Theoretical detection limit of saturated absorption cavity ring-down spectroscopy (SCAR) and two-photon absorption cavity ring-down spectroscopy

Giusfredi et al. (Phys Rev Lett 104, 110801, 2010), have developed a new approach to cavity ring-down spectroscopy where a saturable sample absorption is determined simultaneously with the cavity loss, providing immunity to changes in cavity loss, thereby allowing for lower analyte detection limit. This paper presents an error analysis that provides predictions of the ultimate sensitivity limits that can be realized with this detection method. In particular, the sensitivity is strongly dependent upon the initial degree of saturation of the sample, and optimal values for this are determined both for photon detector and shot-noise-limited detection of both inhomogeneous and homogeneous broadened spectroscopic lines. Also presented are sensitivity limits expected for two-photon absorption spectroscopy determined by cavity ring-down spectroscopy.