Fig. 1. (a) Relationships between 2f signal intensity or temperature sensitivity and temperature; (b) relationships between 2f signal intensity or pressure sensitivity and pressure

Fig. 2. Experimental setup (inserted picture is light spot distribution of novel multi-pass cell)

Fig. 3. (a) Stability of pressure in absorption cell; (b) stability of temperature; (c) stability of gas flow; (d) variation of absorption signal amplitude with time

Fig. 4. (a) 2f signal of CO2 absorption spectrum, background signal acquired by filling the same pressure N2 and differential signal obtained by subtracting the background signal from the absorption signal (insert graph is the definition of signal-to-noise ratio); (b) spectra and signal-to-noise ratio before and after wavelet denoising

Fig. 5. Relation between 2f signal intensity and absorption gas concentration

Fig. 6. (a) Noise amplitude and measurement precision before and after Kalman filtering; (b) Allan variance of measured concentration

Fig. 7. Measured concentration of CO2 in laboratory