Spatiotemporal control of nonlinear effects in multimode fibers for two-octave high-peak-power femtosecond tunable source

Multimode fibers (MMFs) have recently reemerged as an attractive avenue for nonlinear effects due to their high-dimensional spatiotemporal nonlinear dynamics and scalability for high power. High-brightness MMF sources with effective control of the nonlinear processes would offer new possibilities for a wide range of applications from high-power fiber lasers, to bioimaging and chemical sensing, and to novel physics phenomena. Here we present a simple yet effective way of controlling multimodal nonlinear effects at high peak power levels: by leveraging not only the spatial but also the temporal degrees of freedom in step-index MMFs using a programmable fiber shaper. This method represents the first method that enables simultaneous access to the spatial and temporal degrees of freedom of multimodal nonlinear pulse propagation, achieving high tunability and broadband high peak power. Its potential as a nonlinear imaging source is further demonstrated by applying the MMF source to multiphoton microscopy, where widely tunable two-photon and three-photon imaging is achieved with adaptive optimization. These demonstrations highlight the power of spatiotemporal control to enhance tunability and optimize high spectral brilliance in MMFs, providing new possibilities for technology advances in nonlinear optics, bioimaging, spectroscopy, optical computing, and material processing.