We report on the measurement of nonlinear refraction in ion-doped solids with a method that combines the single-beamZ-scan technique and a Fourier analysis of the transmittance time evolution. The laser beam is modulated at a frequencyfand the Fourier components atfand2fare shown to be related, respectively, to linear and nonlinear refractions. Their ratio is used to eliminate spurious linear effects as a way of increasing the sensitivity of the measurement. With this method we are able to measure nonlinear phase changes of a few tens of mrad, corresponding to wave front distortions smaller than&lgr;/105.Moreover, the technique can discriminate nonlinear processes with different relaxation times. ©1997 American Institute of Physics.