  Dissipative Kerr soliton (DKS) frequency combs - also known as microcombs -
have arguably created a new field in cavity nonlinear photonics, with a strong
cross-fertilization between theoretical, experimental, and technological
research. Spatiotemporal mode-locking (STML) not only add new degrees of
freedom to ultrafast laser technology, but also provide new insights for
implementing analogue computers and heuristic optimizers with photonics. Here,
we combine the principles of DKS and STML for the first time to demonstrate the
STML DKS by developing an unexplored ultrahigh-quality-factor Fabry-Perot
microresonator based on graded index multimode fiber (GRIN-MMF). Using the
intermodal stimulated Brillouin scattering, we can selectively excite either
the eigenmode DKS or the STML DKS. Furthermore, we demonstrate an ultralow
noise microcomb that enhances the photonic flywheel performance in both the
fundamental comb linewidth and DKS timing jitter. The demonstrated fundamental
comb linewidth of 400 mHz and DKS timing jitter of 500 attosecond represent
improvements of 25x and 2.5x, respectively, from the state-of-the-art. Our
results show the potential of GRIN-MMF FP microresonators as an ideal testbed
for high-dimensional nonlinear cavity dynamics and photonic flywheel with
ultrahigh coherence and ultralow timing jitter.
