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As the necessity of storing the data in larger quantities has increased rapidly in day to day life. The total storing capacity has been doubling every year. From past thirty years of research and development work, first holographic disks and disk drives are scheduled to release in next few months. A holographic memory which is sized almost as a DVD has 300gigabytes of storing capacity, with 20 mega byte per second. According to the developers the next generation has a large increase of storage capacity with 800 gigabytes and then to 1.6 terabyte. This type of storage helps people to store millions of pages information and high definition multimedia into a small and inexpensive disk. It has been demonstrated already with, hundreds of movies and millions of books in digitally stored small volumetric space. The present technology is far away from the expectations, which is depended on magnetic storage of bits on three-dimensional surfaces to store information in smaller space and lets data to transfer fastly.
Holographic Memory is a developing technology which has promised with all revolutionalised storage capabilities to store data up to 1TB in sugar cube sized crystal. It has a capacity to store 1000 CDs data into a holographic memory system. The idea of storage developed in early 1960's a decade later, scientists at RCA laboratories demonstrated the technology of 500 holograms in an iron doped lithium-niobate crystal and 550 high resolution images in a light sensitive polymer material. The holographic data storage requires lack of cheap parts in advancement of magnetic and semiconductor memories.
It is a bock or sheet material which stores the interference of two light sources. To generate one hologram the laser light beam splits into two source beam and reference beam. The source beam manipulates and sent into the photosensitive material. Once the material inserted in the reference beam results interference of laser light recording in a photosensitive material which represents a hologram. The recorded hologram will be viewed only with reference beam. Finally, The reference beam is projected on the hologram and exact angle is projected during the recording. When this light hits the recorded diffraction pattern the source beam is regenerated into a refracted light. The same kind of copy is even sent out of a hologram and sent out to a hologram and can be read by optical sensor. Holography was invented in 1947 by Hungarian-British physicist Dennis Gabor (1900-1979), who won a 1971 Nobel Prize for his invention.
It is a memory that can store information in form of holographic image. It 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. Like other media, holographic media is divided into write once (where the storage medium undergoes some irreversible change), and rewritable media (where the change is reversible). Rewritable holographic storage can be achieved via the photo refractive effect in crystals.
Holographic Data Storage Technology
Classical hard disk systems, CDs and DVDs store data bit by bit on a magnetical surface. However, the storage technologies behind these products are close to upper physical limits of data density (Thompson and Best, 2000).
The main problem is the following: As storage capacity increases , the magnetic grains that hold data get closer together in a limited area. Eventually, magnetic fields of the neighbouring grains begin to interfere, which reduces the quality and reliability of stored data (Burr et.all, 2001). Optical storage technology also suffers from similar problems related to the limited size of the recording light beams. Holographic storage technology is based on writing data bits vertically instead of horizontally. The advantage is that, the entire volume of recording media is used for storing information. The actual storage takes place in a light-sensitive crystal material .The interference of laser light is utilized for storing millions of bits at a time. During this process, a single laser beam is split into two beams: The signal beam and the reference beam. The signal beam carries the data and a hologram is formed when these two beams cross each other, creating an interference pattern in the recording medium. The signal beam travels through a spatial-light-modulator (SLM) that translates electronic data of zeros and ones into an array of light and dark pixels. When the two beams meet, the interference pattern that is created stores the data as a hologram image. The chemical reactions that occur during the process cause the holographic image to be recorded in the light-sensitive storage medium. An important characteristic of this technology is the ability to store multiple holographic images in the same volumetric space. This can be achieved by varying major components of the system such as the beam angle, laser wavelength and media position, Data retrieval also is based on the same mechanism: When the data in a hologram is needed, the reference beam is sent into the material in the same way as when it was written. This process reconstructs the original signal beam, whose data contents can be converted into computer readable form by using a special digital camera system. A critical element here is the angle of the reference beam. It must match exactly the original beam angle. Even a few thousandths of a millimetre difference will result in failure to reconstruct the recorded data (Bonsar, 2006). The choice of storage medium is very important for the success of holographic recording and retrieval systems. There have been considerable research and development work for obtaining suitable materials for this purpose. The materials that are used currently are inorganic crystal or special polymer. The final media products satisfy stringent optical quality and stability requirements of reliable holographic storage devices.
There are a number of companies working in various fields of holographic storage technology. Two leaders in holography related research and development are InPhase Technologies Optware. The companies plan to ship their first products soon. Both InPhase and Optware are supported by powerful multinational companies including; Bell Labs, Bayer, Mitsubishi, Fuji, Toshiba Hitachi, Maxell and others.
Recording of Data in Holographic Memory System
Light from a single blue-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 process for encoding data onto the signal beam is accomplished by a device called a spatial light modulator (SLM). The SLM translates the electronic data of 0s and 1s into an optical "checkerboard" pattern of light and dark pixels. The data is arranged in an array (or page) of approximately one million bits. The exact number of bits is determined by the pixel count of the SLM.
At the point of intersection of the reference beam and the data-carrying signal beam, the hologram is recorded in a light sensitive storage medium. A chemical reaction occurs in the medium when the bright elements of the signal beam intersect the reference beam, causing the hologram to be stored. By varying the reference- beam angle, wavelength or media position, many different holograms can be recorded in the same volume of material.
Retrieval of Data From Holographic Memory System
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. 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.
Potentials and Problems
Unlike current storage technologies that record one data bit a time, holography allows more than a million bits of data to be written and read in parallel with a single flash of light. High 3D storage densities, fast transfer rates and durable media make holography the most likely choice for next generation data storage and processing needs. Only magnetic tape can compete with holographic storage capacities to some extent, but tapes are less durable and harder to access. A prototype holographic system has lately been demonstrated to have a recording capacity of 515 gigabits per square inch, which is highest ever for any storage technology. By the end of 2006, the producer plans to introduce 300 gigabytes capacity on an ordinary DVD-size disk with a transfer rate of 20 megabytes per second. The prototype could store up to 150 million pages, which is more than the capacity of 60 DVDs or 1000 CDs. The speed also is about 10 times faster than conventional DVD recording.
An important advantage of holographic storage is that the system does not spin like a hard disk or DVD. But, although the light source and camera detector are fixed, the reflective mirrors and lenses have to move to change the beam angles. This requires extremely high precision positioning and movements of the components. Some other properties that make holographic storage attractive include the followings
- Per megabyte cost of the media is low.
- The life expectancy of recorded data is more than 50 years.
- Random access to stored data is the standard method.
- Capabilities for improving information security are higher than current Technologies.
- The cost levels are not yet competitive for large scale market acceptance of holographic memory products.
- The expected shelf and archival lifetime of the products are not well-known.
- The sensitivity of media varies over time and the light source power is difficult to keep stable.
The opponents of the holographic storage technology raise some doubts about the immediate success of end user products. Scepticisms focus especially on the following points:
Since the media is photosensitive, maintaining reliability will be difficult as both the drive and cartridge have to be lightproof for preventing exposure to light. The level of precision has to be extremely high for proper interoperability of complex optical, electronic and mechanical systems. For example, slightest vibrations could throw the intersecting beams off-target.
Despite these and similar negative points, the researchers and producers of the Holographic technology are confident about success in the near future.
During the retrieval of data the reference beam has to be focused at exactly the same angle at which it was projected during recording. A slight error can cause a wrong data page to be accessed. It is difficult to obtain that much of accuracy. The crystal used as the photographic filament must have exact optical characteristics such as high diffraction efficiency, storage of data safely without any erasure and fast erasure on application of external stimulus light ultra violet rays. With the repeated number of accesses the holograms will tend to decay.
The arrival of holographic storage technology is imminent, made possible by recent advances in materials, multiplexing architectures and components. As the rate of increase in hard disk drive capacity levels out, holographic storage will begin to emerge as the clear choice for near-line and archive applications.
InPhase Technologies has already demonstrated capacity and transfer rates of 300 GB and 20 MB/s respectively, both of which will increase markedly in the next few years as the technology matures. Bringing with it lower storage costs and longer media archive life, holographic data storage will help to create a new tapeless era in the video production and broadcast industry.