###### Abstract  
It is important to monitor the temperature and H\({}_{2}\)O 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 H\({}_{2}\)O absorption signals were used in the tunable diode laser absorption spectroscopy (TDLAS) to measure the temperature and H\({}_{2}\)O concentration in a premixed CH\({}_{4}\)/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 H\({}_{2}\)O absorptions were 1.3492 and 1.34927 \(\upmu\)m. 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 CH\({}_{4}\)/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 H\({}_{2}\)O 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 H\({}_{2}\)O concentration measured at an equivalence ratio of 1.15 was the highest at 18.92%. t 28 March 2021 19 April 2021 20 April 2021 20 April 2021 20 April 2021 20 April 2021 20  
simultaneously measure both the internal temperature and gas concentration in a large combustion system because of the severe environment. In general, a thermocouple (T/C) and dew point sensor are used for measuring the temperature and H\({}_{2}\)O concentration. However, it is difficult to predict the overall reaction inside the combustion environment using the results provided due to physical probing and sampling method at fixed points. In addition, these methods have short durability due to the severe environment and high temperatures, and cause flow disturbances in the combustion field. To overcome these limitations, researchers have developed a non-intrusive method using tunable diode laser absorption spectroscopy (TDLAS), which is an optical measurement method that utilizes absorption with a narrow linewidth. It is also highly selective, durable, and fast responsive, and is widely applied in various combustion environments such as IC engine, biomass, scramjet, and coal gasifier [2; 3; 4; 5; 6; 7]. From the theory of TDLAS, a single absorption signal and the ratio of two absorption signals are used to measure the gas concentration and flow field temperature in combustion environments [3; 8]. In this case, as H\({}_{2}\)O molecules are present in a considerable proportion in the combustion environment, the use of their two absorption signals is advantageous to derive the temperature. Temperature measurements using TDLAS have been previously performed in various combustion environments. [PERSON] et al. (2000) obtained the temperature from two H\({}_{2}\)O absorption signals in each scan range by using two lasers (1.343 and 1.392 \(\upmu\)m) and wavelength division multiplexing to measure the temperature and concentration in the C\({}_{2}\)H\({}_{4}\)/Air flame [9]. [PERSON] et al. (2002) measured the temperature and H\({}_{2}\)O concentration of silane/hydrogen/oxygen/argon flames using the H\({}_{2}\)O absorption wavelength region of 1.392-1.394 \(\upmu\)m [10]. Here, [PERSON] et al. used two H\({}_{2}\)O absorption signals separated from each other within the single scan range of one laser.