New perspectives on spatiotemporal mode-locked fiber lasers: multimode vector soliton dynamics and on-demand output parameters

Multimode nonlinear dynamics in spatiotemporal mode-locked (STML) fiber lasers enable exploration of novel solitary waves. However, complex intermodal coupling in multimode fibers prevents precise control over the output parameters of multimode solitons (e.g., mode component and center wavelength), limiting their applications in fiber amplifiers and optical communications. Additionally, vector soliton dynamics have been overlooked due to polarization-dependent saturable absorbers (SAs) in prior STML systems, many unique nonlinear dynamics remain inadequately explained. This work presents a method to customize the output parameters of STML pulses by a spatial alignment structure (SAS). This polarization-independent SAS functioning as an SA, spatial filter, and attenuator. Adjusting the SAS enables controllable multimode soliton characteristics (e.g., dominant mode transition from LP21 to LP11 and LP01 modes) and diverse STML dynamics, including soliton rain, tri-color solitons (TCS), and ultra-broad bandwidth spectrum (UBS) pulses (28 nm bandwidth, 94 fs duration) within the same cavity. Experimental results reveal the critical role of vector soliton dynamics in generating parametric sidebands, TCS, and UBS pulses. This work demonstrates, to our knowledge, the first multimode vector solitons and achieves the broadest bandwidth/shortest pulse duration in 1.5 μm STML lasers. Further research into multifunctional SAS and multimode vector soliton dynamics may uncover new spatiotemporal nonlinear effects, enabling compact, cost-effective STML fiber lasers with on-demand outputs.<p></p>