High slope efficiency single-mode surface-emitting DFB lasers with optimized λ/4 phase shift position.

Single-mode surface-emitting lasers are crucial for emerging mega- and hyper scale data centers, providing a cost-effective solution for high-bandwidth, long-reach data communication systems. However, grating-coupled surface-emitting lasers, which are commonly used to achieve single-mode surface emission, often suffer from low slope efficiency and high thresholds. In this study, we report a high slope efficiency single-mode surface-emitting distributed feedback laser featuring an optimized λ/4 phase shift position within the second-order grating region and an unpumped distributed Bragg reflector. The specific positioning of the λ/4 phase shift relative to the second-order grating significantly influences the interaction between the second-order grating and the longitudinal optical field, thereby affecting its diffraction performance. When the λ/4 phase shift is placed in the first-order grating etching region adjacent to the second-order gratings, we achieved the highest vertical upward diffraction from the second-order gratings. This optimization resulted in a slope efficiency of 0.3 mW/mA for the fabricated device. Incorporating a bottom DBR mirror in subsequent designs could potentially enhance the slope efficiency to 0.6 mW/mA. Additionally, due to a surface grating fully filled with spin-on-glass (SOG), we obtained a high grating coupling coefficient of 560 cm. The device also demonstrated a SMSR of 50 dB and a thermal resistance of 0.62 K/mW, indicating stable single-mode characteristics and good thermal performance. Using a rectangular oxidation aperture and benzocyclobutene, we achieved a 3-dB modulation bandwidth of 22 GHz at a bias current of 8 mA. Furthermore, with further optimization of parasitic parameters and fabrication, a 3-dB modulation bandwidth of 34 GHz is predicted.