This IT Engineering Seminar Topic deals with the following:
As processors and buses roughly double their data capacity every three years (Moore’s Law), data storage has struggled to close the gap. CPUs can perform an instruction execution every nanosecond, which is six orders of magnitude faster than a single magnetic disk access. Much research has gone into finding hardware and software solutions to closing the time gap between CPUs and data storage. Some of these advances include cache, pipelining, optimizing compilers, and RAM. As the computer evolves, so do the applications that computers are used for. Recently large binary files containing sound or image data have become commonplace, greatly increasing the need for high capacity data storage and data access. A new high capacity form of data storage must be developed to handle these large files quickly and efficiently. Holographic memory is a promising technology for data storage because it is a true three dimensional storage system, data can be accessed an entire page at a time instead of sequentially, and there are very few moving parts so that the limitations of mechanical motion are 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 of an object or it contains data in the form of light and dark areas. The nature of the photosensitive material is such that the recorded interference pattern can be reproduced by applying a beam of light to the material that is identical to the reference beam. The resulting 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. Many holograms can be recorded in the same space by changing the angle or the wavelength of the incident light. An entire page of data is accessed in this way.
The three features of holographic memory that make it an attractive candidate to replace magnetic storage devices are redundancy of stored data, parallelism, and multiplexing. Stored data is redundant because of the nature of the interference pattern between the reference and signal beams that is imprinted into the holographic medium. Since the interference pattern is a plane wave front, the stored pattern is propagated throughout the entire volume of the holographic medium, repeating at intervals. The data can be corrupted to a certain level before information is lost so this is a very safe method of data storage. Also, the effect of lost data is to lower the signal to noise ratio so that the amount of data that can be safely lost is dependent on the desired signal to noise ratio.
Stored holograms are massively parallel because the data is recorded as an optical wave front that is retrieved as a single page in one access. Since light is used to retrieve data and there are no moving parts in the detector array, data access time is on the order of 10 ms and data transfer rate approaches 1.0 GB/sec. Multiplexing allows many different patterns to be stored in the same crystal volume simply by changing the angle at which the reference beam records the hologram.
Currently, holographic memory techniques are very close to becoming technologically and economically feasible. The major obstacles to implementing holographic data storage are recording rate, pixel sizes, laser output power, degradation of holograms during access, temporal decay of holograms, and sensitivity of recording materials. An angle multiplexed holographic data storage system using a photo refractive crystal for a recording medium can provide an access speed of 2.4 s, a recording rate of 31 kB/s and a readout rate of 10 GB/s, which is between the typical values for DRAM and magnetic disk. At an estimated cost of between $161 and $236 for a complete holographic memory system, this may become a feasible alternative to magnetic disk in the near future.
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