ABSTRACT
This paper provides an overview of holographic memory, a developing
three-dimensional data storage system for computers. Devices that use
light to store and read data have been the backbone of data storage for
nearly two decades. In early 1980’s CD’s (Compact Discs)
revolutionarised the storage and in later part of 19th century DVD’s
(Digital Versatile Disc) were invented which improved the storage
capacity by storing 8.5GB of data on a single disc. These conventional
storage mediums meet
today's storage needs, but storage technologies have to evolve to keep pace with increasing consumer demand. CD’s, DVD’s and magnetic storage devices stores information as bits. In order to increase storage capabilities we introduce a new storage method called HOLOGRAPHIC MEMORY. Holographic memory uses total volume of recording medium for storage unlike CD’s or DVD’s which use surface area only. 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. Holographic memory offers the possibility of storing 1 terabyte (TB) of data in a sugar-cube-sized crystal. Keywords Holographic Memory, Optical Memory, Spatial Multiplexing, Angular Multiplexing 1. INTRODUCTION With its omnipresence computers, all connected via the Internet, the Information Age has led to an explosion of information available to users. The decreasing costs of storing data, and the increasing storage capacities of the same small device footprint, have been key enablers of this revolution. While current storage needs are being met, storage technologies must continue to improve in order to keep pace with the rapidly increasing demand. However,bothmagnetic and conventional optical 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 beyond which individual bits may be too small or too difficult to store. Storing information throughout the volume of a medium—not just on its surface— offers an intriguing high-capacity alternative. Holographicdata 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 memory offers the possibility of storing 1 terabyte (TB) of data in a sugar-cube-sized crystal. A terabyte of data equals 1,000 gigabytes, 1 million megabytes or 1 trillion bytes. Data from more than 1,000 CDs could fit on a holographic memory system. Most computer hard drives only hold 10 to 40 GB of data, a small fraction of what a holographic memory system might hold 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 memory systems. A brief overview of properties of holograms will be presented first. This will cover the way in which data can be stored in a hologram with the diffraction of laser light. Applying these properties to computer memory systems will follow, including the description of page data, a method in which another dimension is added to the accessing of stored data. Error correction and applications to computer systems are then covered, with the future of holographic memory presented as a conclusion. 2. BASIC PRINCIPLES OF HDSS (Holographic Data Storage System) Fig. 1 Interference patterns for reading and writing into Holographic Memory A hologram is a block or sheet of photosensitive material which records the diffraction 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 diffraction 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 and 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. 3. WORKING 3.1 RECORDING DATA ON MEDIUM Light from a single laser beam is split into two beams, the signal beam (which carries the data) and the reference beam. The hologram is formed where these two beams intersect in the recording medium. The object beam, gets expanded so that it fully illuminates a spatial light modulator (SLM). An SLM is simply an LCD panel that displays a page of raw binary data as an array of clear or dark pixels. The object beam finally interacts with the reference beam inside a photosensitive crystal. The ensuing interference pattern--the substance of the hologram--gets stored as a web of varying optical a chemical reaction occurs in the medium when the bright elements of the signal beam intersect the reference beam, causing the hologram stored. By varying the reference beam angle, wavelength, or media position many different holograms can be recorded in the same volume of material. Characteristics inside this crystal Fig. 2 Recording data on a Holographic Memory 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. It's how holographic storage achieves its high data densities. 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. 3.2 READING DATA FROM HOLOGRAM When reading out the data, the reference beam has to hit the crystal at the same angle that's used in recording the page. To read out the data, the reference beam again illuminates the crystal. The stored interference pattern diffracts the reference beam's light so that it reconstructs the checkerboard image of the light or dark pixels. The image is directed upon a charge-coupled device (CCD) sensor array that reads the data in parallel, and it instantly captures the entire digital page. The binary information can now be read from this CCD and the originally stored data is retrieved. This parallel read out of data provides holography with its fast data transfer rates. Fig. 3 Reading data from a holographic memory The success of holographic storage lies in the ability to accurately focus the reference laser on the exact position within the crystal to retrieve that page of .You would be unable to locate the data if there’s an error of even a thousandth of an inch. Also, the crystals used in the fabrication of the storage element need to exhibit very exacting optical characteristics to store the data correctly and there are very few substances that adequately and economically meet these needs. 4. ADVANTAGES HVD (Holographic Versatile Disc) offers several advantages over traditional storage technology. HVDs can ultimately store more than 1 terabyte (TB) of information -- that's 200 times more than a single-sided DVD and 20 times more than a current double-sided Blue-ray. This is partly due to HVDs storing holograms in overlapping patterns, while a DVD basically stores bits of information side-by-side. HVDs also use a thicker recording layer than DVDs ---a HVD stores information in almost the entire volume of the disc, instead of just a single, thin layer. Fig. 4 DVD vs. HVD: Recording-layer depth Fig. 5 Volumetric recording method Fig. 6 AN OVERVIEW OF A HVD (HOLOGRAPHIC VERSATILE DISC) 5. DISADVANTAGES Manufacturing cost HDSS is very high and there is a lack of availability of resources which are needed to produce HDSS. 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 amount of change. Ultimately, the images become so dim that noise creeps into the read-out operation, thus limiting the material's storage capacity. 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. 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. 6. APPLICATIONS Data mining is one of the important applications. 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. 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 CONCLUSIONS The future of HOLOGRAPHIC DATA STORAGE SYSYEM is 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. References [1] Demetri Psaltis, Fai Mok. Holographic Memories. Scientific American vol. 273 no. 5 November 1995 [2] Brad J. Goertzen, and P. A. Mitkas, Volume Holographic Storage for Large Relational Databases, Optical Engineering, Volume 35, Number 7, pp. 1847, 1996. [3] P. A. Mitkas and L. J. Irakliotis. Three-Dimensional Optical Storage for Database Processing Optical Memory and Neural Networks, Volume 3, Number 2, 1994 [4] Creating Holographic Storage Byte Magazine, April 1996 [5] L. J. Irakliotis, G. A. Betzos, and P. A. Mitkas, "Interfacing optical memories and electronic computers," Proceedings of the Third IEEE International Conference on Electronics, Circuits, and Systems (ICECS '96), 1996. [6] Joshua L. Kann, Brady W. Canfield, Capt, USAF,Albert A. Jamberdino,Bernard J. Clarke, Capt, USAF, Ed Daniszewski, and Gary Sunada, Capt, USAF Mass Storage and Retrieval at Rome Laboratory Proceedings of the 5th NASA Goddard Mass Storage Systems and Technologies Conference [7] Wenhai Liu, Ernest Chuang and Demetri Psaltis Holographic Memory Design for Petaflop Computing Proceedings of HTMT meeting, July, 1998, Princeton [8] Glenn T. Sincerbox Holographic storage: oct 2000 [9] R. Winn Hardin Optical Storage Stacks the DeckOE-Reports Number 187, July 1999
today's storage needs, but storage technologies have to evolve to keep pace with increasing consumer demand. CD’s, DVD’s and magnetic storage devices stores information as bits. In order to increase storage capabilities we introduce a new storage method called HOLOGRAPHIC MEMORY. Holographic memory uses total volume of recording medium for storage unlike CD’s or DVD’s which use surface area only. 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. Holographic memory offers the possibility of storing 1 terabyte (TB) of data in a sugar-cube-sized crystal. Keywords Holographic Memory, Optical Memory, Spatial Multiplexing, Angular Multiplexing 1. INTRODUCTION With its omnipresence computers, all connected via the Internet, the Information Age has led to an explosion of information available to users. The decreasing costs of storing data, and the increasing storage capacities of the same small device footprint, have been key enablers of this revolution. While current storage needs are being met, storage technologies must continue to improve in order to keep pace with the rapidly increasing demand. However,bothmagnetic and conventional optical 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 beyond which individual bits may be too small or too difficult to store. Storing information throughout the volume of a medium—not just on its surface— offers an intriguing high-capacity alternative. Holographicdata 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 memory offers the possibility of storing 1 terabyte (TB) of data in a sugar-cube-sized crystal. A terabyte of data equals 1,000 gigabytes, 1 million megabytes or 1 trillion bytes. Data from more than 1,000 CDs could fit on a holographic memory system. Most computer hard drives only hold 10 to 40 GB of data, a small fraction of what a holographic memory system might hold 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 memory systems. A brief overview of properties of holograms will be presented first. This will cover the way in which data can be stored in a hologram with the diffraction of laser light. Applying these properties to computer memory systems will follow, including the description of page data, a method in which another dimension is added to the accessing of stored data. Error correction and applications to computer systems are then covered, with the future of holographic memory presented as a conclusion. 2. BASIC PRINCIPLES OF HDSS (Holographic Data Storage System) Fig. 1 Interference patterns for reading and writing into Holographic Memory A hologram is a block or sheet of photosensitive material which records the diffraction 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 diffraction 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 and 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. 3. WORKING 3.1 RECORDING DATA ON MEDIUM Light from a single laser beam is split into two beams, the signal beam (which carries the data) and the reference beam. The hologram is formed where these two beams intersect in the recording medium. The object beam, gets expanded so that it fully illuminates a spatial light modulator (SLM). An SLM is simply an LCD panel that displays a page of raw binary data as an array of clear or dark pixels. The object beam finally interacts with the reference beam inside a photosensitive crystal. The ensuing interference pattern--the substance of the hologram--gets stored as a web of varying optical a chemical reaction occurs in the medium when the bright elements of the signal beam intersect the reference beam, causing the hologram stored. By varying the reference beam angle, wavelength, or media position many different holograms can be recorded in the same volume of material. Characteristics inside this crystal Fig. 2 Recording data on a Holographic Memory 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. It's how holographic storage achieves its high data densities. 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. 3.2 READING DATA FROM HOLOGRAM When reading out the data, the reference beam has to hit the crystal at the same angle that's used in recording the page. To read out the data, the reference beam again illuminates the crystal. The stored interference pattern diffracts the reference beam's light so that it reconstructs the checkerboard image of the light or dark pixels. The image is directed upon a charge-coupled device (CCD) sensor array that reads the data in parallel, and it instantly captures the entire digital page. The binary information can now be read from this CCD and the originally stored data is retrieved. This parallel read out of data provides holography with its fast data transfer rates. Fig. 3 Reading data from a holographic memory The success of holographic storage lies in the ability to accurately focus the reference laser on the exact position within the crystal to retrieve that page of .You would be unable to locate the data if there’s an error of even a thousandth of an inch. Also, the crystals used in the fabrication of the storage element need to exhibit very exacting optical characteristics to store the data correctly and there are very few substances that adequately and economically meet these needs. 4. ADVANTAGES HVD (Holographic Versatile Disc) offers several advantages over traditional storage technology. HVDs can ultimately store more than 1 terabyte (TB) of information -- that's 200 times more than a single-sided DVD and 20 times more than a current double-sided Blue-ray. This is partly due to HVDs storing holograms in overlapping patterns, while a DVD basically stores bits of information side-by-side. HVDs also use a thicker recording layer than DVDs ---a HVD stores information in almost the entire volume of the disc, instead of just a single, thin layer. Fig. 4 DVD vs. HVD: Recording-layer depth Fig. 5 Volumetric recording method Fig. 6 AN OVERVIEW OF A HVD (HOLOGRAPHIC VERSATILE DISC) 5. DISADVANTAGES Manufacturing cost HDSS is very high and there is a lack of availability of resources which are needed to produce HDSS. 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 amount of change. Ultimately, the images become so dim that noise creeps into the read-out operation, thus limiting the material's storage capacity. 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. 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. 6. APPLICATIONS Data mining is one of the important applications. 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. 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 CONCLUSIONS The future of HOLOGRAPHIC DATA STORAGE SYSYEM is 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. References [1] Demetri Psaltis, Fai Mok. Holographic Memories. Scientific American vol. 273 no. 5 November 1995 [2] Brad J. Goertzen, and P. A. Mitkas, Volume Holographic Storage for Large Relational Databases, Optical Engineering, Volume 35, Number 7, pp. 1847, 1996. [3] P. A. Mitkas and L. J. Irakliotis. Three-Dimensional Optical Storage for Database Processing Optical Memory and Neural Networks, Volume 3, Number 2, 1994 [4] Creating Holographic Storage Byte Magazine, April 1996 [5] L. J. Irakliotis, G. A. Betzos, and P. A. Mitkas, "Interfacing optical memories and electronic computers," Proceedings of the Third IEEE International Conference on Electronics, Circuits, and Systems (ICECS '96), 1996. [6] Joshua L. Kann, Brady W. Canfield, Capt, USAF,Albert A. Jamberdino,Bernard J. Clarke, Capt, USAF, Ed Daniszewski, and Gary Sunada, Capt, USAF Mass Storage and Retrieval at Rome Laboratory Proceedings of the 5th NASA Goddard Mass Storage Systems and Technologies Conference [7] Wenhai Liu, Ernest Chuang and Demetri Psaltis Holographic Memory Design for Petaflop Computing Proceedings of HTMT meeting, July, 1998, Princeton [8] Glenn T. Sincerbox Holographic storage: oct 2000 [9] R. Winn Hardin Optical Storage Stacks the DeckOE-Reports Number 187, July 1999
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