# Research on Narrow Linewidth External Cavity Semiconductor Lasers
## 5 Conclusions  
In this paper, the three main device structures of NLECSLs are expanded upon. By comparing a large number of data, we conclude that the confocal F-P cavity structure of NLECSLs is the best structure to achieve a narrow linewidth, and could obtain the narrowest linewidth, which is more precise and more suitable for applications that require a high accuracy of the linewidth. NLECSLs are developing towards high power and narrower linewidth. Through the continuous development of new optical feedback elements and optical resonator design, the ultra-narrow linewidth laser below 20 Hz has been realized. Combined with its characteristics of small volume, light weight, high conversion efficiency and wide spectral range, it will be widely used in the fields of ultra-high precision lidar, inter satellite communication, coherent optical communication, laser spectroscopy, atomic clock pumping, atmospheric absorption measurement and optical fiber communication. How to realize the wide tuning range, narrow linewidth laser output is a main research direction for the future development of NLECSLs. In addition, a narrow linewidth laser is critical for its application as a pump source for generating an extremely narrow linewidth Brillouin output [52]. Currently, different approaches to narrow linewidth lasers have distinct characteristics. In the future, new technologies will lead to further compression of the laser linewidth, improvement of frequency stability, expansion of wavelength, and increase in power, which will pave the way for human beings to explore the unknown world. Conceptualization, K.D. and L.W. ; methodology, Y.M., X.L. and J.S. ; writing--original draft preparation, K.D. and Z.L. ; writing--review and editing, L.Z. and L.L. ; visualization, Y.Q., Z.Q. and G.L. ; supervision, Z.Q. ; funding acquisition, G.L. All authors have read and agreed to the published version of the manuscript. This work was supported in part by specific research fund for Innovation Platform for Academicians of Hainan Province under Grant YSPTZX202034 and Grant YSPTZX202127; in part by the Major Science and Technology Program of Hainan Province of China under Grant ZDKJ 2019005; in part by Scientific Research Projects of Higher Education Institutions in Hainan Province under Grant hnky2020-24, Grant Hnjg2021 ZD-22, Grant hnky2020 ZD-12; in part by the Hainan Provincial \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Central** & **Tuning Range** & **Line Width** & **Output Power** & **Publication time** \\ \hline
657 nm & 0.5 GHz & 432 kHz & - & 2011 [41] \\ \hline
689 nm & 3.97 GHz & 6.8 kHz & 20 mW & 2011 [42] \\ \hline
689 nm & 4 MHz & 100 Hz & - & 2012 [43] \\ \hline
635 nm & 5–20 GHz & 15 MHz & 5 mW & 2014 [44] \\ \hline
780 nm & - & 15.7 Hz & 50 mW & 2015 [10] \\ \hline
780 nm & - & 31 Hz & 38 mW & 2015 [45] \\ \hline
1064.49 nm & - & 630 Hz & 4 mW & 2017 [46] \\ \hline
852 nm & 1.5 GHz & 187 kHz & 56 mW & 2018 [47] \\ \hline
1555 nm & 17 nm & 8 kHz & - & 2018 [48] \\ \hline
698.45 nm & 40 GHz & 180 kHz & 36 mW & 2020 [49] \\ \hline
369.5 nm & 14.7 Hz & 20 MHz & - & 2021 [50] \\ \hline
1547 nm & 20 nm & 100 MHz & - & 2021 [51] \\ \hline \hline \end{tabular} Note: “-” denotes that the data are not available.  
\end{table} 
Table 5: Performance of confocal F-P cavity structure of NLECSLs. Natural Science Foundation of China under Grant 622 RC671, Grant 120 MS031, Grant 2019 RC190, Grant 2019 RC192; in part by the National Natural Science Foundation of China under Grant 61774024, Grant 61864002, Grant 11764012, Grant 62174046,Grant 62064004 and Grant 61964007; in part by the Key Research and Development Projects in Hainan Province under Grant ZDYF2020020, Grant ZDYF2020036, and Grant ZDYF2020217; in part by the Open Fund for Innovation and Entrepreneurship of college students under Grant 202111658021X, Grant 202111658022X, Grant 202111658023X, Grant 202111658013. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable.