Holographic memory is a technique that can store information at high density inside crystals or photopolymers. As current storage techniques such as DVD reach the upper limit of possible data density (due to the diffraction limited size of the writing beams), holographic storage has the potential to become the next generation of storage media. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface. This 3D aspect allows for a phenomenon known as Bragg volume selectivity to be utilised, whereby many information laden holograms can be superimposed or multiplexed in the same volume of medium. It is necessary to Bragg detune each hologram recorded with respect to its neighbours. This can be achieved by a number of methods, e.g. rotation of the media with respect to the recording object and reference beams or changing the wavelength or phase of the recording laser beams for each hologram.
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.