Transverse mode instability suppression through partial fiber bending

The transverse mode instability (TMI) is a nonlinear phenomenon that leads to coupling between the fundamental and higher-order modes (HOMs) in high-power fiber amplifiers when the pump power exceeds a threshold. This mode instability diminishes beam quality. Bending of the gain fiber has been shown to be an effective technique for suppressing TMI. We use the phase-matched model to simulate TMI within bent fibers using a co-propagating pump. We separately include the contributions of leakage loss and the mode deformation due to bending. Our results indicate that while both effects contribute to the suppression process, the bending loss plays a more important role. We also find that it is advantageous to only bend the initial part of the fiber near the input, which maximizes suppression of the HOMs while minimizing loss of the fundamental mode. We then numerically optimize the bend radius and the length of the bent portion of the fiber to maximize TMI suppression. Using the counter-propagating pump, the TMI threshold is less sensitive to the location of the bend portion of the gain fiber, as compared with the co-propagating pump.