Abstract : To study the contribution of nonlinear effects to laser damage or predict ultrafast pulses evolution, nonlinear refractive index has to be measured accurately. To study the different contribution to nonlinear index we have implemented several sensitive measurement benches based on the Z-scan method.
First, we have developed a general simulation algorithm. Thanks to it the signal obtained by this method is shown highly dependent on the spatial beam profile at the beam waist and on its evolution around the waist. Then, we present the experimental set-up including the tools needed to verify and characterize the experimental beam and reduce errors on the determination of the nonlinear index.
Finally, this set-up is applied to the study of the nonlinear index of silica for different pulses durations. The nonlinear index of fused silica is found highly dependent on the pulses duration. Thermal effects are negligible and the contribution of electrostriction is important for nanosecond pulses.