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 in terest in photorefractive fixing, 144-168 degrees C. The decay curves are non-exponential and can be fitted to a sum of three simple exponen tial components, whose effective activation energies and preexponentia l factors have been determined. The decay constants have also been fou nd to depend on the time the crystal is kept at the setting temperatur e before writing the grating. The difficulties for establishing a mode l taking into account the occurrence of several sites for protons in L iNbO3 are discussed.