Narrow linewidth, fundamental transverse mode semiconductor laser devices are ideal optical sources for laser communication applications. To achieve a narrow linewidth and fundamental transverse mode output from semiconductor lasers, this study introduces lateral coupled gratings and lateral loss structures on both sides of the ridge waveguide of a broad-area semiconductor laser. These modifications aim to narrow the linewidth and suppress higher-order transverse modes. The device features etched loss structure areas on both sides of the 50 μm wide ridge waveguide, with a length of 150 μm and a lateral etching depth of 10 μm. The length of the lateral coupled grating is 700 μm, with a period of 6.3 μm and a duty cycle of 0.4. Experimental results indicate that at an injection current of 0.6 A, the output beam of the laser device maintains a “single-lobe” state, with a divergence angle of 5.92° and a spectral linewidth of 46 pm and the wavelength is 1 058.7 nm. Compared to the wide ridge waveguide laser (WR-LD), these values represent reductions of 16.5% and 59.6%, respectively. The laser device achieves an output power of 259.19 mW and a slope efficiency of 0.52 W/A, which corresponds to increases of 25.6% and 33.3% compared to the WR-LD.Based on the finite-difference time-domain (FDTD) method, this study utilizes higher-order lateral coupled gratings to narrow the linewidth and analyzes the relationship between limiting factors and etching depth. The lateral loss structure enhances the gain difference between the fundamental mode and higher-order transverse modes. By employing a loss clipping mechanism, it raises the gain threshold of the higher-order transverse modes, enabling fundamental transverse mode output at high current levels.The threshold current of the LM-LC-DFB with the added lateral loss structure is approximately 120 mA, while the threshold current of the WR-LD is 90 mA. At a drive current of 0.3 A, the LM-LC-DFB achieves an output power of 85.17 mW. As the drive current increases further to 0.6 A, the LM-LC-DFB obtains an output power of 259.19 mW and a slope efficiency of 0.52 W/A. Although the threshold current has increased, the output power and slope efficiency of the LM-LC-DFB are enhanced by 25.6% and 33.3%, respectively, compared to the WR-LD, which has output power and slope efficiency of 206.3 mW and 0.39 W/A, respectively. At a drive current of 0.5 A, the device with lateral coupled gratings has a center wavelength of 1058 nm, exhibiting stable single-peaked output with a 3 dB spectral linewidth of 46 pm. In contrast, the output spectrum of the WR-LD device displays two small peaks at the peak position, with its 3 dB spectral linewidth increasing to 114 pm, resulting in a linewidth narrowing of approximately 59.6%.We have analyzed and developed a high-order laterally coupled grating DFB semiconductor laser with a loss structure. By simultaneously fabricating the loss structure and the laterally coupled grating structure on both sides of the wide ridge waveguide, we effectively achieved suppression of high-order transverse modes and narrowing of the spectral linewidth. At an injection current of 0.6 A, the LM-LC-DFB device can maintain stable near-fundamental mode output while achieving an output power of 259.19 mW. Compared to the WR-LD device, the output power is enhanced by 25.6% while improving the transverse mode characteristics. The overall design of the LM-LC-DFB laser structure is straightforward, and the process for high-order gratings is relatively easy, with no stringent requirements for the epitaxial structure.