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This manuscript presents a data-processing technique which improves theaccuracy and precision of absorption-spectroscopy measurements by isolating themolecular absorbance signal from errors in the baseline light intensity (Io)using cepstral analysis. Recently, cepstral analysis has been used withtraditional absorption spectrometers to create a modified form of thetime-domain molecular free-induction decay (m-FID) signal which can be analyzedindependently from Io. However, independent analysis of the molecular signatureis not possible when the baseline intensity and molecular response do notseparate well in the time domain, which is typical when usinginjection-current-tuned lasers (e.g., quantum cascade lasers) and other lightsources with pronounced intensity tuning. In contrast, the method presentedhere is applicable to all light sources since it determines gas properties byleast-squares fitting a simulated m-FID signal to the measured m-FID signal inthe time domain. This method is insensitive to errors in the estimated Io whichvary slowly with optical frequency and, therefore, decay rapidly in the timedomain. The benefits provided by this method are demonstrated viascanned-wavelength direct-absorption measurements acquired with adistributed-feedback (DFB) quantum-cascade laser (QCL). The wavelength of a DFBQCL was scanned across CO's P(0,20) and P(1,14) absorption transitions at 1 kHzto measure the gas temperature and concentration of CO. Measurements wereacquired in a gas cell and in an ethylene-air flame at 1 atm. The measuredspectra were processed using the new m-FID-based method and two traditionalmethods which rely on inferring the baseline error within the spectral-fittingroutine. The m-FID-based method demonstrated superior accuracy in all cases anda measurement precision that was 1.5 to 10 times smaller than that providedusing traditional methods.

Cepstral Analysis for Baseline-Insensitive Absorption Spectroscopy Using Light Sources with Pronounced Intensity Variations