Material-dependent TMI and SBS threshold power simulations of Yb-doped fiber amplifiers

The influence of engineered glass compositions for the enhancement of the TMI and SBS threshold powers in kW class Yb-doped fiber amplifiers is explored with validated simulations. A reduction in the thermo-optic coefficient and an increase in the thermal conductivity raises the TMI threshold power while the SBS threshold power is increased by a reduction in the Brillouin gain. A quasi-analytic model is extended to include the local bend loss of thermally loaded coiled gain fibers and is validated with well-documented experimental results. The range of material coefficients is determined by recent material investigations. It is found that a 55% reduction in the thermo-optic coefficient results in a 75% increase in the TMI threshold power and a 43% increase in the thermal conductivity results in a 30% increase in the threshold power. A 75% reduction in the Brillouin gain is found to increase the SBS threshold by 200%. Simulated threshold powers are in qualitative agreement with recent experimental results. These simulations provide a quantitative measure of the threshold power improvements that can be achieved with engineered glass compositions.