Impact of Macrobend Loss on the Bandwidth of Standard and Bend-Optimized Multimode Fibers

Abstract

10 Gigabit Ethernet (GbE) demands faster optical sources to support high modulation rates. At the same time, the allowable margin in the 10 GbE link power budget is decreasing. This means that a 10 GbE system is unable to support as many tight bends, and it is more difficult to avoid the costly downtime that results when the allowable margin is exceeded. The recent introduction of bend-optimized (BO) multimode fiber (MMF) provides a clear solution. 850 nm vertical cavity surface emitting lasers (VCSELs) and MMFs have long been the most cost effective choice for short reach premise applications. As will be shown, the combination of BO-MMF with VCSELs is even more attractive.Historically, MMF systems operating at low bit rates of 10-100 Mbps used light-emitting diodes (LED) sources, which launch nearly equal power into every fibermode. This launch is approximated by the overfilled launch (OFL), which is still used to characterize the core diameter and numerical aperture of MMF. Unlike LEDs, VCSELs typically underfill the fiber core and are better represented by an encircled flux launch (EFL). Using OFL to evaluate a VCSEL-based MMF system could therefore produce inaccurate and misleading results. A recent study [1] characterized the macrobend loss of MMF with overfilled and restricted mode offset launch conditions. In this study, the MMFs performance with an EFL is evaluated, which is a more relevant launch condition for laser transmission. The impact of both launch conditions, OFL and EFL, on MMF performance is studied and compared.We characterize macrobend losses at small bend radii and their impact on thebandwidth for both standard 50/125 um MMF and a newly introduced BO-MMF.In addition, the 10 GbE link performance is also evaluated using the IEEE link model P802.3ae3.The simulation results illustrate that both macrobend loss and bandwidth are vital to the overall optical link performance. The 10 GbE link performance of the standard fiber deteriorates with macrobends, while the bend-optimized fiber is insensitive to the deployment conditions.