Multimode fibres (MMFs) are attracting interest for complex spatiotemporaldynamics, and for ultrafast fibre sources, imaging and telecommunications. Thisnew interest is based on three key properties: their high spatiotemporalcomplexity (information capacity), the important role of disorder, and complexintermodal interactions. To date, phenomena in MMFs have been studied only inlimiting cases where one or more of these properties can be neglected. Here westudy MMFs in a regime in which all these elements are integral. We observe aspatial beam-cleaning process preceding spatiotemporal modulation instability.We show that the origin of these processes is a universal unstable attractor ingraded-index MMFs. Both the self-organization of the attractor, as well as itsinstability, are caused by intermodal interactions characterized by cooperatingdisorder, nonlinearity and dissipation. The demonstration of adisorder-enhanced nonlinear process in MMF has important implications fortelecommunications, and the multifaceted complexity of the dynamics showcasesMM waveguides as ideal laboratories for many topics and applications incomplexity science.
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