Experimental observation of a novel effect of long-term mode shape degradation in a high peak power ytterbium-doped pulsed fiber lasers based on large mode area step-index fibers has been reported for the first time. It is shown, that the degradation is caused by power coupling from LP₀₁ to LP₁₁ mode, occurring on a long period refractive index grating (LPG) formed in the active fiber core. The photo-darkening process was found to play a significant role in the formation of a LPG.

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Konstantin K. Bobkov, Mikhail M. Bubnov, Svetlana S. Aleshkina, and Mikhail E. Likhachev, "The first experimental observation of long-term mode degradation in high peak power Yb-doped amplifiers," Proc. SPIE 10083, Fiber Lasers XIV: Technology and Systems, 100830T (Presented at SPIE LASE: January 31, 2017; Published: 22 February 2017); https://doi.org/10.1117/12.2251734.