Fig. 1. Experimental setup diagram of off-beam LR-QEPAS system

Fig. 2. Schematics of acoustic detection modules. (a) Off-beam QEPAS; (b) P-QEPAS; (c) C-QEPAS; (d) off-beam LR-QEPAS

Fig. 3. Specific laser beam paths of off-beam LR-QEPAS. (a) Laser beam passes through mR for the first time; (b) laser beam passes through mR for the second time; (c) laser beam passes through mR for the third time; (d) laser beam passes through mR for the fourth time

Fig. 4. Off-beam LR-QEPAS signal as a function of the times that laser beam passes through the mR

Fig. 5. The second harmonic signals of off-beam LR-QEPAS, P-QEPAS, C-QEPAS, and off-beam QEPAS as a function of the modulation depth

Fig. 6. The second harmonic signals of off-beam QEPAS, C-QEPAS, P-QEPAS, and off-beam LR-QEPAS at the optimal modulation depth

Fig. 7. Allan variance analysis of each system. (a) Off-beam QEPAS; (b) C-QEPAS; (c) P-QEPAS; (d) off-beam LR-QEPAS