Define the principle optic axes for a biaxial crystal that are a function of the phi spatial coordinate

Hello,

I would like to simulate a special ring resonator in which the principle optic axes of a biaxial crystal rotate as a function of the spatial coordinate phi around the ring.

Essentially I have a 2D axisymmetric simulation and I would like to have an index of refraction that is a function of the spatial coordinate phi.

Is there a way to do this in Comsol?

Thanks, JB