  Microresonators (MRs) are key components in integrated optics. As a result,
the estimation of their energy storage capacity as measured by the quality
factor (Q) is crucial. However, in MR with high/ultra-high Q, the surface-wall
roughness dominates the intrinsic Q and generates a coupling between
counter-propagating modes. This splits the usual sharp single resonance and
makes difficult the use of classical methods to assess Q. Here, we
theoretically show that an interferometric excitation can be exploited in a
Cavity Ring-Down (CRD) method to measure the ultimate Q of a MR. In fact, under
suitable conditions, the resonant doublet merges into a single Lorentzian and
the time dynamics of the MR assumes the usual behavior of a single-mode
resonator unaffected by backscattering. This allows obtaining a typical
exponential decay in the charging and discharging time of the MR, and thus,
estimating its ultimate Q by measuring the photon lifetime.
