Technical aspects Holographic data storage

mutually coherent light 2 sources creates interference pattern in media. these 2 sources called reference beam , signal beam.
where there constructive interference light bright , electrons can promoted valence band conduction band of material (since light has given electrons energy jump energy gap). positively charged vacancies leave called holes , must immobile in rewritable holographic materials. there destructive interference, there less light , few electrons promoted.
electrons in conduction band free move in material. experience 2 opposing forces determine how move. first force coulomb force between electrons , positive holes have been promoted from. force encourages electrons stay put or move came from. second pseudo-force of diffusion encourages them move areas electrons less dense. if coulomb forces not strong, electrons move dark areas.
beginning after being promoted, there chance given electron recombine hole , move valence band. faster rate of recombination, fewer number of electrons have chance move dark areas. rate affect strength of hologram.
after electrons have moved dark areas , recombined holes there, there permanent space charge field between electrons moved dark spots , holes in bright spots. leads change in index of refraction due electro-optic effect.

when information retrieved or read out hologram, reference beam necessary. beam sent material in same way when hologram written. result of index changes in material created during writing, beam splits 2 parts. 1 of these parts recreates signal beam information stored. ccd camera can used convert information more usable form.

holograms can theoretically store 1 bit per cubic block size of wavelength of light in writing. example, light helium–neon laser red, 632.8 nm wavelength light. using light of wavelength, perfect holographic storage store 500 megabytes per cubic millimeter. @ extreme end of laser spectrum, fluorine excimer laser @ 157 nm store 30 gigabytes per cubic millimeter. in practice, data density lower, @ least 4 reasons:

the need add error-correction
the need accommodate imperfections or limitations in optical system
economic payoff (higher densities may cost disproportionately more achieve)
design technique limitations—a problem faced in magnetic hard drives wherein magnetic domain configuration prevents manufacture of disks utilize theoretical limits of technology.

despite limitations, possible optimize storage capacity using all-optical signal processing techniques

unlike current storage technologies record , read 1 data bit @ time, holographic memory writes , reads data in parallel in single flash of light.