**1. Introduction**

Laser absorption measurements provide reliable characterization of high-temperature reactive flows such as practical combustion processes [1]. In addition to measurements of the concentration of specific target species, numerous schemes for the determination of temperature, pressure, and gas-phase velocity have been reported, and successful results are well-documented in a variety of review articles [1–5]. The work cited in these reviews shows that the development of solid-state semiconductor diode and quantum-cascade lasers has enabled a wide variety of small portable instruments applicable to numerous practical combustion applications, including a wide range of research and industrial facilities. Use of such instruments in existing industrial facilities and large-scale industrial research test rigs can present significant engineering challenges for the survivability of the sensor in the quest for low-noise laser absorption measurements [1,4].

The purpose of the gas sensor designed here is to enable in situ measurements of the post-combustion gases (CO, CO2, H2O) as well as temperature in a full-size high-pressure gas turbine burner test rig. The test facility offers a quite harsh environment both inside and outside the combustor. Inside, the combustion chamber is located inside a high-pressure vessel (up to 25 bar) with inlet hot compressed air (up to 770 K) similar to the intake of an aircraft gas turbine combustor can. In the combustion chamber, the temperature is even higher (2000 K) due to the combustion heat release. These conditions challenge sensitive

**Citation:** Shi, L.; Endres, T.; Jeffries, J.B.; Dreier, T.; Schulz, C. A Compact Fiber-Coupled NIR/MIR Laser Absorption Instrument for the Simultaneous Measurement of Gas-Phase Temperature and CO, CO2, and H2O Concentration. *Sensors* **2022**, *22*, 1286. https://doi.org/ 10.3390/s22031286

Academic Editors: Krzysztof M. Abramski and Piotr Jaworski

Received: 27 December 2021 Accepted: 4 February 2022 Published: 8 February 2022

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species-selective laser absorption measurements, as the high-temperature distributes the population for any gas species over a very wide range of quantum states and the highpressure broadens each absorption transition, resulting in a complex spectrum with few (if any) isolated lines. Outside the test facility, the walls of the test rig are quite hot (>150 ◦C) due to the flames near the combustor walls, which challenges the design of optical access windows and components near the combustor housing. The hot walls of the test rig produce an ambient temperature near 60 ◦C. Even more challenging is the extreme acoustic noise (up to 120 dB) surrounding the test rig. This acoustic noise exceeds specification of common electronic equipment, produces strong vibrations, and requires remote operation of the equipment. Thus, the thermal and optical engineering solutions for sensitive, speciesselective laser absorption measurements that overcome the challenges of the harsh ambient and test environments are an important part of the research reported here.

The design of the sensor reported here builds on a vast library of previous literature. In the past 30 years, there has been an enormous amount of research developing absorption spectroscopy for practical gas-sensing applications [1–5], and the subsequent research literature is exploited to develop the sensor reported here. A complete review of the relevant research is beyond the scope of this introduction, and only a few selected research citations important for the new sensor design are highlighted here.
