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From the beginning generation of computer, speed of the processors has been increasing tremendously approximately twice the capacity for every 3 years. Today, we can access large amount of data with in few seconds using CPU and data storage. While the computer evolves many applications for large binary files like sound or image data for high capacity storage and data access. But, there is no high capacity form of data storage to handle these large files quickly and efficiently. So, Holographic memory is a hopeful technology in the next generation over conventional storage system because it is a three-dimensional data storage system that can store information at high density inside the crystal or photopolymer which has a capacity to Storage system, the data of entire page can be accessed at a time instead of sequential method and there are very few moving parts so the limitations of the mechanical motion can be minimized. Holographic memory uses a photosensitive material to record interference patterns of a reference beam and a signal beam of coherent light, where the signal beam is reflected off and an object or which contains the data in the form of light and dark areas. The natural history of the photosensitive material is to record the interference pattern which reproduces it after applying a beam of light to the material that is identical to the reference beam. The resultant light that is transmitted through the medium will take on the recorded interference pattern and will be collected on a laser detector array that encompasses the entire surface of the holographic medium. Lots of holograms are recorded in the same space by changing the angle or the wavelength of the incident light. The whole page of data can be accessed in this way. Hence holographic storage system has the potential to became the next storage generation over conventional storage
2. Definition of Holographic Memory
It is a storage device that records binary information in the form of holograms, which are produced (as interfering patterns) on photographic or photo chromic media by the way of laser beams, and can be read by means of low-power laser beams. It is a technique that can store information at high density inside the crystals or photopolymers. The advantage of a hologram is the way in which the image is dispersed over the recording surface so that dust or scratches do not totally obscure data though they may reduce the contrast. Several projects have attempted to apply this technology but none have yet been commercially successful.
3. What is HVD?
HVD known as Holographic Versatile Disc is an optical disc technology still in the research stage which would hold up to 4 terabyte (TB) of information. It employs a technique known as collinear holography, whereby two lasers, one red and one blue-green, are collimated in a single beam. It employs a technique known as collinear holography, whereby two lasers, one red and one blue-green, are collimated in a single beam. The blue-green laser reads data encoded as laser interference fringes from a holographic layer near the top of the disc while the red laser is used as the reference beam and to read servo information from a regular CD-style aluminium layer near the bottom.
These discs have the capacity to load up to 4 terabyte of information, which is approximately 6,000 times the capacity of a CD-ROM, 830 times the capacity of DVD, 160 times the capacity of single-layer Blu-ray-Discs and approximately 8 times the capacity of standard computer hard drives as of 2007. These discs also had a transfer rate of 1GB/S.
4. Structure of HVD
The structure of HVD is shown in the above figure and components with respect to figure are shown below:
1. Green writing/reading laser (532 nm)
2. Red positioning/addressing laser (650nm)
3. Hologram (data)
4. Polycarbon layer
5. Photo polymeric layer (data containing layer)
6. Distance layers
7. Diachronic layer (reflecting green light)
8. Aluminium reflective layer (reflecting red light)
9. Transparent base
(i). Working of HVD
A holographic data storage system consists of a recording medium, an optical recording system, and 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 photorefractive crystal such as LiNbO3 or BaTiO3 that has certain optical characteristics. These characteristics are high diffraction efficiency, high resolution, and permanent storage until erasure, and fast erasure on the application of external stimulus such as UV light. A 'hologram' is simply the three-dimensional interference pattern of the intersection of the reference and signal beams at 90 degrees to each other. This interference pattern is imprinted into the crystal as regions of positive and negative charge. 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 the reference beam. One characteristic of the recording medium that limits the usefulness of holographic storage is the property that every time the crystal is read with the reference beam, the stored hologram at that "location" is disturbed by the reference beam and some of the data integrity is lost. With current technology, recorded holograms in Fe- and Tb-doped LiNbO3 that use UV light to activate the Tb atoms can be preserved without significant decay for two years.
(ii). Writing Data
The process of writing information onto an HVD begins with encoding the information into binary to be stored in the SLM. These data are turned into ones and zeroes represented as opaque or translucent areas on a "page". The below is the image that the information beam is going to pass through.
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. Thus the data is stored as a hologram.
(iii). Reading Data
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 millimetre will result in failure to retrieve that page of data.
5. Advantages 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.
It also had the following advantages:
Resistance to damage - If some parts of the medium are damaged, all information can still be obtained from other parts.
Efficient retrieval - All information can be retrieved from any part of the medium.
These discs have the capacity to hold up to 3.9 terabyte (TB) of information, which is approximately 6,000 times the capacity of a CDROM, 830 times the capacity of a DVD, 160 times the capacity of single-layer Blu-ray-Discs, and about 48 times the capacity of standard computer hard drives.
The HVD also has a transfer rate of 1 Gigabit/s
6. Applications of Holographic Memory
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 provides 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 rock mounted of petabytes storage capacity.
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.
7. Comparison with Other Storage Devices
Data Transfer Rate
Main Memory (RAM)
10 - 40 ns
5 - 20 MB/s
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.
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.