**4. Conclusions**

A series of In0.15Ga0.85 N/In0.02Ga0.98N MQW laser diodes (LDs) with di fferent barrier layers are investigated with the simulator Crosslight. It is found that when the first (or last) In0.15Ga0.85N barrier layer is no more than 45 nm (or 100 nm), due to the increase in the thickness of the first (or last) barrier layer, the optical field is limited better in the MQW, and the optical absorption loss is reduced. Subsequently, a low threshold current and high output power are achieved. As the thickness of the barrier layer becomes larger, the output powers of the lasers gradually increase, and the positive e ffects of the reduced optical absorption loss are partially compensated by the negative e ffects of the OCF reduction. However, when the FQB (or LQB) layer is thicker than 225 nm (or 300 nm), the photoelectric performance of LDs become worse. It is due to the rapid decrement of the OCF, which is not enough to be compensated by the reduction of the optical absorption loss. Nevertheless, compared to the FQB structure, the thick LQB layer will significantly reduce the vertical electron leakage current leaking from quantum wells to p-type region, especially the LQB-300 structure. As a result, the maximum output power of the LQB-300 is 3.87 times that of the reference structure.

**Author Contributions:** Conceptualization, W.W.; Methodology, W.W.; Validation, W.W.; Formal Analysis, W.X.; Investigation, W.W., W.X., M.L. and Z.D.; Resources, W.W.; Data Curation, W.W.; Writing—Original Draft Preparation, W.W.; Writing—Review and Editing, W.W.

**Funding:** This research is funded by the Science Challenge Project (No. TZ2016003-2-1), National Key R&D Program of China (No. 2017YFB0403103), and National Natural Science Foundation of China (No. 61504126).

**Acknowledgments:** The authors would like to thank Liang Feng and Sun Song for their helpful suggestions and comments.

**Conflicts of Interest:** The authors declare no conflict of interest.
