Fig. 1. Structure parameter diagram of MPC and spot distribution on the surface of the incident mirror, where the green circle represents the location of the incident perforation. (a) Structure parameter diagram of MPC. (b) Simulation based on the established computational model. (c) Spot distribution obtained with a He-Ne laser.

Fig. 2. The features of two QTFs. (a) A photograph of the QTFs. (b) Frequency response of the commercial QTF (line in green) and the trapezoidal-tip QTF (line in red).

Fig. 3. Schematic configuration of LITES sensor based on a multi-pass cell with dense spot pattern.

Fig. 4. System characteristics at different optical powers. (a) Peak value of 2f signal of the commercial QTF based system. Inset: 2f signal waveform. (b) Peak value of 2f signal of the trapezoidal-tip QTF based system. Inset: 2f signal waveform. (c) Noise and SNR of the system when commercial QTF and trapezoidal-tip QTF were adopted, respectively.

Fig. 5. (a) 2f signal at different concentrations when commercial QTF was used. (b) 2f signal at different concentrations when trapezoidal-tip QTF was used. (c)The function relationship between different concentrations and the peak value of the 2f signals when commercial QTF was used. (d) The function relationship between different concentrations and the peak value of the 2f signals when trapezoidal-tip QTF was used.

Fig. 6. Background noise of the LITES sensor system when different QTFs were used as the detector.

Fig. 7. Allan deviation analysis. (a) Allan deviation analysis for commercial QTF based C₂H₂-LITES sensor. (b) Allan deviation analysis for trapezoidal-tip QTF based C₂H₂-LITES sensor.