Impact of random facet phases on modal properties of partly gain-coupled distributed-feedback lasers

A first comprehensive study of impact of random phase at cleaved facet on modal properties of partly in-phase gain-coupled distributed-feedback (GC DFB) lasers is carried out both theoretically and experimentally. It is shown that, variation of grating phase of antireflective (AR) facet has a relatively small impact on lasing properties, while that of cleaved facet has a significant influence not only on modal properties such as threshold, efficiency, front-to-back facet efficiency ratio (FB ratio), side-mode-suppression-ratio (SMSR), and transient SMSR (TSMSR), but also on DFB lasers' long term stability against longitudinal mode-switching. Correlation among different laser properties is revealed for GC DFB lasers, and compared with that of index-coupled ones. It is found that FB ratio provides a good description of long-term longitudinal mode stability against CL facet phase variation and equivalent external feedback in GC DFB lasers, while TSMSR reveals their dynamic properties for a fixed facet phase. It is shown that devices with a possible stable operation will exhibit both a large TSMSR and a large FB ratio in GC DFB lasers. Lasers that are located within the transition region between left Bragg mode region and the stably operated right Bragg mode region in phase-space will have either a low TSMSR or a small FB ratio, and, therefore, are susceptible to longitudinal Bragg mode-switching. The combination of TSMSR and FB ratio as a screening criteria provides a simple and yet powerful tool to single out the potentially unstable devices due to random facet phase distribution in AR/CL GC DFB lasers.