Pump RIN reduction of the TMI threshold powers in Yb-doped kW class fiber amplifiers

A 3D spatially resolved coupled mode and perturbation analysis for the transverse mode instability threshold powers in Yb-doped fiber amplifiers is extended to include threshold power reduction induced by the pump laser relative intensity noise (RIN). A static intensity grating combines with the fluctuating pump intensity to generate an inversion standing wave. The forward component of the standing wave contributes to the nonlinear coupling between the fundamental mode and the higher-order mode (HOM), thereby reducing the threshold power. It is estimated that a pump RIN level <-150dB/Hz is required to achieve quantum limited performance. The model also quantifies the impact of unwanted laser power in the HOM on the threshold power reduction. The threshold power calculations as a function of RIN and laser power in the HOM are in qualitative agreement with recently published experimental measurements.