HOLOGRAPHIC MEMORY
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This paper describes holographic data storage as a viable alternative to magnetic disk data storage. Currently data access times are extremely slow for magnetic disks when compared to the speed of execution of CPUs so that any improvementin data access speeds will greatly increase the capabilities of computers, especially with large data and multimedia files. Holographic memory is a technology that uses a three dimensional medium to store data and it can access such data a page at a time instead of sequentially, which leads to increases in storage density and access speed. Holographic data storage systems are very close to becoming economically feasible. Obstacles that limit holographic memory are hologram decay over time and with repeated accesses, slow recording rates, and data transfer rates that need to be increased. Photorefractive crystals and photopolymers have been usedsuccessfully in experimental holographic data storage systems. Such systems exploit the optical properties of these photosensitive materials along with the behavior of laser light when it is used to record an image of an object. Holographic memory lies between main memory and magnetic disk in regards to data access times, data transfer rates, and data storage density.
INTRODUCTION
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.
This post was written by: Rajendra Prasad
Rajendra Prasad is a professional blogger, web designer and front end web developer. Follow him on Facebook
