Optimization of Geometric Parametric Instability Using Beam Shaping

Multimode fibers (MMF) have recently attracted significant interest in studying complex nonlinear spatio-temporal dynamics [1]–[3]. In this context, the parabolic refractive index of graded-index (GRIN) MMFs induces unique dy-namics that enhance nonlinear frequency conversion. This is because GRIN fibers support a high number of modes with nearly equidistant propagation constants that interfere constructively periodically. This periodic interference creates a spatial grating allowing for phase-matching of nonlinear mixing between the different modes, resulting in the generation of distinct geometric parametric instability (GPI) sidebands far-detuned from the pump spec-trum [3]. The characteristics of GPI are influenced by several factors, including the injected optical power, fiber geometry, and the ellipticity of the input beam [4], [5]. Understanding and controlling these parameters provide a pathway to optimizing frequency conversion and supercontinuum generation in MMFs, and advancing applications in high-power broadband light sources with customizable spectral features.