Nonlinear mixing of spatial frequencies in photorefractive thermal fixing of holograms in LiNbO3

The nonlinear combinations of gratings appearing in fixed multiplexed configurations during the recording as well as during the fixing of holograms in LiNbO3 has been investigated. We have obtained significant differences in comparison with the previously studied case of non-fixed holograms. Moreover, we have found that the relative strength of the combinational gratings depends on the particular experimental procedure used for thermal fixing. The first observation of fixed combinational gratings is also reported obtaining diffraction efficiencies of similar to0.01% for the sum and difference gratings. Our results should be relevant for applications involving fixed multiplexed holograms. (C) 2001 Elsevier Science B.V. All rights reserved.