Laser absorption spectroscopy tomography is widely applied to measure the two-dimensional distribution information of complex combustion flow fields. A projection measurement is usually required in tomography, and laser absorption spectroscopy requires integrated absorbance. Direct absorption spectroscopy can directly measure the integrated absorbance, however, fitting the absorbance curve is difficult due to the distortion of the baseline in harsh environments. In contrast, wavelength modulation spectroscopy has higher sensitivity and noise resistance， so it is often used to obtain the integrated absorbance in laser absorption spectroscopy tomography. Conventional calibration-free wavelength modulation spectroscopy generally requires complex absorption spectrum simulations in combination with spectral databases and laser modulation parameters, placing high demands on the accuracy of a priori spectral parameters and hardware parameters. Meanwhile, inappropriate initial values can increase the computation time and even lead to local optimum solutions. In order to improve the computational efficiency, a rapid calibration-free wavelength modulation spectroscopy technique to obtain the integrated absorbance is presented in this paper. First, the method is computationally efficient, requiring only algebraic calculations using the 2nd, 4th, and 6th harmonic center peak height parameters to obtain the integrated absorbance, eliminating the need for computationally intensive harmonic fitting calculations. Secondly, the method has a low dependence on the spectral database, requiring only line intensity and low-state energy level spectral parameters. Finally, the method is highly adaptable and does not require scanning the complete absorption spectral line shape, which solves the problem of incomplete harmonic signals caused by the conventional method at high temperature and high pressure due to the broadening of the absorption spectral line. The method has previously been used only for line-of-sight measurements at low-frequency experimental signals, stable environments, and calculating the integrated absorbance at average temperature, concentration and pressure states. In this paper, the method was applied to non-uniform complex combustion field tomography and combined with the proposed tomographic system to achieve online reconstructing temperature and concentration distributions. The accuracy and computational efficiency of the method in obtaining the integrated absorbance were verified by numerical simulations and experiments on the butane burner flame. The results showed that the presented method was consistent with the reconstructed distribution compared to the conventional wavelength modulation method, with a maximum relative deviation of only 0.94% for the measurement and 3.5% for the thermocouple measurements, which verified the accuracy of the method. The computational efficiency of the two methods for obtaining the integrated absorbance was analyzed. The average calculation time per path for the presented method and the conventional method are 0.15s and 21.10s, respectively. The calculation efficiency of the presented method is at least two orders of magnitude higher than that of the conventional method, which provides a fast and reliable research method and technical means to realize industrial-grade online reconstruction of temperature and concentration distribution of combustion fields.
