All-step-index-fiber spatiotemporally mode-locked laser

Spatiotemporal mode-locking (STML) in multi-mode fiber (MMF) lasers has extended the concept of temporal dissipative solitons into spatiotemporal dissipative solitons. To date, all reported STML in MMF lasers has used graded-index (GRIN) MMFs either solely or hybridly with other fibers. Compared to GRIN MMFs, step-index (STIN) MMFs have much larger intermode dispersion on both group and phase velocities. Building all-STIN MMF lasers can provide a new platform to explore the spatiotemporal dissipative soliton dynamics. Here, we report experimental and numerical observation of STML in an all-STIN MMF laser. Distinct from GRIN MMF lasers, the large intermode dispersion in the all-STIN MMF laser cannot be balanced by Kerr nonlinearity, and significant walk-off between mode-resolved pulses was observed experimentally. Simulations suggest that this walk-off is counteracted by spatial coupling in the laser, and a mother-child coupling mechanism is proposed to understand it. This mother-child coupling can enable STML with a single repetition rate with infinitely large intermode dispersion. Our work enriches MMF laser architectures for STML in a parameter regime that has not been considered, to our knowledge.