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Cavity ring-down spectrometer for high-fidelity molecular absorption measurements



Vincent T. Sironneau, Hong Lin, Zachary Reed, Joseph T. Hodges


We present a cavity ring-down spectrometer which was developed for near-infrared measurements of laser absorption by atmospheric greenhouse gases. This system has several important attributes that make it possible to conduct broad spectral surveys and to determine line-by-line parameters with wide dynamic range, and high spectral resolution, sensitivity and accuracy. We demonstrate a noise-equivalent absorption coefficient of 4$\times$10$^-12}$ cm$^-1}$ Hz$^-1/2}$ and a signal-to-noise ratio of 1.5$\times$10$^6}$ in an absorption spectrum of carbon monoxide. We also present high-resolution measurements of trace methane in air which span more than 1.2 THz, with a frequency axis having an uncertainty of less than 100 kHz. Finally, we discuss how this system enables stringent tests of advanced line shape models. To illustrate, we measured an air-broadened carbon dioxide transition over a wide pressure range and analyzed these data with a multi-spectrum fit of the partially correlated, quadratic speed-dependent Nelkin-Ghatak profile. This analysis resulted showed that the line shape depends upon collisional narrowing, speed dependent effects and partial correlations between velocity- and phase-changing collisions. We obtained a quality-of-fit parameter in the multispectrum fit equal to 36,000, thus quantifying small-but-measurable limitations of the model profile.
Journal of Quantitative Spectroscopy and Radiative Transfer


cavity ring-down spectroscopy, greenhouse gases, methane, carbon dioxide, line shape, speed dependent broadening, Dicke narrowing


Sironneau, V. , Lin, H. , Reed, Z. and Hodges, J. (2015), Cavity ring-down spectrometer for high-fidelity molecular absorption measurements, Journal of Quantitative Spectroscopy and Radiative Transfer, [online],, (Accessed May 30, 2023)
Created March 31, 2015, Updated October 12, 2021