Fig. 1. Principle of QEPAS and LITES

Fig. 2. Simulation results for the bare QTF in the QEPAS system

Fig. 3. The amplitude of swing for the bare QTF in the LITES system

Fig. 4. The simulation mode of cylindrical cavity-based QEPAS system

Fig. 5. Simulation results for the QTF in cylindrical cavity-based QEPAS system

Fig. 6. Simulation results for the QTF in cylindrical cavity-based LITES system

Fig. 7. Schematic diagram of the cylindrical cavity-based QEPAS/LITES sensor systems

Fig. 8. Response curve for bare QTF

Fig. 9. The relationship between 2f signal amplitude of bare QTF based QEPAS system and laser modulation depth

Fig. 10. 2f signal for QEAS sensor with and without cylindrical cavity

Fig. 11. Noise for QEAS sensor with and without cylindrical cavity

Fig. 12. 2f signal for LITES sensor with and without cylindrical cavity

Fig. 13. Noise for LITES sensor with and without cylindrical cavity