SPATIOTEMPORAL INSTABILITY OF ULTRASHORT PULSES IN LARGE-CORE GRADED-INDEX MULTIMODE FIBERS

: We present a numerical study of the spatiotemporal nonlinear dynamics of ultrashort pulses in large-core graded-index multimode fibers operating in both normal and anomalous dispersion regimes. Using a generalized multimode nonlinear Schrödinger framework with spatially periodic nonlinearity, we investigate the generation of spatiotemporal instability and dispersive waves under varying spatial excitation conditions. In the normal dispersion regime, we observe the formation of spatiotemporal instability (STI) sidebands with frequency shifts exceeding 76 THz, and show that excitation of higher-order modes significantly enhances spectral broadening through quasi-phase-matched intermodal interactions. In the anomalous dispersion regime, strong spectral expansion and multi-peak dispersive wave (DW) generation are observed, with the number and bandwidth of DWs increasing as the spatial input profile becomes broader. Our results reveal that both fiber core size and input beam shape critically influence nonlinear frequency generation. This study establishes spatial excitation as a key parameter for controlling the spectral and temporal evolution of pulses in GRIN MMFs, and provides new insights into engineering complex nonlinear dynamics in multimode waveguides. The findings contribute to a deeper understanding of spatiotemporal processes in multimode nonlinear optics and offer design principles for advanced fiber-based systems in ultrafast photonics.