Low-threshold mode instability in Yb3+-doped few-mode fiber amplifiers: influence of a backward reflection

Spatio-temporal instability of the fundamental mode in Yb3+-doped few-mode PM fiber amplifiers with a core diameter of 8-10 μm was studied experimentally and theoretically. The mode instability threshold was registered at 1-100 Watts pump power. The threshold was found to decrease dramatically in the presence of a backward reflection of the signal from the output fiber end or an external counter-propagating beam; an increase of the signal bandwidth or input power resulted in the increase of the threshold. Numerical simulation revealed a self-consistent growth of the higher-order mode and a traveling electronic index grating accompanying the population grating induced by the mode interference field (due to different polarizability of the excited and unexcited Yb3+ ions).

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