Mitigation of transverse mode instability by polarization rotation in high-power PM fiber amplifiers

We investigate transverse mode instability (TMI) in an in-house large-mode-area polarization maintaining (PM) fiber amplifier. The TMI threshold was systematically measured at different linear polarization input angles with respect to the slow axis of the fiber. At a polarization input angle of 50°, the TMI threshold increased by more than 100% with respect to the threshold of the slow axis and 60% with respect to one of the fast axis. Furthermore, the temporal characteristic of TMI were studied in detail at different polarization input angles but fixed power of 290W, which was above the TMI threshold of the slow and fast axis. This analysis revealed the three different temporal regimes associated to TMI: chaotic fluctuations in the slow-axis, stable at 50°, and periodic fluctuations in the fast axis. These new results provide with valuable insights into the effect of TMI, especially concerning PM fibers, as well as with a relatively simple way of mitigating TMI in these fibers.