Recent developments in the understanding and passive mitigation of transverse mode instability

• Study of the TMI threshold as a function of the modal content. • Study of the TMI threshold as a function of the core size. • Highest extracted average power from a rod-type fiber. • Mitigation of TMI by polarization rotation in PM fibers. • Quasi-static modal energy transfer in PM fiber. In this article we look at the newest developments in the understanding and mitigation of TMI in single-core fibers. This includes recent quantitative measurements that reveal the dependence of the TMI threshold on the modal content of the seed, systematic measurements on the dependence of the TMI threshold on the fiber core size, as well as the study of TMI in PM fibers including a novel passive mitigation strategy and the static modal energy transfer recently observed in these fibers.