Soliton Molecular Chains Induced by Cascaded Self-Injection Trapping

Soliton molecules (SMs), ranging from the simplest soliton pairs to complex multi-soliton patterns, serve as versatile carriers for unveiling intricate nonlinear interactions and developing advanced ultrashort laser pulses. Owing to many-body interactions, SMs composed of multiple solitons display more complex structures and dynamics. Here, a unique type of SMs, termed soliton molecular chains (SMCs), in a passively mode-locked fiber laser is reported and their formation mechanism—the cascaded self-injection trapping—is elucidated. The SMCs comprise equally spaced multi-solitons, increasing progressively with pump strength, and exhibit temporal separation locking and relative phase correlating behaviors. Simulation results fully reproduce and interpret experimental observations, unveiling that the emerging solitons in SMCs are trapped at the local minima of the effective pinning potential in a cascade manner. Specifically, these potential wells originate from the interactions between the emerging solitons and the time-delayed self-injection pulses of the preceding soliton generated through an inherent sub-cavity. Unlike SMs formed through long-range interactions between solitons and background oscillations, this study demonstrates a novel mechanism that provides an alternative approach to synthesizing SMs with desired patterns by artificially introducing self-injection pulses.