Simultaneous detection of CH_(4)and CO_(2)through dual modulation off-axis integrated cavity output spectroscopy

This study established a novel method for the simultaneous detection of two-component gases.Radio frequency(RF)white noise disturbance laser current and wavelength modulation were simultaneously used to improve the off-axis integrated cavity output spectroscopy technique,and a high-precision dual modulation OA-ICOS(RF-WM-OA-ICOS)system was established.The two laser beams were coupled into one laser beam that was applied incident to the cavity of RF-WM-OA-ICOS system.The second harmonic signals of CH_(4)and CO_(2)gas simultaneously appeared in the rising or falling edge of a triangular wave.This method was used to measure CH_(4)and CO_(2)with different concentrations.The results indicated that the proposed system has high stability and can accurately and simultaneously measure the concentrations of CH_(4)and CO_(2),with an optimal integration time of 220 s.The minimum detection limit was 10 ppb for CH_(4)and 1.5 ppm for CO_(2).The corresponding noise equivalent absorption sensitivity values were calculated as 2.67×10^(-13)cm^(-1)·Hz^(-1/2)and 5.18×10^(-11)cm^(-1)·Hz^(-1/2),respectively.The proposed dual-component gas simultaneous detection method can also be used for high-precision simultaneous detection of other gases.Therefore,this study may serve as a reference for developing portable multicomponent gas analyzers.

Calibration-free wavelength modulation spectroscopy for gas concentration measurements under low-absorbance conditions

We derive the expressions of the first and second harmonic signals on the basis of absorption spectral and lock-in theories, and determine the gas concentration according to the ratio of second and first harmonic signals. It is found that the X and Y components of the harmonic signals are influenced by the phase shift between the detection and reference signal, and the phase shift can be any value in a range from 0 to 2π, which is different from the results obtained previously. Meanwhile, an additional item caused by the residual amplitude modulation will make a great contribution to the second harmonic signal, and may not be neglected under low absorbance conditions. Theoretical analysis indicates that subtracting back-ground signal from the second harmonic signal can remove the influence of this item, and can improve the measurement accuracy of gas concentration. On this basis, we select the transition of CO2 at 6527.64 cm-1 to analyse the approximation errors during the derivation by numerical simulation and then measure the CO2 concentration under low absorbance conditions, with absorbance varying from 1‰ to 6‰.

Highly-sensitive NO,NO2,and NH3 measurements with an open-multipass cell based on mid-infrared wavelength modulation spectroscopy

A compact prototype based on mid-infrared wavelength modulation spectroscopy（WMS）is developed for the simul-taneous monitoring of NO,NO2,and NH3 in the urban area.Three quantum cascade lasers（QCLs）with central frequencies around 1900.0 cm^-1,1600.0 cm^-1,and 1103.4 cm^-1are used for NO,NO2,and NH3detections,respectively,by timedivision multiplex.An open-path multi-pass cell of 60-m optical path length is applied to the instrument for high sensitivity and reducing the response time to less than 1 s.The prototype achieves a sub-ppb detection limit for all the three target gases with an average time of about 100 s.The instrument is installed in the Jiangsu environmental monitoring center to conduct performance tests on ambient air.Continuous 24-hour measurements show good agreement with the results of a reference instrument based on the chemiluminescence technique.

Sensitive absorption measurements of hydrogen sulfide at 1.578 μm using wavelength modulation spectroscopy

Sensitive detection of hydrogen sulfide（H2S） has been performed by means of wavelength modulation spectroscopy（WMS） near 1.578 μm. With the scan amplitude and the stability of the background baseline taken into account, the response time is 4 s for a 0.8 L multi-pass cell with a 56.7 m effective optical path length. Moreover, the linearity has been tested in the 0–50 ppmv range. The detection limit achievable by the Allan variance is 224 ppb within 24 s under room temperature and ambient pressure conditions. This tunable diode laser absorption spectroscopy（TDLAS） system for H2 S detection has the feasibility of real-time online monitoring in many applications.

Wavelength modulation spectroscopy for measurements of gas parameters in combustion field

A novel wavelength modulation spectroscopy sensor for studying gas properties near 1.4 μm is developed, validated and used in a direct-connect supersonic combustion test facility. In this sensor there are two H2O transitions near 7185.60 cm^-1 and 7454.45 cm^-1 that are used to enable the measurements along the line-of-sight. According to an iterative algorithm, the gas pressure, temperature and species mole fraction can be measured simultaneously. The new sensor is used in the isolator and extender of the supersonic combustion test facility. In the isolator, the sensor resolves the transient and measured pressure, temperature and H2O mole fraction with accuracies of 2.5%, 8.2%, and 7.2%, respectively. Due to the non-uniform characteristic in the extender, the measured results cannot precisely characterize gas properties, but they can qualitatively describe the distinctions of different zones or the changes or fluctuations of the gas parameters.

Absorption linewidth inversion with wavelength modulation spectroscopy

For absorption linewidth inversion with wavelength modulation spectroscopy（WMS）, an optimized WMS spectral line fitting method was demonstrated to infer absorption linewidth effectively, and the analytical expressions for relationships between Lorentzian linewidth and the separations of first harmonic peak-to-valley and second harmonic zero-crossing were deduced. The transition of CO_2 centered at 4991.25 cm^（-1） was used to verify the optimized spectral fitting method and the analytical expressions. Results showed that the optimized spectra fitting method was able to infer absorption accurately and compute more than 10 times faster than the commonly used numerical fitting procedure. The second harmonic zero-crossing separation method calculated an even 6 orders faster than the spectra fitting without losing any accuracy for Lorentzian dominated cases. Additionally, linewidth calculated through second harmonic zero-crossing was preferred for much smaller error than the first harmonic peak-to-valley separation method. The presented analytical expressions can also be used in on-line optical sensing applications, electron paramagnetic resonance, and further theoretical characterization of absorption lineshape.

Wavelength modulation spectroscopy at 1530.32 nm for measurements of acetylene based on Fabry–Perot tunable filter

Sensitive detection of acetylene（C_2H_2） is performed by absorption spectroscopy and wavelength modulation spectroscopy（WMS） based on Fiber Fabry–Perot tunable filter（FFP-TF） at 1530.32 nm. After being calibrated by Fiber Bragg Grating（FBG）, FFP-TF is frequency-multiplexed and modulated at 20 Hz and 2.5 kHz respectively to achieve wavelength modulation. The linearity with 0.9907 fitting coefficient is obtained by measuring different concentrations in a 100 ppmv–400 ppmv range. Furthermore, the stability of the system is analyzed by detecting 50 ppmv and 100 ppmv standard gases for 2 h under room temperature and ambient pressure conditions respectively. The precision of 11 ppmv is achieved by calculating the standard deviation. Therefore, the measuring system of C_2H_2 detection can be applied in practical applications.

WMS温度测量中谐波信号锁相放大器的设计

在波长调制光谱技术(Wavelength Modulation Spectroscopy,WMS)温度测量中,传统的锁相放大器测量谐波信号的振幅必须对被测信号及参考信号的相位单独进行调整。针对这一问题,设计了一种基于高精度平衡调制器AD630进行同步相敏检测的谐波信号的双锁相放大器。与传统的锁相放大器相比,所提双锁相放大器利用正交双通道的平方和检测,可以避免被测信号与参考信号进行严格的相位匹配问题。利用Multisim软件构建仿真模型,包括前置高通滤波器、正交双通道的相敏检波器、低通滤波器等部分。理论计算和仿真结果表明,所提方法能够准确检测微弱一次、二次、四次谐波信号的幅度,检测误差小于5%,具有电路简单、运行速度快、线性度高等优点,能够满足WMS温度测量工程应用。

可编程门阵列WM-TDLAS气体检测系统设计及应用

本文基于可编程逻辑门阵列(field programmable gate array,FPGA)快速处理数字信号及多线程优势,结合波长调制可调谐半导体激光吸收光谱(WM-TDLAS)技术,研制可编程门阵列WM-TDLAS二氧化碳浓度检测系统。根据应用功能需求,利用FPGA芯片的逻辑门阵列可编程特性,设计了具备信号采集及调制、谐波解调功能的数字化锁相放大器(digital lock-in amplifier,DLIA)。为验证其性能进行谐波提取测试、Q值、抗噪声实验,结果表明目标频率提取线性度达99.99%,Q值可达45。在对不同信噪比信号进行谐波提取实验中,当信噪比为43 dB时,均值最大相对误差仅为0.91%。采用中心波长1572 nm分布式反馈激光器作为光源,覆盖选定的6360 cm−1处吸收线,密集多通气体吸收池有效光程14 m,开展了气体浓度检测实验。系统检测浓度与二次谐波幅值拟合线性度为99.982%,通过提升扫描波长频率,系统可在0.1 s获取浓度值。艾伦(Allan)方差结果表明,当积分时间为44 s时,系统的检测下限为1.86 ppm。实验结果表明,该可编程门阵列WM-TDLAS检测系统具有检测精度高、响应速度快、稳定性强和小型化的特点,可用于实际应用中浓度实时监测。

Analysis of relative wavelength response characterization and its effects on scanned-WMS gas sensing

Our recently proposed three-step method showed the promising potential to improve the accuracy of relative wavelength response(RWR) characterization in the wavelength-modulation spectroscopy(WMS) over the commonly used summation method.A detailed comparison of the three-step method and the summation method,for the wavelength-scanned WMS gas-sensing,was performed with different laser parameters(modulation indexes and scan indexes) and gas properties(pressures and concentrations).Simulation results show that the accuracy of the predicted gas parameters is strongly limited by the RWR characterization with large modulation index and high gas pressure conditions.Both fitting residuals of RWR and errors of predicted gas parameters from the recently proposed three-step method are nearly 2 orders of magnitude smaller than those from the summation method.In addition,the three-step method is further improved by introducing a coupling term for the 2^(nd) harmonic amplitude.Experiments with CO_(2) absorption transition at 6976.2026 cm^(-1) were conducted and validated the simulation analysis.The modified-three-step method presents an improved accuracy in RWR description with at least 5% smaller fitting residual for all conditions compared with the three-step method,although the deviation of the deduced CO_(2) concentrations between these two methods does not exceed 0.2%.

基于WMS光纤气体传感器频率偏差的抑制

利用波长调制光谱(WMS)技术的光纤气体传感器进行痕量气体体积分数检测可以达到很高的检测灵敏度,离散数据采样过程中很难保证严格在吸收线中心频率处采样,频率偏差会使检测结果产生严重偏差。利用傅里叶级数展开分析WMS各次谐波信号,在采样过程中通过测量一次谐波的最大值与最小值取平均,获取与待测气体体积分数无关的一次谐波基线值,抑制频率偏差对系统检测精度的影响。以二次谐波峰值与一次谐波基线的比值作为系统的输出,有效地消除光强波动等因素带来的干扰。

WMS的玻璃药瓶内氧气含量检测及其标定方法改进

应用激光波长调制光谱(WMS)技术,建立了一种开放光路短光程检测玻璃药瓶内氧气含量的方法。选择氧气分子位于760.885(13 142.58cm-1)的吸收谱线,通过多次调试优化了系统相关参数,给出了实时扣背景及实时谱线校正等数据处理的方法和步骤。采集七种不同氧气含量的玻璃药瓶样本,获取相应的二次谐波信号,分别建立二次谐波峰值、半高谱峰面积与浓度的线性回归方程进行定量预测。实验结果表明,其拟合系数分别为0.996 6和0.997 8,后者相比前者的标定方法提高了0.12%。采用完全交互验证的方法来评价两个模型的预测精度,其预测的均方根误差(RMSEP)分别是0.003 1和0.002 0,后者相比前者降低了37.69%。对浓度是4%的气体样品,比较不同时间的20次测量结果,标准差分别为0.002 2和0.0016,后者相比前者降低了27.3%,同时其测量灵敏度分别为0.198%和0.097%,后者相比前者的灵敏度提高了约51%。证明了该系统及数据处理方法对玻璃药瓶内氧气含量检测是可行的,且利用半高谱峰面积更丰富的幅值信息来反演气体浓度可以降低波峰失真影响,检测精度更高,稳定性更好。

WMS-4f/2f气体流速测量的理论分析和数值仿真

气体流速的测量对于航空航天发动机的燃烧诊断等应用非常重要。针对现有可调谐激光吸收光谱测量方法的不足,提出基于波长调制光谱二次谐波归一化四次谐波（WMS-4f/2f）的检测方案。不同于目前采用的直接吸收光谱或波长调制光谱二次谐波检测方法,WMS-4f/2f方案可以得到更尖更窄的吸收峰,有利于更精准地确定多普勒频移。对此方法测量流速的不确定度进行了理论分析和数值仿真,结果表明,选用O2位于760.445nm的吸收线,所得气体流速不确定度为5.66m/s,与WMS-2f或直接吸收光谱方法相比不确定度降低了一个数量级。该方法在高速流场的流速测量领域具有巨大的应用潜力。

波长调制光谱(WMS)谐波信号分析与仿真

概述了采用波长调制光谱(WMS)技术进行痕量气体浓度测量的基本原理,对强度调制和频率调制结合的信号(IM-FM)模型进行了理论描述,对谐波信号随归一化调制振幅变化和信号对称性进行了重点分析以说明强度调制的影响。利用MATLAB软件,对甲烷在6046.9647cm-1处的吸收线进行仿真,以验证理论分析的正确性。

基于WMS技术的机动车尾气CO和CO2同时监测

基于对机动车尾气中CO和CO2同时测量的需求,提出并实现了利用WMS-2f/1f免校准技术实现两种气体的无标测量。结果表明:在选择恰当吸收谱线的基础上,对CO和CO2的混合气体进行浓度检测,并对测量结果进行Allan方差评价,认为在系统积分时间为2 s时,CO和CO2的最低检测限分别为0.76×10^-6和0.89×10^-6,所用激光传感器系统的精度满足机动车尾气的测量要求。

Preliminary study on direct measurements and diagnostics for chemical reaction dynamics of NOx by using laser wavelength modulation spectroscopy

Studies on the kinetics of gas-phase chemical reactions currently rely on calculations or simulations and lack simple,fast,and accurate direct measurement methods.We developed a tunable laser molecular absorption spectroscopy measurement system to achieve direct measurements of such reactions by using wavelength modulated spectroscopy and performed online measurements and diagnostics of molecular concentration,reaction temperature,and pressure change during the redox reaction of ozone with nitrogen oxides(NOx)with 0.1 s temporal resolution.This study provides a promising diagnostic tool for studying gas-phase chemical reaction kinetics.

基于VisuShrink小波阈值变换的甲烷检测降噪技术

在利用可调谐半导体激光吸收光谱技术探测甲烷气体泄漏时,复杂的环境光干扰会降低浓度检测信号的信噪比使波形失真。为了抑制噪声,增强电信号与浓度值的线性相关度,提出一种以VisuShrink方法改善小波阈值函数的小波阈值去噪算法。该算法针对小波阈值去噪算法的软硬度不能平衡信号,存在失真的问题,利用统计方法对阈值进行计算,弱化间断点阈值跳变影响。通过仿真计算对比未加噪声前的信号与滤波后的信号,提出算法的信噪比为28.405 30,优于四阶巴特沃斯滤波器。VisuShrink方法在SNR和NCC参数上性能远优于巴特沃斯滤波器,与软阈值函数相近,在MSE均方误差分析上远小于软阈值精度,与巴特沃斯相近,具有较好的平滑特性。实际搭建检测平台对多个浓度样本采样进行多次采样,采用VisuShrink方法设计的滤波器将系统误差精度从4.14%提升至2.472%,算法可有效用于甲烷检测系统提高光谱数据的平滑性和准确性。

基于TDLAS的近红外甲烷高灵敏检测技术

为增强甲烷气体检测技术的气体吸收率,提高检测灵敏度,利用可调谐二极管激光吸收光谱技术,采用中心波长为1 653.7 nm的分布反馈激光器作为光源,研制了有效光程为14.5 m的Herriott型气体吸收池,并采用波长调制光谱法进行甲烷气体浓度检测。结果表明,二次谐波峰值信号与甲烷气体浓度成较强的线性关系,线性度为0.998 52,检测下限为4.82 ppm;初始积分时间为1 s时的Allan方差为6.37 ppm;积分时间到112 s时,Allan方差为427 ppb,检测灵敏度为4.27×10^(-7)。

基于调制激光光谱吸收的高稳定性气体检测

为了精确测定待测气体的体积分数,提高可调谐二极管激光吸收光谱(TDLAS)的稳定性,优化了TDLAS气体检测系统的扫描信号。基于HITRAN数据库,选定了甲烷气体在1654 nm附近吸收峰,计算出含甲烷、水蒸气和二氧化碳混合气体的吸收系数;使用可视化仿真工具对气体检测系统进行理论仿真,同时通过搭建的气体检测系统验证仿真结果。结果表明,通过优化扫描信号,提高了检测系统的稳定性,偏差值从0.3%降至0.07%;建立了2次谐波信号均值与气体体积分数的反演模型,线性拟合的相关系数R^(2)=99.99%。此研究为提升TDLAS系统的稳定性和准确性,及实现高稳定气体检测具有一定的参考价值。

煤热解中痕量乙烯在线激光吸收光谱检测

为了实现煤热解过程中痕量乙烯(C2H4)标识气体的在线识别与检测,结合波长调制与长光程技术,搭建了煤热解乙烯在线激光吸收光谱检测装置。采用中心波长为1620 nm通信波段DFB激光器作为激光光源,有效光程为15 m的多光程吸收池为样品吸收池,利用SR830进行波长解调,通过二次谐波信号反演得到乙烯浓度。使用高精度流量控制器,利用高纯氮气稀释乙烯配比,制备得到10×10^(-6)到90×10^(-6)的标准乙烯样气,其线性拟合的相关系数R2为0.9989;对浓度为20×10^(-6)的乙烯进行连续4000 s的Allan方差分析,实验结果表明,检测极限为121×10^(-9)。为了研究不同气体氛围下煤热解过程中乙烯浓度的演化规律,控制气体流速为150 mL/min,分别在氮气、空气以及合成空气中对乙烯标识气体的释放过程进行热重分析实验。研究发现,当温度小于500℃时,3种气体环境下乙烯释放量较少且基本一致,当温度在500~700℃时,氮气环境中乙烯释放量要远高于其他两种气体,但空气中乙烯释放的增速最快,最大释放量约为40×10^(-6),温度高于700℃时乙烯释放量均开始减少。这一结果为进一步探索煤热解中乙烯的生成机理提供了理论和实验基础。