This dissertation investigates the development and application of laser absorption spectroscopy (LAS) within the long-wave mid-infrared (LW-MIR) region, defined as 500 – 1500 cm-1 (7 - 20 um), which includes strong vibrational bands of environmental and combustion-relevant species. It introduces a difference-frequency generation (DFG) laser source that employs an orientation-patterned gallium arsenide (OP-GaAs) crystal, providing wide tunability and high spectral resolution suitable for detailed spectroscopic analysis. The laser’s capabilities are showcased through high-spectral-resolution spectroscopy in the 11.56 – 15 μm range and molecular fingerprinting of gases like NH3, C2H2, C2H4, and CO2. Further applications highlighted include environmental monitoring, where the DFG laser detects volatile organic compounds (VOCs) such as benzene, toluene, ethylbenzene, and xylene (BTEX) with ppm-level sensitivity, and combustion diagnostics (e.g., for HCN, furan, and NO) that provide insights into the chemical kinetics of renewable fuels where a novel DFG-based multi-speciation technique is proposed. Furthermore, SO2 time histories were measured in the LW-MIR in H2S oxidation experiments behind reflected shock waves which facilitated proposing a new kinetic model for acid gas combustion. Finally, the dissertation explores a novel application of the LW-MIR region for simultaneous open-path optical communication and gas sensing, leveraging the atmospheric window for dual functionality in industrial environments, disaster management, or remote sensing.

Bio
Ali is currently a PhD candidate in the Mechanical Engineering Program under the supervision of Prof. Aamir Farooq. Ali received his BSc in Mechanical Engineering from King Fahd University of Petroleum and Minerals in 2019 and his MSc in Mechanical Engineering from KAUST in 2020, with a thesis on argon power cycle engines. His PhD research focuses on the development of laser-based sensors in the long-wave mid-infrared region for environmental monitoring and combustion applications, as well as hydrogen production from hydrogen sulfide.