## 5 Conclusions  
A time division multiplexing differential modulation technique is proposed to eliminate the interference caused by the fluctuation of laser source in a single optical path detection system. A new multi-reflection chamber was designed to further improve the system detection precision. A near-infrared C\({}_{2}\)H\({}_{2}\) detection system was developed based on TDLAS using a 1520 nm DFB laser. A laser driver and a high-precision temperature controller were developed, and a lock-in amplifier is  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Reference/Type**} & **Detection** & **Detection** & **Detection** & **Detection** & **System** \\  & **Wavelength** & **Technique** & **Limit (ppmv)** & **Error (\%)** & **Stability (\%)** \\ \hline
[13] & 1533 nm & TDLAS-WMS & 4 & – & – \\
[15] & 1530 nm & TDLAS-WMS & 3.97 & 3 & 3 \\
[19] & 1534 nm & TDLAS-WMS & 200 & 5 & 2 \\
[24] & 1530 nm & TDLAS-WMS & 1.46 & – & – \\ Single-pass un-modulation & 1520 nm & DAS & 710 & – & – \\ Single-pass modulation & 1520 nm & TDLAS-WMS & 142 & – & – \\ Multi-pass un-modulation & 1520 nm & DAS & 1.5 & – & – \\ Multi-pass modulation & 1520 nm & TDLAS-WMS & 0.3 & \(\pm\)2 & \(\pm\)1.2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Performance comparison between the proposed C\({}_{2}\)H\({}_{2}\) detection system and the reported C\({}_{2}\)H\({}_{2}\) detection system.  
Figure 9: (**a**) Results of the stability test of 50 ppmv C\({}_{2}\)H\({}_{2}\). (**b**) Analysis of Allan deviation for long-term characteristics of the detection system.  
used to extract the 2f signal. A number of experiments were performed using the detection system to study its performance. The experimental results indicated that the detection system has good performance indicators. The MDL is 0.3 ppmv, and the precision was 2% in the detection range of 10 to 100 ppmv and contains errors caused by dynamic gas preparation. A 10-h stability observation experiment was performed on 50 ppmv C\({}_{2}\)H\({}_{2}\), and the fluctuation was less than \(\pm\)1.2%. The Allan deviation value was 0.3 ppmv when the initial integration time was 1 s, according to the Allan deviation analysis experimental data. When the integration time increased to 362 s, Allan deviation value was 0.0018 ppmv, which indicated the system's good stability. In addition, the time division multiplexing differential modulation technique and the chamber structure adopted by the system were suitable for the precision improvement of other near-infrared single optical path gas detection systems. Therefore, the proposed approach was flexible and practical.  
Conceptualization, B.W. Methodology, B.W. and H.L. ([PERSON]). Software, B.W. Validation, B.W., H.L. ([PERSON]), and C.C. Formal analysis, L.C. Investigation, H.L. ([PERSON]). Writing--original draft preparation, H.L. ([PERSON]). Writing--review and editing, H.L. ([PERSON]), B.W., and C.C. Visualization, H.L. ([PERSON]), T.D., and Y.C. Project administration, L.C. Funding acquisition, B.W.  
The National Key R&D Program of China, grant number 2017 YFB0405303, funded this research.  
The author wishes to express his gratitude to the National Key R&D Program of China (2017 YFB0405303) for their generous support toward this work.  
The authors declare no conflicts of interest.