Stimulated Brillouin scattering (SBS) is often an unwanted loss mechanism in both active and passive fibers. Highly multimode excitation of fibers has been proposed as a novel route toward efficient SBS suppression. Here, we develop a detailed, quantitative theory which confirms this proposal and elucidates the physical mechanisms involved. Starting from the vector optical and scalar acoustic equations, we derive appropriate nonlinear coupled mode equations for the signal and Stokes modal amplitudes and an analytical formula for the SBS (Stokes) gain with applicable approximations, such as the neglect of shear effects. This allows us to calculate the exponential growth rate of the Stokes power as a function of the distribution of power in a highly multimode signal. The peak value of the gain spectrum across the excited modes determines the SBS threshold—the maximum SBS-limited power that can be sent through the fiber. The theory shows that the peak SBS gain is greatly reduced by highly multimode excitation due to gain broadening and relatively weaker intermodal SBS gain. The inclusion of exact vector optical modes in the calculation is crucial in order to capture the incomplete intermodal coupling due to mismatch of polarization patterns of higher-order modes. We demonstrate that equal excitation of the 160 modes of a commercially available, highly multimode circular step index fiber raises the SBS threshold by a factor of 6.5 and find comparable suppression of SBS in similar fibers with a 𝖣-shaped cross section.

中文翻译：

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用于高度多模激发的纤维中受激布里渊散射理论


受激布里渊散射 （SBS） 通常是主动和被动纤维中不需要的损失机制。纤维的高度多模激发已被提议作为实现有效 SBS 抑制的新途径。在这里，我们开发了一个详细的定量理论，证实了这一提议并阐明了所涉及的物理机制。从矢量光学和标量声学方程开始，我们推导出信号和斯托克斯模态振幅的适当非线性耦合模态方程，以及 SBS （Stokes） 增益的解析公式和适用的近似值，例如忽略剪切效应。这使我们能够计算斯托克斯功率的指数增长率，它是高度多模式信号中功率分布的函数。激励模式的增益频谱峰值决定了 SBS 阈值，即可通过光纤发送的最大 SBS 限制功率。该理论表明，由于增益展宽和相对较弱的多模态 SBS 增益，高度多模激励会大大降低峰值 SBS 增益。在计算中包含精确的矢量光学模式对于捕获由于高阶模式的偏振模式不匹配而导致的不完全模态耦合至关重要。我们证明，市售的高度多模圆阶跃折射率光纤的 160 种模式的相等激发将 SBS 阈值提高了 6.5 倍，并在具有 D 形横截面的类似光纤中发现了 SBS 的类似抑制。