【作者】 彭勇；

【导师】 于清旭；

【作者基本信息】 大连理工大学 ， 光学工程， 2011， 博士

【摘要】 在工农业生产中,众多种类的气体承担着重要角色,其中有些气体对人类生存环境安全的影响至关重要,因此,气体定量分析已成为现代检测技术的重要部分。痕量气体光声光谱检测技术具有：选择性强、灵敏度高、响应时间短、可连续采样检测等优点,是实现痕量气体实时监测的理想方法,在众多领域得到广泛的研究和应用。本文针对C2H2和NH3检测的需要,研制了高性能的近红外痕量气体光声光谱检测系统,其内容主要有：从气体光声效应的经典理论出发,详细分析了光声信号强度与光声池结构和工作模式的内在联系。针对一阶纵向圆柱形共振光声池典型结构进行了详细分析。给出了光声池一阶纵向[100]模式的声场分布、光声池常数、共振频率和品质因数表达式,并用数值计算模拟了三个特征参量与光声池几何尺寸的三维关系图,为光声池参数优化和设计提供了理论依据。针对光声光谱检测系统噪声的来源和信号的提取方法,讨论了抑制噪声的方法；从理论上分析了二次谐波检测技术对提高系统检测灵敏度的必要性和可行性。针对吸收气体分子碰撞加宽的洛伦兹线形,采用数值模拟计算方法得到了二次谐波峰值与半宽度的比值和调制系数之间的理论曲线,为频率调制最佳参数的选取提供了理论依据。同时以乙炔气体为例,理论分析得出：在一个大气压背景气体下,样品气体浓度在ppb～ppm范围内变化时,其样品气体浓度的变化对吸收谱线宽度的影响很微小,因而对最佳调制深度的影响可以忽略。详细分析了掺铒光纤中光放大的物理过程,给出了基于FFP-TF的可调谐环形腔掺铒光纤激光器的结构,构建了一个输出功率为3.5 mW,3dB带宽小于0.01nm,覆盖C+L波段的掺铒光纤激光器,为了提高激光功率,增强光声信号,提高系统的检测灵敏度,采用了在可调谐光纤激光器后串接掺铒光纤放大器(EDFA)作为组合光源,系统选择了双包层、双向泵浦结构、具有高饱和增益输出的掺铒光纤放大器,饱和输出功率可达500mW,放大器均衡输出带宽为36nm。在上述工作基础上,使用掺铒光纤激光器结合掺铒光纤放大器作为光源,采用双光程一阶纵向共振式光声池,结合波长调制和锁相放大器的二次谐波检测技术,成功设计组建了高性能的痕量气体近红外光声光谱检测系统。基于对C2H2分子振动泛频v1+v3和NH3分子振动泛频v1+v2屹和2v3在近红外1.5μm谱带吸收线的分析,结合近红外光声光谱检测系统中所选用的光源波长调谐范围,选择C2H2的P9峰(1530.37nm)和NH3的pP3(5)a(1531.7nm)吸收谱线进行光声信号探测。由于NH3分子是极性分子,在光声池内壁上有吸附和解吸附过程,实验中选取低表面能Teflon材料作为池壁材料,实现了对NH3的高灵敏度快速检测。系统性能评定结果表明,常温常压下,对C2H2的检测极限灵敏度为1.3ppb；对NH3的检测极限灵敏度为3ppb, Teflon谐振腔在200SCCM的流量下对低浓度NH3表现出较快的响应速度,检测响应时间约为60s。本论文研制的基于光纤激光器组合掺铒光纤放大器为光源和共振光声池的痕量气体光声光谱系统,在大气环境污染监测,供电系统变压器运行故障诊断,以及医学临床呼吸气体监测等方面都具有广阔的应用前景。更多还原

【Abstract】 Trace gas detection techniques have been playing an important role in the modern analysis technologies. Trace gas measurements base on photoacoustic spectroscopy (PAS) possess the advantages such as high sensitivity, selective detection, fast response in continuous-flow mode and suitable for online multicomponent trace gas analysis, so it has been widely studied and applied in many fields. In this dissertation, a high performance near-infrared photoacoustic spectrometer for trace C2H2 and NH3 detection is developed. The main research works are outlined as follows.The relationship between photoacoustic signal intensity and photoacoustic cell structure parameters is discussed in detail according to the mechanism of gas photoacoustic effect. Through analyzing the typical structure of the first-order longitudinal resonant cylindrical photoacoustic cell the sound field distribution, photoacoustic cell constant, resonance frequency and quality factor of [100] mode are derived, and three dimension diagrams of three characteristic parameters with photoacoustic cell geometric parameters are numerically simulated. The results show that the maximum photoacoustic signal can be obtained when the microphone is installed in the middle of the cell. Furthermore, the basic consideration for determining the geometric parameters of photoacoustic cell is described.By analyzing the noise sources and signal extraction methods of the PA system, it emphasized that the noise suppression method and the second harmonic detection techniques would be key important factor for improving detection limit sensitivity. Based on the broadened Lorentz shape resulted from the collision of the absorbed gas molecules, a theoretical curve between the ratio of the second harmonic peak value to the width at half maximum and modulation index is given by numerical simulation method, which provides a theoretical basis for the selection of the optimal frequency modulation parameters. Using acetylene gas as an example, the theoretical analysis shows that when the gas concentration varies in the range of ppb-ppm, the change of gas concentration has a very small influence on the width of absorption lines under atmospheric pressure. Therefore, the effect on optimal modulation depth can be ignored. The physical mechanism of light amplification in erbium-doped fiber is described. A tunable ring cavity erbium-dopde fiber laser is designed and developed, which obtained an 3.5 mW,3dB bandwidth less than 0.01nm and spectrum over C+L-band. In order to enhance the laser output power, and to improve the photoacoustic signal and detection sensitivity, a combination of the tunable fiber laser and an erbium doped fiber amplifier (EDFA) is deployed. The EDFA has a double-clading, double-direction pump structure, and high saturated gain output. The saturated output power and the 3dB bandwidth of the EDFA are 500mW and 36nm respectively. Based on all above, a high performance photoacoustic spectrometer for trace gas detection is firstly developed with a first-order longitudinal resonant photoacoustic cell with double absorption optical path, the lock-in amplifier based second harmonic detection technique, a tunable fiber laser and an erbium doped fiber amplifier.The near-infrared absorption spectrums of acetylene and ammonia gases are analysed. An acetylene P9 peak (1530.37nm) and an ammonia transitionpP3(5)a (1531.7nm) are selected for the systerm performance evaluation due to the weak interference with H2O and CO2 absorption at these laser lines. Because NH3 molecule shows polar, which can adsorb on the inner wall of photoacoustic cell, a low surface energy material Teflon is used as cell materials. As a result, the fast detection for NH3 with high sensitivity is achieved.The evaluation of system performance showed that the detection limit sensitivity for acetylene detection is 1.3ppb and the detection limit for ammonia under room temperature and atmospheric pressure is 3ppb; the response time of the Teflon resonant cell for low concentration NH3 is up to 60s approximately under the flow rate of 200SCCM.This thesis developed a sensitive photoacoustic spectrometer based on a tunable fiber laser and an erbium doped fiber amplifier. The system have many potential applications such as environmental air pollution detection, dissolved oil gas analysis for power transformer monitoring and human breath gas analysis in medicine.更多还原