  We observe fine structure in the resonance spectra of optical microcavities.
We identify the polarization-resolved modes in the spectrum and find that
resonance frequencies split in accordance with the theoretical prediction. The
observed fine structure is dominantly caused by an optical spin-orbit coupling
and non-paraxial propagation and reflection. Both effects are intrinsic, i.e.
present in an ideal rotation-symmetric system, and scale inversely proportional
to the mirror radius of curvature. For cavities with a small radius of
curvature, the influence of fine structure on the resonance spectrum is
important and unavoidable and should thus be taken into account.
