Two-Dimensional Tapered Optical Fiber Core for Whispering Gallery Mode Excitation

A two-dimensional tapered fiber core mimicking a commonly used tapered fiber coupler is proposed and experimentally demonstrated in a piece of single-mode fiber (SMF) for whispering gallery mode (WGM) excitation. Tapering the fiber core with femtosecond laser ablation can simultaneously enhance the evanescent field and decrease the effective refractive index of the core mode, which lowers the phase mismatch between the core mode and the WGM in a spherical cavity with a diameter less than the fiber cladding radius. Experimental results show that symmetric Lorentzian dip with a quality factor (Q-factor) of
$3.6\times 10 ^{3}$
and asymmetric Fano-like lineshape can be achieved. Such a tapered fiber core provides an effective coupling method for a single WGM based photonic device or WGM array-based lab in-fiber due to its inherent compatibility with fiber optic hardware, high integration, and robustness.