Saturated-Absorption Cavity Ring-Down Spectroscopy

Abstract: A novel approach to cavity ring-down spectroscopy with the sample gas in saturated-absorption regime allows to decouple and simultaneously retrieve empty-cavity background and absorption signal, improving both measurement sensitivity and resolution. OCIS codes: 300.6340 Spectroscopy, infrared; 300.6390 Spectroscopy, molecular; 300.6460 Spectroscopy, saturation The availability of a molecular-spectroscopy technique, able to combine the ultimate performance in terms of sensitivity, resolution and frequency accuracy, can be crucial in many fundamental physical measurements. Indeed, profiting from the strength and ease of saturation of many mid-IR ro-vibrational transitions, this technique could provide new insights in elusive quantum-mechanical effects encoded in molecules. Such a technique could also represent a major step forward in trace-gas sensing. Cavity ring-down (CRD) spectroscopy has already proven to be a good technique to directly provide a sensitive and quantitative measurement of gas absorption coefficient with a simple experimental set-up. In principle, it is not limited by amplitude noise of the laser source, but only by detection shot noise. However, variations of the empty-cavity decay rate always prevent to achieve this ultimate limit and to average measurements over long times. Other techniques (e.g. CRD heterodyne spectroscopy and NICE-OHMS) are even more sensitive than standard CRD, but they are more complex (frequency modulations and/or lockings are needed), less quantitative (calibration procedures are needed) and require fast and sensitiv