  High-Q silicon nitride (SiN) microresonators enable optical Kerr frequency
comb generation on a photonic chip and have recently been shown to support
fully coherent combs based on temporal dissipative Kerr soliton formation. For
bright soliton formation it is necessary to operate SiN waveguides in the
multimode regime so as to produce anomalous group velocity dispersion. This can
lead to local disturbances of the dispersion due to avoided crossings caused by
coupling between different mode families, and therefore prevent the soliton
formation. Here we demonstrate that a single mode "filtering" section inside
high-Q resonators enables to efficiently suppress avoided crossings, while
preserving high quality factors (Q~10^6). We demonstrate the approach by single
soliton formation in SiN resonators with filtering section.
