High-power antimonide semiconductor lasers operating in the mid-infrared spectrum hold great potential for a wide range of applications, including industrial gas detection, material processing, and free-space communication. Although conventional broad-area (BA) lasers benefit from mature manufacturing processes and can deliver high output power, issues such as thermal effects and lateral carrier accumulation lead to increased lateral divergence at high current levels, degrading beam quality. This significantly limits their direct use in high-precision optical systems that demand high energy efficiency. Consequently, achieving narrow beam output while maintaining high power has become an urgent challenge to overcome.

An advanced sawtooth waveguide (ASW) structure has been proposed, which significantly increases the loss differences between higher-order modes and the fundamental mode. Compared to conventional BA lasers, ASW lasers not only achieve comparable output power under the same current but also significantly reduce the lateral divergence angle, from 19.61° to 11.39°, marking a 42% improvement. Additionally, the dependence of the divergence angle on current has been substantially improved, demonstrating stable and effective mode control capacity. The results were published in High Power Laser Science and Engineering, vol. 12, Issue 4 (Jianmei Shi, Chengao Yang, Yihang Chen, et al. Precise mode control of mid-infrared high-power laser diodes using on-chip advanced sawtooth waveguide designs[J]. High Power Laser Science and Engineering, 2024, 12(4): 04000e42).

Figure 1 shows the schematic diagram of the ASW lasers. Based on the near-field distribution characteristics of the BA waveguide, a series of sawtooth structures are placed along the cavity length to achieve precise mode control of higher-order modes. This design also improves carrier accumulation on both sides of the waveguide. Two-dimensional finite-difference time-domain (FDTD) simulations verify the suppression of higher-order lateral modes by the designed structure. As shown in Fig 2, the calculated optical field distribution of lateral modes through the ASW structure demonstrates that the scattering loss of higher-order modes on both sides is significantly increased, while the fundamental mode is minimally affected. The optical field distribution at the cavity facet for each mode is shown in Fig 3, with the microstructure size ratio set to 0.5. At this ratio, the remaining energy of the fundamental mode is about 1.4 times that of the higher-order modes, indicating that the intensity of the higher-order modes is significantly reduced near the waveguide edges while most of the fundamental mode is preserved. This will also result in a better alignment between the carrier profile and the mode profile. As for experiments, both types of devices have the same output aperture, and the ASW device achieves an output power of 1.1 W, comparable to the BA laser. The lateral far-field distribution and divergence angles of ASW and BA lasers as a function of injection current are shown in Fig 4. Compared to the multi-lobed patterns of BA lasers, the far-field distributions of ASW lasers are much more focused. Within the entire dynamic current range, ASW devices demonstrate not only a smaller lateral divergence but also reduced dependence of divergence on current, with the slope decreasing from 2.19°/A to 1.35°/A, underscoring stable and effective mode control capacity.

Fig 1. Schematic diagram of the ASW structure.

Fig 2. The optical field distributions of lateral modes with order numbers m=0, 1, 2, 3, 6 and 8 through the ASW lasers, respectively.

Fig 3. (a) Energy retained after transmitting different lateral modes through ASW structure with microstructure size ratio of 0.4, 0.5, and 0.6. (b)-(f) Simulated optical field distribution of modes with order numbers m=0, 1, 3, 6, and 8 at the facet of conventional BA lasers and ASW lasers with microstructure size ratio of 0.5.

Fig 4. The lateral far field profiles at (a) 1 A and (b) 4.5 A of conventional BA lasers and ASW lasers, respectively. (c) The lateral far field angles depending on the injection current at 288 K of conventional BA lasers and ASW lasers.

This research proposes a novel waveguide design for antimonide semiconductor lasers that selectively introduces mode losses to improve lateral far-field performance. This design enables higher power and narrow, stable beam output without adding fabrication complexity or costs, meeting the demands of high-precision optical applications. This novel design is compatible with other semiconductor materials working in different wavelength ranges, and is one of the ideal technical routes to achieve the practical application of highly integrated and high-brightness laser technology.