We present a technique called differential cavity ring-down spectroscopy (D-CRDS) for reducing etaloning effects in a ring-down spectrometer. The method relies on alternately measuring and then subtracting intra-cavity losses associated with pairwise TEM00 modes that are separated in frequency by an integer number of cavity free spectral ranges. We present proof-of-concept measurements in which we illustrate experimental configurations of the D-CRDS technique. These are chosen to suppress etaloning effects and reduce instrumental artifacts in the spectrum of base losses. We also discuss D-CRDS and conventional frequency-stabilized cavity ring-down spectroscopy (FSCRDS) line measurements of air-broadened CO2 spectra, and we show that D-CRDS gives an accurate measure of line shape parameters that is relatively insensitive to etalon-induced distortions. Based on an average of approximately 100 spectra, this differencing technique suppresses drift in etalons to give a signal-to-noise ratio of 170,000:1 and a minimum detectable absorption coefficient of 4E-12 cm ^-1. These specifications represent an improvement by a factor of approximately 2.5 compared to the respective values obtained with the conventional FS-CRDS technique. Finally, we present theoretical expressions for etalon effects by analyzing coupled cavity interactions between the primary ring-down cavity and other optical elements of the experiment, and we show how the suppression of etalons generally depends upon etalon period and frequency sampling intervals in the acquired spectra.
Physical Review A
cavity ring-down spectroscopy, carbon dioxide, line shape, speed dependence