  Compared with well-developed free space polarization converters, polarization
conversion between TE and TM modes in waveguide is generally considered to be
caused by shape birefringence, like curvature, morphology of waveguide cross
section and scattering. Here, we reveal a hidden polarization conversion
mechanism in X-cut lithium niobate microrings, that is the conversion can be
implemented by birefringence of waveguides, which will also introduce an
unavoidable avoided-mode crossing. In the experiment, we find that this mode
crossing results in severe suppression of one sideband in local nondegenerate
four-wave mixing and disrupts the cascaded four-wave mixing on this side.
Simultaneously, we proposed, for the first time to our best knowledge, one
two-dimensional method to simulate the eigenmodes (TE and TM) in X-cut
microrings, which avoids the obstacle from large computational effort in
three-dimensional anisotropic microrings simulation, and the mode crossing
point. This work will provide an entirely novel approach to the design of
polarization converters and simulation for monolithic photonics integrated
circuits, and may be helpful to the studies of missed temporal dissipative
soliton formation in X-cut lithium niobate rings.
