A laser is based on the electromagnetic modes of its resonator, which
provides the feedback required for oscillation. Enormous progress has been made
in controlling the interactions of longitudinal modes in lasers with a single
transverse mode. For example, the field of ultrafast science has been built on
lasers that lock many longitudinal modes together to form ultrashort light
pulses. However, coherent superposition of many longitudinal and transverse
modes in a laser has received little attention. The multitude of disparate
frequency spacings, strong dispersions, and complex nonlinear interactions
among modes greatly favor decoherence over the emergence of order. Here we
report the locking of multiple transverse and longitudinal modes in fiber
lasers to generate ultrafast spatiotemporal pulses. We construct multimode
fiber cavities using graded-index multimode fiber (GRIN MMF). This causes
spatial and longitudinal mode dispersions to be comparable. These dispersions
are counteracted by strong intracavity spatial and spectral filtering. Under
these conditions, we achieve spatiotemporal, or multimode (MM), mode-locking. A
variety of other multimode nonlinear dynamical processes can also be observed.
Multimode fiber lasers thus open new directions in studies of three-dimensional
nonlinear wave propagation. Lasers that generate controllable spatiotemporal
fields, with orders-of-magnitude increases in peak power over existing designs,
should be possible. These should increase laser utility in many established
applications and facilitate new ones