Abstract

For high power fiber amplifiers with excellent beam quality double clad fibers are an essential tool. They consist of a signal core section, which is doped with a laser active medium and typically centered, and a much larger pump core. The signal which is to be amplified is coupled into the signal core and has typically an excellent beam quality already, while the pump core is the guiding structure for the high power, low brightness pump light which is to be absorbed by the signal core. The theoretical maximum absorption therefore correlates with the core doping concentration and the ratio of signal to pump core area. However due to the pump core being a highly multimode waveguide and all the pump modes having different overlap integrals with the absorbing signal core, the real pump absorption can drastically differ from the theoretical maximum. To ensure optimal absorption, extensive research was already done in the field of cladding pump absorption. The outcome of works such as [1, 2] relies on breaking the symmetry in these fibers by either twisting or coiling them and by utilizing differently shaped pump guiding structures to enhance the pump absorption capabilities.

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