Analysis of third-order nonlinearity effects in very high-Q WGM resonator cavity ringdown spectroscopy

The time domain coupled-mode theory (CMT) is applied to the analysis of the dynamic of third-order optical nonlinear effects in high-finesse whispering-gallery-mode (WGM) resonators. We show that this model is well adapted to the analysis of cavity-ringdown spectroscopy signal under modal-coupling due to Rayleigh backscattering in linear and nonlinear regimes. The experiments are carried out in silica WGM microspheres. Considering thermal and Kerr effects, CMT simulations are in good agreement, with experimental results for input power up to about 1 mW. For well-known optical materials such as silica, this experimental data analysis method can be used to measure the quality factor, the coupling regime, and the mode volume of high-finesse WGM. Furthermore, this technique could be developed to infer linear and nonlinear properties of high-finesse coated WGM microspheres.