In holographic data storage, a photo-sensitive medium is exposed to the interference pattern that is generated when an object beam, with an input data page encoded in the spatial profile of the beam, is intersected by a second, coherent laser beam. The photosensitive medium replicates these interference fringes as a change in optical absorption, refractive index, or thickness. Data are retrieved from the medium by exposing it to light from just one of the beams, which is then diffracted from the stored fringe pattern to reconstruct the other beam, including all the information that had been in the input data page. For a material of sufficient thickness, a large number of interference patterns, each identified by a different grating vector, can be stored or “multiplexed” in the same volume element, with negligible crosstalk between the individual interference patterns. Multiplexing of a large number of pages in the same volume element of the recording medium can be accomplished in several ways—for example by varying the angle between object and reference beam or the wavelength of both beams. Given no other limiting factors, the number of holograms that can be multiplexed in one volume element is directly proportional to the product of the thickness of the medium and its refractive index—that is, materials with optical thicknesses of the order of several millimeters are desirable.By its very nature, the holographic-storage mechanism distributes the stored information redundantly throughout the recording volume.
Frequency characters of Holographic data storage system's data page And Description of optical system