Abstract
High-precision cavity-enhanced spectroscopic measurements are commonly compromised by interferences caused by external etalons. Here, we present the differential cavity ring-down spectroscopy (D-CRDS) technique for reducing these perturbations. We discuss how etalons are caused by coupled-cavity interactions between the primary ring-down cavity and other optical elements of the experiment, and we model and experimentally verify how drift in cavity base loss correlates with barometric pressure and laboratory temperature. D-CRDS measurements of near-infrared CO2 spectra that are insensitive to etalon-induced distortions are then presented. Based on an average of ∼100 spectra, these results yield a signal-to-noise ratio of 170,000∶1 and a minimum detectable absorption coefficient of 4×10−12  cm−1.
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