洛伦兹线型近似引起的甲烷检测误差研究

2ν₃波段R3分支(6 046.95 cm^-1)是近红外甲烷检测领域最常用波段。R3分支三条谱线相距很近,通常用一条谱线的洛伦兹线型去描述其谱形,校正温度、压强引起的二次谐波峰值误差,然而洛伦兹线型本身引起的误差并没有得到足够的重视。对TDLAS系统建模分析,以低频锯齿波叠加高频正弦波调制激光,经待检测气体吸收后,再利用数字锁相放大及低通滤波实现解调,最终通过旋转坐标系得到一次谐波归一化的二次谐波信号。通过分别比较单条谱线洛伦兹线型与三条谱线Voigt线型对二次谐波的影响,分析温度、压强变化条件下,由单条谱线洛伦兹线型近似带来的二次谐波误差。结果表明：(1)压强、温度变化时,洛伦兹线型二次谐波峰值误差较平均极小值误差更小;(2)洛伦兹线型二次谐波峰值的误差随着压强降低而显著增加,温度为298 K、压强降低至0.2 atm时,由洛伦兹线型近似带来甲烷气体二次谐波峰值的误差达65.5%;(3)以峰谷率、谐波宽度等参数衡量二次谐波谱形,在温度为298 K、压强小于0.8 atm条件下,峰谷率误差大于4.5%,压强为1 atm、温度大于380 K条件...     

基于吸收光谱的光程长度测量方法研究

基于吸收光谱的基本原理,通过计算比尔-朗伯定律数学表达式中的参数实现了光程长度的测量。分析了高斯线型、洛伦兹线型和Voigt线型,采用了Voigt线型对光谱信号进行拟合。研究了Voigt线型峰值计算方法、洛伦兹线宽计算和误差函数求解三个内容。利用可调谐半导体激光吸收谱技术(TDLAS)中的直接吸收谱技术测量了氧气的吸收光谱,得到拟合的光谱峰值数据。将峰值数据带入比尔-朗伯定律数学表达式中,计算出实验光程长度为66.55 cm。对比测量值66.04 cm,测量精度为0.78%,该方法用于光程长度测量是可行的。     

可调谐半导体激光吸收光谱技术检测痕量乙烯气体的系统研制

可调谐半导体激光吸收光谱技术(TDLAS)利用激光器的窄线宽和波长调谐特性,使其扫描被测气体的单个吸收峰,实现痕量气体的高分辨率、高灵敏度快速检测。通过分析近红外波段的乙烯吸收谱线特性,选取1 626.8 nm附近的吸收峰作为检测谱线,研制了基于white池结构的TDLAS检测系统,结合波长调制和二次谐波检测,对体积分数为20～1 200 ppmv的乙烯气体进行了测量,推算该系统的检测下限约为10 ppmv。     

开放型TDLAS-WMS技术CO2痕量气体检测

采用窄线宽、边模抑制高的DFB激光器研制一套开放型TDLAS波长调制技术气体检测装置.选取2004 nm处CO2分子吸收峰作为吸收谱线,采用锁相放大器进行调制、解调后的二次谐波信号幅值检测气体浓度大小.设计基于开放环境中的Herriott型气体吸收池,使用ZEMAX非序列模式进行吸收池仿真,光线追迹后理论光程可达到13...     

非标定波长调制吸收光谱气体测量研究

为消除可调谐激光调制吸收光谱气体测量技术对于标定过程的依赖,研究了二次谐波信号的非标定波长调制气体测量方法.通过对测量的二次谐波线型进行分析,给出相同工况下二次谐波模拟信号,并利用测量与模拟二次谐波信号进行线性拟合直接计算气体浓度.实验室内采用非标定波长调制气体测量方法,利用 6336.24 cm^-1处特征吸收谱线对10 cm长气体吸收池内的CO₂进行了测量.结果表明,非标定波长调制气体测量方法可适应各种不同条件,适合于现场气体在线测量.当调制系数在1.8—3. 关键词： 波长调制 二次谐波 吸收光谱 半导体激光器     

大气环境条件下波长调制光谱无标测量的压强修正

可调谐激光吸收光谱技术(TDLAS)由于其高灵敏度、高选择性等优势广泛用于痕量气体检测领域。然而其测量结果容易受到目标气体压力波动的影响,特别是在大气环境下尤为明显,现有方法多为在现场安装压力传感器,对测量结果进行校正。提出了一种无需压力传感装置的气体浓度修正方法。选取碰撞展宽占主导地位的气体吸收谱线,分别建立谱线展宽与波长调制光谱一次谐波(WMS-1f)信号的峰谷值间距和二次谐波(WMS-2f)过零点间距的解析表达式,通过测量一次谐波峰谷值间距或二次谐波过零点间距直接得到被测气体压强,进而利用波长调制光谱一次谐波归一化的二次谐波(WMS-2f/1f)技术补偿测量环境中压力波动对气体浓度测量结果的影响。实验以浓度为1 980 mg·m~(-3)的CO_2为目标气体,选取其位于4 989.97 cm~(-1)的吸收作为目标谱线,在大气压附近进行不同调制深度的变压力测量实验,通过实验分析了压强变化对二氧化碳吸收谱线谐波信号的影响,利用一次谐波峰谷值间距和二次谐波过零点间距分别反演了气体压强,并与气体压强传感器测得的压强数据进行对比,压强偏差在1%以内,验证了通过谐波间距解析表达式计算压强的正确性及通过测量谐波间距对浓度补偿的可行性。最后利用WMS-2f/1f技术和通过谐波间距测得的压强数据对气体浓度进行压强补偿修正,结果表明通过测量谐波间距修正后的浓度与通过高精度压力表补偿后浓度相比误差小于2%,与通过谐波间距推导得出的压力不确定度(小于2%)一致,验证了该方法的可行性和有效性,进一步提高了TDLAS技术在压强波动较大环境下进行气体浓度检测的测量精度。利用谐波间距对气体浓度补偿的方法无需额外的气体压力传感器,简单易行,特别适合于大气环境中气体成分的高灵敏高精度开放光路遥测,也可用于气体浓度和压强的同时测量。     

基于可调谐半导体激光吸收光谱的氧气测量方法的研究

O₂是工业过程中广泛应用的重要气体, 在工业生产环境下实现O₂浓度的快速在线检测对提高燃烧效率和节能减排具有重要的意义。可调谐半导体激光吸收光谱谐波探测技术是一种具有高灵敏、高选择性、快速响应等特点的气体检测新技术,该技术利用了半导体激光器的可调谐和窄线宽特性,通过精心选择待测气体的某条吸收线可排除其他气体的干扰,实现待测气体浓度的高灵敏快速在线检测。文章以可调谐分布反馈(Distributed feedback, DFB)半导体激光器作为光源,通过波长调制方法对760 nm附近氧气某一吸收线的二次谐波信号测量,从而实现了对氧气浓度的快速在线检测。系统指标达到：检测范围0.01%～20%;检测精度0.1%;长期稳定性1%。     

全频域下窄线宽激光器光谱线宽的分析

窄线宽激光器的线宽表征方式通常采用延时自外差法测量技术。该技术是通过延时光纤差拍产生一个与待测激光线宽相关的洛伦兹频谱,因此该频谱只具有单一的线宽表现形式。为了能够观察到激光器的线宽和频率噪声在其傅里叶频率分布下的完整特性,报道了一种基于β算法计算窄线宽激光器线宽的方法。该方法是结合频率噪声中的白噪声和1/f噪声分别诱导不同激光线型的理论,从而确定激光线宽。首先,对β算法的基本原理进行了详细的分析说明。通过基于维纳-辛钦定理,分析了窄线宽激光器不同频率范围内的频率噪声和激光线宽的依赖关系。阐明了在截止频率趋于0和无穷大的两个范围条件时,激光频谱特性从高斯线型向洛伦兹线型演变。同时推导出使两种线型转换的截止频率表达式,并将其转换为频率噪声函数,该函数定义为β分子线。此时频率噪声分量中高斯线型的总和即为激光线宽计算公式;其次,对窄线宽激光器的频率噪声和激光线型进行数值仿真。将通过OEwaves公司的OE4000互相关零差相位/频率噪声自动测试系统测得的频率噪声谱密度,带入β算法理论公式中。结果显示：1/f噪声导致激光呈现高斯线型,线宽随截止频率的增加而增大。而白噪声将导致洛伦兹线型,线宽不再随截止频率而改变。此外,在低频区域,频率噪声电平远大于其傅里叶频率,噪声调制系数较高,该部分噪声可以决定线宽大小。因此,高斯线型区域对应的频率噪声的积分,即为待测激光器的线宽;在高频区域,频率噪声电平与其傅里叶频率相差较小,频率波动较快,噪声对线宽影响可以忽略。并且频率带宽在截止频率范围内,计算的线宽误差较小。最后,实验上运用β算法对RIO公司的1 550 nm低噪声窄线宽激光器的频率噪声功率谱密度进行积分计算,成功获得了其不同傅里叶频率分布下对应的激光线宽值。其中β分子线将频率噪声中的白噪声和1/f噪声分隔两部分：当频率噪声谱密度大于β分子线时,激光即为高斯线型,线宽随频率积分带宽的增加而减少;而频率噪声谱密度小于β分子线时,激光呈现洛伦兹线型,线宽为定值不再改变。同时为了对β算法进行实验验证,搭建了延迟光纤为50 km、移频频率为60 MHz的延时自外差法测量系统。对注入电流为110 mA的RIO 1 550 nm低噪声窄线宽激光器的线宽进行实验测量,测量结果表明激光线宽为1.8 kHz,与上述β算法中2.8 kHz的频率带宽积分结果一致。充分证明了此算法的准确性。β算法可以对任意类型的窄线宽激光器进行线宽表征,对窄线宽激光器的研究具有重要意义。     

CO和CO2的1.58μm波段可调谐二极管激光吸收光谱的二次谐波检测研究

红外可调谐二极管激光吸收光谱技术作为新的痕量气体监测分析方法,在大气化学研究和污染气体监测中得到了应用.文章介绍利用可调谐二极管激光吸收光谱学(TDLAS)技术对常压下的CO和CO2的近红外波段光谱静态测量研究和洛伦兹线型下的CO2二次谐波光谱与调制度的关系.     

TDLAS测量CO2的温度影响修正方法研究

可调谐二极管激光吸收光谱(TDLAS)技术测量CO₂浓度时,由于测量氛围温度变化的影响引起气体吸收谱线的线强和线型发生变化,最终导致浓度测量存在较大误差。为了克服温度变化对浓度测量的影响,选用中心波长在1 580 nm的DFB激光器,基于直接吸收法,模拟电厂尾部烟道内的高浓度二氧化碳气体环境,研究了在常温(298 K)和变温(298~338 K、间隔10 K)不同温度工况下CO₂浓度的测量。结果显示,常温浓度测量的最大相对误差为-5.26%,最小相对误差为1.25%,相对误差均方值为3.39%,验证了TDLAS测量系统在常温下有着良好的测量精度和稳定性,但其在变温测量时浓度测量结果误差较大,其最大相对误差已经超过25%。为了修正温度变化对浓度测量结果的影响,适应工业测量的需要,在变温测量基础上利用最小二乘法拟合出测量系统在不同温度下的浓度与气体吸收的修正关系式。经过修正后,CO₂浓度测量的相对误差降到5%以下,相对误差均方值降到3.5%以下。修正结果表明,所提出的修正方法可以有效抑制温度变化对浓度测量结果的影响,显著提高了测量系统在变温环境下的测量精度和稳定性,为TDLAS系统测量CO₂浓度的现场应用提供了理论支持和技术保障。     

基于可调谐激光吸收光谱技术的二硫化碳中红外光谱参数测量

采用4.6μm附近的量子级联激光器作为光源,搭建了一套二硫化碳(CS2)吸收光谱测量系统,结合可调谐二极管激光吸收光谱技术,对光谱范围为2178.99—2180.79 cm-1的CS2吸收光谱展开了深入研究,重点测量了2180.5—2180.74 cm~(-1)的四条吸收谱线,利用基于非线性最小二乘的多元线性回归算法对CS2吸收光谱进行拟合,精确得到了该范围内谱线的中心波长、线强以及空气展宽系数等光谱参数.经计算,对应谱线线强不确定度小于5%,空气展宽系数不确定度小于15%,这个结果可作为免标定CS2红外光谱探测的基础光谱参数,对痕量CS2气体传感具有重要意义.未来我们将进一步开展2170—2200 cm-1整个谱段的CS2谱线参数的测量,以期填补其在HITRAN和GEISA数据库光谱参数的空白.     

Mode-locked pulses in multi-atmosphere CO2 lasers

The present paper deals with AM mode locking of multiatmosphere CO₂ lasers. We measure the effective linewidth in the laser mixtures and deduce from these data the pulse width. The method is based on determining the maximum frequency shift from the synchronized frequency. The advantage of this method is that the results are not influenced by time constant of detector and display. The results were verified by direct measurements of the pulse widths in mixtures up till three atmospheres. The pulse widths and effective line widths of the P(20) transition are determined for the usual CO₂ laser mixture up till 6 atm. We observe that helium has a large effect on the pulse width. The width decreases with increasing helium percentage at all pressures investigated. We suggest that this has to do with the overlap of a sequence-band transition and that the effect of sequence bands depend ion the helium percentage.     

Temporal evolution of plasma from a highly ionized helium capillarydischarge

The generation of a highly ionized helium capillary plasma and the study of its temporal evolution are discussed. A 30-cm-long and 1-mm-diameter helium plasma was created with well-terminated kiloamp current pulses of 90-ns full width at half-maximum (FWHM). Emission spectroscopy was used to study the recombination of totally stripped ions into hydrogenic helium ions and to measure the evolution of the plasma density from the Stark broadening of HeII transitions. A 1.2-kA discharge current pulse was observed to create a plasma density of 8×10¹⁶ cm^-3 in 1 torr of helium. The maximum intensity of HeII transitions occurs in the afterglow of the discharge pulse, following the collisional recombination of totally stripped ions with plasma electrons when the plasma cools. The study is of interest in relation to the possibility of obtaining amplification in the 164.0-nm line of HeII in a capillary discharge     

基于自吸收量化的激光诱导等离子体表征方法

为了表征激光诱导等离子体的定量特征参数,提出了一种谱线自吸收量化的方法,通过获得分析元素谱线的半高全宽来量化谱线自吸收程度,进而得到等离子体的特征参数,包括电子温度、元素含量比以及辐射物质的绝对数密度.与传统激光诱导击穿光谱定量分析方法相比,新方法由于计算过程与谱线强度弱相关,所以分析结果基本不受自吸收效应的影响,同时也无需额外的光谱效率校准.基于铝锂合金的实验结果表明,该方法能够实现精确的相对定量分析和等离子体的特性诊断.     

Line parameters of HCl obtained by simultaneous analysis of spectra

Line parameters of the 1-0 band of HCl have been retrieved from spectra of finite resolution (0.1 cm^-1) obtained with a Fourier transform spectrometer which gives the spectral transmittance of the sample at intervals larger than the Lorentzian widths of the lines, by using a non-linear, least-squares method to analyze two spectra simultaneously. The accuracy of this technique is demonstrated by comparing the retrieved parameters with previously reported values. Line positions are obtained with a precision of 0.005 cm^-1 and the intensities and widths agree within the estimated accuracy of 5%. Evidence of line shifts due to N₂ perturbation has been obtained.     

Thermal infrared lines of methane broadened by nitrogen at low temperatures

Measurements of spectral transmittance in the v₄-fundamental band of ¹²CH₄ have been performed at low temperatures using a Fourier transform spectrometer with apodized spectral resolution of 0.06 cm^-1. With applications to lines formed in the atmospheres of Titan and Earth in mind, N₂ has been used as the broadening gas. Comparisons of observed and computed spectral transmittances on a line-by-line basis have yielded line strengths, N₂-broadened half-widths and their variation with temperature. Best agreement between measured and computed spectra was obtained when the absolute intensity of the band was taken as 128 cm^-2-atm^-1 at 296 K. Line widths were found to vary as Tⁿ with n = -1.0 for lines of the F-species and 0.63 for the A-species. Our measured line widths are considerably larger than those used in the AFGL compilation.     

三种二元水溶液体系中氢键对拉曼线宽的影响

采用显微共聚焦拉曼光谱对三种二元水溶液(乙腈/水、二甲基亚砜(DMSO)/水和丙酮/水)进行测量,得到含有氢键作用的水溶液体系拉曼频移和线宽随浓度的变化规律。应用混合模型和相互作用体系的线宽经验公式对实验结果进行了分析。结果表明,含有氢键作用的水溶液体系中,氢键作用越强线宽越大,并且线宽随单位浓度变化率大;同种氢键体系中,线宽不仅受浓度浮动的影响,氢键作用也是一个重要的影响因素,定量分析证明线宽与浓度、氢键作用关系很好地符合Ojha等提出的线宽公式。     

13CH4分子v3振动带空气和氮气加宽系数温度依赖性研究

采用中红外波段连续可调谐二极管激光器和自行研制的低温吸收池, 测量了温度为296 K, 252 K, 213 K, 173 K时, 3.38 μm附近¹³CH₄分子的四条跃迁谱线的氮气和空气加宽光谱; 首次通过实验获得空气和氮气对¹³CH₄分子的碰撞加宽系数, 以及谱线加宽系数的温度依赖系数. 实验过程中, 利用Voigt线型对所测量的光谱进行拟合. 实验结果表明, 氮气和空气对¹³CH₄分子的碰撞诱导加宽系数随温度的降低而增大; 相同温度下, 氮气对¹³CH₄分子的碰撞加宽系数普遍大于空气加宽系数. 实验数据为地球和外星体大气遥感探测提供了依据.     

Some effects of radiative and collisional broadening on line emission in a spherical,laser-heated plasma

We have generalized our treatment of resonance-line radiation from a spherical, laser-heated plasma by including the effects of radiative and collisional line broadening in representing the line-emission and absorption profiles by a Voigt function. We find that the emitted profiles are broadened by factors of five or more, depending on the magnitude of the damping constant. It is also shown that the Al(XIII) to Al(XII) line ratios are seriously affected by the different escape probabilities associated with Doppler and Voigt profiles. We also find that plasma cooling via resonance-line radiation is accelerated relative to that calculated with a pure Doppler profile, and that use of the Voigt profile decreases the density of excited states relative to the Doppler values. These phenomena are analyzed in terms of the differing profile shapes and widths.     

Effects of pulse energy ratios on plasma characteristics of dual-pulse fiber-optic laser-induced breakdown spectroscopy

Laser-induced plasmas of dual-pulse fiber-optic laser-induced breakdown spectroscopy with different pulse energy ratios are studied by using the optical emission spectroscopy(OES)and fast imaging.The energy of the two laser pulses is independently adjusted within 0–30 m J with the total energy fixed at 30 m J.The inter-pulse delay remains 450 ns constantly.As the energy share of the first pulse increases,a similar bimodal variation trend of line intensities is observed.The two peaks are obtained at the point where the first pulse is half or twice of the second one,and the maximum spectral enhancement is at the first peak.The bimodal variation trend is induced by the change in the dominated mechanism of dual-pulse excitation with the trough between the two peaks caused by the weak coupling between the two mechanisms.By increasing the first pulse energy,there is a transition from the ablation enhancement dominance near the first peak to the plasma reheating dominance near the second peak.The calculations of plasma temperature and electron number density are consistent with the bimodal trend,which have the values of 17024.47 K,2.75×10¹⁷cm;and 12215.93 K,1.17×10¹⁷cm;at a time delay of 550 ns.In addition,the difference between the two peaks decreases with time delay.With the increase in the first pulse energy share,the plasma morphology undergoes a transformation from hemispherical to shiny-dot and to oblate-cylinder structure during the second laser irradiation from the recorded images by using an intensified charge-coupled device(ICCD)camera.Correspondingly,the peak expansion distance of the plasma front first decreases significantly from 1.99 mm in the single-pulse case to 1.34 mm at 12/18(dominated by ablation enhancement)and then increases slightly with increasing the plasma reheating effect.The variations in plasma dynamics verify that the change of pulse energy ratios leads to a transformation in the dual-pulse excitation mechanism.