Control of modal energy exchange in transverse mode instability with travelling waves

In this work we experimentally demonstrate an innovative approach to control the direction of the modal energy exchange during transverse mode instability (TMI) in high-power fiber amplifiers. Our technique involves seeding the amplifier with a traveling wave created by exciting two modes with slightly different frequencies. The resulting modal interference pattern moves along the amplifying fiber in either direction (depending on the sign of the frequency difference) at a speed determined by the absolute value of the frequency difference. By carefully selecting the traveling speed, it is possible to generate a constant phase shift between the modal interference pattern (travelling wave) and the thermally-induced index grating. This situation enables consistent energy transfer from higher-order modes to the fundamental mode (or vice versa). Additionally, the travelling wave can be forced to move so fast that it can even wash out the inversion profile, thus preventing any modal energy transfer and mitigating TMI. Both working regimes can effectively increase the TMI threshold of the high-power fiber amplifiers.