Cascaded Kerr beam self-cleaning in graded-index multimode fibers.

• We firstly propose a cascaded Kerr beam self-cleaning scheme that combines multiple MMFs to further improve multimode beam quality. • Radial offset between fiber 1 and fiber 2 of the cascaded multimode fiber system should be as small as possible for cascaded beam self-cleaning. • Our results also suggest that an increased number of HOMs with negligible energy along with a higher number of cascaded fibers contribute to superior multimode beam quality. Spatial beam self-cleaning has shown a nonlinear improvement in beam quality in multimode fibers (MMFs), proving its value in high-power fiber lasers and nonlinear biomedical imaging. In this Letter, we propose a novel cascaded Kerr beam self-cleaning scheme that combines multiple MMFs to further improve beam quality. Through this scheme, the initially speckled beam gradually evolves into a well-defined bell-shaped, primarily dominated by the fundamental mode, after undergoing two or three cascaded self-cleaning processes. Simulation results also suggest that an increased number of high-order modes (HOMs) with negligible energy along with a higher number of cascaded fibers contribute to superior beam quality. This phenomenon is attributed to the new excited HOMs acting as attractor and facilitating an energy cascade towards the fundamental mode. [ABSTRACT FROM AUTHOR]