Analysis of Erbium-Doped Ultralarge-Core Segmented-Cladding Fibers for Optical Amplification

We analyze numerically an erbium-doped segmented-cladding fiber (SCF) for optical amplification. The fiber has a uniform high-index core and a leaky cladding consisting of alternating high and low refractive-index angular segments of equal size, where erbium is doped in the core and the high-index cladding segments. By choosing the fiber parameters properly, the fiber can offer effective single-mode operation at both the signal (1530 nm) and pump (980 nm) wavelengths with an ultralarge core. We calculate the mode-field distributions in the fiber at these two wavelengths with a full-vector finite-element method. We obtain the gain characteristics and the pump threshold of the fiber by solving the propagation rate equations. With a typical erbium-ion concentration of 0.7 times 10²⁵ ions/m³ , an input pump power of 280 mW, and an input signal power of 0.1 muW, the maximum gain achievable varies from ~ 14 dB to ~ 37 dB with an optimal fiber length of several meters, when the core diameter varies from 40 mum to 28 mum. The threshold pump power required is of the order of 100 mW at 0.1-muW input signal power, and the saturation output signal power is of the order of 100 mW at 280-mW input pump power. The noise figure is ~ 4 dB. Our analysis suggests that, by using a high enough pump power, a few meters of erbium-doped ultralarge-core SCF can provide a small-signal gain comparable with that of a conventional erbium-doped fiber.