Polarization instability in high power fiber amplifiers

We study a phenomenon related to TMI in PM fiber amplifiers: polarization instability. Hereby the polarization of the Fundamental Mode (FM) and of the Higher Order Mode (HOM) in a PM fiber fluctuate during TMI, but the HOM shows significantly stronger polarization changes than the FM. In this work, we will explain the reasons for this behavior and also show that these instabilities, or at least the increase of the sensitivity of the polarization of the HOM to external perturbations, occur even below the TMI threshold.