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Mitigation of mode instabilities in high-power fiber laser systems by active modulation of the pump power

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


Institute of Electrical and Electronics Engineers -IEEE-:
Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference, CLEO/Europe-EQEC 2017 : 25-29 June 2017, Munich, Germany
Piscataway, NJ: IEEE, 2017
ISBN: 978-1-5090-6736-7
ISBN: 978-1-5090-6737-4
Conference on Lasers and Electro-Optics Europe (CLEO) <2017, Munich>
European Quantum Electronics Conference (EQEC) <2017, Munich>
Conference Paper
Fraunhofer IOF ()

Summary form only given. In recent years the evolution of the average output power emitted by high-power fiber-lasers and amplifiers has reached a level at which the onset of thermal effects has been observed. The most detrimental of these effects is the phenomenon of transverse mode instabilities (TMI) [1]. These instabilities are characterized by a sudden onset of spatial and temporal fluctuations in a formerly stable beam once a certain average output power threshold has been reached [2]. TMI are currently the most limiting effect for the further average output power scaling of fiber-laser systems and amplifiers with nearly diffraction-limited beam quality. Therefore, the development of mitigation strategies for TMI is critical to further enhance the performance of this technology.In this work we present a new active approach to mitigate TMI in which the pump power is periodically modulated by a function generator. In good agreement with our simulations, we have been able to demonstrate a very high stabilization of the beam fluctuations up to a power ~ 1.5 times above the TMI threshold, which corresponds to an average output power of 407 W in our system. To achieve this result the pump was modulated with 720 Hz and a modulation amplitude of ~ 68 % peak-to-peak. Fig. 1. illustrates the standard deviation of the beam stability as a function of the average output power (a) of the free-running system (red dots) and the one stabilized with the pump modulation (blue dots). Additionally, exemplary beam profiles at 407 W are depicted for the non-stabilized case (b) and for the stabilized one (c). It can be seen that a significant suppression of TMI and a substantial improvement in stability are achieved by applying the pump modulation technique to the system. Furthermore, it was still possible to stabilize the beam at an output power of nearly 600W which is higher than two times the TMI threshold, albeit outbursts of the fluctuations became observable at some moments. Nevertheless, the pump modulation technique has already resulted in the highest average output power reported from a rod-type fiber laser system emitting a high-quality stabilized beam. This method is very easy to incorporate in already existing systems as there is no need for any additional optical components.