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