Optical Amplifiers for Mode Division Multiplexing

Abstract

Space division multiplexing (SDM) has attracted considerable attention from the optical fiber communication community as a promising means to increase the transmission capacity per fiber and, more importantly, to reduce the associated cost per transmitted information bit by utilizing multiple spatial channels within a single strand of glass. Various SDM transmission fibers, for example, few-mode fibers, multicore fibers, and few-mode multicore fibers, have been proposed, and the possibility to support capacities beyond that of conventional single-mode fiber technology has now been proven. However, in order to realize the potential energy and cost savings offered by SDM systems, the individual spatial channels should be simultaneously multiplexed, transmitted, amplified, and switched with associated SDM components and subsystems. In particular, SDM amplifiers can simultaneously amplify multiple spatial channels in a single device and can provide significant space, cost, and energy savings from component sharing and device integration. In this chapter, we will review the current state of the art in optical amplifiers for the various SDM approaches under investigation – with particular focus on few-mode fiber amplifiers for mode division multiplexing. In these amplifiers, differential modal gain (or mode-dependent gain) is the most important property, and various mitigation strategies are introduced/highlighted in this chapter. Afterwards, we will focus in particular on both core-pumped and cladding-pumped 6-mode erbium-doped fiber amplifiers as practical implementation examples and will further discuss differential modal gain control for both pumping configurations. Finally, the remaining challenges to realizing practical few-mode fiber amplifiers are discussed, and future prospects for the few mode fiber amplifier are envisioned.