Abstract

Early holographic storage experiments in photorefractive materials have shown that efficient thermal fixing is achieved in lithium niobate crystals with moderate concentration of iron doping (LiNbO₃:Fe).^1-2 However, this doping generates scattering centers that allow the buildup of spurious gratings that contribute to beam fanning.³ This effect can be deleterious in holographic storage since the scattered light is usually super-imposed on the diffracted signal and significantly decreases the sig- nal-to-noise ratio. In this work, we report on a comparative study of various configurations that reduce the effect of beam-fanning on the quality of a diffracted image which has been fixed in a 0.010% molar concentration iron-doped LiNbO₃ crystal. A given configuration is determined by both the direction of the c-axis of the crystal and that of the linear polarization of the writing/ readout beams relative to the plane of incidence. Only the isotropic scattering case is considered in the following. The main result is that the scattering noise in the image plane is drastically reduced when the c-axis is perpendicular to the plane of incidence. This is explained by the fact that beam-fanning occurs along the direction of the c-axis, and therefore does not overlap spatially with the diffracted image.

© 1994 Optical Society of America