Abstract

Mode instability (MI) in Yb-doped fiber amplifiers attracted great interest in the last years [1,2]. This effect was referred to nonlinear transformation of power from the intense fundamental mode LP₀₁ to the initially-weak LP₁₁ mode. The MI was observed both in large mode area fibers with large core diameter and in few-mode fibers with a relatively small core [1-5]. Two mechanisms of formation of refractive index gratings caused by population inversion (due to polarizability difference of the excited and unexcited Yb³⁺ ions) and temperature distributions induced by the modes interference field were discussed as the main reason for the MI [1-3]. This paper is devoted to the detailed experimental investigation and theoretical modeling of the MI effect in 10-um core Yb-doped fibers.

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