  We observe strong modal coupling between the TE00 and TM00 modes in Si3N4
ring resonators revealed by avoided crossings of the corresponding resonances.
Such couplings result in significant shifts of the resonance frequencies over a
wide range around the crossing points. This leads to an effective dispersion
that is one order of magnitude larger than the intrinsic dispersion and creates
broad windows of anomalous dispersion. We also observe the changes to frequency
comb spectra generated in Si3N4 microresonators due polarization mode and
higher-order mode crossings and suggest approaches to avoid these effects.
Alternatively, such polarization mode-crossings can be used as a novel tool for
dispersion engineering in microresonators.
