【作者】 韩文念；

【导师】 汪曣；

【作者基本信息】 天津大学 ， 生物医学工程， 2012， 博士

【摘要】 激光腔内吸收光谱自被提出以来因其高灵敏度特性一直为各国学者所关注,被广泛应用于气体、液体和等离子体等介质中粒子弱吸收的探测。光纤技术的发展使分布式腔内吸收光谱检测成为可能,但目前还没有成熟的商品仪器出现。本文基于光纤环腔激光器构建腔内吸收传感系统研究痕量气体的全光纤、准分布式测量。在对系统器件进行分析和简化传输函数的基础上,阐述了腔内吸收光谱的形成条件,理论推导了光纤环腔腔内吸收传感系统工作的稳态模型和瞬态模型,据此计算模拟了光纤环腔激光器的放大自发辐射谱和气体腔内吸收光谱,验证了本系统光谱波长扫描测量的可行性。针对放大自发辐射噪声和标准具噪声等对腔内吸收光谱检测的干扰问题,将低频波长扫描和波长调制技术相结合应用于腔内吸收气体传感系统,通过理论分析和实验确定调制参数。对1%浓度的乙炔气体进行传感,在1526.5~1537 nm波长区间检测到17条吸收谱线,证明该方法可有效提高系统气体传感的灵敏度。为增强传感性能,对波长扫描方式加以优化并再次实验乙炔传感,未采用波长调制技术便在1525~1545 nm波长区间获得27条吸收谱线,传感灵敏度明显提高,证实了系统改进的有效性。制作不同长度的气室研究了本系统中气体传感的最佳吸收光程长问题。对腔内吸收光谱谱线提取方法进行了研究,选取小波零交叉法、小波变换迭代法和小波变换形态学结合法等三种方法对不同实验方法获取的扫描谱进行谱线提取;对腔内吸收调制光谱,通过相关分析推出当调制频率较低时,各阶次的谐波谱与调制光谱的各阶傅里叶变换成正比,应用虚拟仪器技术由离散傅里叶法直接计算谐波谱,并提出将二次谐波谱作为反演目标气体浓度的依据,标定谱线吸收波长则采用三次谐波谱。分别采用单谱线法和多谱线平均法计算浓度反演的误差,分析了本传感系统的定量能力。为解决可调滤波器驱动电压与对应波长值漂移导致难以精确定位谱线波长问题,提出引入布拉格光栅作为参考的谱线波长标定方法:采用高斯拟合定位谱峰峰值,寻找对应的F-P滤波器驱动电压,以经过温度补偿的布拉格光栅中心波长的精确值作为标准,由多项式拟合得到驱动电压与波长的关系曲线,对吸收谱线波长进行标定。在此工作基础上评价了传感系统用于多气体传感的定性能力,实验表明传感系统采用该方法后波长标定误差降至0.1 nm,达到多气体组分谱线分析的要求。更多还原

【Abstract】 Laser intra-cavity absorption spectroscopy has been of concern to scholars from various countries since it was proposed. Because of its high sensitivity, it is widely applied to the detection of weak absorption in gas, liquid and plasma. With the development of optical fiber technology, it is possible to measure intra-cavity absorption spectroscopic signal using a distributed structure, but there are no mature commercial instruments available yet. In this paper, an all-fiber, quasi-distributed intra-cavity absorption spectroscopic gas sensing system based on the fiber ring laser is presented, tested and applied to trace gas analysis.Based on the analysis of the components and simplification of the transfer functions, the generation conditions of intra-cavity absorption spectroscopic signal are discussed, and the work models of sensing system including steady-state model and transient model are derived. According to the models, the amplified spontaneous emission spectrum of fiber ring laser and intra-cavity absorption spectrum of gas are calculated and simulated. It is confirmed that using wavelength sweep to acquire intra-cavity absorption spectroscopic signal with the sensing system is feasible.To decrease the interference caused by amplified spontaneous emission and etalon noise, low-frequency wavelength sweep combined with wavelength modulation technique is applied to the system. The modulation parameters are determined through theoretical analysis and systematic investigations. Seventeen absorption lines for 1% acetylene gas are observed in the wavelengths region from 1526.5 nm to 1537 nm. To enhance the sensor performance, the selection of the tunable F-P filter and the mode of wavelength scanning are optimized. Twenty-seven absorption lines are observed between the wavelengths of 1525 nm to 1545 nm in wavelength sweep sensing experiments with the new system. The obvious improvement of sensitivity indicates that the optimization is efficient. The optimal optical path length of gas absorption for this system is studied using gas cells with different lengths.Spectra extraction is studied with four methods including wavelet transform zero-crossing, wavelet transform iteration and wavelet transform combined with morphology to extract absorption spectra from experimental spectral data. The correlation analysis of low-frequency modulation spectrum indicates that the harmonic spectrum is proportional to the Fourier component of modulation spectrum, which can be calculated by the discrete Fourier transform. For intra-cavity absorption modulation spectroscopy, the calibration method is proposed that the calculation of gas concentration and wavelength of absorption line are performed on the basis of the second harmonic and third harmonic, respectively. The quantitative measurement ability of the system is analyzed by using single-line method and multi-line average method to calculate concentration calibration error respectively.To avoid the uncertainty of the wavelength values of the absorption lines, the method to calibrate the wavelength value by introducing the Bragg gratings as reference for intra-cavity absorption spectroscopy is investigated. Firstly, introduce the Bragg grating array as wavelength reference. The peaks of gas absorption spectrum and grating reflection spectrum are found through Gaussian fitting. Secondly, use known grating wavelengths to create the calibration curve between driving voltages of F-P optic filter and the wavelengths by polynomial fitting. Finally, calculate the center wavelengths of absorption lines according to the driving voltages corresponding to the peaks. The experimental results show that the error in wavelength calibration is reduced to 0.1 nm, which meets the requirement for multi-component gas analysis.更多还原