基于倏逝场和微结构光纤光栅的高灵敏度VOCs气体传感器研究

To fitting the requirements of in-line and real-time detection of VOCs, a kind of VOCs sensor based on LPFG in index-guided microstructured optical fiber (MOF) and the evanescent field will be studied in this project. The index-guided MOF is used as working fibers, the air holes in cladding is used as sensitive element, Femtosecond pulse laser is applied to fabricate LPFG into MOF. The sensor key questions in theory and experiment will be studied in details. The influence of MOF structure parameters and length on the propagation characteristics,the VOCs species also will be researched.The evanescent field in the cladding will be enhanced due to a LPFG couples light resonantly from the fundamental core mode to a co-propagating higher order cladding mode. VOCs will be direct measured using LPFG without extra sensitive chemical nano-film through the evanescent field interacts with molecules of VOCs. It will open up a new way to detect VOCs in the environment. At present, the VOCs fiber sensors are usually based on LPFG coated with a polymer thin film. The field of cladding mode interacts with VOCs through the film, but the energy distributes outside the cladding is very weak and the sensitivity is limited. Meanwhile the polymer film is a key element to absorb the gas molecules. The resonant spectrum of LPFG is strongly dependent on the polymer film whose effective index is usually higher than that of fiber cladding. Moreover, the film thickness must be strictly controlled at the level of nanometer. The fabrication process is complicated. We apply LPFG written into MOF by femtosecond pulse laser to activate the higher order cladding modes. The interaction of organic gas with the evanescent field in the cladding is remarkably enhanced through the molecules of VOCs diffuse into the air holes of MOF. So it is hopeful to improve the sensitivity of VOCs gas sensor. The significance is important in many field such as environmental protection, food safety, chemical industry, house and home.

本项目以挥发性有机物(VOCs)的在线实时连续检测和痕量分析的应用需求为基本背景，以建立一种高灵敏度VOCs气体检测方法为目标，对基于微结构光纤长周期光栅(LPFG)和倏逝场的高灵敏度VOCs传感器中的科学问题进行较为深入的理论和实验研究。拟采用全反射型微结构光纤作为VOCs传感器的工作光纤，以光纤包层中分布的大量空气孔作为敏感区域，以飞秒激光制作LPFG作为激发倏逝场的有效手段，研究传感器的相关理论、器件结构和测量方法中的关键问题。研究不同VOCs气体的传感器特性，以及光纤结构参数和长度、LPFG的结构参数等对传感器的影响。结合微结构光纤和LPFG两者的优点，利用光纤空气孔中的倏逝场能量分布较强，与VOCs气体分子相互作用距离长的特点，研究VOCs与倏逝场的相互作用机制，实现VOCs气体的高灵敏度检测和痕量分析,无需镀任何额外的化学敏感膜，开辟一种新的VOCs测量方法和传感器结构。

挥发性有机物（VOCs）对生态环境和人类健康都具有严重的危害。为了实现对VOCs的有效监测，需要一系列新型有效的传感和测量技术，需要灵敏度高、性能稳定的传感器。近年来，国外投入巨资研究开发用于识别未知气体污染源和连续监测已知气体污染源变化的新型传感器。我们以VOCs的在线实时监测的应用需求为基本背景，建立一种高灵敏度VOCs气体检测方法，主要研究成果：（1）用飞秒激光在微结构光纤上制作出低插入损耗的LPFG，激发包层的高阶模式，增强倏逝场的强度。在柚子型微结构光纤、全固态光纤、单模光纤上刻写LPFG，有限元法模拟光纤的模式特性和光纤光栅的耦合特性。（2）分析了不同结构微结构光纤（包括空芯光子晶体光纤、悬芯微结构光纤、全固态光子晶体光纤、七芯微结构光纤、柚子型悬芯光纤、毛细管、柚子型微结构大孔光纤等）的传导模场分布，研究光纤的端面或改变孔道折射率，对模场分布及其干涉光谱的影响，并进行了相关温度、空气压力、扭曲、应变、折射率、VOCs气体传感特性研究；（3）把微结构光纤和光纤干涉型传感器相结合，研制出两种新型测量微量、痕量气体的VOCs传感器。一种基于悬芯光子晶体光纤的内反射型传感器和一种基于高双折射锥形光纤的Sagnac干涉环VOCs传感器。这两种传感器不需要额外镀敏感聚合物膜，对VOCs气体如甲醇、乙醇、四氢呋喃、丙酮、甲苯具有高灵敏度。传感器的结构新颖，性能稳定对温度不敏感。与传统的气体传感器相比，它结构简单、易于实现、灵敏度高、可以实现在线测量和重复测量。