腐蚀薄包层布拉格光纤光栅折射率传感研究

Study on Refractive Index Sensing of Etched Thin Cladding Fiber Bragg Grating

摘要: 本文对腐蚀薄包层光纤布拉格光栅折射率传感的理论进行了分析，并利用该理论进行了折射率测量的实验研究。首先在理论上，基于光纤三层结构模型，分析了Bragg波长与光纤直径的关系，同时计算了不同光纤直径下，Bragg波长与环境折射率的变化关系，得到不同直径的薄包层光纤光栅的折射率响应曲线。模拟计算结果表明，Bragg波长与环境折射率变化呈非线性关系，直径更小的光纤传感器拥有更高的灵敏度，在1.333~1.462的折射率范围中，腐蚀到纤芯的薄包层光纤光栅折射率传感器灵敏度约为22 nm/RIU，这是在理论上，该类传感器在特定折射率范围内的最大灵敏度。随后在实验中，通过对一个布拉格光纤光栅分三个阶段腐蚀，得到了三个不同直径(32 μm, 14 μm, 8.45 μm)的薄包层光纤布拉格光栅折射率传感器，记录了腐蚀过程中反射光谱的变化，并分别进行了折射率传感实验。实验结果表明，在1.33300~1.42789的折射率范围内，传感器的平均灵敏度分别为0.246 nm/RIU、1.319 nm/RIU，6.332 nm/RIU，考虑到0.02 nm光谱分辨率，计算得到的折射率分辨率值为0.08 RIU、0.015 RIU、0.00316 RIU。最后，对于理论与实验结果存在微小的差异，即理论模型没有能够预测Bragg波长红移这一趋势，本文也提出了新的见解。

Abstract: In this paper, the theory of refractive index sensing of etched thin-cladding optical fiber Bragg grating is analyzed, and the experimental study of refractive index measurement is carried out using the theory. Firstly, in the theory, the relationship between Bragg wavelength and fiber diameter is analyzed based on the three-layer structure model of the fiber, and the variation of Bragg wavelength with ambient refractive index is also calculated for different fiber diameters to obtain the refractive index response curves of thin-cladding fiber gratings with different diameters. The simulation results show that there is a nonlinear relationship between the Bragg wavelength and the change of the ambient refractive index. The smaller diameter of the fiber sensor has a higher sensitivity. In the refractive index range of 1.333~1.462, the sensitivity of the thin-cladding fiber grating refractive index sensor etched to the core is about 22 nm/RIU, which is theoretically the maximum sensitivity of this kind of sensor in the specific refractive index range. Then in the experiment, three thin-clad fiber grating refractive index sensors with different diameters (32 μm, 14 μm, 8.45 μm) are obtained by etching a Bragg fiber grating in three stages. The change of reflection spectrum during the etching process is recorded, and the refractive index sensing experiments are carried out respectively. The experimental results show that the average sensitivity of the sensor is 0.246 nm/RIU, 1.319 nm/RIU and 6.332 nm/RIU in the refractive index range of 1.33300~1.42789. Considering the spectral resolution of 0.02 nm, the calculated refractive index resolution is 0.08 RIU, 0.015 RIU and 0.00316 RIU. Finally, a new opinion is put forward for the slight difference between the theoretical and experimental results, that is, the theoretical model cannot predict the trend of red shift of Bragg wavelength.