Thermal modeling of high-power Yb-doped fiber lasers with irradiated active fibers

With both radiation effects and thermal effects taken into consideration, a multiphysics thermal model concerning high-power Yb-doped fiber lasers operated with post-irradiated active fibers is established. Radiation-related parameters, including propagation losses, refractive indexes and lifetime, are considered. And, with the temperature profile of the active fiber, temperature-dependent parameters, including absorption and emission cross-sections, refractive indexes and lifetime, are updated every loop to simulate the output parameters. Simulation results show that radiation induces great changes to the thermal profiles of the active fiber. And severe performance degradation of high-power Yb-doped fiber lasers are recorded, featuring a remarkable drop in output power and an even steeper decline in the transverse mode instability threshold, which is a predominant limitation at high radiation doses. With a deposited radiation of 100 Gy, an output decline of about 50% and a mode instability threshold drop over 85% are observed. And it's shown that, with the exploited active fiber, it's hardly possible for the investigated fiber laser to generate stable single-mode output at kilowatt levels with accumulated radiation doses beyond 50 Gy. At low radiation doses within 20 Gy, to maintain safe and stable single-mode operation of the laser system, longer active fibers with lower absorption coefficients are preferred despite a small rollover of the output power.