Fig. 1. Schematic of near-infrared CO₂ sensor system

Fig. 2. Selection of CO₂ absorption line. (a) Absorption spectra of CO₂; (b) absorbance of CO₂ and H₂O, and driver current as a function of laser emission wavenumber at operation temperature of 28 ℃

Fig. 3. Relationship between modulation depth and amplitude of second harmonic signal at CO₂ volume fraction of 2×10^-3

Fig. 4. Relationship between amplitude of second harmonic signal and measurement time for different CO₂ concentration

Fig. 5. Fitting curve between volume fraction of gas and amplitude of second harmonic signal

Fig. 6. Retrieved result versus time when volume fraction of CO₂ is 0

Fig. 7. Allan deviation curve of near-infrared CO₂ sensor

Fig. 8. Measured response time curve of sensor

Fig. 9. Observed CO₂ concentration in laboratory environment