Concentric layers with heterogeneous doping for cladding-pumped L-band fiber amplifiers

Abstract

. Cladding pumping in optical amplifiers has recently seen rising interest in the context of multicore amplifiers. The use of multicore erbium-doped fibers (EDFs) can decrease the complexity of amplifier subsystems since it allows pumping with a single high-power multimode laser diode for multiple parallel amplification channels in the same fiber cladding. Due to the lower pump intensity in cladding-pumped amplifiers, high population inversion is harder to achieve compared to core-pumped amplifiers, which favors L-Band applications over C-Band applications. The various designs of cladding-pumped fiber amplifiers that have been proposed to amplify the L-Band tend to use either homogeneous erbium doping or homogeneous erbium/ytterbium co-doping in the core. In this paper, we demonstrate that a carefully engineered combination of erbium (Er) doped layers and erbium-ytterbium (Er/Yb) co-doped layers in a single core (concentric layers with heterogeneous doping) can improve the power conversion efficiency (PCE) compared to homogeneous doping designs The main idea behind this investigation is to use this additional degree of freedom in the design to better control the erbium ion population inversion along the fiber, which helps to minimize the amplified spontaneous emission and thus increases the PCE. Through numerical simulations, we compare the PCE and noise figure (NF) of four types of homogeneous doping geometries in the core (annular Er, annular Er/Yb, central Er and central Er/Yb) with a design based on concentric layers with heterogeneous doping. For all the scenarios, the doping geometry and fiber length are carefully optimized to maximize the PCE. The input signal power, input pump power, pump propagating direction, refractive index profile and other not-doping-related parameters are kept constant for all the scenarios.