基于长周期光纤光栅的高灵敏度液体折射率传感器研究

Refractive index is one of the most important physical parameters for those substances such as liquid, gas and transparent material. Thus the accurate measurement of refractive index with fast response and high resolution is significantly valuable for the applications in environment monitoring, biomedical diagnosis, petrochemical industry and so on. Most of the traditional refractive index sensors are mainly based on various fiber gratings. Actually they usually exhibit low sensitivity and hysteretic response to external refractive index variation, besides high manufacture cost and weak structure stability is evitable. Especially, around the refractive index of 1.333 which is marked as the value of pure water, the sensitivity of refractive index is not so good and need to be emergently enhanced. Therefore the restriction of measurement scale becomes a main disadvantage of such refractive index sensors based on fiber gratings. This project would propose a new method to measure refractive index of liquid from 1.33 to 1.38 with high sensitivity, which is mainly based on a designed M-Z interferometer with two cascaded long period gratings (LPGs). Two optimized LPGs with same parameters are directly inscribed in a single mode fiber by use of infrared femtosecond laser. They are cascaded one by one and compose of the in-fiber M-Z interferometer as the sensing head for external refractive index. The response sensitivity and spectra resolution of the M-Z interferometer are found to be dramatically enhanced by use of cladding chemical etching and fiber tapers, since these techniques can reduce the cladding diameter and enhance the evanescent field of cladding mode. In a range of 1.33-1.38, the proposed M-Z interferometer based on two cascaded LPGs are very attractive candidate for the refractive index sensing of liquid, due to their compact structure, high sensitivity and excellent resolution.

折射率是自然界中液体、气体和透明材料等物质重要的基本物理量之一，实现折射率的快速和精确测量对环境监测、医学诊断和石油化工等领域有重要意义。传统的基于光纤光栅的折射率传感器，不仅制作成本高，结构稳定性差，而且普遍存在灵敏度低，响应速度慢等问题。特别是在水的折射率1.333附近，光纤光栅类传感器对折射率的灵敏度和分辨率均不够理想，测量范围因此受到限制。本项目提出一种基于级联长周期光纤光栅（LPG）的新型结构实现对1.33-1.38范围内液体折射率的高灵敏度传感。通过飞秒激光在单模光纤中刻写两个结构相同的长周期光纤光栅串联构成M-Z干涉仪，同时采用包层腐蚀和光纤拉锥等辅助手段有效减小光纤包层半径，增加包层模作用场，大大提高了M-Z干涉仪在1.33-1.38水窗范围内的折射率灵敏度和分辨率，最终实现对液体折射率的快速和精确测量。

折射率是物质重要的物理量之一，实现折射率的快速和精确测量是近年研究热点之一。传统的基于光纤光栅的折射率传感器，不仅成本高，稳定性差，而且普遍存在灵敏度低等问题，特别是在水的折射率1.33 附近，灵敏度和分辨率比较低。本项目采取理论计算结合实验研究的方法，利用飞秒激光在单模光纤中刻写两个结构相同的长周期光纤光栅（LPG）级联构成马赫-曾德尔（M-Z）干涉仪，实现水窗范围内对折射率的高灵敏度和高分辨率测量。理论研究方面，利用耦合模理论深入分析了基于级联LPG的M-Z干涉仪的物理机制，利用有限元软件建立了仿真模型，计算了光栅周期、光栅长度、光栅间距等结构参数对输出光谱的影响。结果表明，改变光栅周期会影响谐振峰的波长位置和干涉拍长，改变光栅长度会影响谐振峰的透射率，改变光栅间距会影响谐振峰的波长位置和透射率。实验研究方面，搭建了飞秒激光刻写LPG的实验系统，采用逐点法在单模光纤上制备了15种不同参数的M-Z干涉仪样品，在1.300-1.395范围内测得折射率灵敏度为26nm/RIU。随后利用HF酸溶液腐蚀光纤包层至半径为20µm时，测得在1.33-1.38范围内灵敏度为385nm/RIU。随后利用氢氧焰对光纤熔融拉锥至半径为8µm时，测得在1.33-1.38范围内灵敏度为557nm/RIU。综合考虑上述因素，选用光栅周期为500µm，光栅长度为7.5mm，光栅间距为10cm的M-Z干涉仪样品，先通过HF酸溶液腐蚀至包层半径为20µm，再将腐蚀后的样品熔融拉锥至包层半径为8µm，最终获得了在1.33-1.38范围内对折射率的灵敏度最大值为1023nm/RIU。本项目研究的折射率传感器，结构紧凑，制备简单，成本较低，可实现在1.33-1.38水窗范围内对折射率的高灵敏度测量，在环境监测、医疗诊断以及石油化工等领域有潜在的应用和推广价值。