Narrowing of spectral linewidth in epitaxial quantum dot lasers on silicon: Comparison of short- and long-cavity feedback conditions

Narrow linewidth quantum dot (QD) lasers are in demand for practical applications that require a high degree of temporal coherence. Toward achieving narrow linewidth laser, this work numerically investigates the impact of external optical feedback on the frequency noise (FN) and the intrinsic spectral linewidth characteristics of QD lasers epitaxially grown on silicon (Si) substrate. Modified Lang-Kobayashi rate equations have been used to calculate the effect of feedback ratio, the feedback phase, and the non-radiative recombination lifetime in both short- and long-cavity regimes. We found that although in the short-cavity regime, the spectral linewidth can be either narrowed or broadened depending on the feedback phase. However, in the long-cavity regime, effective linewidth reduction is available for sufficiently high feedback ratios (leading to C > 1) for any feedback phase. The linewidth is reduced from 630 kHz in the free-running case to around 8 kHz, when the feedback ratio is increased from zero to −10 dB, this is equal to a reduction of 19 dB in the linewidth. These findings provide a simple and cost-effective way for producing narrow linewidth Si-based QD lasers appropriate for advancing coherent optical technology on silicon.