To overcome the capacity of the limitation in current optical data storages, including (CD and DVD), bit-data are optically recorded and read on the surface of disk medium. To exceed the data capacity of such surface-recorded memory, three-dimensionally recording/reading bit-data has been investigated in photosensitive media such as photopolymeric material, photochromic molecules, and fused silica.

Although photorefractive ferroelectric crystals such as BaTiO3, LiNbO3, and Bi12SiO20 with external electric field, can be good candidates of bit-oriented three-dimensional data storage, there are not report with these crystals except our works. Since photorefractive crystals have been historically used for real-time holographic materials for the use of phase conjugators and two-wave couplers, they have been naturally used as multiplexed holographic memory media. However holographic memory requires an extremely large dynamic range in materials at every position to record the Fourier transformed analog data of the entire bit-data, while bit-oriented memory records binary data. The data redundancy is guaranteed with an addition of error bits in bit-oriented memory, while it is essentially and automatically made in hologram. There are many other advantages of bit-oriented volumetric three-dimensional memory, including the random access capability, no higher-order diffraction with nonlinear response of the material, and highest data density/capacity in three-dimension because of unnecessity of reference beam and small field of view.

The major problem with the use of photorefractive crystals, whichever holographic or bit-oriented, is that the laser beam erases the data recorded in different addresses. To reduce this problem, the beam power is to be controlled to regulate the data modulation taking account of erasure in recording. However this trick is not a solution for not unnecessary erasing of data.