Chapter 4 Saturated-Absorption Cavity Ring-Down (SCAR) for High-Sensitivity and High-Resolution Molecular Spectroscopy in the Mid IR P. Cancio, I. Galli, S. Bartalini, G. Giusfredi, D. Mazzotti, and P. De Natale Abstract A non-conventional cavity ring-down spectroscopic technique is de- scribed. When the light intensity is well above the saturation level for the molecular species inside a high-finesse cavity, each single cavity ring-down event simultane- ously measures both the background losses from the cavity mirrors and the linear absorption from the gas. Such a differential scheme acting on very short time scales (a few tens of microseconds) can improve the sensitivity of conventional cavity ring-down by more than one order of magnitude, while achieving sub-Doppler res- olution, if needed. Applications to optical detection of very rare molecular species like radiocarbon dioxide and resolved molecular hyperfine structure in 17 O 12 C 16 O are presented. 4.1 Sensitivity and Resolution Limits of Linear Cavity Ring-Down Molecular Spectroscopy with CW Laser Sources in the Mid IR Since the first demonstrations of Cavity Ring Down (CRD) spectroscopy with CW laser sources as driving light (CW-CRD) [1–3], the improvements in terms of sensi- tivity and resolution with respect to pulsed CRD were quickly noticed. In CW-CRD only one longitudinal mode of the high-finesse non-degenerate Fabry-Pérot (F-P) cavity is excited in shorter build-up times and in a more efficient cavity coupling than pulsed version. In these conditions, a simple first order exponential decay is observed if the response of the intracavity medium is linear. This is because op- tical losses are determined by the well defined region of the cavity mirrors and of the intracavity sample, which is illuminated by that light mode. Indeed, lower shot-to-shot ring-down rate fluctuations than in pulsed CRD are measured for CW- CRD. As a consequence shot-noise-limited sensitivity can be in principle reached with this technique. In addition, the narrower linewidth of CW coherent sources al- lows residual Doppler and sub-Doppler resolution in CW-CRD spectroscopy [4–6]. P. Cancio (B ) · I. Galli · S. Bartalini · G. Giusfredi · D. Mazzotti · P. De Natale Istituto Nazionale di Ottica, Consiglio Nazionale delle Ricerche, Via N. Carrara 1, 50019 Sesto Fiorentino, Italy e-mail: pablo.canciopastor@ino.it G. Gagliardi, H.-P. Loock (eds.), Cavity-Enhanced Spectroscopy and Sensing, Springer Series in Optical Sciences 179, DOI 10.1007/978-3-642-40003-2_4, © Springer-Verlag Berlin Heidelberg 2014 143