A lensing effect arising from the Kerr nonlinearity.

When a short optical pulse propagates through a nonlinear medium, the Kerr Effect leads to a phase delay which is largest on the beam axis (where the Intensity is highest) and smaller outside the axis. As a consequence, the wavefronts are deformed, and the pulse is focused (assuming a positive n2 coefficient). This effect is called self-focusing and has important implications for passive Mode locking of lasers and for optical damage of media.