  Multimode fibers (MMFs) have recently reemerged as attractive avenues 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 nonlinear effects at high peak power levels: by
leveraging not only the spatial but also the temporal degrees of freedom of the
multimodal nonlinear pulse propagation in step-index MMFs using a programmable
fiber shaper. This method represents the first method that enables modulation
and optimization 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
effectiveness of directly modulating multimodal nonlinear pulse propagation to
enhance the high-dimensional customization and optimize the high spectral
brightness of MMF output. These advancements provide new possibilities for
technology advances in nonlinear optics, bioimaging, spectroscopy, optical
computing, and material processing.
