Simultaneous measurement of nitric oxide concentration and temperature using overlapping absorption signals in direct absorption spectroscopy at 5181.2 nm

Simultaneous measurement of temperature and nitric oxide (NO) concentration is essential for reliable combustion diagnostics. In this study, a single quantum cascade laser (QCL) scanning the range of 1929.4–1931.3 cm⁻ 1 was used to concurrently retrieve temperature and NO concentration, specifically addressing the spectral overlap of NO and H 2 O signals. To minimize baseline fitting errors inherent in these overlapping features, we developed an analysis algorithm incorporating a Householder reflection–based orthogonalization process. The proposed method determines temperature and NO concentration by comparing baseline-corrected measured spectra with reconstructed virtual spectra. We validated the retrieved temperature against thermocouple measurements and compared H 2 O concentration results with 1.3 µm TDLAS data. Additionally, NO measurements were evaluated against an electrochemical sensor. Experimental results demonstrated consistent agreement across the tested range (603–653 K), confirming that this single-laser approach reduces system complexity while providing enhanced diagnostic reliability in high-humidity environments.