Analysis of transform from self-defocusing to selffocusing of LN crystal based on coupling theory

Z-scan experiment for measuring three-order nonlinear refractive coefficient and time-resolved scan experiment for measuring photorefractive nonlinear refractive coefficient were made with lithium niobate crystal doped with Fe and Tb. To explain the characteristics of the time-resolved scan curves, based on the coupling effects of photorefractive and third-order nonlinearity in the nonlinear medium, together with two analogic refractivity-timed expressions, an analysis on the dynamic evolving process of spatial bright soliton is made and eventually the physical mechanism of dynamical evolution from self-defocusing effect to self-focusing one under specific experimental condition is presented, which is completely coincident with the experimental phenomenon. The analysis has also validated the existing possibility of spatial dark soliton in self-focusing photorefractive crystal.