TIME EVOLUTION OF PHOTOREFRACTIVE FIXING PROCESSES IN LINBO3

Decays of diffraction efficiency of gratings written in Fe:LiNbO3 have been studied in connection with photorefractive fixing. Measurements have been carried out at different temperatures within the range of interest in photorefractive fixing, 144-168 degrees C. The decay curves are non-exponential and can be fitted to a sum of three simple exponential components, whose effective activation energies and preexponential factors have been determined. The decay constants have also been found to depend on the time the crystal is kept at the setting temperature before writing the grating. The difficulties for establishing a model taking into account the occurrence of several sites for protons in LiNbO3 are discussed.