Multimode Fibers for Data Centers

Abstract

Data centers (DCs) have evolved rapidly to deliver higher data rates, higher density, and longer distances while staying as economical as possible. Multimode fiber (MMF) operated at 850 nm is the leading optical medium now used in DCs for distances up to 100–150 m, enabling utilization of verticalcavity surface-emitting lasers (VCSELs) to provide low-cost optical connectivity compared to single-mode fiber solutions. However recent trends in DC drive the MMF-based systems toward several new fronts, including wavelength division multiplexing involving longer wavelengths (BiDi and SWDM), extended reach through engineered links, and variations in the core dimensions or operating wavelength of the MMF. In this chapter, the role of MMFs in DCs will be reviewed, beginning with a discussion of the fundamental aspects of light propagation, modal bandwidth and other fiber characteristics, and link models. Various approaches to address transmission limitations due to chromatic and modal dispersion are then summarized. One such approach is to operate the MMF at longer wavelengths to take advantage of the lower chromatic dispersion. The concept of a “universal” fiber that bridges the gap between the multimode and single-mode transmission is then introduced. Recent trends in DC are also reviewed, and one clear conclusion is that the role of MMF in DC is still evolving to meet the increased needs for scalability, density, data rate, and economic requirements.