Investigation of transverse mode instability threshold on the modal content of the seed light in fiber amplifiers

We experimentally investigate the impact of the modal content of the seed beam on the transverse mode instability (TMI) threshold in high-power fiber amplifiers. Theoretical models have predicted that the TMI threshold should decrease with more higher-order mode (HOM) content in the seed. However no systematic experiment has been done so far to probe such a statement. In this work we have built a system comprising a spatial mode multiplexer that allows manipulating the power content of the fundamental mode (FM) and the HOM in the seed beam. Such beams, with the same power but different modal contents, were coupled into a multimode active rod-type fiber and the evolution of the TMI threshold was studied.