Controlling the optical-feedback-instability of VCSELs through inter-mesa coupling

The rapid advancement of artificial intelligence and cloud computing has led to a strong demand for high-speed vertical-cavity surface-emitting lasers (VCSELs). Omitting the optical isolator in short-range modules based on VCSELs reduces costs but requires higher optical-feedback tolerance. Thus, exploring VCSELs with strong resistance to optical feedback is pivotal for the development of high-speed short-range optical modules. This study proposes coupled-mesa VCSELs (CM-VCSELs) as a robust strategy against external feedback. We investigated the feedback effects in VCSELs experimentally and numerically by systematically varying the inter-VCSEL distance. Compared to VCSELs without lateral coupling, we found that the inter-VCSEL coupling leads to significantly improved stability with reduced linewidth and relative intensity noise. Furthermore, there is an optimum inter-VCSEL distance where the linewidth is the smallest for a given optical-feedback strength. To verify the conventional understanding, we extracted the linewidth enhancement factor (LEF), confirming that the stability improvement stems from the lower LEF. Our study demonstrates that CM-VCSELs offer a large design tolerance window for VCSELs, and that feedback instability can be controlled through inter-mesa distance. Our strategy provides a low-cost manufacturable approach to VCSEL-based short-range high-speed modules.