  We construct a classification of dispersion relations for the electromagnetic
waves non-minimally coupled to the space-time curvature, based on the analysis
of the susceptibility tensor, which appears in the non-minimal Einstein-Maxwell
theory. We classify solutions to the Fresnel equation for the model with a
trace-free non-minimal susceptibility tensor according to the Petrov scheme.
For all Petrov types we discuss specific features of the dispersion relations
and plot the corresponding wave surfaces.
