Fig. 1. Schematic of Beer-Lambert law

Fig. 2. Schematic of Abel inverse transformation for axisymmetric flame

Fig. 3. Typical images of CH₄/air flame （left）， CH₄/NH₃/air flame （middle） and NH₃/air flame （right）

Fig. 4. Schematic of the mid-infrared absorption tomographic spectroscopy for flame measurements， DFB： distributed feedback laser： PD： photodetector；CVL： convex lens； CCL： concave lens； FM， flat mirror NBF： narrow bandpass filter

Fig. 5. Output power （a） and emitted wavelength （b） as a function of injection current at different operational temperature

Fig. 6. Reconstructed results for flames with different central temperature

Fig. 7. Typical raw baseline and absorption signals

Fig. 8. Representative experimental data and Voigt-fitting profiles in CH₄ （100%） （a） and CH₄ （80%）/NH₃ （20%） （b） flame

Fig. 9. Repeatable measurement results at at a fixed HAB for methane-ammonia flame

Fig. 10. Measured radial temperature distribution in methane-ammonia flames with an ammonia proportion of 20%

Fig. 11. CH₄ flames with different NH₃ blending ratios