Geometrically modified fiber optic sensors (FOS), particularly U-bent FOS, have gained significant attention due to their remarkably high refractive index (RI) and evanescent wave absorbance (EWA) sensitivity, as well as their ergonomic design and ease in handling. In this article, we present a theoretical model for the U-bent FOS probes to predict the sensor behavior by numerically simulating the light propagation in an equivalent 2D semi-circular ring using ray tracing approach. In addition to the effects due to the modification of geometry, this article presents a thorough investigation of the influence of the bend-induced material deformation on the nature of light propagation and refractive losses. We introduce “bend ratio” (ratio of bend radius to fiber core radius) to explain the influence of geometry modification and the bend-induced inhomogeneity in RI (BIRI) of the fiber core on RI sensitivity. The bend ratio of bent plastic optical fiber sensors falls under one of the four bending regimes, namely, gentle, geometric, saturation, and plastic, for which the bend ratios are less than ∼35, ∼25, 17, and 7, respectively. The results also show that for bend ratios less than 7, BIRI inhomogeneity is responsible for the high RI sensitivity observed with U-bent probes as opposed to the simple geometric modification. This article also indicates the existence of an optimum bend ratio (for a given value of RI of the surrounding medium) where RI sensitivity is maximum. These findings were validated with previously reported experimental results.

中文翻译：

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使用射线光学研究 U 型弯曲光纤传感器的折射率灵敏度

几何改进的光纤传感器 (FOS)，尤其是 U 型弯光纤传感器，由于其极高的折射率 (RI) 和渐逝波吸收 (EWA) 灵敏度以及符合人体工程学的设计和易于操作而受到极大关注。在本文中，我们提出了 U 型弯头 FOS 探头的理论模型，通过使用光线追踪方法对等效 2D 半圆环中的光传播进行数值模拟来预测传感器行为。除了由于几何形状的修改而产生的影响之外，本文还对弯曲引起的材料变形对光传播和折射损耗的性质的影响进行了彻底的调查。我们引入“弯曲比”（弯曲半径与纤芯半径的比值）来解释几何修改和纤芯 RI (BIRI) 弯曲引起的不均匀性对 RI 灵敏度的影响。弯曲塑料光纤传感器的弯曲率属于四种弯曲方式之一，即温和、几何、饱和和塑性，其弯曲率分别小于~35、~25、17和7。结果还表明，对于小于 7 的弯曲比，与简单的几何修改相反，BIRI 不均匀性是使用 U 型弯曲探针观察到的高 RI 灵敏度的原因。该文章还指出存在最佳弯曲比（对于给定的周围介质 RI 值），其中 RI 灵敏度最大。这些发现得到了先前报告的实验结果的验证。