基于空芯光纤气室的痕量农田N2O气体在线监测技术研究

Trace N2O gas online monitoring is a difficult issue in the agricultural pollutants detection. Tunable diode laser absorption spectroscopy that developed in recent years has the advantages of fast response and good selectivity, but the limited length of the gas absorption chamber and the complex environment of farmland scene, result to the detection of low accuracy and poor reliability test results, it's difficult to meet the low concentration of N2O gas monitoring needs. In this project the N2O molecules' specificity of the spectrum absorption principle is applied, as wel as the all-fiber optical system, the long pathlength hollow-core photonic crystal fibers gas chamber is used in the project, and the problem of farmland Trace N2O monitoring such as high sensitivity, high reliability are resolved. The project is to carry out: 1) Study of ring cavity infrared laser technology based on double grating and erbium-doped fiber, to eliminate complex environment interference from farmland impurity gases through dual-wavelength spectroscopy absorption; 2) Study of hollow-core photonic crystal fiber coupling and micro-structure of production technology, to increase the accuracy of monitoring through hollow fiber long optical path chamber; 3) Study of lock-in amplifier and programmable amplifier technology, to achieve effective extraction of strong clutter weak optical signals. By studying these questions, key technology of trace N2O gas monitoring based on hollow fiber gas chamber is solved， to provide a high-precision, highly reliable, interference-line monitoring method for the long-term research and control of agricultural N2O emissions, and a theoretical basis to rational fertilization in agriculture.

痕量N2O气体在线监测是农业污染物分析方面的一个难题。近年发展起来的可调谐激光吸收光谱技术具有检测响应快、稳定性好等优点，但由于吸收气室长度有限而影响了监测精度，难以满足农田复杂气象条件下超低浓度N2O气体在线监测的需求。针对上述问题，本项目利用N2O气体分子对光谱的特异性吸收原理，采用全光纤光路系统，引入长光程空芯光子晶体光纤吸收气室，重点开展：1）研究双光栅环形腔掺铒光纤红外激光技术，通过双波长吸收峰监测消除农田复杂环境下杂质气体的干扰；2）研究空芯光子晶体光纤耦合方式及微结构制作技术，通过空芯光纤超长光程气室来增加监测精度；3）研究微弱信号锁相放大与程控放大技术，实现强杂波背景下微弱光电信号的有效提取。通过以上问题的研究，突破基于空芯光纤气室的痕量N2O气体监测关键技术，为长期研究与控制农业N2O排放提供一种高精度、高可靠、抗干扰的在线监测方法，为农业合理施肥提供理论基础与数据支持。

农业温室气体排放总量大、局部浓度低，所以农田痕量N2O在线监测是农业污染物检测与分析方面的一个难题。近年发展起来的可调谐激光吸收光谱技术（TDLAS）具有响应快、选择性好等优点，但由于其吸收气室多为玻璃、金属等材料制成，受物理长度限制影响了监测精度，难以满足农田复杂气象条件下低浓度N2O在线监测的需求。针对上述问题，本项目利用N2O分子对光谱的特异性吸收原理，采用全光纤光路系统，引入长光程空芯光子晶体光纤吸收气室，重点开展了以下三个方面的研究：1）光纤耦合与降低光损耗问题研究：空芯光纤与单模光纤、多模光纤的耦合及其连接损耗，设计空芯光纤耦合结构，解决空芯光纤气室的光损耗问题；2）研究空芯光纤波导气室参数对气体扩散、光子传输性能的影响，构建空芯光纤波导气室物理模型，解决N2O吸收强度与系统噪声相矛盾的问题；3）研究光纤光路去噪与微弱电信号提取方法，建立双光纤光栅差分光路和嵌入式微弱信号解析电路，解决强杂波背景下微弱信号提取问题。项目通过以上问题的研究，突破了基于空芯光纤气室的痕量气体监测关键技术，达到以下目标：①精确测量：通过超长光程空芯光子晶体光纤吸收气室、双光栅差分光路去噪与强杂波背景下微弱电信号解析，实现痕量农田N2O气体的在线监测，监测分辨率达到100ppb, 监测范围为0-200ppm;②稳定监测：利用双波长涵盖N2O气体两个典型光谱吸收峰，通过双波长差分检测，排除光源信号不稳定、探测器暗电流、农田常见杂质气体等不确定因素的干扰；③快速响应：优化设计空芯光子晶体光纤与单模光纤、多模光纤的耦合方式，建立空芯光子晶体光纤波导气室数学模型，在保证光纤连接损耗最优的同时探索提升气体扩散速度的途径，达到监测时间延迟<1分钟。. 本项目的研究，建立了一套农田痕量温室气体高精度、高可靠的在线监测方法，为长期研究与控制农业温室气体排放提供了检测方法与数据支撑。项目研究过程中，发表学术期刊论文3篇，学术会议论文5篇，申请专利12项，其中授权7项。