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Origin and evolution of phase-shifts in high-power fiber laser systems: Detailed insights into TMI

: Jauregui, C.; Stihler, C.; Tünnermann, A.; Limpert, J.


Carter, A.L. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Fiber Lasers XVI: Technology and Systems : 4-7 February 2019, San Francisco, California, United States
Bellingham, WA: SPIE, 2019 (Proceedings of SPIE 10897)
ISBN: 978-1-5106-2437-5
ISBN: 978-1-5106-2436-8
Paper 1089704, 6 S.
Conference "Fiber Lasers - Technology and Systems <16, 2019, San Francisco/Calif.>
Industrial Laser, Laser Source and Laser Applications Conference (LASE) <2019, San Francisco/Calif.>
Photonics West Conference <2019, San Francisco/Calif.>
European Commission EC
617173; ACOPS
Advanced Coherent Ultrafast Laser Pulse Stacking
Fraunhofer IOF ()

In this work, we study the generation and evolution of phase-shifts between the modal interference pattern and the thermally-induced index grating due to pump-power changes. This study is not only important to understand new mitigation strategies based on controlling such phase-shifts, but also to comprehend how pump/signal noise can trigger TMI. Understanding how such a phase-shift can develop from a pump/signal change is not trivial, since the movement of both the MIP and the RIG are thermally driven and, therefore, should have similar time constants. Our simulations show unequivocally that a change of the pump power will lead to the generation of a phase-shift and the physical reason for this behavior is unveiled. The main reason is an increased sensitivity of the MIP to temperature variations because the local beat-length changes of the MIP are accumulated along the whole fiber length. Therefore, the further downstream the fiber a MIP maximum is (i.e. closer to the pump end in a counter-pumped configuration), the stronger and faster its position shift will be. This insight shows a way to obtain more TMI-resilient fiber designs and may help understanding the core area dependence of TMI.