The lifetimes of non-fixed holograms in LiNbO<sub>3</sub> crystals with different dopants, doping levels and oxidation states have been measured in the temperature range from 30 to 180° C. Two dark decay mechanisms have been identified: proton compensation and electron tunneling. For LiNbO<sub>3</sub> crystals with low doping levels, the dominant mechanism of dark decay is proton compensation with the activation energy of 1.0 eV. This mechanism is independent of dopant, doping level and oxidation state. For LiNbO<sub>3</sub> with high doping levels, the prevailing mechanism is electron tunneling with the activation energy much less than that of proton compensation, e.g., 0.28 eV for LiNbO<sub>3</sub>:0.25 wt% Fe<sub>2</sub>O<sub>3</sub>. The dark decay of holograms stored in crystals with medium doping levels is the result of a combination of both effects.
© 2001 Optical Society of America