Efficient coupling of free propagating light into Whispering Gallery Modes

Whispering Gallery Mode resonators are dielectric structures with cylindrical symmetry. They are typically excited with an evanescent field leaking out of a tapered fiber or a waveguide. It is also known that they can be excited with free propagating beams. In this work, we use a recently developed analytical model which quantitatively describes the coupling of free propagating beams into Whispering Gallery Modes for spherical particles. Using this model, we have been able to theoretically quantify the mode purity and the coupling efficiency of a resonant Whispering Gallery Mode of an order j * = 1456. We have observed that the transverse position of the beam plays a crucial role in determining the mode purity and coupling efficiency. Last but not least, we have verified that the coupling efficiency as well as the Q-factor predicted by our model are in an outstanding agreement with the experimental values measured on a microresonator of the same dimensions as the simulated one.