Self-similarity of time evolution of photodarkening losses induced in Yb-doped fibers and photodarkening figure of merit

In this paper we report and summarize an extensive investigation of photodarkening in Yb-doped silica fibers. We made a set of similar fibers in order to cover a large Yb concentration range. The results show that once uniform inversion is reached all along the test fiber it is possible to show self-similarity of the time evolutions. The stretching parameter has a little influence. We can therefore provide two unique numbers to define photodarkening performance: the saturation loss and the time scale parameter. We also propose a possible figure of merit to compare different fibers. This may allow to evaluate the impact of photodarkening on laser/amplifier devices.