Holographic memory

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Holographic Versatile Card

Holographic data storage is a technique with which information in a very high density of crystals inside or photo - polymers can be saved.

technology

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Since today's storage technologies, such as. If, for example, the DVD has reached the upper limit of the data density due to the physical limits of the write laser set by diffraction , a holographic memory has the potential to become the next generation of storage medium (see also Holographic Versatile Disc ). The advantage of this data storage device is that the entire volume of the recording material can be used, not just the surface. This aspect allows phenomena such as Bragg volume addressing to be exploited, whereby a great deal more information can be accommodated in the same volume of storage medium. For this, every hologram has to be Bragg detuned against its neighbors . This can be achieved by several methods, e.g. B. by rotating the storage medium taking into account the recording medium and the reference radiation or by changing the wavelength or phase of the recording laser beam for each hologram .

As with other data carriers, holographic memories are divided into write-once memories (the storage medium is changed irreversibly) and rewritable memories (changes are reversible). Rewritable holographic memories can be achieved through the photorefractive effect in crystals:

  • Double-sided coherent light from two light sources creates 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 amplification of the amplitude, one speaks of constructive interference and the light appears brighter. This means that enough energy is available to transport electrons from the valence band over the band gap into the conduction band . The resulting "holes" can be seen as a positive charge. For use as a holographic memory, these so-called defect electrons must be fixed in place.
  • Electrons in the conduction band can move freely within the medium. Their movement is influenced by two opposing effects: the Coulomb force and diffusion . According to Charles Augustin de Coulomb's law, the electrons strive for a charge balance and will therefore remain as close as possible to one of the electron holes or occupy it. This is counteracted by the urge to distribute the electrons homogeneously . Depending on how strong Coulomb's law is or how big the spatial difference in concentration is, the electrons linger or migrate to locations with a lower electron concentration.
  • Immediately after ascending into the conduction band, there is a possibility that the electron will occupy a defect electron again. The higher the rate for this, the lower the probability of a diffuse charge equalization. This is a decisive criterion for determining the shelf life of holographic memories.
  • After some electrons have migrated to the locations of lower concentration and have occupied the electron holes there, an electric field exists between the migrated electrons and the holes at locations of higher concentration. Due to the Kerr effect, this electric field affects the refractive index of the medium and changes it.

If information is to be retrieved or read from a hologram , only the reference beam is necessary. The beam is sent into the medium with the same properties as when writing. Due to the changed optical properties of the medium mentioned at the beginning, the refractive index deviates locally from the expected value and two rays leave the medium, one on the expected path and another on a different route. An optical sensor captures this beam and determines its properties. These provide information about the original signal beam used when writing and its information.

Holograms can theoretically store a bit in a cube with the edge length of the wavelength of the light used for writing. The light z. B. a helium-neon laser is red (exact wavelength: 632.8 nm ). Now, using light at this wavelength, one square inch of perfect holographic memory would be able to store 1.61 x 10 9 bits, which is approximately 201.4 megabytes (2.5 x 10 8 bits per square centimeter ). One cubic inch of such memory would have a storage capacity of 8.1 terabytes (493 GB per cm³). In practice, however, the storage density is orders of magnitude lower, since bits are still required for error correction and the deficiency of the optical system has to be compensated for.

On the other hand, holography offers the possibility of writing several holograms in a given storage volume under different irradiation conditions and of superimposing them largely without interference. This technique is called "holographic multiplexing". For this purpose, for example, light of different wavelengths can be used (wavelength multiplexing), the interference patterns can be generated with light beams at different angles (angle multiplexing), or the storage medium can be rotated around an axis of symmetry (rotation multiplexing). Several multiplexing techniques can also be combined. The possible degree of multiplexing depends on the technology chosen and the storage material used. In this way, the memory limit described in the previous section can be multiplied, so that holographic memories can actually achieve very high memory densities with the corresponding effort.

history

In the spring of 1999 it was published that the Heidelberg research facility European Media Laboratory and the Hamburg Tesa manufacturer Beiersdorf AG had signed a cooperation agreement for the further development of a so-called "T-ROM" (also called Tesa-ROM ).

In the spring of 2009 it was reported that the research division of the American conglomerate General Electric had developed a holographic memory with a capacity of up to 500 gigabytes.

See also

literature

Web links

Individual evidence

  1. ↑ "Tesa-ROM" is being developed commercially - article at heise online , April 27, 1999
  2. ^ The Tesa-ROM at CeBIT - article at the University of Mannheim , from March 12, 1999
  3. ↑ Holographic memory with 500 GByte capacity - article at heise online , from April 28, 2009