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Encyclopedia > Holographic data storage

Contents

Wikipedia does not have an article with this exact name. ... Holographic memory is a technique that can store information at high density inside crystals or photopolymers. ... This Manual of Style has the simple purpose of making things easy to read by following a consistent format — it is a style guide. ...

Theory behind Two-Colour Holographic Recording

Creating holograms is achieved by means of two coherent beams of light split from one laser source, one being the reference beam and the other the signal beam. When both these beams interfere with one another, a resulting interference pattern is formed which encompasses the pattern both in amplitude and phase information of the two beams. When an appropriate photorefractive material is placed at the point of interference, the interference patterns are recorded inside the material. When the reference beam illuminates the material in the absence of the signal beam, the hologram causes the light to be diffracted in the same direction of the initial signal beam and all the information of the original signal beam is reconstructed. Holography (from the Greek, Όλος-holos whole + γραφή-graphe writing) is the science of producing holograms, an advanced form of photography that allows an image to be recorded in three dimensions. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength[1]. The elementary particle that defines light is the photon. ... For other uses, see Laser (disambiguation). ... A reference beam is a laser beam used to read and write holograms. ... Amplitude is a nonnegative scalar measure of a waves magnitude of oscillation, that is, magnitude of the maximum disturbance in the medium during one wave cycle. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... The photorefractive effect is a nonlinear optical effect seen in certain crystals and other materials that respond to light by altering their refractive index. ... Interference of two circular waves - Wavelength (decreasing bottom to top) and Wave centers distance (increasing to the right). ...

Set up for holographic recording
Set up for holographic recording

For two-colour holographic recording, the additional fact is that the beams would be defined as follows, the reference and signal beams shall be fixed to a particular wavelength (green, red or IR) and the sensitizing/gating beam shall be of another shorter wavelength (blue or UV). The sensitizing/gating beam is used to sensitize the material before and during the recording process, while the information is recorded in the crystal via the reference and signal beams. It shall be shone intermittently on the crystal during the recording process for measuring the diffracted beam intensity. Readout is achieved by illumination with the reference beam alone. Hence the readout beam with longer wavelength would not be able to excite the recombined electrons from the deep trap centres during readout, as they need the sensitizing light with shorter wavelength to erase them. Quartz crystal Synthetic bismuth crystal Insulin crystals Gallium, a metal that easily forms large single crystals A huge monocrystal of potassium dihydrogen phosphate grown from solution by Saint-Gobain for the megajoule laser of CEA. In chemistry and mineralogy, a crystal is a solid in which the constituent atoms, molecules... Lighting refers to either artificial light sources such as lamps or to natural illumination of interiors from daylight. ... e- redirects here. ...


Usually, for two-colour holographic recording, two different dopants are required to promote trap centres, which belong to transition metal and rare earth elements and are sensitive to certain wavelengths. By using two dopants, more trap centres would be created in the Lithium Niobate crystal. Namely a shallow and a deep trap would be created. The concept now is to use the sensitizing light to excite electrons from the deep trap farther from the conduction band to the conduction band and then to recombine at the shallow traps nearer to the conduction band. The reference and signal beam would then be used to excite the electrons from the shallow traps back to the deep traps. The information would hence be stored in the deep traps. Reading would be done with the reference beam since the electrons can no longer be excited out of the deep traps by the long wavelength beam.


Sensitivity refers to the extent of refractive index modulation produced per unit of exposure. Diffraction efficiency n is proportional to the square of the index modulation times the effective thickness. The dynamic range determines how many holograms may be multiplexed in a single volume. The refractive index (or index of refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium. ... In telecommunications, multiplexing (also muxing or MUXing) is the combining of two or more information channels onto a common transmission medium using hardware called a multiplexer or (MUX). ...


Recording the Data

A single laser beam is split into two beams, the signal (or object) beam for data transfer and a reference beam. A hologram is formed when the two beams interset the recording medium. The data is encoded into the signal beam using a spatial light modulator (SLM) device that translates electronic data (0's and 1's) into an optical pattern of light and dark pixels. The data is arranged in an array similar to a checkerboard of usually 1M (million) bits. The hologram is recorded in a light sensitive storage medium where the reference and signal beams intersect. A chemical reaction in the storage medium occurs when the light elements of the signal beam diffracts with the reference beam and creates a volumetric hologram. By varying the angle of the reference beam, wavelength or media position, many holograms can be stored in the same volume of storage material. The data is decoded by reflecting the reference beam off the hologram and thus, reconstructing the stored information. The hologram is then projected onto a detector that reads the data in parallel.


Effect of Annealing

For a doubly doped LiNbO3 crystal there exists an optimum oxidation/reduction state for desired performance. This optimum depends on the doping levels of shallow and deep traps as well as the annealing conditions for the crystal samples. This optimum state generally occurs when 95 – 98% of the deep traps are filled. In a strongly oxidized sample holograms cannot be easily recorded and the diffraction efficiency is very low. This is because the shallow trap is completely empty and the deep trap is also almost devoid of electrons. In a highly reduced sample on the other hand, the deep traps are completely filled and the shallow traps are also partially filled. This results in very good sensitivity (fast recording) and high diffraction efficiency due to the availability of electrons in the shallow traps. However during readout, all the deep traps get filled quickly and the resulting holograms reside in the shallow traps where they are totally erased by further readout. Hence after extensive readout the diffraction efficiency drops to zero and the hologram stored cannot be fixed. Anneal may refer to: Annealing (metallurgy), a heat treatment wherein the microstructure of a material is altered, causing changes in its properties such as strength and hardness. ...


External links

  • Daewoo Electronics Develops the World’s First Holographic Storage Device
  • First commercial application of HDS by InPhase

 
 

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