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

• We present a near-infrared cavity ring-down system for measurements of greenhouse gases.
• We show a CO spectrum with a signal-to-noise ratio of 1.5 million.
• We show CO2 line shape measurements with quality of fits greater than 35,000:1.
• We show a noise equivalent absorption coefficient of 4×10−12cm−1/Hz1/2.
• We achieve an uncertainty of the frequency axis of 100 kHz over 1.2 THz range.

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 4×10−12 cm−1 Hz−1/2 and a signal-to-noise ratio of 1.5×106:1 in an absorption spectrum of carbon monoxide. We also present high-resolution measurements of trace methane in air spanning more than 1.2 THz and having a frequency axing with an uncertainty 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. 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. This analysis showed that the line shape depends upon collisional narrowing, speed dependent effects and partial correlations between velocity- and phase-changing collisions.