It's how holographic storage achieves its high data densities. By changing either the angle of the reference beam or its frequency, you can write additional data pages in to the same volume of crystal. However, all the holograms appear dimmer because their patterns must share the material's finite dynamic range. In other words, the additional holograms alter a material that can support only a fixed amount of change. Ultimately, the images become so dim that noise creeps into the read-out operation, thus limiting the material's storage capacity.
Current work uses iron-doped lithium niobate, strontium barium niobate, or barium titanate crystals.
While a defect in the medium for disk or tape storage might garble critical data, a defect in a holographic medium doesn't wipe out information. Sincerbox believes that it will take several more years to refine the technology enough to build small desktop HDSS units. Such devices might be ready by about the year 2003. While holographic storage appears to be a radically new technology, actually it's not. The basic concepts were worked out almost 30 years ago.