*1.3. Simultaneous Detection of Multiple Species at Pressures Higher Than Atmospheric*

Open literature presentations of solid-state-laser-based simultaneous measurement of multiple species and temperature in the harsh environment of turbulent combustion at high temperature and pressure are scarce. Spearrin et al. [21] recorded two of the major hydrocarbon combustion species, CO and CO2 in a scram-jet combustor using both direct absorption and wavelength modulation spectroscopy with a temporal resolution 200 μs (for DA) or 5 ms (for WMS). Through the selection of suitable temperature-sensitive transitions in the MIR at 2059.91 cm−<sup>1</sup> (4.854 μm) and 2394.6 cm−<sup>1</sup> (4.176 μm) for CO and CO2, respectively, they extended the temperature-sensitive detection range to 800–2400 K in this high-velocity hydrocarbon combustion flow. The laser beams for both CO and CO2 detection were coupled into a bifurcated hollow-core optical fiber for transfer to the engine site, where the collimated and collinear beams traversed the optically accessible test section before being split and focused on two separate detectors. Mounting the lasers on vibration-insulated breadboards and the detectors to water-cooled plates and purging all beam paths with N2 (including the hollow-core fibers) minimized disturbances from the harsh environment of the flow facility.

In the work of Peng et al. [22], a diode laser absorption instrument was developed combining four diode lasers at NIR and MIR wavelengths for the measurement of water vapor temperature (3920.1 and 4029.5 cm−1), CO (P(21) at 2055.4 cm−1), and CO2 (R(92) at 2394.4 cm−1) in a single-ended beam configuration. The beams were directed through the combustion flow channel, while the back-scattered light from the opposite wall was captured by a receiving hollow-core fiber. To enable the beams of all four diode lasers to pass collinearly through the reactive flow, they were fiber-coupled in free space onto a 4-to-1 multimode hollow-core silver-coated glass fiber bundle.

Cassady et al. [23] developed a very compact and shielded MIR-absorption instrument for the simultaneous measurement of water vapor temperature (via a low- and high-*E*" transition of 3920.06 cm−<sup>1</sup> (2.551 m) and 4029.59 cm−<sup>1</sup> (2.482 μm), respectively) and H2O, CO (2059.92 cm−<sup>1</sup> (4.854 μm)), and CO2 (2390.52 cm−<sup>1</sup> (4.175 μm)) concentrations with a high sampling rate of 44,000 samples pre second from the reactive flow in the annular gap of a rotating detonation engine (RDE, operating pressure 2–8 bar) using four co-aligned MIR laser beams and WMS. They were able to perform time-resolved measurements while running the engine at lean to stoichiometric equivalence ratios.
