Self-focusing collapse distance in ultrashort pulses and measurement of nonlinear index

We analyze the method of moving focus to determine the critical power for self-focusing by means of numerical simulation and a semianalytical model. It is shown that the original interpretation of a moving focus experiment does not hold in general and that inclusion of defocusing effects due to free electrons is necessary to relate the measured data to critical power. (C) 2011 Optical Society of America