Fitting of calibration-free scanned-wavelength-modulation spectroscopy spectra for determination of gas properties and absorption lineshapes

Abstract

The development and initial demonstration of a scanned-wavelength, first-harmonic-normalized, wavelength-modulation spectroscopy with nf detection (scanned-WMS-nf/1f) strategy for calibration-free measurements of gas conditions are presented. In this technique, the nominal wavelength of a modulated tunable diode laser (TDL) is scanned over an absorption transition to measure the corresponding scanned-WMS-nf/1f spectrum. Gas conditions are then inferred from least-squares fitting the simulated scanned-WMS-nf/1f spectrum to the measured scanned-WMS-nf/1f spectrum, in a manner that is analogous to widely used scanned-wavelength direct-absorption techniques. This scanned-WMS-nf/1f technique does not require prior knowledge of the transition linewidth for determination of gas properties. Furthermore, this technique can be used with any higher harmonic (i.e., n>1), modulation depth, and optical depth. Selection of the laser modulation index to maximize both signal strength and sensitivity to spectroscopic parameters (i.e., gas conditions), while mitigating distortion, is described. Last, this technique is demonstrated with two-color measurements in a well-characterized supersonic flow within the Stanford Expansion Tube. In this demonstration, two frequency-multiplexed telecommunication-grade TDLs near 1.4 μm were scanned at 12.5 kHz (i.e., measurement repetition rate of 25 kHz) and modulated at 637.5 and 825 kHz to determine the gas temperature, pressure, H2O mole fraction, velocity, and absorption transition lineshape. Measurements are shown to agree within uncertainty (1%-5%) of expected values.

Effects of variation in background mixing ratios of N<sub>2</sub>, O<sub>2</sub>, and Ar on the measurement of <i>δ</i><sup>18</sup>O–H<sub>2</sub>O and <i>δ</i><sup>2</sup>H–H<sub>2</sub>O values by cavity ring-down spectroscopy

“…To achieve this objective, it may be useful to introduce spectral fitting strategies in which all of the lineshape parameters are treated as free rather than fixed variables. Such "calibration-free" spectral fitting strategies have been recently been developed for high-temperature and high-pressure applications in energy research, and these might serve as models for lowertemperature and lower-pressure applications in environmental research (Sun et al, 2013;Goldenstein et al, 2014Goldenstein et al, , 2017bSur et al, 2015).…”

In situ H2O and temperature detection close to burning biomass pellets using calibration-free wavelength modulation spectroscopy

“…1) is based on the approach introduced by Sun et al [24] and extended to spectral lineshape fitting by Goldenstein et al [25]. This CF-WMS method offers the well documented advantages of (i) being valid at any modulation amplitude and optical depth (as long as Beer-Lambert's law is valid), (ii) incorporating the actual, measured frequency dependent background signal into the simulated absorption, which avoids the need to model the scanned and modulated laser intensity, and (iii)…”

“…The phase shift, θ, is obtained from the algorithm prior to the fit by comparison between the simulated and analytical laser intensities. In case the collisional broadening parameters are not well known, the integrated absorbance of the simulated DAS signal can be used as suggested by Goldenstein et al [25].…”

“…Recently, several approaches for calibration-free WMS (CF-WMS) have been suggested [19][20][21][22][23][24][25], usually based on normalizing nf-WMS with 1f-WMS signals to account for instrumentation factors and a wavelength dependent laser intensity. CF-WMS systems were successfully employed for temperature and concentration measurements in shock-tubes [26], engines [27], scram-jet combustors in the presence of inhomogeneously distributed temperature and species concentration [28], and in coal-fired high pressure combustion facilities [29].…”

Resolution-doubled one-dimensional wavelength modulation spectroscopy tomography for flame flatness validation of a flat-flame burner

“…Particularly, gas properties and absorption lineshapes can be retrieved by fitting the first harmonic-normalized second harmonic, i.e., WMS-2f/1f, of the modulated absorption signals, even in harsh environments with flame fluctuation, emission and non-absorption attenuation [24,25]. By combining WMS with one-dimensional tomography, the resolution-doubled 1D-WMST technique with a designed novel geometry of the parallel laser beams is introduced and used to reconstruct the distributions of temperature and mole fractions of chemical species of the axisymmetric flame in the radial direction.…”