Evanescent Light Coupling Between Waveguides and Optical Microparticles with Whispering Gallery Resonances

Optical microparticles with spherical, cylindrical, or elliptical symmetry are known to possess Whispering Gallery Mode (WGM) resonances. Interactions of these microparticles with an evanescent light field of, for example, a tapered optical fiber, opens up a possibility for the resonant propelling of such microparticles with high selectivity by wavelength and by size. In order to optimize this sorting process, in the present work we develop a computational model that describes the evanescent light propagation along the tapered fiber and its coupling to a WGM resonator. This model is based on the commercial software package COMSOL. It is capable of predicting various experimental parameters. In particular, it allows to estimate the distribution of the evanescent field around the tapered optical fiber, to calculate WGM resonance spectra of light absorption and transmission by the resonator, and to determine the optimal distance between particle and waveguide. The high quality of the model is demonstrated by the excellent agreement between the measured and simulated WGM resonance spectra.