systems (HDSS) are based on the same concept [1, 8, 11, 12, 13]. Figure 1: Storing information in a holographic data storage system [1] 234
Figure 2: Retrieval of information from a holographic data storage system [1] The basic components that are needed to construct an holographic data storage systems HDSS are: 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 When the blue-green argon laser is fired, a beam splitter creates two beams (refer Figure 1). 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. 1. Recording data Holographic data storage contains information using an optical interference pattern within a thick, photosensitive optical material. Light from a single laser beam is divided into two separate optical patterns of dark and light pixels. By adjusting the reference beam angle, wavelength, or media position, a multitude of holograms (theoretically, several thousand) can be stored on a single volume [1]. 2. Reading data The stored data is read through the reproduction of the same reference beam used to create the hologram. The reference beam s light is focused on the photosensitive material, illuminating the appropriate interference pattern, the light diffracts on the interference pattern, and projects the pattern onto a detector. The detector is capable of reading the data in parallel, over one million bits at once, resulting in the fast data transfer rate. Files on the holographic drive can be accessed in less than 0.2 seconds [14, 20, 21, 23]. 235
An advantage of a holographic memory system is that an entire page of dataa 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 willl 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. 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. 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 [1]. 3. Longitivity Holographic data storage can provide companies a method to preserve and archive information. The write-once, read many (WORM) approach to data storage would ensure content security, preventing the informationn from being overwritten or modified. Manufacturers believe this technology can provide safe storage for content without degradation for more than 50 years, far exceeding current data storage options. Counterpoints to this claim are that the evolution of data reader technology has in the last couple of decades changed every ten years. If this trend continues, it therefore follows thatt being able to store data for 50 100 years on one format is irrelevant, because you would migrate the data to a new format after only ten years [4, 15]. V. INTERESTING FACTS This section summarizes some of the facts illustrate the impact of holographic memories. It has been estimated that all the books in the U.S. Library of Congress, could be stored on six (6) HVD's. The pictures of every landmass on Earth (Google Earth for example) can be stored on two (2) HVD's. With MPEG4 ASP encoding, a HVD can hold between 4,600 to 11,9000 hours of video, which is enough for non-stop playing for a year. Figure 3: Conventional and Holographic [5] 236
VI. COMPARISON OF CONVENTIONAL AND HDV In conventional optical disc, the data is burned onto the surface one bit at a time. It gets stores at a rate of 1 bit/pulse. As bit per bit is saved onto it, the space in between the data is left unused. This makes wastage of the space to a larger extent. But when it comes to Holographic optical disc or Holographic Versatile Disc (HVD) the data is stored as a page data and it is generally recorded into the volumetric recording layer. It can store at a rate of 60,000 bits/pulse when Spatial Laser Modulator (SLM) is passed onto it. This is illustrated in the figure 3. With three-dimensional recording and parallel data readout, holographic memories can outperform existing optical storage techniques. In contrast to the currently available storage strategies, holographic mass memory simultaneously offers high data capacity and short data access time (Storage capacity of about 1TB/cc and data transfer rate of 1 billion bits/second). The capacity of DVD is generally of 9.4 GB and of BLU-RAY disc is of 50GB. But the capacity of HVD ranges from 300 GB to 3.9 TB. Scientists are planning to push future storage densities of optical mass storage over 40,000 Terabits/cu.cm [10]. When it comes to read/write speed DVD is said to have 11.08 Mbps, BLU-RAY of 36 Mbps but that of HVD is 1Gbps [5]. This is summarized in the figure 4. 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. Figure 4: Comparison between storage devices [5] But still each and every development has some defects with it. They are not limitations regarding technical aspect but it is mostly regarding the cost. The manufacturing cost of HDV is very high and there is a lack of availability of resources which are needed to produce HDV. 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 a fixed become so dim that noise creeps into the read-out operation, thus limiting the material's storage capacity. But then when Blu-ray was introduced in 2006, a 25-gigabyte disc cost nearly $1 a gigabyte. It is about half the cost now. Overtime, the overall cost of holographic data storage should decrease to an acceptable amount. So, even the HDV s price can go down as time passes [10]. 237
A difficulty with the HDV 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. In the past, this has limited the number of reads that can be made before the signal-to - noise ratio becomes too low. Moreover, writes in the same fashion can degrade previous writes in the same region of the medium. 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 s an error of even a thousandth of an inch. VII. FUTURE OF HOLOGRAPHIC DATA STORAGE There are many possible applications of holographic memory. Holographic memory systems can potentially provide the high speed transfers and large volumes of future computer system. One possible application is 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 which can t be distinguished by human eyes due to the vast amount of data. Some current computer system implement data mining, but the mass amount of storage required is pushing the limits of current data storage systems. 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. Optical storage such as holographic memory provides a viable solution to the extreme amount of data which is required for a petaflop computing [12, 16, 17, 18, 19, 20]. Experts also note the possible introduction of "hybrid" holographic media. Just as magnetic hard drives are starting to incorporate significant quantities of flash or Ram within the disc, nearterm holographic storage media may add some amount of flash memory in the cartridge to provide a degree of re-write ability until a suitable rewritable media is developed and productized [15, 23, 24]. VIII. CONCLUSION The holographic disk will be the next technological revolution and its future is very promising. Recent research has demonstrated that holographic storage systems with desirable properties can be engineered. The next step is to build these systems at costs competitive with those of existing technologies and to optimize the storage media. It will most likely be used in next generation supercomputers where cost is not as much of an issue. 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. As there is a limitation of more cost but as time goes on this magnetic memory device will be cheaper as that was in the case of BLU-RAY. IX. REFERENCES [1] www.computer.howstuffworks.com. [2] Steve Redfield and Jerry Willenbring "Holostore technology for higher levels of memory hierarchy," IEEE potentials, 1991, PP. 155-159. [3] M. Imlau, M. Fally, G. W. Burr, and G. T. Sincerbox, "Holography and optical storage," Springer Handbook of Lasers and Optics, 2rd edition, ed., F. Trager, Springer-- Verlag (2012). 238
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