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 (∼10^7) 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.