Evolution from dissipative soliton resonance to four-wave mixing-assisted supercontinuum in the spatiotemporal mode-locked fiber laser

To the best of our knowledge, the nonlinear evolution characteristics are first systematically investigated by increasing the pump power without altering the cavity architecture in the isolator-free spatiotemporal mode-locked (STML) Yb-doped fiber laser allowing counter-propagating fields, which reveals the staged activation of multiple nonlinear effects in the experiment. As the pump power increases from 236 mW to 568 mW, self-phase modulation (SPM), stimulated Raman scattering (SRS) and four-wave mixing (FWM) are successively triggered to form the mode-locked properties of dissipative soliton resonance (DSR), SRS-induced dual-wavelength and FWM-assisted supercontinuum. At low pump power of 236 mW, the DSR features triangular spectra, rectangular nanosecond pulses and strong stability under the nonlinear effect of SPM. When the pump power exceeds Raman threshold, a distinct Raman peak emerges near 1070 nm, forming a dual-wavelength STML. The main and Raman pulses exhibit synchronous oscillation and stable STML characteristics, indicating that SRS plays a key role in energy redistribution and mode stabilization. At higher pump power of 568 mW, enhancing nonlinear interactions leads to producing a supercontinuum with a bandwidth exceeding 50 nm, which is attributed to the enhanced FWM effect. Throughout the evolution, the pulse train remains stable with a repetition rate of 4.48 MHz. The results provide a comprehensive understanding of nonlinear evolution in the multimode STML fiber lasers and highlight the significant roles of SPM, SRS, and FWM in the ultrafast multimode fiber laser dynamics.