A spatio-temporal model of a self-mode-locked laser is presented. It uses a full wave-optics approximation of the propagation in the laser resonator and includes dispersion, self-phase modulation, and self-focusing of the intracavity radiation. It is shown that the self-consistent evolution of the pulse toward steady state imposes strong space–time focusing in the crystal, where both the space and time foci are located. This combined focusing is inherent for Kerr-lens mode-locked lasers with linear and ring cavities, and with and without intracavity saturable absorbers. It is considered as a general mechanism for compensation of space–time astigmatism within the nonlinear resonator. The predictions of the model agree very well with the experiment.
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