Optical fatigue of undoped lithium niobate crystals caused by irreversible photorefractive damage at high-intensity illumination

It has been established by Raman scattering study that the optical damage in nominally pure LiNbO₃ crystals presents an irreversible part. This irreversible fraction of the optical damage is induced only by the high-intensity (I ≥ 25 kW/cm²) exposition for a long time, and it has accumulative character with exposure time. This irreversible effect has been shown to be related to the optical fatigue of the material. The consequent needle-like domain inversion was considered to cause accumulative deterioration of the material, i.e., a fatigue due to the continuous electric field application, relating to the high space-charge field induced by inhomogeneous high-intensity illumination.