Light coupling between the whispering gallery modes of a coated microsphere and a tapered fiber

Spherical microresonators were coated using a 70SiO2 - 30HfO2 glass doped with 0.3 mol% Er3+. The sphere was characterized by coupling a 1480 nm pump laser inside the sphere using a tapered optical fiber and observing the 1530-1580 nm Er3+ emission outcoupled with the same tapered fiber. The measurements were done for different coating thicknesses and compared with theoretical calculations to understand the relationship of the detected signal with the whispering gallery mode electric field profiles.