Modeling Light Propagation and Amplification Efficiency in Highly Multimode, Yb-doped Fiber Amplifiers

Abstract: 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 continuous-wave 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, pump depletion and mode-dependent gain. We consider a frequency domain, field based model, with modal gain being dependent on both intramodal gain and gain-induced mode coupling. We derive coupled equations for the evolution of signal modal amplitudes, pump power and population inversion, and numerically solve these equations using a finite-difference method. For highly multimode excitations, the optical intensity in the fiber is speckled and various modes grow at different rates, due to differential overlap with the gain medium and spatial hole burning. Our analysis is applied to Yb-doped fibers, with a quasi-quantitative analysis of the specific case of Yb, identifying different regimes in which either spontaneous emission (SE) or amplified spontaneous emission (ASE) limit amplifier efficiency, especially for larger core and multimode fibers. Finally, we incorporate ASE and spectrally resolved optical channels into our model and demonstrate the experimentally verifiable phenomenon of ASE suppression with sufficient input signal power. 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.