Asymmetrical Cross-Polarization Coupling in a Whispering-Gallery Microresonator

Abstract

Cross-polarization coupling between transverse electric (TE) and transverse magnetic (TM) whispering-gallery modes in an optical microresonator produces effects such as coupled-mode induced transparency (CMIT). The detailed analytical theory of this coupling indicates that the TE-to-TM and TM-to-TE couplings may have different strengths. Using an experimental setup centered around a hollow bottle resonator and polarization-sensitive throughput detection, that had been used in previous CMIT experiments, this asymmetry was confirmed and studied. By fitting the throughput spectra of both polarizations to the numerical output of a basic model, the asymmetry parameter defined as the ratio of the coupling amplitudes was determined from the output power in the polarization orthogonal to that of the input. The results of many experiments give a range for this ratio, roughly from 0.2 to 4, that agrees with the range predicted by the detailed theory. An analytical approximation of this ratio shows that the main reason for the asymmetry is a difference in the axial orders of the coupled modes. In some experimental cases, the orthogonal output is not well fitted by the model that assumes a single mode of each polarization, and we demonstrate that this fitting discrepancy can be the result of additional mode interactions.