Non-adiabatic tapered fiber-based robust and efficient coupler for a WGM microresonator

We propose and experimentally demonstrate a robust and efficient coupling method for whispering-gallery-mode (WGM) microresonators using a non-adiabatic tapered fiber. An ultrashort fiber with a total length of ∼1<i>m</i><i>m</i> and a minimum waist diameter of ∼5µ<i>m</i> was fabricated through a two-step process combining arc discharge and oxyhydrogen flame heating. The fiber excites cladding modes and achieves coupling efficiencies of up to 92%, while offering improved mechanical robustness compared with conventional adiabatic tapered fibers, which are typically 20-30 mm long with fragile micron-scale waists. Experiments show that the proposed fiber excites both Lorentzian and Fano resonances and exhibits high sensitivity to axial displacement. Efficient coupling was further demonstrated with SNAP, bottle, and sausage resonators, confirming the universality of this approach. The compact structure, low fabrication cost, and enhanced robustness make the proposed fiber a promising candidate for the development of miniaturized, high-sensitivity, and multifunctional WGM optical sensors.