Multimode optical fibers are attracting a growing interest for their capability to transport high-power laser beams, coupled with novel nonlinear optics-based applications. However, optical fiber breakdown occurs when beam intensities exceed a certain critical value. Optical breakdown associated with irreversible modifications of the refractive index, triggered by multiphoton absorption, has been largely exploited for fiber material micro-structuration. Here we show that, for light beam intensities slightly below the breakdown threshold, nonlinear absorption strongly affects the dynamics of a propagating beam as well. We experimentally analyze this subthreshold regime and highlight the key role played by spatial self-imaging in graded-index fibers for enhancing nonlinear optical losses. We characterize the nonlinear power transmission properties of multimode fibers for femtosecond pulses propagating in the near-infrared spectral range. We show that an effective N-photon absorption analytical model is able to describe the experimental data well.

中文翻译：

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多模光纤中的飞秒非线性损耗

多模光纤因其传输高功率激光束的能力以及基于非线性光学的新型应用而受到越来越多的关注。然而，当光束强度超过某个临界值时，就会发生光纤击穿。由多光子吸收引发的与折射率不可逆改变相关的光学击穿已被广泛用于纤维材料的微结构化。在这里，我们表明，对于略低于击穿阈值的光束强度，非线性吸收也会强烈影响传播光束的动力学。我们通过实验分析了这种亚阈值机制，并强调了渐变折射率光纤中空间自成像在增强非线性光学损耗方面的关键作用。我们表征了多模光纤在近红外光谱范围内传播的飞秒脉冲的非线性功率传输特性。我们表明，一个有效的N-光子吸收分析模型能够很好地描述实验数据。