Fig. 1. Principle of direct absorption spectroscopy

Fig. 2. Absorption lines of CO₂ and H₂O between 1‒3 μm, respectively

Fig. 3. Simulate absorption lines for 5×10^-4 CO₂ and 2% H₂O at normal temperature and pressure based on HITRAN database

Fig. 4. Testing operational parameters of DFB laser. (a) Relationship between output wavenumber of DFB laser and operating current and temperature; (b) curves of laser power-current-voltage at operating temperature of 42 ℃

Fig. 5. Schematic diagram of CO₂ gas sensing system based on TDLAS

Fig. 6. Schematic diagram of TDLAS system in laboratory

Fig. 7. Calibration process of volume fraction for XENSIV^TMPAS CO₂ sensor

Fig. 8. Fitting of measured CO₂ transmittance spectrum. (a) Transmission spectrum (solid line) and fitted baseline (dashed line) of CO₂ gas molecules from 4989.36 to 4990.53 cm^-1; (b) R(16) absorption line of CO₂

Fig. 9. Continuous monitoring of indoor CO₂ volume fraction trends for 8 consecutive days. (a) TDLAS CO₂ sensor and XENSIV^TMPAS CO₂ sensor monitored results of CO₂ volume fraction for 8 consecutive days in laboratory under influence of personnel changes; (b) daily CO₂ volume fraction change. Bottom whisker, bottom box line, top box line and top whisker respectively represent minimum, 25%th, 75%th, and maximum values of CO₂ volume fraction at the same time. Solid black line inside box represents median of CO₂ volume fraction, while black curve represents the average CO₂ volume fraction

Fig. 10. Analysis of the correlation between 24-hour indoor CO₂ measurement results and sensor readings. (a) TDLAS CO₂ sensor and XENSIV^TMPAS CO₂ sensor continuously measure indoor CO₂ volume fraction for 24 consecutive hours;(b) correlation analysis of measurement results from two sensors