Fig. 1. Schematic of the f0 shift for QTF in H2-enhanced LITES technique.

Fig. 2. Schematic diagram of the H2-enhanced LITES sensor.

Fig. 3. Frequency response of the QTF in H2 with different concentrations. (a) The f0 curves of the QTF. (b) The obtained f0 and Q-factor of the QTF.

Fig. 4. The Q-factor of the QTF in different gas surroundings. (a) The Q-factor as a function of gas concentration; (b) 1/Q as a function of gas viscous damping and viscosity.

Fig. 5. 2f signal and noise of LITES sensor when the QTF was in pure N2 and H2, respectively.

Fig. 6. Allan deviation analysis of LITES sensor with or without H2-enhancement.

Fig. 7. Schematic diagram of the ultra-highly sensitive C2H2-LITES sensor based on self-designed round-head QTF with low f0 and MPC with 40 m optical length.

Fig. 8. The f0 response of the self-designed round-head QTF with or without H2-enhancement.

Fig. 9. LITES signal amplitude as a function of the modulation current.

Fig. 10. 2f signal and modulation signal of LITES sensor when the QTF was in pure N2 and H2, respectively.

Fig. 11. 2f signal amplitude recorded as a function of time for C2H2 concentration varying from 0 to 20 ppm.

Fig. 12. The Allan deviation analysis of the H2-enhanced C2H2-LITES sensor based on self-designed round-head QTF with low f0 and MPC with 40 m optical length.