## 4 Conclusions  
In this paper, we demonstrated a high-power surface grating 1550 nm tunable DFB laser based on the gain-coupling effect. The gain coupling effect was realized by forming a periodic gain difference with periodic p-electrode windows. A tapered waveguide was introduced to improve the output power, and a multiple quantum well structure formed of AlGaInAs material was used in the active region to reduce the linewidth of the laser. Our device achieved consistently excellent performance. This was exhibited by CW output power of 401.5 mW at 20 \({}^{\circ}\)C, a maximum side-mode rejection ratio exceeding 55 dB, a measured 3 dB linewidth was 18.86 MHz, and a wavelength quasi-continuous tuning range of 6.156 nm from 10 \({}^{\circ}\)C to 50 \({}^{\circ}\)C. Our device does not use complex manufacturing techniques such as secondary epitaxial growth and can be manufactured by i-line lithography, which  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Year**} & \multirow{2}{*}{**Pout**} & **Tunable Range** & \multirow{2}{*}{**SMSR**} & \multirow{2}{*}{**Linewidth**} \\ \hline
2012 [32] & 210 mW & - & \textgreater{}45 dB & 64 kHz (3 dB) \\
2012 [33] & \textgreater{}15 mW & \textgreater{}15 nm & \textgreater{}40 dB & - \\
2013 [34] & \textgreater{}13 mW & 5.88 nm & \textgreater{}45 dB & - \\
2014 [35] & 6 mW & 3.78 nm & \textgreater{}52 dB & \textless{}170 kHz (3 dB) \\
2014 [36] & 7 mW & \textless{}4 nm & \textgreater{}50 dB & \textless{}185 kHz (3 dB) \\
2014 [37] & \textgreater{}3 mW & 9 nm & \textgreater{}42 dB & 1.67 MHz (3 dB) \\
2015 [38] & \textgreater{}4 mW & 30 nm & \textgreater{}40 dB & - \\
2015 [39] & \textgreater{}1.5 mW & 25 nm & \textgreater{}40 dB & - \\
2018 [40] & \textgreater{}15 mW & \textgreater{}9 nm & \textgreater{}35 dB & - \\
2020 [41] & \textgreater{}4 mW & 40 nm & \textgreater{}45 dB & 0.68 nm (20 dB) \\
2020 [42] & \textgreater{}25 mW & 10 nm & \textgreater{}60 dB & - \\
2021 [43] & \textgreater{}10 mW & \textgreater{}3.5 nm & \textgreater{}55 dB & - \\ This paper & 400 mW & 6.156 nm & \textgreater{}55 dB & 18.86 MHz (3 dB) \\ \hline \hline \end{tabular}
\end{table}
Table 1: The parameters of the device in this paper are compared with other reported DFB lasers.  
Figure 7: (**a**) 3 dB linewidth measurement result at 20 \({}^{\circ}\)C; (**b**) 3 dB linewidth measurement results at different temperatures.  
improves yield and achieves stable working conditions. This provides a low-cost, high-efficiency, high-yield DFB laser for use in the fields of space laser communication, lidar, and laser ranging.  
Conceptualization, L.L. and L.Q.; methodology, Y.L. and Y.C.; software, P.J. and C.Z.; validation, X.L. and L.L.; formal analysis, Y.W., Y.S., H.T., and C.Q.; investigation, P.J., Y.C., Y.W., C.Y., and Y.L.; resources, L.Q. and L.L.; data curation, X.L. and L.L.; writing--original draft preparation, X.L.; writing--review and editing, L.L.; visualization, X.L.; supervision, L.L. and L.Q.; project administration, L.Q.; funding acquisition, L.Q. and L.W. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Key R & D Program of China [2021 YFF0700500]; the National Natural Science Foundation of China [11874353, 61904179, 62090054, 61934003, 62090052, 62090051]; the Science and Technology Development Project of Jilin Province [202001069X, 20200501006 GX, 20200501007 GX, 20200501008 GX]; and the Dawn Talent Training Program of CIOMP.  
Not applicable.  
The authors would like to thank [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] for helping with this article.  
The authors declare no conflict of interest.