Modeling and measuring the quality factor of whispering gallery mode resonators

In this paper, we study an experimental method of evaluation of efficiency of an evanescent field coupling of light to a dielectric whispering gallery mode resonator. We show that change of the relative contrast of the observable optical resonance provides an accurate and complete information about the loaded as well as unloaded bandwidth of the resonator mode. We introduce a model of the coupling that allows for an interpolation of the resonator quality factor using experimental data taken without usage of a frequency tunable narrow line laser, a fast photodiode, as well as a frequency reference. The method is also immune to imperfect geometrical mode matching frequently occurring in a realistic system. We validate this model experimentally at both 1.5 and 4.5 mu M.