Inversion behavior in core- and cladding-pumped Yb-doped fiber photodarkening measurements

Yb-doped fibers are widely used in applications requiring high average output powers and high power pulse amplification. Photodarkening of the Yb-doped silicate glass core potentially limits the lifetime or efficiency of such fiber devices. In many studies of photodarkening, two principal methods of controllably inducing an inversion are used, namely, cladding pumping and core pumping of the sample. We present simulation results describing the key differences in the inversion profiles of samples of different physical parameters in these two cases, and we discuss the problems and possibilities that arise in benchmarking fibers of various physical parameters. Based on the simulation and experimental work, we propose guidelines for photodarkening benchmarking measurements and show examples of measurements made within and outside of the guidelines.