Enhancing single-mode stability and optical power of AlGaN-based DUV lasers by 1/4λ phase-shifted surface DFB gratings

AlGaN-based distributed-feedback (DFB) laser diodes (LDs) operating at 280 nm and incorporating surface gratings with different phase shifts were designed and investigated. Compared to Fabry–Pérot (FP) and other phase-shifted designs, the DFB LD with a 1/4 phase shift exhibits the best performance, achieving a threshold current of 17.39 mA (25.4% lower than FP), a slope efficiency of 1.254 W/A (411.1% of FP), and a side-mode suppression ratio of 25 dB at 0.10 A. Systematic analysis reveals that the 1/4 phase shift introduces a localized defect mode at the stop band center, thereby lifting Bragg-mode degeneracy and strengthening longitudinal-mode selectivity, leading to a significant improvement in slope efficiency. Furthermore, scattering-loss analysis indicates that the design can tolerate a dislocation density of up to 2.2 × 109 cm−2, highlighting fabrication feasibility on AlN/sapphire substrates. This design outlines a promising route toward low-threshold, high-power, single-mode, and cost-effective AlGaN Deep-ultraviolet DFB LDs.