Fig. 1. Structure of the cylindrical mirrors multi-pass cell

Fig. 2. Different spot distributions which create by adjusting the angle of the incident light and the relative position of the two mirrors

Fig. 3. Steps for NGO-VMD wavelet denoising

Fig. 4. Simulated signals and the signals after denoising using different denoising algorithms

Fig. 5. Schematic diagram of the experimental setup

Fig. 6. Detected CH₄ absorption signal （black curve） at a concentration of 825 ppm and the baseline signal （red curve）

Fig. 7. Direct absorption signal（a） and absorption spectrum of CH₄（b）

Fig. 8. Denoised progress of the NGO-VMD wavelet algorithm

Fig. 9. Integrated area at different CH₄ concentration

Fig. 10. Measured inverted concentration for continuous measurement and Allan deviation analysis of 120 ppm CH₄ before and after filtering