Photodarkening-induced increase of temperature in ytterbium-doped fibers

We study the temperature of an Yb-doped large-mode-area (LMA) fiber during an accelerated photodarkening experiment. In these measurements, photodarkening is optically induced by IR irradiation (i.e. 915 nm) while the fiber temperature is measured by a thermal camera. Fiber temperature is observed to exceed 120 °C under conditions of 10.5 W of pump power and unforced air cooling. We show evidences that this temperature increase is caused by the lost pump power due to photodarkening. A thermal model is used to explain the fiber temperature in terms of pump power absorbed by photodarkening-induced defects. Furthermore, the effect of temperature on the rate of photodarkening and saturation of the losses is studied. Both the photodarkening saturation level and the photodarkening rate are observed to show significant temperature dependence that result on a variation of the photodarkening rate ion dependency. The use of an air cooling system and low inversion measurements is shown to reduce the ion dependency from 7 to 4.5.