Adaptive multimode fiber source for deep nonlinear microscopy

Multimode optical fibers (MMFs) are efficient for high-dimensional data transmission and high-power delivery by supporting multiple spatial modes, with broad applications in optical computing and bioimaging. However, inevitable random mode coupling degrades their spatiotemporal properties, despite their high spectral energy, hindering practical applications in imaging, particularly in nonlinear microscopy. Here, we report the observation of nonlinear localization in a step-index MMF at megawatt peak power levels and demonstrate its application in nonlinear microscopy. Under this nonlinear localization condition, the MMF uniquely supports the formation and propagation of this localized wave packet over long distances and flexible paths with high stability, which cannot be achieved in either single-mode fibers or condensed bulk media.