Nonlinear dynamics of ultrashort pulses in solid-state lasers mode-locked by self-focusing

An investigation is reported of nonlinear dynamics of a four-mirror Z-shaped cavity with mode locking by self-focusing. It is shown that the mode-locking region is nonuniform: it is characterised by the appearance of ‘windows’ of irregular behaviour of the system as the result of a change in the pump intensity or in the distance between the active element and a spherical mirror. Conclusions drawn from a theoretical analysis are confirmed by the results of a numerical experiment based on a fluctuation model.