Ultrafast Pulse and Continuous Wave Resonant Sidebands in Spatiotemporal Mode-Locked Fiber Laser

We demonstrate the first stable coexistence of multimode ultrafast pulses and continuous waves (MM-CWs) accompanied by CW-induced resonant spectral sidebands in a spatiotemporal mode-locked (STML) fiber laser. This breakthrough is achieved through a strategically designed saturable absorber based on nonlinear multimode interference (NL-MMI), fabricated by splicing graded-index multimode fibers with 50/125 μm and 62.5/125 μm core diameters into an SMF-MMF-SMF structure. The STML laser achieves record L-band wavelength tuning spanning 19 nm (1568–1587 nm), the broadest range reported in 1.5μm STML systems, with pulse width shortening from 1.59 ps to 1 ps spectral during redshift. Furthermore, we discover phase-matched resonant sidebands generated through coupling between multimode continuous waves (MM-CWs) and dispersive waves (DWs), which dissipate excess energy to stabilize mode competition. Crucially, these resonant sidebands establish a topological energy transfer channel that enforces phase synchronization between STML pulses and MM-CWs, enabling their long-term coexistence. Beam profile evolution further confirms spatiotemporal redistribution during operation. Our work enriches dispersive wave dynamics in multimode environments and opens avenues for multichannel ultrafast photonic systems.