Resonant response and mode conversion of the microsphere coupled with a microfiber coupler.

We experimentally investigate the resonant response in a microsphere coupled with a biconical microfiber coupler (MFC) composed of single- and/or few-mode fibers. The evolution between Lorentz and Fano resonances dependent on the phase difference of the supermodes is demonstrated along the coupling region of the MFC. A flexible narrow-linewidth add-drop filter can be simultaneously achieved with the advantage of easy alignment and efficiently evanescent coupling. In addition, the mode conversion process of the microsphere is demonstrated, and the purity of the LP11 mode in a coupler at the whispering gallery mode resonance is analyzed. The proposed MFC coupling system holds great promising applications in optical filtering and sensing, mode switching, and micro-resonator-based combs.