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Single mode low-NA step index Yb-doped fiber design for output powers beyond 4kW

: Beier, F.; Proske, F.; Hupel, C.; Kuhn, S.; Hein, S.; Sattler, B.; Nold, J.; Haarlammert, N.; Schreiber, T.; Eberhardt, R.; Tünnermann, A.


Robin, C.A. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Fiber Lasers XIV. Technology and Systems : 30 January - 2 February 2017, San Francisco, California, United States
Bellingham, WA: SPIE, 2017 (Proceedings of SPIE 10083)
ISBN: 978-1-5106-0607-4
ISBN: 978-1-5106-0608-1
Paper 100830Z
Conference "Fiber Lasers - Technology and Systems" <14, 2017, San Francisco/Calif.>
Fraunhofer IOF ()

Fiber amplifiers are representing one of the most promising solid state laser concepts, due to the compact setup size, a simple thermal management and furthermore excellent beam quality. In this contribution, we report on the latest results from a low-NA, large mode area single mode fiber with a single mode output power beyond 4 kW without any indication of mode instabilities or nonlinear effects and high slope efficiency. Furthermore, we quantify the influence of the bending diameter of our manufactured low NA fiber on the average core loss by an OFDR measurement and determine the optimal bending diameter in comparison to a second fiber with a slightly changed NA. The fibers used in the experiments were fabricated by MCVD technology combined with the solution doping technique. The investigation indicates the limitation of the step index fiber design and its influence on the use in high power fiber amplifiers. We demonstrate, that even a slightly change in the core NA crucially influences the minimum bending diameter of the fiber and has to be taken into account in applications. The measured output power represents to the best of our knowledge the highest single mode output power of an amplifier fiber ever reported on.