06-12-2012, 05:36 PM
OPTICAL DATA SECURITY full report
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INTRODUCTION
Optical data storage is an 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 improvement in data access speeds will greatly increase the capabilities of computers, especially with large data and multimedia files. Optical 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. Optical data storage systems are very close to becoming economically feasible. Photo-refractive crystals and photopolymers have been used successfully in experimental optical 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. Optical memory lies between main memory magnetic disk in regards to data access times, data transfer rates, data storage density.
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.
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.
Optical memory uses the basic principles of holography for the recording purposes and hence it is also called as holographic memory system. Optical memory is a promising technology for data storage because it is 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. Optical 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.
Currently, optical memory techniques are very close to becoming technologically and economically feasible. The major obstacles to implementing optical data storage are recording rate, pixel sizes, laser output power, degradation of holograms during access, temporal decay of holograms, and sensitivity of recording materials. At an estimated cost of between $161and $236 for a complete optical memory system, this may become a feasible alternative to magnetic disk in the near future.
LASER
Light amplification by stimulated emission of radiation is abbreviated as LASER. Laser is a device for the generation of coherent, nearly monochromatic and highly directional electromagnetic radiation emitted, somewhere in the range from sub-millimeter through ultraviolet and X-ray wavelengths. More than two hundred types of lasers have been fabricated which range in power, size, performance, use and cost. Fundamental attributes of a laser are directionality, monochromaticity, coherence and brightness. These attributes make it ideal for optical recording. To record holograms on the crystals usually argon ion lasers, krypton lasers and diode lasers are used.
Lens and Mirrors
Mirrors are used to reflect laser beams to the desired direction. Lenses are usually used to converge the laser to a point. A special type of lens is used in the case of optical recording called the Fourier lens. The lens has the property of obtaining the Fourier transform and the inverse transform system is described below.
Spatial Light Modulators (SLM)
SLM is an optical device that is used to convert the real image or data into a single beam of light that will intersect with the reference beam during recording. It basically consists of an array of pixels which are usually microscopic shutters or LCD displays. These can be controlled by a computer. The computer sends binary data to the SLM. Each pixel of the SLM corresponds to bit of data. So depending on whether the bit is a 1 or a 0 the pixel will go dark or transparent in the case of a LCD, or will be open or shut in the case of microscopic shutters.
Figure 3 show a 3d model of a spatial light modulator. The white pixels represent a binary 1 while the black pixels denote a binary zero. The white or the transparent pixels allows the light beams incident on it to pass through it while the dark or opaque pixels restrict the transmission of light through it. In effect the light beam coming out of the SLM contains the binary information transmitted to the SLM by the computer. Another important point to be noted here is that a complete page of binary date is converted to a single beam at a time. The access of a complete page at a time accounts for the increase in the access speeds of the optical storage system.