Pulse-onset dynamics through instabilities in a bidirectional mode-locked fibre laser
in: Communications Physics (2020)
Real-time observation of the emergence of coherent structure from noises via instabilities is of particular interest across disciplines ranging from biology to astrophysics. In the context of photonics, ultrafast fibre lasers provide an ideal test-bed for an experimental observation of dynamical instabilities and generation of coherent structures of ultrashort pulses. Here, for the first time, we present experimentally obtained switch-on dynamics of counter-propagating ultrashort pulses in a bidirectional mode-locked fibre laser with delayed pulse formation via instabilities. The pulses exhibit transitions between Q-switched instabilities and single or multiple-soliton formation. We define a localisation parameter using round-trip resolved pulse autocorrelation function to quantify the extent of the pulse formation, which indicates a complex energy interchange between coherent features and background radiation. The round-trip time-resolved intensity and spectral measurements also reveal a pronounced drift of the central wavelength during the build-up process. Furthermore, we report the formation of synchronised and unsynchronised dispersion waves which has been numerically predicted recently. Our results reveal the complexities of the establishment of coherent features and their interaction with background radiation, contributing further towards the understanding of nonlinear systems in general.