In hologiraphic data storage, pages of information are overlapped in the voume of the recording medium. Due to destructive read-out of hologirams in photorefractive crystals such as LiNbO_3:Fe, hologirams are recorded with an exposure schedule in order to equalize diffraction efficiency. This leads to a final diffraction efficiency proportional to 1/M~2, where M is the number of exposures. Coherent erasuer of a particular page also erases all the other pages stored in the msae volume. We believe to have found a technique that does not require an exposure schedule and that can record M holograms with diffraction efficiency following a 1/M dependence. Our technique is based on non-destructive read-out in doubly-doped LiNbO_3 (1). The technique is based on the recording of localized holograms in thin layers across the voolume of the crystal.
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