Fig. 1. CO₂ gas detection system based on hollow waveguide fiber

Fig. 2. Relationship between the peak value of the harmonic signal and different CO₂ volume fraction. (a) 2f signal; (b) 2f/1f signal

Fig. 3. CO₂ volume fraction changing with time under different measurement conditions. (a) Locked frequency and non-locked frequency measured results under 2f condition; (b) locked frequency and non-locked frequency measured results under 2f/1f condition

Fig. 4. CO₂ volume fraction frequency distribution histogram and Gaussian line fitting under different measurement conditions. (a) 2f condition; (b) 2f-lock condition; (c) 2f/1f condition; (d) 2f/1f-lock condition

Fig. 5. Allan variance of the system obtained by four methods

Fig. 6. Long-term measurement resuls of CO₂ volume fraction under 2f/1f-lock condition and Kalman filtering result

Fig. 7. Frequency distribution histogram and Gaussian line fitting graph of CO₂ volume fraction after Kalman filtering