Frequency resolved transverse mode instability in rod fiber amplifiers

Frequency dynamics of transverse mode instabilities (TMIs) are investigated by testing three 285/100 rod fibers in a single-pass amplifier setup reaching up to ~200W of extracted output power without beam instabilities. The pump power is increased well above the TMI threshold to uncover output dynamics, and allowing a simple method for determining TMI threshold based on standard deviation. The TMI frequency component is seen to appear on top of system noise that may trigger the onset. A decay of TMI threshold with test number is identified, but the threshold is fully recovered between testing to the level of the pristine fiber by thermal annealing the fiber output end to 300°C for 2 h.