Abstract
We used coupled-mode theory in an Yb-doped multimode fiber amplifier to compute the effects of gain saturation, nonlinear index, and fiber curvature on the evolution of the field. A positive nonlinear index results in power transfer to lower-order modes, usually the fundamental LP<sub>10</sub> mode, and for negative nonlinear index the reverse is predicted. The nonlinear interaction between modes breaks the core’s cylindrical symmetry, resulting in recombination of degenerate LP mode pairs into super-modes: consisting of an expected in- and anti-phase pair, but also a quadrature of super-mode that reflects an increase of “information” capacity associated with nonlinearity. Convergence to all three super-modes was observed in our simulations, but the last more often. We also present observed evidence of mode phasing in experiments with two fiber amplifiers.
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