  Spatiotemporal solitons (STSs) are localized solitary waves in both space and
time that involve complex linear and nonlinear processes. Optical STSs have
been observed in various media, but they are difficult to be realized in
multimode fibers due to their large modal dispersion. Here, we report STS
mode-locking and spatiotemporal nonlinear dynamics in a step-index multimode
fiber mediated by gain. Gain competition and energy redistribution among
different spatial modes help nonlinearity effectively cancel both temporal
splitting and spatial diffracting of the entire pulse (multicomponent soliton)
and thereby maintain its shape during propagation. Optical STSs in multimode
fibers therefore open a new way to clarify the fundamental science of
spatiotemporal solitary waves and also imply important applications in
multi-channel communications, optical switches and mode-area power scaling of
fiber laser pulses.
