This paper describes the results of an investigation into techniques for obtaining erasure resistant holograms in electro-optic crystals. The most successful approach made use of thermally activated ionic drift during or after recording. The samples are heated for about 30 min at 100°C to obtain optically nonerasable holograms with as much as 50% diffraction efficiency in LiNbO3 or in doped Ba2NaNb5O15.

Holograms were recorded and fixed simultaneously in heated (∼160°C) crystals of Fe−doped LiNbO3. With this procedure the crystals have the erase/write asymmetry required for multiple storage of high−diffraction−efficiency holograms. Five hundred fixed holograms, each with more than 2.5% diffraction efficiency, were recorded.

This paper considers two effects for directly studying thick-phase holograms: (a) coupling between the two laser beams used to record a hologram and (b) interference between a readout beam and the diffracted beam within the hologram. Both effects were observed in experiments using single crystals of undoped LiNbO3. The results demonstrate that the holograms arise from electric field patterns caused by either diffusion or drift of photogenerated free electrons.

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