Fig. 1. The absorption spectrum of CH₄, CO₂, and H₂O in the vicinity of 1 653 nm

Fig. 2. Experimental system installation diagram

Fig. 3. The relationship diagram of laser emission wavelength, current tuning and temperature tuning

Fig. 4. Quadrature lock-in amplifier working principle diagram

Fig. 5. Digital quadrature lock-in amplifier input and output signal simulation diagram

Fig. 6. The structure diagram design of narrow-band low-pass filter

Fig. 7. Simulation diagram of laser drive signal and analog gas absorption signal

Fig. 8. Simulation diagram of TDLAS-WMS system based on digital quadrature lock-in amplifier

Fig. 9. Second harmonic signal simulation diagram and fitting result diagram

Fig. 10. Output diagram of the second harmonic signal of CIC filter and LPF filter

Fig. 11. The relationship between the concentration inversion value and the set value of the TDLAS system based on CIC and LPF

Fig. 12. Experimental detection results of methane gas with concentration of 1~100 ppm

Fig. 13. Concentration inversion effect diagram of NIPALS-ELM and CLS algorithm models

Fig. 14. Stability test result graph and Allan analysis of variance graph