Self-consistent soliton evolution in single–two-mode fiber lasers

Ultrafast few-mode fiber lasers have received increasing attention from basic research to practical applications due to their unique pulse performance and intriguing nonlinear dynamics. Here, we experimentally and numerically reveal the formation and evolution behaviors of a soliton in a mode-locked fiber laser composed of two-mode and single-mode fibers. The LP11 pulse walks away from the LP01 pulse in the two-mode fiber due to modal dispersion and then transforms into an auxiliary LP01 pulse after entering the single-mode fiber. After re-entering the two-mode fiber, the LP01 pulse excites the LP11 pulse via mode coupling; therefore, the LP11 pulse also consists of dominant and auxiliary pulses. Such a soliton fiber laser converges to an asymptotic steady state with unlocked spatial modes arising from the interplay between the strong modal dispersion and weak mode coupling.