Comparison of transverse mode instability optical fiber models

As a first-of-its-kind study, three optical fiber laser amplifier models of the transverse mode instability (TMI) nonlinearity are directly compared to one another in order to elucidate the impacts of the assumptions and approximations that lead to the reduced models. These three models can be, respectively, considered to be of a high-, mid-, and low-fidelity in regard to their ability to capture the physics behind the TMI. Ultimately, this effort demonstrates that even though the mid- and lowest-fidelity models do predict the correct direction of the relative changes in the nonlinear threshold as compared to a base fiber design and configuration, they are not matching the higher-fidelity model in the amount of relative change to the threshold. Although not the primary thrust of this article, a unique and highly efficient TMI model is presented, and an important insight is delineated as to how the TMI nonlinearity is related to the fact that the fiber amplifier becomes a mode phase modulator during high-power operation.