Photorefractive Damage in congruent LiNbO3. Part I. Zinc doped Lithium Niobate Crystals

The experimental measurements of the photorefractive damage in a series of zinc doped congruent lithium niobate are reported and discussed. The first experimental technique used is based on the measurement with time of the photoinduced distortion of an HeNe beam spot at 632.8 nm. The second method is based on the direct measurement of the photoinduced birefringence variation with time, using a powerful laser beam at 514.5 nm as gating radiation, and a laser weak beam at 632.8 nm as probe. The dependences of the photorefractivity and of the photosensitivity on the power and dopant concentration have been investigated. The photorefractive damage is highlight by the electro-optic behaviour with the zinc concentration and the defect structure of the doped crystals. We conclude that doping above a second threshold concentration with divalent Zn ions leads to a significant decrease of the photorefraction with respect to pure congruent crystals. In this concentration range, LiNbO3:Zn are strongly photorefractive damage resistant and thus present an interesting alternative for modulating devices to the stoichiometric lithium niobate. Link to the high electro-optic coefficients, these crystals are very suitable for Q-switching applications.