Influence of the spatial confinement on the self-focusing of ultrashort pulses in hollow-core fibers.

The collapse of a laser beam propagating inside a hollow-core fiber is investigated by numerically solving different nonlinear propagation models. We have identified that the fiber confinement favors the spatial collapse, especially in case of pulses with the input peak power close to the critical value. We have also observed that when using pulses in the femtosecond range, the temporal dynamics plays an important role, activating the spatial collapse even for pulses with input peak powers below the critical value. The complex self-focusing dynamics observed in the region below the critical power depends on the temporal evolution of the pulse and, also, on the interaction between the different spatial modes of the hollow-core fiber.