Self-defocusing

Self-defocusing is the spreading tendency caused when a beam lowers the refractive index most strongly where its own intensity is largest. In the local optical Kerr effect model $n=n_0+n_{2}I$, a beam brightest on its axis produces this behavior for $n_2<0$: its transverse phase profile resembles that of a diverging optical lens.

Compare the beam width after the same sample at several input powers while holding the input profile fixed. A power-dependent change can reveal nonlinear spreading, but ordinary diffraction, optical absorption and heating must also be considered. Self-defocusing modifies a beam's propagation rather than removing the ordinary diffraction it already experiences.