Using diffraction theory

we have analyzed Z-scan measurement with thin samples and given the suitable condition for normal Z-scan technique.By theory calculating

we found only very small system error would be produced when detector was in near field substituted far field.We have discussed the Zscan technique for sample with strong fifth order nonlinearity or saturated nonlinear refractive index. The methods for measuring nonlinear refractive index and other nonlinear coefficients in these condition have been obtained.