Fig. 1. Flowchart of the numerical analysis to inverse gas concentration

Fig. 2. Exhaled CO₂ gas detection system based on hollow waveguide fiber

Fig. 3. Absorption signal simulation for CO₂ and H₂O gases with volume fraction of 5% around 3661.637 cm^-1

Fig. 4. Measured spectrum (solid line) and fitted spectrum (dotted line) of 2f-WMS, as well as fitting residual

Fig. 5. Relationship between retrieved CO₂ concentration by CF-WMS and standard CO₂ gas concentration

Fig. 6. Relationship between the WMS-2f peak value and the CO₂ concentration

Fig. 7. Histogram plot of CO₂ concentration. (a) Distribution of CO₂ concentration with measurement time obtained by CF-WMS; (b) distribution of CO₂ concentration with measurement time obtained by WMS

Fig. 8. Allan deviation plot for measurement system

Fig. 9. Comparison of real-time measurement results of exhaled CO₂ gas detected by CF-WMS and WMS