Modeling light propagation and amplification efficiency in highly multimode, Yb-doped fiber amplifiers.

Multimode fibers have been proposed for mitigating nonlinear effects in high-power fiber amplifiers, allowing for significant power scaling. Most previous studies on light propagation in fiber amplifiers focus on single-mode or few-mode fibers. Here we develop a tractable numerical model to simulate light propagation in narrowband, highly multimode fiber amplifiers, which takes into account gain saturation and spatial hole-burning, pump depletion, and mode-dependent gain. We consider a frequency domain, field-based model which incorporates both gain-induced mode coupling and amplified spontaneous emission (ASE) from first principles. Our analysis is applied to Yb-doped fibers, with a semi-quantitative analysis identifying regimes in which either spontaneous emission or ASE limits amplifier efficiency. Our model can be combined with existing models for various nonlinear effects, providing a useful tool for quantitatively studying nonlinearity mitigation and power scaling in multimode fiber amplifiers.