Determination of coupling regime of high-Q resonators and optical gain of highly selective amplifiers

We present a simple method to determine simultaneously the main characteristics of passive or active high-Q optical resonators. The method is based on cavity ringdown spectroscopy, where the probe wavelength is rapidly swept across the resonance. It has already been shown that this technique allows the loaded cavity lifetime of passive resonators to be obtained. We show that we can also infer the coupling regime for passive resonators and the resonant gain for active resonators. The method is tested on Er3+ doped fiber resonators and also applied to determine the intrinsic and external Q-factors of an MgF2 whispering gallery mode resonator.