Geometric parametric instability in periodically modulated graded-index multimode fibers

We present a theoretical and numerical study of light propagation in graded-index (GRIN) multimode fibers where the core diameter has been periodically modulated along the propagation direction. The additional degree of freedom represented by the modulation permits us to modify the intrinsic spatiotemporal dynamics which appears in multimode fibers. More precisely, we show that modulating the core diameter at a periodicity close to the self-imaging distance allows us to induce a moire-like pattern, whereas a longer periodicity induces an adiabatic modulation or the self-imaging pattern. This complex dynamics modifies the geometric parametric instability gain observed in homogeneous GRIN fibers by generating new spectral peaks.