Bright and dark spatiotemporal localized structures in ytterbium-doped long-cavity multimode-fiber lasers

Multimode fiber lasers have emerged as a promising platform for exploring exotic high-energy spatiotemporal localized structures. However, the influence of long-cavity configurations on such systems has received little attention, despite their significant potential for accessing new regimes of mode-locking dynamics. Here, we demonstrate a long-cavity multimode fiber laser capable of delivering μJ-level bright and dark spatiotemporal localized structures, as well as spontaneous transitions between them. The long-cavity configuration provides microsecond-scale roundtrip time and supports partial gain recovery between successive roundtrips. In parallel, the nonlinear polarization rotation (NPR) introduces a nonlinear transmission that enables the formation of bright dissipative soliton resonance pulses and dark pulses. The synergy of a long-cavity configuration and NPR triggers the spontaneous transitions between dark and bright structures. Numerical simulations qualitatively reproduce key experimental phenomena, validating the indispensable role of NPR-induced nonlinear transmission in governing the types of output pulses. Our work unveils a unique pulse shaping mechanism associated with long-cavity configurations, sparking interest in investigating high-dimensional dissipative dynamics in mode-locked laser systems.