*1.1. Selected Background Literature for Absorption Sensing of CO, CO2, and H2O*

The absorption of water vapor in the 2ν<sup>1</sup> and 2ν<sup>3</sup> vibrational overtone bands and ν<sup>1</sup> + ν<sup>3</sup> combination band overlaps in wavelength with the emission range of mature diode lasers developed in the near-infrared (NIR) for telecommunication. Water vapor, a primary product of hydrocarbon combustion, was a natural target of the early diode laser sensor development for combustion, and there is a rich literature concerning it. More than 20 years ago, Allen's review [6] highlighted numerous combustion applications of water vapor sensing. In the work reported below, two frequently used and well-studied NIR transitions near 7185.59 cm−<sup>1</sup> (1391.7 nm) and 6806.03 cm−<sup>1</sup> (1469.3 nm) are exploited. More recently, Goldenstein et al. demonstrated high-temperature sensitivity using this line pair under high-pressure and high-temperature conditions realized in shock-heated H2O/N2 mixtures [7], and that work provides the fundamental spectroscopy needed to design the sensor reported here. In addition, the ratio of absorption of water vapor using this pair of laser wavelengths was used to monitor temperature in a rotating detonation engine, illustrating its suitability for harsh environments [8]. The lower-state energy *E*" of this pair of transitions differs by 2246 cm<sup>−</sup>1, and thus the ratio of absorption is sensitive to temperature in the 700 to 2400 K range needed for the gas turbine combustion sensing.

CO is an intermediate species, as hydrocarbons are oxidized on the way to becoming the primary product CO2. Thus, the CO/CO2 ratio is an important indicator for the complete combustion of hydrocarbon fuel. This ratio can increase dramatically if temperature and combustion time is not sufficient for full oxidation in specific operating scenarios of practical devices leading to unwanted toxic emissions of CO. Therefore, measurements of the CO/CO2 ratio are invaluable to the designer of new combustor concepts. The second overtone vibrational band of CO (near 1600 nm) also overlaps with the telecommunications wavelength lasers, the first overtone vibrational band (near 2300 nm) overlaps with extended NIR diode lasers, and research on sensors using transitions from both of these bands appear in the literature (e.g., Wagner et al. [9] used the R20 line of CO near 2313 nm to measure spatially resolved CO profiles in atmospheric laminar counter-flow diffusion flame). Unfortunately, these transitions are relatively weak, and the concentration of the intermediate combustion product CO is often much lower than the primary product species CO2. Thus, the much stronger transitions in the fundamental vibrational band near 4.6 μm are used here for CO detection. Wavelength-tunable solid-state lasers in the mid-infrared (MIR) with a tuning range that overlaps the CO fundamental band transitions have become available with the development of quantum cascade lasers [10]. CO detection is also complicated by the strong overlap of the fundamental band transitions of CO with those

of CO2. In the sensor described below, wavelength ranges with different amounts of CO2 interference are used to infer the CO absorption.

These laser wavelengths have been used previously to detect CO, e.g., by Spearrin et al. [11], who characterized a QC laser for CO detection in the harsh environment of a pulsed detonation combustor at 4.85 μm (the P(20) transition at 2059.91 cm<sup>−</sup>1) using wavelength-modulation spectroscopy (WMS) with the normalized second-harmonic signal detection (WMS-2f) technique, rendering the measured signal independent of the incoming laser intensity. The encountered gas pressures varying between around 6 and more than 20 bar. With the same technique, the same research group developed a NIRabsorption instrument for the detection of carbon dioxide (CO2) with an isolated transition (R(26)) in the ν<sup>1</sup> + ν<sup>3</sup> band at 3733.48 cm−<sup>1</sup> (2.68 μm), with the laser first characterized in shock-tube experiments with pressures between 3 and 12 bar and a large temperature range between 1000 and 2600 K [12]. The instrument was then utilized for CO2 detection in a pulse detonation engine (PDE) using ZrO2 optical fibers for beam delivery.

Nwaboh et al. [13] used a quantum-cascade laser operated in the intrapulse mode to probe the P(1) line of CO at 2139.4 cm−<sup>1</sup> (4.67 μm) in gravimetrically prepared gas mixtures to perform an uncertainty analysis for direct absorption spectroscopy-based mole fraction measurements of CO. A reproducibility of 1% and an uncertainty of 4% were demonstrated.

Vanderover et al. [14] used a single MIR QCL covering four selected rovibrational transitions of CO (R(9), R(10), R(17), and R(18) at 2179.77, 2183.22, 2179.24, and 2182.36 cm<sup>−</sup>1, respectively) in the fundamental vibrational band for temperature and concentration measurements from peak absorbance ratios and peak absorbance, respectively. The fractional temperature sensitivity and specific detectivity were demonstrated for gas mixtures prepared in a gas cell (298 K) and a shock tube (1000–3600 K).

Previously, a preliminary version of the sensor used a single quantum-cascade laser (QCL) near 2190.02 cm−<sup>1</sup> (4566 nm) to detect CO in a full-scale gas turbine burner test rig [15,16]. The CO concentrations varied considerably after ignition, and the laser absorption measurements were in qualitative agreement with the temporal behavior observed by a sampling probe mounted in the free-space exhaust flow just downstream of the optical access port.
