Fig. 1. Relationship between the wave numbers of CH₄, CO₂, and H₂O absorption lines and the laser driving current based on HITRAN2016 simulation, as well as the absorption lines of H₂O

Fig. 2. Schematic diagram of HPSDS experimental setup for H₂O detection

Fig. 3. Linearity of volume fraction at different modulation frequencies and the optical system of the HPSDS signal at 1.2 GHz. (a) Linear relationship between HPSDS output phase peak-to-peak values and gas volume fraction at different modulation frequencies; (b) HPSDS signals measured at different H₂O volume fractions when the modulation frequency is 1.2 GHz

Fig. 4. Results and volume fraction histograms of continuous measurement for 2 h at three different modulation frequencies

Fig. 5. Allan variance plot for the system with three different modulation frequencies

Fig. 6. Comparison of the linearity of the phase peak-to-peak values of the HPSDS signal output with the WMS-2f signal at different volume fraction

Fig. 7. Allan variance of HPSDS and WMS system

Fig. 8. Segmented monitoring of actual atmospheric H₂O measurements in May 2024

Fig. 9. Measured values for TSP01. (a) TSP01 measures temperature and relative humidity profiles over 4 h; (b) TSP01 measures temperature and relative humidity profiles over 6 h