High quality (Q) factor resonators are of great interest in the domains of applied or fundamental physics [1]. The optimization fabrication processes of very high-Q resonators need a thorough measurement of all their optical and physical characteristics. In this purpose, it has been proposed by Savchenkov et al. [2] to use a cavity-ring down spectroscopy technique consisting in measuring the ringing envelop decay when the probe laser carrier frequency is rapidly swept across the resonance [2]. This method is well adapted to high finesse single mode resonator and allows the loaded Q-factor to be measured. In this presentation we would like to show that a detailed analysis of the transient profile can reveal additional resonator characteristics. The comparison of the non-stationary transmission and a simple model gives: - the coupling and dispersive properties, - the strength of the intracavity Rayleigh scattering causing modal coupling, - an estimation of the actual power launched in the high-Q mode. We check and apply our method to fused silica passive whispering-gallery-mode (WGM) microspheres coupled via a tapered fiber. Glass or silica spherical WGM resonators can be more conveniently made by utilizing melting techniques, but their Q-factor is intrinsically limited to a few 10^8 due to surface contamination [3]. One way to overcome this limit is to include some gain in the resonator to compensate for optical losses [4]. We experimentally demonstrate a Q-factor enhancement up to Q ≈ 2 × 10^9 when the losses in the active microresonator are compensated, as well as the generation of a very high gain (22 dB) in erbium-doped fluoride glass microspheres in the wavelength-selective gain regime [5]. References [1] K. Vahala, Optical Microcavities (World Scientific, 2004) [2] A. A. Savchenkov, A. B. Matsko, V. S. Ilchenko, and L. Maleki, Opt. Express 15, 6768 (2007) [3] A. B. Matsko and V. S. Ilchenko, IEEE J. Sel. Topics Quantum Electron. 12, 3 (2006) [4] J. M. Choi, R. K. Lee, and A. Yariv, Opt. Lett. 26, 1236 (2001) [5] A . Rasoloniaina, S. Trebaol, V. Huet, E. Le Cren, G. Nunzi Conti, H. Serier-Brault, M. Mortier, Y. Dumeige, and P. Féron, Opt. Lett. 37, 4735 (2012)