Holographic Data Storage - Technical Aspects

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:

• 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 vacancies 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.

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 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.

Despite those limitations, it is possible to optimize the storage capacity using all-optical signal processing techniques

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.