The analysis of substances outside the cladding has challenged the optical fiber sensors community for decades. A common solution relies on the propagation of light in cladding modes. However, the coupling of light to/from these modes is typically based on permanent gratings in specific locations, which restrict the sensors to point measurements only. In this work, we present dynamic, random-access coupling of light between core and cladding modes of standard fibers, in arbitrarily located short sections. Coupling is based on the stimulation of Brillouin dynamic gratings by two coded pump waves and intermodal scattering of a third optical probe wave. All waves are launched and collected in the core mode. No permanent gratings are inscribed. Distributed sensing of surrounding media is demonstrated with 2 m range and 8 cm resolution. Measurements distinguish between water and ethanol outside the fiber. The measurement accuracy of the local index outside the cladding is 0.004–0.0004 refractive index units.

中文翻译：

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分布式包层模式光纤传感器

数十年来，对包层外部物质的分析一直对光纤传感器领域构成挑战。常见的解决方案取决于光在包层模式下的传播。但是，与这些模式之间的光耦合通常基于特定位置的永久光栅，这将传感器限制为仅进行点测量。在这项工作中，我们介绍了在任意放置的短截面中，标准光纤的纤芯和包层模式之间的动态，随机访问光耦合。耦合基于两个编码泵浦波对布里渊动态光栅的刺激以及第三光学探测波的模态散射。所有波浪都是在核心模式下发射和收集的。没有刻有永久性光栅。演示了以2 m的范围和8 cm的分辨率对周围介质进行分布式感测。测量可以区分纤维外部的水和乙醇。包层外部的局部折射率的测量精度为0.004-0.0004折射率单位。