Modal Bandwidth and Single-Mode VCSEL Transmission Capability over Multimode Fibers

Abstract

Single-mode VCSELs have been demonstrated with great transmission capability over multimode fibers in the literature through restricted launch, which is beyond what is allowed by the effective modal bandwidth (EMB) defined for multimode transmission with the encircled flux launch condition. However, the impact of fiber index profile errors and their interactions with launch conditions have not been thoroughly studied. We conduct a detailed experimental study to gain insight on the impacts of the fiber index profiles and launch conditions on the system performance of single-mode VCSELs transmitting over multimode fibers. Experimentally, our results show that a launch spot roughly matching the fiber LP01 mode field leads to very high bandwidth and robust performance, largely independent of the imperfection of the index profiles and therefore can perform well in very high data rates. For a restricted launch without mode matching, the performance highly depends on the quality of the fiber, in line with EMB. We found that the center offset tolerance is around $2~\mu \text{m}$ for a mode matched launch. We also conducted single-mode VCSEL transmission experiments over 600-m and 1000-m multimode fibers, which agree well with the modal bandwidth results.