Broad-area edge-emitting diode lasers have been widely applied in laser pumping, laser ranging, material processing, display technology, and medical fields owing to advantages such as high power, high efficiency, high brightness, compact size, and long lifetime. However, the broad transverse waveguide dimensions of these lasers typically result in significant variations in the optical field under high drive currents, which excites multiple transverse modes. Consequently, the “multilobe” phenomenon occurs in the near-field beam profile, thus severely deteriorating the transverse-beam quality. Additionally, it causes increased far-field divergence angles and reduced brightness, thus limiting their applicability in various fields. Therefore, achieving high power while maintaining high beam quality has become a priority.

First, a 100 μm wide mesa is formed via photolithography and inductively coupled plasma (ICP) etching with an etching depth of 850 nm. Subsequently, side grating structures are etched on both sides of the ridge waveguide using photolithography and wet etching, with an etching depth of 150 nm. A 300-nm-thick SiO₂ electrical insulation layer is deposited using plasma-enhanced chemical vapor deposition (PECVD). Subsequently, an 80 μm current injection window is fabricated via ICP etching. Next, a p-side metal contact is deposited, and the wafer substrate is thinned and polished. The n-side metal contact is subsequently deposited, followed by rapid thermal annealing to complete the alloying process. The distance from the etched grating grooves to the mesa is defined as x, and lasers with different values of x (0, 5, 10, 15, and 20 μm) are fabricated on the same wafer for comparison. After wafer fabrication, cleaving is performed using a dicing machine and a cleaver to separate the wafer into bar-shaped devices. The cavity surfaces are left uncoated and the bars are further cut into single-tube chips. The fabricated chips have a cavity length of 2 mm. Finally, the single-tube chips are bonded to the heat sink, with the p-side facing upward, using a chip mounter. The temperature is controlled using a thermoelectric cooler (TEC). The resolution of the spectrometer used for testing is 0.05 nm, and near-field testing of the device is performed using a 10× magnification objective lens.

As shown in Fig. 4, the single-side output power and voltage characteristics of devices with different x under continuous wave operation at 25 ℃ are presented. First, the voltage performances of different devices are almost identical. However, under different x, the single-sided output powers of the devices exhibit significant differences. As the x increases, the output power of the devices increases as well. The laser spectra at a bias current of 2 A are shown in Fig. 5. When x=20 μm, the device has a 3 dB bandwidth of 2.223 nm at 2 A bias current, whereas for x=0 μm, the 3 dB bandwidth is only 1.882 nm, which is 15% lower. Figure 6 shows a comparison of the far-field divergence angles, which is defined by the 95% power, of the lasers at different x. At a current of 1 A, the slow-axis divergence angle is 17.2° when x=0 μm, and it increases to 19.9° when x=20 μm. Figure 7 shows the near-field distributions of lasers with different x at bias currents of 1 A and 2 A. An analysis of these near-field distribution curves reveals that when x=20 μm, the near field exhibits multiple peaks, thus indicating the presence of significant higher-order mode patterns, and the beam waist width is slightly broader. When x=0 μm, the near-field beam waist is slightly narrower and the near-field light field distribution is more concentrated.

We propose a lateral grating structure and demonstrate its effectiveness in controlling the transverse modes and spectrum of broad-area semiconductor lasers, where a reduced spectral bandwidth is achieved. This structure increases the loss of higher-order modes and enables injection-insensitive transverse divergence without significantly reducing the output power. Owing to its low cost and compatibility with the semiconductor laser manufacturing technology, this structure can be applied to the development of high-power broad-area semiconductor lasers with low divergence and high beam quality.