高灵敏度和大动态范围光纤折射率传感器研究与实现

All-optical fiber refractive index sensor, as one of the research hotspots of optical fiber sensors, has important applications in physical, biochemical and environmental monitoring. At present, most studies only focus on how to improve the sensitivity of the sensor, but the high-sensitivity sensor has a narrow measurement range and lacks universality. Focusing on the application requirements of high sensitivity and large dynamic range optical fiber refractive index sensors, the applicant's team used femtosecond laser to fabricate a composite optical fiber Fabry-Perot structure sensor and verified its effectiveness, but the dynamic range needs to be further broadened. Based on our previous work, this project intends to achieve the coexistence of high sensitivity and large dynamic range by using highly localized fiber Bragg grating. In theory, the propagation characteristics of HL-FBG are numerically studied by full-vector beam propagation theory to reveal the sensitivity of the position of cut-off wavelength and the intensity of cladding mode resonance peak near the cut-off wavelength to refractive index. Meanwhile, femtosecond laser direct writing technology will be used to fabricate HL-FBG to meet the requirements, so as to achieve the coexistence of high sensitivity (resolution 10-7 magnitude) and large dynamic range (refractive index range 1~1.45). The research results of this project can not only provide a new solution to the problem of poor universality of high sensitivity sensors, but also offer strong support for applications such as biochemical sensing and environmental monitoring.

全光纤折射率传感器在物理、生化和环境监测等方面具有重要应用，是光纤传感器的研究热点之一。大多数研究只关注如何提高灵敏度，但是高灵敏度传感器存在测量范围窄、缺乏普适性的问题。围绕如何兼顾高灵敏度和大动态范围传感器的应用需求，申请人团队采用飞秒激光制备复合式光纤法布里-珀罗结构传感器，并验证了其有效性，但其动态范围还有待拓宽。在前期工作基础上，本项目拟采用高局域布拉格光纤光栅（HL-FBG）实现高灵敏度和大动态范围共存。通过全矢量光束传输理论，数值研究HL-FBG传输特性，揭示HL-FBG截止波长位置和附近包层模谐振峰强度对折射率敏感的规律。同时，拟采用飞秒激光直写技术制备满足要求的HL-FBG，实现高灵敏度（10-7量级）和大动态范围（1~1.45）共存的测量目标。本项目的相关研究成果不仅为解决高灵敏度传感器普适性差的问题提供一个新思路，而且为生物化学传感和环境监测等应用提供有力支持。

在各行各业中，能够同时进行高灵敏度（高分辨率）和大动态范围的测量是极其重要的。然而，目前绝大多数的传感器都存在高灵敏度和大动态范围难以同时共存的问题，简单来说就是，传感器的灵敏度越高，其动态范围越小。本项目采取理论分析、模拟仿真结合实验研究的方法，提出了一种利用飞秒激光直写技术在单模光纤中制备高局域布拉格光纤光栅的结构，来实现高灵敏度和大动态范围同时共存的测量。首先，在理论分析研究的方面，在掌握纯数学分析的高局域布拉格光纤光栅理论基础上，进一步建立基于全矢量分析软件的高局域布拉格光纤光栅分析理论，研究高局域布拉格光纤光栅中包层模场分布和透射光谱的特性，以及高局域布拉格光纤光栅的参数对外界环境折射率的传感性能的影响。其次，搭建飞秒激光直写技术的实验系统，探索在光纤中制备包层模谐振强深、包层模谐振分布范围宽、光谱质量好的高局域布拉格光纤光栅的工艺条件，研究加工参数对高局域布拉格光纤光栅传输光谱的影响。最后，对制备的高局域布拉格光纤光栅开展连续且小间隔的SRI测量实验，得到了在折射率1.331到1.334范围内，分辨率为3.4×10-7。由于超宽和深包层模式共振，强度解调方法可以在从空气到光纤芯的有效折射率的宽范围内实现超高强度灵敏度，这在环境监测、生物化学、石油化工等领域的传感中具有潜在的应用和推广价值。