Fig. 1. Absorption line intensity of CO₂ gas molecules was calculated using the HITRAN database as well as partial absorption lines of CO₂ and H₂O in the tuning range of 2 μm mid-infrared DFB laser. (a) Absorption line strength; (b) partial absorption lines

Fig. 2. Curve of laser power

Fig. 3. Model of fiber-coupled all-solid-state mid-infrared QEPAS photoacoustic detection module

Fig. 4. Fiber-coupled all-solid-state mid-infrared QEPAS acoustic detection module. (A) Design drawing of the module; (b) front view of the module; (c) side view of the module; (d) picture of the module

Fig. 5. Experimental setup diagram

Fig. 6. Frequency response of quartz tuning fork

Fig. 7. QEPAS signals of CO₂ with volume fraction of 0.01 at different modulation depths as well as relationship between QEPAS signal amplitude and modulation depth. (a) QEPAS signals at different modulation depths; (b) relationship between QEPAS signal amplitude and modulation depth

Fig. 8. Relationship between QEPAS signal amplitude and CO₂ concentration

Fig. 9. 2f signal. (a) 2f signal of CO₂ with volume fraction of 0.001; (b) 2f signal of CO₂ in our laboratory

Fig. 10. Allan deviation analysis of the sensor system