  Chip-scale optical frequency combs have attracted significant research
interest and can be used in applications ranging from precision spectroscopy to
telecom channel generators and lidar systems. In the time domain,
microresonator based frequency combs correspond to self-stabilized soliton
pulses. In two distinct regimes, microresonators have shown to emit either
bright solitons in the anomalous dispersion regime or dark solitons (a short
time of darkness in a bright background signal) in the normal dispersion
regime. Here, we investigate the dynamics of continuous-wave-laser-driven
soliton generation in the zero-group-velocity-dispersion (GVD) regime, as well
as the generation of solitons that are spectrally crossing different dispersion
regimes. In the measurements, zero-dispersion solitons with multi-peak
structures (soliton molecules) are observed with distinct and predictable
spectral envelopes that are a result of fifth-order dispersion of the
resonators. Numerical simulations and the analysis of bifurcation structures
agree well with the observed soliton states. This is the first observation of
soliton generation that is governed by fifth-order dispersion, which can have
applications in ultrafast optics, telecom systems and optical spectroscopy.
