Observation of mode-locked spatial laser solitons

A stable nonlinear wave packet, self-localized in all three dimensions, is an intriguing and much sought after object in nonlinear science in general and in nonlinear photonics in particular. We report on the experimental observation of mode-locked spatial laser solitons in a vertical-cavity surface-emitting laser with frequency-selective feedback from an external cavity. These spontaneously emerging and long-term stable spatio-temporal structures have a pulse length shorter than the cavity round trip time and may pave the way to completely independent cavity light bullets.