Cross-polarization coupling of whispering-gallery modes due to the spin-orbit interaction of light

Light can couple between two orthogonally polarized whispering-gallery modes of a microresonator; the effect is easily observable when those modes are frequency degenerate, and can result in coupled-mode induced transparency (CMIT). Experimental observations of CMIT show that the cross-polarization coupling (CPC) strength is typically 10^-8-10^-7 per round trip. It is shown in this Letter that polarization rotation resulting from optical spin-orbit interaction through the experimentally realistic asymmetry of a microresonator about its equator can produce CPC with strengths in the same range as observed in experiments.