Saturated-absorption cavity ring-down (SCAR) for high-sensitivity and high-resolution molecular spectroscopy in the mid IR

A non-conventional cavity ring-down spectroscopic technique is described. 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 simultaneously 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 resolution, if needed. Applications to optical detection of very rare molecular species like radiocarbon dioxide and resolved molecular hyperfine structure in 17O12C16O are presented.