Accurate Modeling of Transverse Mode Instability in Thulium-Doped Optical Fiber Amplifiers

We accurately modelled measured TMI thresholds in Tm-doped fiber amplifiers for the first time and with much improved efficiency, demonstrating the significance of the background loss at ∼2 μm and, consequently, marked differences in behavior compared to Yb-doped fiber amplifiers. We identified that the main contributors to heat load are different from those to TMI in Tm-doped fiber amplifiers. Our study shows that the high TMI thresholds predicted in the earlier study cannot be achieved considering the effect of the background loss, especially at longer wavelengths. These findings provide significant new guidance for the future direction of high-power fiber lasers.