Pure-quartic solitons in multimode mode-locked fiber lasers

Ultrashort pulse generation in mode-locked cavities hinges on a precise balance between dispersion and nonlinear effects. While conventional soliton lasers predominantly rely on second-order dispersion for soliton formation, this approach intrinsically restrains energy-scaling capabilities. In recent years, pure-quartic solitons (PQS) governed by fourth-order dispersion have been proposed, emerging from the balance between negative fourth-order dispersion and self-phase modulation in single-mode cavities. In this paper, by leveraging dispersion-managed strategies, we report the characterization of PQS dynamics in a multimode mode-locked fiber cavity, for the first time to the best of our knowledge. The results show that spectral-sideband positions of PQS of each spatial mode exhibit a slight difference in the multimode cavity because of different nonlinearity in their mode channels. PQS in the multimode cavity shows higher energy thanks to plenty of modes in multimode fibers. Furthermore, by comparing single-mode and multi-mode cavities, we demonstrate that intermodal crosstalk in multimode fiber lasers directly induces spectral symmetry breaking, sideband suppression, and rich pulse dynamics. Moreover, the nonlinear energy transfer mechanisms in pulsating soliton states, and the mechanisms that manipulate the excitation of a chaotic soliton bunch by using modal dispersion in the multimode cavity are comprehensively investigated. These results not only present avenues for designing high-energy soliton lasers, but also demonstrate the use of higher-order dispersion for optical pulse control, enabling innovations in nonlinear optics and its applications.