High-power single-mode distributed-feedback (DFB) semiconductor lasers operating at 1.06 μm are essential for applications such as free-space optical communication, laser ranging, biomedical systems, and frequency doubling. These applications necessitate sources with stable wavelength emission and high single-mode output power. Conventional approaches often utilize low-order buried gratings, which require complex epitaxial regrowth processes, increasing costs and risking contamination or oxidation issues, particularly with Al-containing materials. Furthermore, fabricating these fine gratings typically involves expensive electron-beam lithography (EBL), prone to stitching errors in long cavities. Sidewall-grating structures, fabricated directly onto the ridge-waveguide sides without regrowth, offer a promising alternative. Specifically, high-order sidewall gratings are compatible with cost-effective stepper photolithography, enhancing reliability and reducing manufacturing expenses. Despite successful demonstrations in other material systems, research on 1.06 μm sidewall-grating DFB lasers remains limited. This work aims to investigate and demonstrate a simplified, cost-effective fabrication route for high-power, single-mode 1.06 μm DFB lasers by employing a ninth-order sidewall grating, fabricated using stepper lithography, integrated onto an optimized high-efficiency epitaxial structure. The objective is to achieve robust single-mode operation with high output power and slope efficiency, validating this approach as a practical alternative to traditional methods.

An optimized double-asymmetric large-optical-cavity (LOC) epitaxial structure with a single In_0.26GaAs strained quantum well was designed for low internal loss and high efficiency. For longitudinal-mode control, a ninth-order sidewall grating was implemented. The grating parameters—including period (Λ=1.42 μm), duty cycle (50%), etch depth (H=1.25 μm), wider ridge segment (w₁=4.0 μm), and narrower ridge segment (w₂=2.0 μm), resulting in a corrugation D=1.0 μm—were determined using Lumerical Mode FDE simulations. These simulations were employed to analyze effective-refractive-index dependencies, ensure single-lateral-mode conditions (verified by mode profiles, Fig. 4), and achieve a target coupling-strength?length product of κL≈0.75 for a 3000 μm cavity. An asymmetrically placed λ/4 phase shift (at 1/3L from the rear facet) was incorporated to suppress spatial hole burning. Devices were fabricated on MOCVD-grown wafers. Stepper photolithography defined the grating pattern, followed by ICP etching for the ridge and gratings. Standard P- and N-electrode metallization, wafer thinning, annealing, and facet coatings were performed. Chips were mounted P-side down on AlN heat sinks. SEM analysis confirmed the fabricated grating dimensions and etch quality.

Fabry?Pérot (FP) lasers fabricated from the same wafer validated the epitaxial quality, exhibiting a low internal loss α_i=0.85 cm?1, high internal quantum efficiency η_i=0.92 (Fig. 6), and a slope efficiency (SE) of 0.88 W·A^-1 for 3 mm cavities [Fig. 7(a)], with emission around 1.063 μm [Fig. 7(b)]. The ninth-order sidewall-grating DFB lasers demonstrated stable CW single-mode operation. At 20 °C, a maximum single-mode output power of 355 mW was achieved with a corresponding side-mode suppression ratio (SMSR) of 44.3 dB. The threshold current was approximately 0.1 A, and the SE was 0.45 W·A?1 [Fig. 8(a),(c)]. The lasing wavelength tuned with current and temperature, with a temperature coefficient of 0.079 nm ℃?1 [Fig. 8(b),(c)]. Far-field divergence angles were 30.08° (fast axis) and 6.87° (slow axis) at 1 A, indicating stable fundamental lateral-mode operation [Fig. 8(d)]. The lower SE of the DFB laser compared to the FP laser is primarily attributed to scattering losses from the high-order grating. Nevertheless, the results confirm the viability of the cost-effective high-order sidewall-grating approach using stepper lithography.

We have successfully designed and demonstrated high-power, single-mode 1.06 μm DFB semiconductor lasers utilizing a ninth-order sidewall-grating structure fabricated via cost-effective stepper photolithography. The lasers were based on an optimized double-asymmetric LOC single-quantum-well epitaxial structure designed for low internal loss and high efficiency. The fabricated DFB lasers exhibited stable single-mode operation with a maximum single-mode output power of 355 mW and a high SMSR of 44.3 dB. The slope efficiency was 0.45 W·A?1. Verification measurements on FP lasers from the same wafer confirmed the high quality of the epitaxial material, showing a low internal loss of 0.85 cm?1 and high internal quantum efficiency of 0.92. This study validates the high-order sidewall-grating approach, combined with careful epitaxial and device design (including an asymmetric phase shift), as a viable and economically advantageous pathway for producing high-power, single-frequency semiconductor lasers at 1.06 μm. While grating-induced scattering losses currently limit the slope efficiency compared to FP devices, future work focusing on optimizing grating-fabrication techniques to reduce these losses, along with enhanced thermal management, could further improve the output power and overall performance, broadening the applicability of these devices.