Holographic data storage

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Holographic data storage captures data using laser beams shining on photorefractive material. 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.

1 Terms used
2 Technical aspects
3 Two-color recording
4 Recording the data
5 Effect of annealing
6 Development and marketing
7 See also
8 References
9 External links

[edit] Terms used

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 data.

[edit] Technical aspects

Like other media, holographic media is divided into write once (where the storage medium undergoes some irreversible change), and rewritable media (where the change is reversible). Rewritable holographic storage can be achieved via the photorefractive effect in crystals:

Image:Hologram maken (1).svg

Mutually coherent light from two sources creates an interference pattern in the media. These two sources are called the reference beam and the signal beam.
Where there is constructive interference the light is bright and electrons can be promoted from the valence band to the conduction band of the material (since the light has given the electrons energy to jump the energy gap). The positively charged atoms they leave are called holes and they must be immobile in rewritable holographic materials. Where there is destructive interference, there is less light and few electrons are promoted.
Electrons in the conduction band are free to move in the material. They will experience two opposing forces that determine how they move. The first force is the coulomb force between the electrons and the positive holes that they have been promoted from. This force encourages the electrons to stay put or move back to where they came from. The second is the pseudo-force of diffusion that encourages them to move to areas where electrons are less dense. If the coulomb forces are not too strong, the electrons will move into the dark areas.
Beginning immediately after being promoted, there is a chance that a given electron will recombine with a hole and move back into the valence band. The faster the rate of recombination, the fewer the number of electrons that will have the chance to move into the dark areas. This rate will affect the strength of the hologram.
After some electrons have moved into the dark areas and recombined with holes there, there is a permanent space charge field between the electrons that moved to the dark spots and the holes in the bright spots. This leads to a change in the index of refraction due to the electro-optic effect.

Image:Hologram lezen.svg

When the information is to be retrieved or read out from the hologram, only the reference beam is necessary. The beam is sent into the material in exactly the same way as when the hologram was written. As a result of the index changes in the material that were created during writing, the beam splits into two parts. One of these parts recreates the signal beam where the information is stored. Something like a CCD camera can be used to convert this information into a more usable form.

Holograms can theoretically store equal to one bit per cubic block the size of the wavelength of light in writing. For example, light from a helium-neon laser is red, 632.8 nm wavelength light. Using light of this wavelength, perfect holographic storage could store 4 gigabits per cubic millimetre. In practice, the data density would be much lower, for at least four reasons:

The need to add error-correction
The need to accommodate imperfections or limitations in the optical system
Economic payoff (higher densities may cost disproportionately more to achieve)
Design technique limitations--a problem currently faced in magnetic Hard Drives wherein magnetic domain configuration prevents manufacture of disks that fully utilize the theoretical limits of the technology.

Unlike current storage technologies that record and read one data bit at a time, holographic memory writes and reads data in parallel in a single flash of light.^[1]

[edit] Two-color recording

Image:Holographic Data Storage.svg

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.

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 valence 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.

[edit] 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 intersect 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.

[edit] 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.

[edit] Development and marketing

At the National Association of Broadcasters 2005 (NAB) convention in Las Vegas, InPhase conducted the first public demonstrations of the world’s first prototype of a commercial storage device at the Maxell Corporation of America booth.

The three main companies involved in developing holographic memory, as of 2002, were InPhase, Polaroid spinoff Aprilis, and Optware of Japan.^[2] Although holographic memory has been discussed since the 1960s,^[3] and has been touted for near-term commercial application at least since 2001,^[4] it has yet to convince critics that it can find a viable market.^[5] As of 2002, planned holographic products did not aim to compete head to head with hard drives, but instead to find a market niche based on virtues such as speed of access.^[2]