Low-threshold mode instability in few-mode Yb3+-doped fiber amplifiers: an overview of recent results

We present an overview of theoretical analysis and experimental observation of the spatio-temporal transverse mode instability in polarization-maintaining high-gain fiber amplifiers with a high concentration of Yb3+ ions and core diameters of 8-10 μm. The nonlinear power transformation of the LP01 fundamental mode into the high-order modes is explained by a traveling refractive index grating accompanying a population grating induced by the mode interference field. The origin of the refractive index gratings is discussed. The threshold power for the transverse mode instability is found to depend on the fiber core diameter, numerical aperture, dopant concentration and the signal bandwidth. The threshold power is observed to decrease dramatically in the presence of a backward reflection of a signal from the output fiber facet.