Holographic data storage

Holographic memory is a technique that can be stored within a crystal or photo - polymer, with the information in a very high density.

Technology

Because current memory technologies, such as the DVD, the upper limit of the density of data have reached due to the limits of diffraction, physical limitations of the write laser, a holographic memory has the potential to be the next generation recording medium to be (see Holographic Versatile Disc). The advantage of this data storage is that the entire volume of the recording material can be used and not just the surface. This aspect allows phenomena such as Bragg volume addressing can be utilized, whereby a lot more information can be placed in the same volume of storage media. For each hologram against its neighbors must be Bragg tune. This can be accomplished by several methods, for example by rotation of the recording medium in consideration of the recording medium and the reference beam, or by changing the wavelength or phase of the recording laser beam for each hologram.

As with other media holographic memory are once in writable memory ( the storage medium is changed irreversibly ) and rewritable memory ( change is reversible ) divided. Rewritable holographic memory can be achieved by the photorefractive effect in crystals:

  • On both sides of coherent light from two light sources produces an interference pattern in the medium. The two light sources are referred to as the reference beam and the signal beam.
  • In places where superimposed waves lead to an increase of the amplitude, it is called constructive interference and the light appears brighter. Characterized is enough energy available to convey the electrons from the valence band via the band gap to the conduction band. The thus created "holes" can be regarded as quasi positive charge. For use as a holographic memory, these so-called defect electrons must be localized.
  • Electrons in the conduction band are free to move within the medium. Their movement is influenced by two opposing effects: the Coulomb force and the diffusion. After Charles Augustin de Coulomb's law, the electrons are aiming for a charge balance and therefore will remain as close as possible to one of the electron holes or fill them. This is counteracted by the urge for homogeneous distribution of the electrons. Depending on how much it affects the Coulomb's law or how large the spatial concentration difference is, the electrons stay or migrate to places of lower electron concentration.
  • Directly from the rise to the conduction band, there is the possibility that the electron re-occupied a defect electron. The higher the rate of this, the lower is the probability of a diffuse charge equalization. This is a decisive criterion for determining shelf life of holographic memory.
  • After some electrons have migrated to places of lower concentration and have occupied the local electron holes, there is an electric field between the added migrated electrons and electron holes in places with higher concentration. This electric field acts due to the Kerr effect on the refractive index of the medium, and changes this.

If information is to be accessed or read from a hologram, only the reference beam is necessary. The beam is sent with the same properties as when writing into the medium. Due to the aforementioned altered optical properties of the medium, the refractive index deviates locally from the expected value and two beams leave the medium, an expected along the way and another on a different route. An optical sensor captures this beam and determines its properties. These provide information on the original used in describing signal beam and its information.

Holograms can theoretically store a bit in a cube having an edge length of the wavelength of the light that was used for writing. Light, for example, a helium -neon laser is red ( exact wavelength: 632.8 nm). Now if one uses light of this wavelength, a holographic memory of perfect square inch 1.61 × 109 bits would be, which is about 201.4 MB (2.5 x 108 bits per square centimeter ) can store. One cubic inch of such storage would have a storage capacity of 8.1 terabytes ( 493 GB per cm ³). But the storage density is in practice lower by orders of magnitude, there's bits are needed for error correction, and the defectiveness of the optical system must be balanced.

History

In the spring of 1999 was published, that the Heidelberg research facility European Media Laboratory and the Hamburg Tesa maker Beiersdorf AG have signed a cooperation agreement on the development of a so-called "T- ROM" (also called Scotch -ROM).

And about 10 years later ( in early 2009 ) was published, the research branch of the U.S. conglomerate General Electric has developed a holographic memory with a capacity of up to 500 gigabytes.

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