# The Influence of Laser Linewidth on the Brillouin Shift Frequency Accuracy of BOTDR
## 5 Conclusions  
In this paper, the influence of the laser linewidth on BFS accuracy in the FS-BOTDR was analyzed. The BGS broadening effect and the SNR of the FS-BOTDR was numerically simulated by taking phase fluctuation as the major noise contribution. The simulation results presented that the RMSE of BFS decreases when the laser linewidth narrows. In the experiments, we utilized five different lasers as the seed source, respectively, to measure the BFS change. The linewidth of the five lasers were, correspondingly, 1.05 MHz, 101 kHz, 10.2 kHz, 3.1 kHz, and 98 Hz. As the pulse width increased from 12 ns to 42 ns, the BGS width and BFS accuracy were further evaluated and discussed in detail.  
The experimental results indicate that the BFS accuracy improves with the laser linewidth narrowing. However, the BFS accuracy will deteriorate when the laser linewidth decreases to a certain value, such as 98 Hz in this paper. The exception may arise from the increasing CRN related closely with the narrowing linewidth. Therefore, how the CRN affects the BFS accuracy needs to be further theoretically explained and experimentally verified in future, if an extremely-narrow-linewidth laser is utilized in a FS-BOTDR. The results in this paper will be helpful to choose an appropriate laser for BOTDR, and provide potential techniques for improving the measurement accuracy of temperature or strain based BOTDR.  
[PERSON] proposed the idea; [PERSON] performed the theoretical analysis and wrote the paper; [PERSON] performed the temperature experiments; [PERSON] tested the laser linewidths; [PERSON] and [PERSON] analyzed the data; [PERSON], [PERSON] and [PERSON] revised the manuscript.  
This research was funded in part by the National Natural Science Foundation of China (61805167), in part by the Coal-Bed Methane Joint Research Fund of Shanxi Province, China (2015012005 and 2016012011), and in part by the Social Development Project of Shanxi Province Key Research Plan (201703D321034).  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Laser**} & **Peak** & \multicolumn{2}{c}{**12 ns**} & \multicolumn{2}{c}{**22 ns**} & \multicolumn{2}{c}{**32 ns**} & \multicolumn{2}{c}{**42 ns**} \\ \cline{2-10}  & **Power** & **RMSE** & \(\mathbf{W_{\text{BGS}}}\) & **RMSE** & \(\mathbf{W_{\text{BGS}}}\) & **RMSE** & \(\mathbf{W_{\text{BGS}}}\) & **RMSE** & \(\mathbf{W_{\text{BGS}}}\) \\ \hline \multirow{2}{*}{**Laser linewidth**} & **dBm** & **MHz** & **MHz** & **MHz** & **MHz** & **MHz** & **MHz** & **MHz** & **MHz** \\ \hline
1.05 MHz & 23.09 & 1.090 & 108.6 & 0.524 & 93.6 & 0.399 & 92.2 & 0.360 & 89.5 \\
101 kHz & 23.09 & 1.084 & 106.9 & 0.521 & 91.6 & 0.397 & 90.2 & 0.358 & 89.2 \\
10.2 kHz & 23.09 & 1.021 & 105.8 & 0.460 & 90.9 & 0.335 & 90.5 & 0.272 & 8  
**Acknowledgments:** The 1.05-MHz-linewidth laser and the 3.1-kHz-linewidth laser used in the experiments were freely provided by the Tianjin Opeak Technology Co. Ltd and Laser Institute of Shandong Academy of Science (LISD) in China, respectively. They are very helpful to the experiments in this paper. The authors thank the two companies very much.  
**Conflicts of Interest:** The authors declare no conflict of interest.