Spatial Instability of Fundamental Mode in Step-Index Multimode Fiber

Research on nonlinear optics in multimode optical fibers is a hot emerging field, motivated by applications to spatial division multiplexing in long distance communications, novel high energy laser sources, as well as for the fundamental interest in complex physical systems. A spectacular nonlinear effect which has been recently revealed in highly multimode graded-index fibers is beam self-cleaning, whereby a speckled beam transforms into a bell-shaped beam, typically dominated by the fundamental mode of the fiber. This transition is driven by the Kerr effects, and it occurs as the input power is increased above a certain value. In this paper we discuss a new type of spatial beam instability in few-mode step-index fibers, whereby a femtosecond laser beam coupled to the fiber's fundamental mode loses its stability at high peak powers. As a result, the fundamental beam becomes fully depleted, in favor of the higher-order modes of the fiber. The resulting multimode beam pattern is now dominated by the modes of highest order, and it remains remarkably stable even in the presence of external fiber perturbations. In this respect, the fundamental mode instability has a strong similarity with beam self-cleaning, but it acts in reverse.