Multiple Photorefractive Holographic Storage

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Multiple photorefractive holographic storage, the various pieces of the diffraction efficiency of the hologram with the increase in the number stored holograms decreased, a large amount of storage, the respective pieces of hologram diffraction efficiency is very low, the readout light cabinet is required for enhanced to improve the diffraction efficiency and storage capacity.

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Photorefractive holographic storage accompany light scattering, this light scattering light, is a part of the signal noise, need to adopt appropriate measures fiction noise.

Multiple photorefractive holographic storage processes are both writing is erased dynamic process; therefore need proper exposure techniques and fusing technology (raster fixed).

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Crosstalk exists multiple holographic storage, requires the use of appropriate encoding method, or read mode to eliminate crosstalk. Meanwhile, on the holographic storage mechanism, multiplexing, recording materials and other aspects of research work has also made great progress. Optical database, optical neural networks, optical interconnection, associative storage and pattern recognition research progress, but also sparked interest for holographic storage; while laser photorefractive recording materials, group of pages (such as spatial light modulator SLM) and high performance photodetectors (such as charge-coupled device CCD) and other areas of real progress.

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Optical information storage technology introduction

Disc storage.

Information technology is the key technology of the 21st century, the information industry is the pillar industry of the 21st century. Information is stored in the national economic construction and modernization of military science and technology has a very important position. With the computer technology and multimedia technology, processing and storage require a substantial increase in the amount of data, it is for large capacity and high rate of information storage technologies are more demanding. Disk storage is a fairly mature technology, in the coming period will continue to play an important role. However, the surface density of the magnetic memory being physically superparamagnetic limitations: that is, when the magnetic domain volume is too small, so that magnetic domains magnetized maintaining anisotropy energy of orientation will be close to the internal energy of the magnetic domains, which cause spontaneous to demagnetization. Over the past 10 years, the hard disk's surface density of 60% per year rate of increase, and soon approaching superparamagnetic limit (40Cb/in2). For now, the magnetic storage technology continues to improve in order to meet people's greater storage capacity, faster data transmission requirements.

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Holographic storage system

Volume holographic storage principle

When the recording medium thickness ratio greater recorded interference fringe spacing, the two coherent beams in the medium to form the grating-shaped three-dimensional hologram, called a volume hologram. Such a hologram, its absorption coefficient and refractive index variation period, it acts as a three-dimensional diffraction of light diffraction grating, reproduction, only if the Bragg condition, before the maximum diffraction intensity. Therefore, the characteristics of the volume hologram Bragg's law to determine its display illumination beam angle and wavelength selection response, making it a high storage capacity main way multiple holographic storage.

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The volume hologram is formed by the object beam (0) and the reference light (R) of the recording medium within the mutual interference. For convenience, assume that object and reference beams are plane-wave case, according to the light interference theory, object light and reference light interfere with each other is formed inside the recording medium such as a plane spaced family structure, which is called the body grating.

Volume hologram diffraction of light on the crystal in Bragg's interpretation of X-rays is very similar to the phenomenon, which often borrow "Bragg's Law" to discuss the body hologram wavefront reconstruction, the angle 0 is called "Bragg grid corner. " Only when reproducing light fully satisfy the Bragg condition is to get the strongest diffraction.

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Holographic storage ultimately need the reproduced image is read out with the reproduction reference light, the detector can also be received by the detector into an image received by the monitor f observations used in this experiment detector is a charge coupled device (CCD) . CCD has a small size, light weight, sensitivity l Island, Ruming long, low power consumption and dynamic range, etc..

Addressing device performance and random access time available resolution for quantitative description, resolution and storage density has a close relationship. Currently, the main beam deflector is addressable device, this time the resolution is defined as the maximum angle of deflection angle divided by the diffraction limit, this ratio gives the angular position can be resolved or equivalently the total number of points that can be resolved given the total Points N. Addressable devices are generally divided into mechanical motion and no mechanical moving components addressing two major categories, no mechanical moving addressable devices can provide a shorter random access time.

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Other single components

Storage system to work properly in the storage system is also used in some of the following basic components.

Fourier transform lens and lens system used in a collimating lens and a pair of Fourier transform lens, the focal length of the collimator lens, = 15cm; Fourier transform lens factory = 30cm, mark an important quality parameter lens the relative aperture, also known as the F number. Let the lens through the application of the maximum aperture diameter of _D, focal length, the relative aperture is defined as D / f, F number is the reciprocal of the relative aperture. Relative aperture the greater the higher the resolution of the lens, allowing the group page has a more number of pixels. For the detector, it can allow a shorter exposure time, increase the data read-out rate.

Photorefractive holographic storage multiplexing

Because now the same volume holographic storage class can be complex variables Jane them to implement the body of holography.

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Angle multiplexing

Angle-multiplexed holographic storage multiplexing always use the earliest, a technology research to the fullest. It is long enough in Zhouding Bo. Sui state, by changing the incident angle of reference light, so that each piece of the volume hologram the reference light from different angles, in order to achieve the same position of the recording medium stores a plurality of holograms. Meanwhile, the completion of multiplexing is required to change the incident angle of reference light, so its implementation of the system hardware is not very high.

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Wavelength multiplexing

Readout wavelength of reproduction light can also cause changes in the Bragg mismatch, which can be recorded using different wavelengths of the recording medium in the same volume hologram is recorded, this kind of multiplexing is known as wavelength multiplexing. Generally, the wavelength-multiplexed light of each reference hologram reproducing light are the same angle, the difference between different wavelengths. Although the wavelength-multiplexed holographic storage is the main multiplexing mode, but because of the development of the laser can not meet the requirements of wavelength multiplexing, multiplexing is mostly so this study was to investigate theoretically pure practical wavelength-multiplexed with technological development is relatively slow.

Phase Multiplexing

For this multiplexing, each hologram is fixed wavelength and the angle of the main beam, but each piece of stored images using respective different phase coded reference optical writing and reproducing optical readout. Different hologram recording reference light is no longer simply a plane wave or a spherical wave, the wavefront are incompatible with each phase distribution network, therefore, on this point, the phase angle multiplexing multiplexing and relatively similar. But it is according to certain encoding rules deterministic phase encoding, phase encoding rational design, which can effectively suppress crosstalk between different holograms. Is commonly used in quadrature phase coded.

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