Jeremie B. Courtois, Katarzyna E. Bielska, Joseph T. Hodges
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.
, Bielska, K.
and Hodges, J.
Differential cavity ring-down spectroscopy, Physical Review A, [online], https://doi.org/10.1364/JOSAB.30.001486, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=912785
(Accessed February 29, 2024)