Build-up dynamics in bidirectional soliton fiber lasers

Bidirectional ultrafast fiber lasers present an attractive solution, enabling the generation of two mutually coherent ultrashort pulse trains in a simple and turnkey system. Still, the lack of a comprehensive numerical model describing steady-state bidirectional generation, and even less ultrafast soliton breakdowns and collisions, is obstructing the achievement of the performance compared with unidirectional lasers. In this paper, we have experimentally investigated real-time build-up dynamics of counter-propagating solitons in an ultrafast ring Er-doped fiber laser via the dispersive Fourier transform methodology. We parade that counter-propagating pulses experience independent build-up dynamics from modulation instability, undergoing breathing dynamics and diverging subordinate pulse structure formation and annihilation to a stable bidirectional pulse train. Yet, the interaction of pulses in the cavity presents the key underlying phenomenon driving formation evolution distinct from unidirectional pulse build-up. Our findings will provide physical foundations for bidirectional ultrafast fiber laser design to carry forward their application.