Distributed feedback (DFB) lasers incorporating narrow-ridge waveguides and grating structures can effectively achieve fundamental-mode laser emission, which have broad applications in fields such as optical communication and laser ranging. Conventional DFB semiconductor lasers use buried gratings, which increase the complexity and cost of device fabrication. Researchers have employed surface gratings, such as the laterally coupled DFB (LC-DFB) laser, and enhanced laser performance by improving the grating structures and utilizing novel dielectric materials. Although a narrow-ridge waveguide can effectively confine the lateral modes of the laser and facilitate fundamental-mode operation, it results in a relatively small mode area, leading to lower output power. Therefore, narrow-ridge waveguides are often integrated with optical amplifiers to enhance the output power. However, optically integrated lasers involve complex fabrication processes and incur higher costs. Fabricating a broad-ridge waveguide is a simple and effective method for enhancing output power. However, owing to its weaker lateral mode confinement, mode competition between the fundamental and higher-order modes can reduce the output power. The lateral diffusion of carriers provides a higher gain for higher-order modes, increasing the number of lateral modes and reducing the injection efficiency, which is one of the key factors contributing to mode degradation and output power reduction. This paper proposes an LC-DFB semiconductor laser with a surface slit structure (SS-LC-DFB). Introducing the surface slit structure mitigates the accumulation of carriers in the lateral grating regions and enhances the ability of the ridge waveguide to confine higher-order lateral modes. The SS-LC-DFB laser exhibits higher output power than the LC-DFB laser and effectively suppresses the lateral modes.

The optical field distributions of both devices were simulated using the Lumerical MODE solver. With an LC-DFB laser, the fundamental mode typically concentrates its energy at the center of the waveguide, whereas higher-order modes gradually shift away from the waveguide center and extend towards the grating region as the mode order increases [Fig. 2(b)]. After introducing the slit structure [Figs. 2(c) and (d)], the energy distribution of the optical field moves further away from the center of the waveguide. As the slit widthincreases, the optical field intensity near the sides of the grating decreases, and the coupling feedback becomes insufficient, hindering higher-order modes in reaching lasing conditions. This indicates that the introduction of the slit structure improves the lateral mode discrimination of the laser. The carrier distributions and concentrations in both the LC-DFB and SS-LC-DFB lasers were simulated using the Pics3D simulation software. With the LC-DFB laser, after the current injection, significant carrier diffusion occurs in the lateral grating region [Fig. 3(a)]. In this area, the intensity of higher-order modes is relatively strong, and their coupling with carriers can lead to the lasing of higher-order modes. In contrast, with the SS-LC-DFB laser, the carrier diffusion is suppressed by the slit structure, resulting in the reduction of carriers flowing towards the side of the grating and flowing downwards. Consequently, more carriers are concentrated beneath the ridge waveguide [Fig. 3(b)]. From the perspective of the carrier concentration distribution in the active region, the carrier concentration in the ridge waveguide region of the SS-LC-DFB laser is higher than that of the LC-DFB laser [Fig. 3(c)]. This demonstrates improved carrier injection efficiency and enhanced output power of the device.

The fabricated SS-LC-DFB laser improves the lateral mode characteristics and enhances the output power. As the injection current increases from 0.16 A to 0.8 A, the far-field optical spot profile of the SS-LC-DFB laser maintains a well-defined near-single-lobe shape (Fig. 7). By contrast, as the current increases, the mode confinement capability of the LC-DFB laser weakens, resulting in the appearance of multiple modes, indicating that the slit structure effectively suppresses the lateral modes. Figure 6 shows that the SS-LC-DFB laser exhibits superior performance over the LC-DFB laser in terms of lasing spectral mode characteristics. Figure 5 shows the continuous wave power-current-voltage (P-I-V) characteristics of the SS-LC-DFB laser at 25 ℃. At an injection current of 0.8 A, the output power of the SS-LC-DFB laser reaches 335.27 mW, which represents an increase of approximately 18.3% compared to that of the LC-DFB laser (283.01 mW). This improvement is attributed to the introduction of the slit structure, which reduces lateral carrier leakage and provides sufficient gain for the laser.

An LC-DFB semiconductor laser featuring a surface slit structure is fabricated. The experimental results demonstrate that the slit structure effectively confines higher-order lateral modes, improves the modal characteristics of the device, and reduces the multilobe phenomenon in the far-field optical spot while simultaneously increasing the output power. As the injection current increases, the far-field optical spot distribution of the LC-DFB laser exhibits multiple side lobes, indicating an inability to suppress higher-order lateral modes. In contrast, the far-field optical spot distribution of the SS-LC-DFB laser remains close to a single-lobe output. At 0.8 A, the output power of the SS-LC-DFB laser reaches 335.27 mW, representing an approximate 18.3% improvement compared to that of the LC-DFB laser.