It is important to monitor the temperature and H₂O concentration in a large combustion environment in order to improve combustion (and thermal) efficiency and reduce harmful combustion emissions. However, it is difficult to simultaneously measure both internal temperature and gas concentration in a large combustion system because of the harsh environment with rapid flow. In regard, tunable diode laser absorption spectroscopy, which has the advantages of non-intrusive, high-speed response, and in situ measurement, is highly attractive for measuring the concentration of a specific gas species in the combustion environment. In this study, two partially overlapped H₂O absorption signals were used in the tunable diode laser absorption spectroscopy (TDLAS) to measure the temperature and H₂O concentration in a premixed CH₄/air flame due to the wide selection of wavelengths with high temperature sensitivity and advantages where high frequency modulation can be applied. The wavelength regions of the two partially overlapped H₂O absorptions were 1.3492 and 1.34927 μm. The measured signals separated the multi-peak Voigt fitting. As a result, the temperature measured by TDLAS based on multi-peak Voigt fitting in the premixed CH₄/air flame was the highest at 1385.80 K for an equivalence ratio of 1.00. It also showed a similarity to those tendencies to the temperature measured by the corrected R-type T/C. In addition, the H₂O concentrations measured by TDLAS based on the total integrated absorbance area for various equivalent ratios were consistent with those calculated by the chemical equilibrium simulation. Additionally, the H₂O concentration measured at an equivalence ratio of 1.15 was the highest at 18.92%.

Exhaust gases generated by fossil fuel combustion significantly contribute to air pollution and climate change, emphasizing the need for efficient combustion diagnostics. OH radicals are key indicators of flame behavior; however, conventional laser-induced fluorescence (LIF) techniques are impractical for industrial monitoring due to their complexity. In this study, wavelength modulation spectroscopy (WMS) employing a near-infrared laser at 1.49 μm was used to measure OH radical concentrations, incorporating a correction for spectral interference from H ₂ O. A wavelength division multiplexer (WDM) was used to enable the simultaneous operation of two lasers at 1.49 μm (OH) and 1.39 μm (H ₂ O, temperature), allowing for correction of water interference. OH concentrations were determined using both WMS and direct absorption spectroscopy (DAS), and the results were evaluated through comparison with CHEMKIN simulations. The proposed dual-laser system demonstrated reliable and interference-corrected quantification of OH radicals in methane (CH ₄ )/air premixed flames over a range of equivalence ratios. Comparisons with thermocouple-based temperature measurements and CHEMKIN-predicted species concentrations confirmed the reliability of the proposed technique. This study highlights the robustness and applicability of WMS-based OH diagnostics for combustion monitoring and demonstrates the potential for future implementation in industrial burner systems.