Fig. 1. Schematic diagram of experimental system based on dual-spectroscopy gas detection technique

Fig. 2. Infrared absorption lines of CH₄，CO，CO₂，H₂O，O₂

Fig. 3. Relationship between laser emission wavelength and driving voltage

Fig. 4. Resonance profiles of QCTF1 and QCTF2 detectors

Fig. 5. Functional relationship between QCTF signal amplitude and excitation position

Fig. 6. Functional relationship between modulated signal phase and bispectral signal amplitude

Fig. 7. Photoacoustic，photothermal and dual-spectroscopy signals under optimal conditions

Fig. 8. Variation of WMS-2F signal amplitude with laser power