Fig. 1. Simulation of CO₂ absorption based on the HITRAN2023 database. (a) CO₂ absorption line intensity in the range of 6000–6450 cm^-1; (b) CO₂ absorption line located at 6339.706 cm^-1.

Fig. 2. Laser characteristics. (a) The relationship between the output wavelength and injected current at different temperatures; (b) the relationship between the output power and injected current at different temperatures.

Fig. 3. The schematic diagram of the CO₂-LITES sensor’s experimental setup.

Fig. 4. The frequency response of the self-designed QTF.

Fig. 5. The relationship between the 2f signal amplitude and current modulation depth.

Fig. 6. Relationship between the 2f signal and CO₂ concentration. (a) The 2f signal under different CO₂ concentrations; (b) the peak value of the 2f signal at various CO₂ concentrations and the associated linear fitting.

Fig. 7. Noise determination of CO₂-LITES sensor.

Fig. 8. Allan deviation analysis of CO₂-LITES sensor.