NA-dependent absorption in double clad multicore amplifier fibers

We investigate cladding pump light absorption in double-clad multicore amplifier fibers in dependence of the pump numerical aperture (NA). Results indicate that for the investigated case the assumptions for Beers law break down and need to be revised to include an NA dependent absorption. In this work we present a method for NA resolved absorption measurements and focus on the absorption behavior of a double clad multicore fibers, where experimental results show that the cladding absorption is changing drastically with the NA of the pump light. Additionally pump modes are investigated numerically to assist these findings. These results have not only implications for amplifier performance but impact characterization of cladding pump light absorption for such fibers as well. Lastly it is also expected that such behavior can be mitigated or exploited by fiber design.