Mode Instability in coiled fiber amplifiers

Results of numerical simulations of transverse mode instability (TMI) in Ytterbium-doped coiled large mode area step index fiber amplifiers are presented and discussed. The TMI thresholds for counter-pumped amplifiers were found to be significantly greater than for co-pumped amplifiers. A greater dependence of the TMI threshold on amplifier length was observed in the counter-pumped case than in the co-pumped case. Furthermore the onset of TMI was found to be localized in a specific critical position along the amplifier in the co-pumped case. In this configuration, amplifiers could be pushed slightly beyond the TMI threshold and still produce stable output due to bend loss filtering of the higher order modes. The TMI thresholds were found to have exhibit a non-monotonic behavior with respect to coiling diameter and only weakly dependent on core launch misalignment. Power stripped from the core into the cladding was in some cases found to interfere with the signal within the core leading to small power fluctuations that appear unrelated to the TMI process. In some co-pumped cases the beam was distorted at the amplifier output only during a small window within the overall simulated time period. This indicates that all of the different components of the broadened frequency spectrum are coherent. At this time, the physical explanation for this effect is unknown.