Spatio-temporal evolution of ultrashort pulses in graded-index multimode fiber at normal dispersion

A novel nonlinear phenomenon at normal dispersion regime inside of graded-index multimode fiber (GRIN MMF), geometric parametric instability (GPI), can be observed while propagating beam experiences spatio-temporal evolution. Longhi theoretically predicted this GPI and associated with periodic refocusing inside GRIN MMF [1]. Later his work is verified by Krupa et al. [2] and Lopez-Galmiche et al. [3] and discrete GPI sidebands are reported with more than 120 THz detuning from to pump frequency. These studies focused on quasi-continues pulse evolution in GRIN MMF and the spatio-temporal evolution of femtosecond pulses at normal dispersion regime remains unknown. Here, we present the first experimental observation of GPI sidebands with ultrashort pulses in GRIN MMF. To gain detailed understanding, we perform numerical simulations and theoretical calculations and obtained results are well-aligned with experimental observations.