Mode coupling strength in a microsphere cavity coupled with fiber taper

Abstract

With the coupled-mode theory, we analyzed the energy transmission between the microsphere cavity and waveguide system, and revealed the mode coupling strength in a bare microsphere coupled with a fiber taper theoretically and experimentally. In a practical taper-microsphere coupling system, the traveling wave clockwise (CW) and counterclockwise (CCW) whispering gallery modes (WGMs) should existed simultaneity. Those two modes are coupled in the microsphere cavity with a coupling strength (beta). In experiment, the reflection spectra of the taper-microsphere system were investigated, By modifying various coupling condition for microspheres, different groups of data were obtained from the spectra, and used as fitting data for coupling strength estimation. The theory will be valuable in evaluating the upper limit of quality factor (Q) of a microcavity basing on the WGMs. Referring to the result and consequence of beta, it enables multiple ways to improve the Q-factor of a real cavity like annealing. Because the experiment device is compact and sensitive, such method is also promising for detecting the propagating loss in fiber instead of traditional way.