研究了去包层U型弯曲光纤的折射率传感特性.首先根据模间干涉理论, 分析了U型光纤传感器的传感原理, 指出干涉谱损耗峰波长与环境折射率和弯曲半径有关.利用单模光纤(SM-28)实验制作不同曲率半径的U型光纤传感器, 把传感器的U型部分浸入不同折射率的液体中, 研究其折射率传感特性.当U型光纤曲率半径为2.5~5.0 mm时, 传输光谱中均能观察到明显的模间干涉现象; 当液体折射率从1.30RIU变到1.43RIU时, 光谱损耗峰波长发生红移, 且弯曲半径越大, 折射率传感灵敏度越高; 在曲率半径为5 mm时灵敏度为207 nm/RIU(折射率1.30~1.40RIU)和1 220 nm/RIU (折射率1.40~1.42RIU).干涉峰的波形参量(半高宽、对比度)决定于包层模和纤芯导模之间的比例, 当曲率半径为4 mm时, 损耗峰半高宽最小达3.2 nm.综合半高宽和灵敏度两个参量, 得出曲率半径4.5 mm的U型光纤传感器品质因素最高, 分别为43.1RIU-1(折射率1.30~1.40RIU)和191.2 RIU-1(折射率1.40~1.42RIU), 可直接由SM-28单模光纤制成, 且制作工艺简单、成本低、机械强度高不需要任何特殊处理, 具有很好的应用前景.

To develop a kind of easy-fabrication and stable optical fiber sensor, a new method for fabricating the fiber sensors was proposed, and the sensing performance of a U-shape bending single mode fiber was investigated. A section of SM-28 single mode fiber was bent into U-shaped structure with variety of radii of curvature, and then the U-shaped structure is fixed. After that, the U-shaped portion of the fiber was immersed in liquid with different refractive indices respectively. Meanwhile, the transmission spectrum of the U-shaped optical fiber was recorded. The modal interference theoretical analysis shows that the core guiding mode and the cladding mode, which propagate independently in the bending section, interferes each other in the straight region, and that the interference wavelengths depend on the bending radius and surrounding refractive index. Obvious interference spectra can be clearly observed on the U-shape bending fiber with radius ranging from 2.5 mm to 5.0 mm. The interference wavelengths red-shift as the surrounding refractive index increases from 1.30 to 1.43RIU. Higher sensitivity could be achieved when the bending fiber has a lager bending radius. In the case of the bending radius being 5.0 mm, the sensitivity reaches to 207nm/RIU and 1 220 nm/RIU in the SRI region of 1.30~1.40 RIU and 1.40~1.42RIU. The characteristics of the interference dip, such as full width at half maximum, contrast are determined by the ratio of core guiding mode and cladding mode. The narrowest full width at half maximum was achieved when the bending radius was 4.5 mm of U-bend single mode fiber were discussed. A low cost and easy making sensor was proposed. The figure of merit, which integrates the sensitivity and the full width at half maximum, reach the maximal value of 43.1 RIU-1 and 191.2 RIU-1 in the surrounding refractive index region of 1.30~1.40 RIU and 1.40~1.42 RIU, when the bending radius is 4.5 mm. This sensor is fabricated with a common single mode fiber with simply bending process, and shows potential in the fiber sensing application with the features of low cost and high mechanical strength.