Cavity-ringdown-spectroscopy-based study of high Q resonators in add-drop configuration

An optical resonator like a fiber ring (FR) or a whispering gallery mode (WGM) resonator with two couplers along its loop is referred to be in the add-drop configuration, in analogy with the add-drop multiplexer in telecom networks. Both for practical applications as well as in several fundamental studies involving high-Q resonators, this configuration is of great interest and the assessment of the intrinsic properties of the resonator and of its interaction with the coupling systems is extremely important. We developed an original method able to fully characterize high-Q resonators in an add-drop configuration. The method is based on the study of the two cavity ringdown (CRD) signals, which are produced at the transmission and drop ports by wavelength sweeping a resonance in a time interval comparable with the photon cavity lifetime. All the resonator parameters can be assessed with a single set of simultaneous measurements. We implemented the model describing the two CRD profiles from which a best fit process of the measured profiles allows deducing the key parameters. We successfully validated the model with an experiment based on a FR resonator of known characteristics. Finally, we fully characterized a high-Q, home-made, MgF₂ WGM disk resonator in the add-drop configuration, assessing its intrinsic and coupling parameters.