Observation of Critical Coupling in a Fiber Taper to a Silica-Microsphere Whispering-Gallery Mode System

We present the observation of critical coupling in a high- $Q$ fused-silica microsphere whispering-gallery mode resonator coupled to a fiber taper. Extremely efficient and controlled power transfer to high- $Q$ $\left(\sim {10}^7\right)$ resonators has been demonstrated. Off-resonance scattering loss was measured to be less than $0.3\%$. On-resonance extinction in transmitted optical power through the fiber coupler was measured as high as 26 dB at the critical coupling point. This result opens up a range of new applications in fields as diverse as near-field sensing and quantum optics.