Overcoming depolarization in TMI-limited polarization-maintaining fiber amplifiers

Recent studies focus on mitigating transverse mode instabilities (TMI) in polarization-maintaining (PM) fiber amplifiers [1]-[3], demonstrating higher TMI-thresholds at linear polarization angles apart from the fibers principle axes. Off-axis operation is feasible, if a stable linear output polarization can be retrieved [4]. So far, the studies [1]-[3] used fibers of short absorption lengths <1.5 m, whilst common kW-power Yb-doped fiber amplifiers typically have absorption lengths around 10 m and longer. In this contribution we investigate how depolarization is hindering the off-axis PM operation of long fiber amplifiers and show how it can be overcome.