  Due to ultra high quality factor ($10^6-10^9$), axisymmetric optical
microcavities are popular platforms for biosensing applications. It has been
recently demonstrated that a microcavity biosensor can track a biodetection
event as a function of its quality factor by using phase shift cavity ring down
spectroscopy (PS-CRDS). However, to achieve maximum sensitivity, it is
necessary to optimize the microcavity parameters for a given sensing
application. Here, we introduce an improved finite element model which allows
us to determine the optimized geometry for the PS-CRDS sensor. The improved
model not only provides fast and accurate determination of quality factors but
also determines the tunneling distance of axisymmetric resonators. The improved
model is validated numerically, analytically, and experimentally.
