Narrow linewidth apodized coupling grating distributed feedback lasers enabled by enhanced uniformity of photon density distribution

Apodized coupling gratings distributed feedback (AC-DFB) lasers have demonstrated their ability to achieve narrow linewidth. However, the detailed impacts of facet coatings and the structural parameters of apodized coupling gratings on the longitudinal spatial hole burning (LSHB) effect, along with comprehensive evaluations of how the LSHB influences linewidth and in-depth optimizations in AC-DFB lasers, have yet to be thoroughly investigated. In this study, we present the physical models for AC-DFB lasers and utilize numerical simulations to investigate the influences of facet coatings and the structural parameters of apodized grating on the LSHB effect. The promoted uniformity of photon density distribution of AC-DFB lasers is achieved through the rational design of the apodized grating coupling coefficient distribution and length, which effectively modulates the optical field of the lasers. The further thorough analysis of AC-DFB laser linewidth, considering the impact of the LSHB effect, demonstrates that optimization can reduce linewidth broadening caused by the LSHB by 80.9%, narrowing the linewidth to 43.4 kHz at 120 mA with HR-AR facet coating. This study provides valuable insights into the precise optimization of AC-DFB laser, contributing to improved narrow linewidth performance and supporting their application in high-speed optical communication systems.