Fig. 1. Spectral line distribution of CO, CO₂ and H₂O in the wavelength range of 1-5 μm at 300 K

Fig. 2. Spectral line distributions of CO, CO₂, CH₄ and H₂O in 4288-4302 cm^-1 and current-wavenumber relationship of DFB laser at 25 ℃

Fig. 3. Second harmonicsignal amplitude of CO gas under different modulation amplitudes

Fig. 4. Schematic of the experimental device

Fig. 5. Second harmonic (2f) signal of CO gas with different volume fractions

Fig. 6. Linearity between second harmonic signal amplitude and CO volume fraction

Fig. 7. Comparison between measured value and know reference value of CO volume fraction

Fig. 8. Measurement results of CO with volume fraction of 3×10^-6. (a) Successive measurement results; (b) histogram plot obtained from time series measurement

Fig. 9. Allan variance analysis of the system