This paper presents a data-processing technique that improves the
accuracy and precision of absorption-spectroscopy measurements by
isolating the molecular absorbance signal from errors in the baseline
light intensity (
I
o
) 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
I
o
. 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., tunable diode
and quantum cascade lasers (QCLs)] and other light sources with
pronounced intensity tuning. In contrast, the method presented here is
applicable to virtually all light sources since it determines gas
properties by least-squares fitting a simulated m-FID signal
(comprising an estimated
I
o
and simulated absorbance spectrum) to
the measured m-FID signal in the time domain. This method is
insensitive to errors in the estimated
I
o
, 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-spectroscopy measurements
acquired with a distributed-feedback (DFB) QCL. The wavelength of a
DFB QCL was scanned across the CO 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 a laminar
ethylene–air diffusion flame at 1 atm. The measured spectra were
processed using the new m-FID-based method and two traditional
methods, which rely on inferring (instead of rejecting) 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.
The design and demonstration of a compact single-ended laser-absorption-spectroscopy sensor for measuring temperature and HO in high-temperature combustion gases is presented. The primary novelty of this work lies in the design, demonstration, and evaluation of a sensor architecture that uses a single lens to provide single-ended, alignment-free (after initial assembly) measurements of gas properties in a combustor without windows. We demonstrate that the sensor is capable of sustaining operation at temperatures up to at least 625 K and is capable of withstanding direct exposure to high-temperature (≈1000 K) flame gases for long durations (at least 30 min) without compromising measurement quality. The sensor employs a fiber bundle and a 6 mm diameter antireflection-coated lens mounted in a 1/8 NPT-threaded stainless-steel body to collect laser light that is backscattered off native surfaces. Distributed-feedback tunable diode lasers (TDLs) with a wavelength near 1392 nm and 1343 nm were used to interrogate well-characterized HO absorption transitions using wavelength-modulation-spectroscopy techniques. The sensor was demonstrated with measurements of gas temperature and HO mole fraction in a propane-air burner with a measurement bandwidth up to 25 kHz. In addition, this work presents an improved wavelength-modulation spectroscopy spectral-fitting technique that reduces computational time by a factor of 100 compared to previously developed techniques.
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