Mode instability in kW-class thulium doped fiber amplifiers

Recent advances in thulium doped fibers have significantly improved the optical conversion efficiencies, indicating their potential utility for a variety of applications. Recently, kilowatt-class fiber amplifiers have been demonstrated with narrow linewidth operation. However, their limitations in scaling to higher powers aren't well understood. While thermal mode instability (TMI) is a well-known limitation in ytterbium doped fiber amplifiers, no experimental data exists for CW thulium doped fiber amplifiers. As such, it's difficult to quantify or predict when this limitation is expected to impact TDFA's. Here, we report on the first measurements of TMI in CW thulium doped fiber amplifiers. We characterize the TMI threshold of both 20μm and 25μm fibers by measuring the beam quality, relative intensity noise (RIN), and polarization extinction ratio (PER). In particular, a strong dependence on the TMI threshold with the fiber length is noted. Counter intuitively, measurements show that short fibers have significantly higher TMI thresholds than longer fiber lengths; indeed doubling the fiber length can reduce the TMI threshold by almost 50%. By optimizing the fiber length, a 20/400μm thulium doped fiber amplifier is scaled to 905W, with 57% optical conversion efficiency and a linewidth 5 GHz.