Interferometric cavity ringdown technique for ultrahigh Q-factor microresonators

Abstract

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 MRs with high/ultrahigh Q, the wall-surface roughness dominates the intrinsic Q and generates a coupling between counter-propagating modes. This splits the usual sharp single resonance and makes the use of classical methods to assess Q difficult. Here, we theoretically show that an interferometric excitation can be exploited in a cavity ringdown (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 us to obtain a typical exponential decay in the charging and discharging time of the MR, and thus, to estimate its ultimate Q by measuring the photon lifetime.