Fig. 1. Absorption spectral lines of NO, CO₂, and H₂O are distributed

Fig. 2. Schematic diagram of FeNO detection system

Fig. 3. Relationship between modulation amplitude and peak value of second harmonic signal

Fig. 4. Peak value and volume fraction curves of CO₂ second harmonic signal based on WMS and DAS technologies

Fig. 5. Relationship between CO₂ volume fraction and peak value of second harmonic signal

Fig. 6. Simulation of second harmonic signals for CO₂ with a volume fraction of 4% and NO with a volume fraction of 2.0×10^-8

Fig. 7. Relationship between NO volume fraction and peak value of second harmonic signal

Fig. 8. Continuous measurement of gas volume fraction variations for 15 min. (a) CO₂; (b) NO

Fig. 9. System response curve

Fig. 10. Continuous measurement results of gas volume fraction and Allan variance. (a) CO₂; (b) NO

Fig. 11. Curves CO₂ and NO volume fractions during single exhalation cycle

Fig. 12. Measurement results of volume fraction of exhaled gas by volunteers. (a) CO₂; (b) NO