Pulsed laser surface fragmentation and mid-infrared laser spectroscopy for remote detection of explosives

Stand-off technology for the remote detection of explosives and their traces on contaminated surfaces is a field of research that has recently gained much interest. Optical methods are well established in applications for counterterrorism because they facilitate analysis without contact between human being and hazardous materials. In this paper, to our knowledge for the first time, a remote stand-off detection system is developed by combination of pulsed laser fragmentation and pulsed mid-infrared laser absorption spectroscopy. Since the absorption of explosives is more efficient for infrared wavelengths laser radiation in the eye safe region around λ=1.47 μm rather than the conventional Nd:YAG laser line at λ=1.06 μm is preferred for the fragmentation. Generated product gases such as nitric oxide are probed by a synchronized distributed feedback quantum cascade laser (DFB-QCL) at λ≈5.3 μm. The ratio of NO and NO₂ is a measure to distinguish between energetic and non-energetic materials. PACS 42.62.Fi; 07.07.DF; 42.55.Px     

Co-laser photoacoustic spectroscopy of gases and vapours for trace gas analysis

This comprehensive study reviews the sensitive and selective detection of trace gases by laser photoacoustic spectroscopy. A computer controlled CO-laser photoacoustic system is presented. The wavelength range between 5.0 and 6.5 μm is of great interest because it coincides with specific absorption bands of various gases and vapours of environmental concern. However, since water-vapour, which is present in most samples, absorbs rather strongly in this wavelength region, its contribution to the total absorption has to be determined with high accuracy. For this purpose, we developed a dual-beam setup with sample- and reference cell. The novel design of these resonant photoacoustic cells is based on a new matrix formalism with distributed acoustic impedances and sources.Our photoacoustic system is calibrated with certified gas mixtures and vapours. In total, the absorption cross sections of 18 gases and vapours have been derived for all CO-laser lines. In addition to the absorption, the relaxation time of vibrationally excited nitric oxide (NO) could be deduced by measuring the phase shift of the photoacoustic signal for a sample of nitrogen containing traces of NO and water-vapour.The main application concerns the detection of multiple components in gas mixtures, particularly in exhaust gases. The high sensitivity achieved permits the detection of trace gases at ppbv concentrations. The problem of interfering absorptions occurring for multicomponent mixtures is studied in detail. We discuss measurements and results on exhausts of various vehicles. The photoacoustic spectra of the exhaust samples are analyzed on the basis of the calibration spectra with the aid of an iterative mathematical procedure.The individual concentrations of 12 of the most important components including nitric oxide, olefines, aromatic hydrocarbons and aldehydes could be derived. In particular, the selective detection of the different isomers of xylene is emphasized.     

基于QCL的红外吸收光谱技术的研究进展

量子级联激光器作为一种新型的单极型半导体激光器,其峰值发射波长处于中红外波段（2.5~25 μm）,具有功率高、线宽窄、响应速率快等传统半导体激光器所没有的独特优势,且具有较高的探测灵敏度,非常适合中红外波段的气体分子的检测。可广泛应用于大气痕量气体、呼吸气体、燃烧气体、生化气体、机动车尾气、工业废气以及农药残留气体等低浓度气体的检测。因此,利用量子级联激光器对气体分子进行探测在非侵入式医学诊断、环境监测以及工农业生产等领域都具有十分重要的意义。自20世纪末量子级联激光器发明以来,室温激光器的性能得到了长足的进步,也出现了多种结构形式的量子级联激光器。这也使得量子级联激光器红外吸收光谱技术得到了很大的发展。事实上,很多光谱技术在量子级联激光器发明之前就已经得到了发展和应用,而利用量子级联激光器作为光源则在很大程度上扩展了可探测波段,也在一定程度上提高了探测极限。这其中就包括了直接吸收光谱技术、波长调制技术、腔衰荡光谱技术、腔增强吸收光谱技术以及光声光谱等。综述了国内外量子级联激光器进行红外吸收光谱技术的研究现状和发展趋势,分析了量子级联激光器红外吸收光谱技术在发展过程中所遇到的瓶颈以及后期得到的解决方案,比较详细地介绍了各种方法的原理、应用,并指出了在吸收光谱测量中的优缺点,同时对外场痕量气体探测作了简要总结。最后,对量子级联激光器红外吸收光谱技术在未来痕量气体探测上的应用和发展进行了展望,指出随着红外吸收光谱技术的快速发展,这些方法可以得到更有效的改进和发展,进而朝着高灵敏度、高集成度以及高时效方向发展。     

激光诱导击穿光谱与拉曼光谱技术在危险物检测中的研究进展

炸药、生物及化学危险物检测在反恐和公共安全领域具有重要应用价值,也是目前亟需解决的问题。激光诱导击穿光谱技术利用高能激光脉冲诱导材料产生等离子体,通过探测等离子体辐射光谱从而分析其组成成分。拉曼光谱技术是基于非弹性光散射的一种光谱检测方法,可以反映分子的振动信息。由于它们都具有快速和非接触遥测的优点,成为最有发展潜力和应用前景的危险物检测技术。介绍了激光诱导击穿光谱、拉曼光谱以及二者联合探测技术在危险物检测中的国内外发展现状,并对各自的优缺点进行了分析。激光诱导击穿光谱信号强、实时性好,但重复性差、基底效应影响显著,在判别组成元素相同而分子结构不同的危险物和干扰物时面临巨大挑战。拉曼光谱能够提供被测物的分子信息,适合于鉴别有机危险物,但信号弱、受荧光干扰大、检测低浓度样品及分析混合物的能力弱,外场使用时受周围杂散光以及环境变化的影响大。将这两种光谱探测技术相融合,发挥各自的优点,可以有效地提高探测危险物的准确度。但两种光谱联合探测系统结构和数据处理复杂,成本高,还有许多技术难点亟需解决。文章最后,对危险物激光诱导击穿光谱和拉曼光谱研究的前景进行了展望。     

Hollow-waveguide-based carbon dioxide sensor for capnography

A CO2 sensor for capnography, based on a hollow waveguide(HWG) and tunable diode laser absorption spectroscopy(TDLAS), is presented; the sensor uses direct absorption spectroscopy and requires neither frequent calibration nor optical filters, giving it a significant advantage over existing techniques. Because of the HWG, the CO2 measurement achieved a concentration resolution of 60 ppm at a measurement rate of 25 Hz, as characterized by Allan variance. The length of the HWG was selected to efficiently suppress the optical fringes. This setup is perfectly suited for the detection of CO2 by capnography, and shows promise for the potential detection of other breath gases.     

基于室温量子级联激光器的脉内光谱技术测量N_2O

中红外激光光源覆盖了大鼍气体的基频吸收带,尤其适合于痕量气体的高灵敏检测.其中具有高输出功率、宽调谐范围、能够在室温工作的量子级联激光器为高灵敏痕量气体检测技术的研究提供了理想的光源.基于脉冲量子级联激光器的脉内光谱技术提供了一种简单而又有效的测量痕量气体的方法.当一个长激发脉冲作用在激光器上时,激光器的频率随着脉冲持续时间的增加而线性减小,从而在单个脉冲上扫描出被测气体分子的特征吸收谱线,实现对目标气体的定性或者定量分析.介绍了基于分布式量子级联激光器的脉内光谱技术,并采用该技术对N_2O进行了光谱测量.一个500 ns的长激发脉冲应用在脉冲量子级联激光器上用于快速波长扫描,达到接近1 cm~(-1)的线性调谐范围,得到了中心在1 274 cm~(-1)附近的N_2O的吸收谱线,与HITRAN数据库相应的N_2O吸收谱线有着良好的一致性.     

基于可调谐激光吸收光谱的大气甲烷监测仪

可调谐二极管激光吸收光谱(TDLAS)技术利用二极管激光器波长调谐特性,获得被测气体的特征吸收光谱范围内的吸收光谱,从而对污染气体进行定性或者定量分析,这种方法不仅精度较高,选择性强而且响应速度快,已经被用于大气痕量气体监测以及工业控制。在对空气中的痕量气体进行检测中,由于气体浓度较低,需要和长吸收光程技术相结合。将可调谐二极管激光吸收光谱与经过108次反射后达到27 m光程的多次反射池相结合研制了用于地面环境空气中甲烷含量监测的便携式吸收光谱仪,并结合了用于微弱信号检测的二次谐波检测技术,从而达到了体积分数低于1×10-7的检测限,并利用不同体积分数的甲烷气体对系统进行了测试,得到了很好的测试结果。     

基于便携式柱面镜多通池的二氧化碳高灵敏度探测研究

二氧化碳作为大气中重要的温室气体，与气候变化和人类活动密切相关，因此对其浓度的探测具有重要意义。利用近红外可调谐二极管激光器结合自主设计的便携式小型化柱面镜光学多通吸收池，实现了二氧化碳气体的高灵敏探测。通过Matlab编写光线传输矩阵，优化设计了基于柱面镜的光学多通吸收池，相比于传统Herriott型多通池，具有腔镜利用面积高、在相同体积内可实现有效光程长等特点，在物理基长为15 cm的情况下，实现了14 m的有效光程。实验中使用中心波长为1.57 μm的DFB二极管激光器，采用直接吸收光谱方法对CO₂气体进行了探测研究，并用Allan方差对系统性能进行了分析。结果表明，在平均时间为5 s时，系统的探测灵敏度为33.1 μL·L-1，平均时间为235 s时，系统的探测灵敏度可达到5.3 μL·L-1。此外，利用该系统实现了大气中CO₂的探测，得到大气中的CO₂浓度为383.4 μL·L-1。基于柱面镜多通池搭建的可调谐激光吸收光谱(TDLAS)系统，结合了柱面镜多通池可在小体积内实现长光程和可调谐激光吸收光谱技术高灵敏度、高分辨率、快速响应的优点，大大减小了系统体积，提高了系统探测灵敏度，在气体探测领域有广泛的应用。     

Laser photoacoustic spectroscopy helps fight terrorism: High sensitivity detection of chemical Warfare Agent and explosives

Tunable laser photoacoustic spectroscopy is maturing rapidly in its applications to real world problems. One of the burning problems of the current turbulent times is the threat of terrorist acts against civilian population. This threat appears in two distinct forms. The first is the potential release of chemical warfare agents (CWA), such as the nerve agents, in a crowded environment. An example of this is the release of Sarin by Aum Shinrikyo sect in a crowded Tokyo subway in 1995. An example of the second terrorist threat is the ever-present possible suicide bomber in crowded environment such as airports, markets and large buildings. Minimizing the impact of both of these threats requires early detection of the presence of the CWAs and explosives. Photoacoustic spectroscopy is an exquisitely sensitive technique for the detection of trace gaseous species, a property that Pranalytica has extensively exploited in its CO2 laser based commercial instrumentation for the sub-ppb level detection of a number of industrially important gases including ammonia, ethylene, acrolein, sulfur hexafluoride, phosphine, arsine, boron trichloride and boron trifluoride. In this presentation, I will focus, however, on our recent use of broadly tunable single frequency high power room temperature quantum cascade lasers (QCL) for the detection of the CWAs and explosives. Using external grating cavity geometry, we have developed room temperature QCLs that produce continuously tunable single frequency CW power output in excess of 300 mW at wavelengths covering 5μm to 12μm. I will present data that show a CWA detection capability at ppb levels with false alarm rates below 1:108. I will also show the capability of detecting a variety of explosives at a ppb level, again with very low false alarm rates. Among the explosives, we have demonstrated the capability of detecting homemade explosives such as triacetone triperoxide and its liquid precursor, acetone which is a common household liquid. This capability, deployed at airports and other public places, will go a long way towards increasing public safety and minimizing inconveniences faced in airline travel.     

调谐半导体激光吸收光谱自平衡检测方法研究

可调谐半导体激光吸收光谱技术(TDLAS)是利用半导体激光器的波长调谐特性,扫描待测气体特征吸收线,从而获得待测气体的浓度信息。基于可调谐半导体激光吸收光谱的自平衡检测方法能够有效地消除激光器光强波动等共模噪声和其他同性干扰的影响。实验表明自平衡检测方法可以获得较理想的结果,检测限低于体积比1.2×10-6,与直接吸收光谱法相比降低了一个数量级。自平衡检测电路简单,自带的电子增益补偿机制能够自动进行平衡探测,该方法不用加信号调制和锁相放大器,直接探测待测气体的吸收光谱,从而降低成本,减小系统装置体积,易于集成为便携式痕量气体检测仪。     

基于中红外DFG光源的甲烷气体光谱检测方法研究

基于中红外光源的气体光谱检测是新的痕量气体监测与分析方法,在大气监测领域具有重要的应用。构建了一套基于中红外DFG光源的甲烷气体光谱检测系统。该系统以1 550 nm和1 060 nm波段可调谐半导体激光器作为基频光源,采用PPLN晶体作为差频非线性变频器件,实现了3.3 μm处的窄线宽可调谐中红外光源输出。实验结果表明,当PPLN晶体工作温度为99.5℃时,闲频光的输出功率为112 μW,差频转换效率达到1.246 mW/W2。晶体的温度接受带宽为4.3℃,泵浦光波长接受带宽为5.3 nm。在此基础上,分别利用直接吸收法和谐波检测法获得了3 028.751 cm-1处的甲烷气体吸收光谱和二次谐波检测信号。     

Nitric oxide detection inside the cylinder of an SI engine by direct UV absorption spectroscopy

The present work is a first step for the implementation and the development of a rapid and quantitative measurement technique using the UV broadband-lamp absorption spectroscopy. This technique is applied to detect nitric oxide molecules inside the combustion chamber of a methane-fueled spark-ignition engine. The ability of this technique to detect NO in very harsh thermodynamic conditions is proved under certain engine operating conditions. In spite of the broadband absorption, mainly due to the hot CO₂ and H₂O, clear NO absorption γ-bands are observed when the engine is skip-fired or when NO is added at the intake port. It was however, not possible to detect any nitric oxide absorption during the combustion time when each cycle was ignited and no nitric oxide was added. Finally, using the skip-ignition mode, the acquired NO absorption spectra allowed to conclude that evacuation of the in-cylinder residual gases can be complete at the end of the sixth cycle.     

基于TDLAS平衡差分技术的CO气体检测

利用可调谐半导体激光吸收光谱(TDLAS)结合平衡差分探测技术测量了1.578 μm附近的CO气体3-0带P(4)跃迁在不同压强和不同浓度下的吸收光谱信号。由于平衡差分探测方法可以有效地抑制激光光强波动、温度漂移和机械振动等共模噪声,从而提高了光谱探测灵敏度。通过与直接吸收信号相比,平衡差分的信噪比提高了3.4倍,探测极限为87 ppmv。测量了浓度为1%压强为40,55,70和85 Torr时的CO气体,结果显示在70 Torr时其光谱信号最强。并且,利用直接吸收和平衡差分技术测量了不同浓度的CO气体在总压强在70 Torr时的光谱信号,发现平衡差分技术光谱强度与浓度的关系线性度符合较好,其测量误差小于5%。为了进一步验证系统的稳定性,连续采集了324 s的光谱信号,最后通过Allan方差分析,发现本实验系统的最佳探测时间为38 s,探测极限为47.8 ppmv。     

基于原子和分子谱线分析的LIBS快速测量CO_2

作为温室气体的主要成分,CO_2的排放控制有利于应对全球气候变暖以及生态环境变化,对CO_2的快速检测具有重要意义。目前检测CO_2的方法有滴定法,电化学法,气相色谱法,红外吸收光谱法等,但对应用于工业现场的在线监测还存在着不足。激光诱导击穿光谱(LIBS)具有远程测量,无需或仅需简单预处理,多组分同步测量等优点,本文提出将其应用于CO_2在线监测,期望发展适用于工业过程碳排放的在线监测技术。利用质量流量控制器控制纯度为99.99%的CO_2和N2配比形成不同CO_2浓度的混合气体模拟烟气环境,经过混气瓶充分混合后送入密封样品池进行LIBS测量实验。研究不同延迟时间下C247.86nm和CN38.34nm谱线的演化规律,验证了等离子体形成过程中存在部分CO_2分子解离反应生成CN分子,在CO_2定量分析时应考虑CN分子谱线的影响,并获得同步测量C原子和分子谱线的最佳延迟时间为800ns。在此基础上,由于等离子体演化过程中,各种信息相互影响,分析指标与多个测量参数存在关系,综合考虑C原子、CN碎片及修正高浓度影响下的自吸收效应,采用多元回归分析方法建立了CO_2定量分析曲线,其拟合度R~2和斜率分别达到了0.978和0.981,结果表明相比单个指标直接定标,该方法提高了定量分析模型的可靠性,验证了LIBS技术快速测量CO_2的可行性。     

Collisional broadening and shifting of ammonia absorption lines at 790 nm

By using a diode laser spectrometer based on a commercial heterostructure diode laser operating in free-running mode, line shape parameters of some ammonia ro-vibrational overtones at 790 nm have been measured at room temperature. These weak absorption lines have been detected by using the wavelength modulation spectroscopy technique with second-harmonic detection. The broadening and shifting coefficients have been obtained for ten absorption lines by fitting the collected second-harmonic absorption features and varying the pressure of different buffer gases. Received 13 February 2002 / Received in final form 18 September 2002 Published online 17 December 2002 RID="a" ID="a"e-mail: alex@ifam.pi.cnr.it     

Non-contact detection of explosives by means of a tunable diode laser spectroscopy

As explosives are unstable, their decay products may be targeted for detection by means of tunable diode laser spectroscopy (TDLS). A model in which an explosive is placed inside an envelope with a hole in it has been studied. It has been shown that outside the envelope the concentration of the decay products may exceed that of the vapors of an explosive, by several orders. Decay products of explosives have been investigated using Fourier-transform spectrometry and TDLS. The following decay products have been identified: NO, N₂O, CO, NH₃. Analysis of the temperature dependencies of their occurrence rates has allowed estimation of the activation energies, which appear to be close to 1 eV. An experimental model of the diode laser spectrometer designed for noncontact detection of explosives has been developed. The possibility of such detection has been demonstrated experimentally. PACS 42.62.Fi; 07.07.Df; 42.55.Px     

痕量气体激光检测系统波长锁定技术研究

利用可调谐半导体激光吸收光谱技术(TDLAS)对痕量气体进行检测时,环境温度变化以及激光器控制电路的噪声常常使得激光器输出波长发生漂移,影响了气体浓度测量的准确性。以开放光路的激光吸收光谱氨气检测系统为例,在分析激光器扫描中心波长随电流变化规律的基础上,提出了基于电流控制的自适应锁定扫描中心波长的方法。研究了基于参考校准光谱的光谱数据对准算法,实现了开放大气中氨气浓度的实时监测。结果表明,波长锁定大大提高了痕量气体浓度反演的准确性和稳定性。氨气浓度具有日变化周期:上下班时段浓度上升,中午达到最大值,夜间浓度降低,系统检测限为3.8mg.m-3.m。     

基于光声光谱技术的NO,NO2气体分析仪研究

NO, NO₂是大气污染源中的常见气体, 对环境具有严重的危害性. 为检测污染源中这两种气体的浓度, 构建了成本较低的基于红外热辐射光源的光声光谱气体检测系统. 分析计算得到了NO, NO₂ 在2500–6667 nm波段吸收谱线. 通过建立光声传输线RLC振荡电路模型和仿真得到品质因数、声压大小与谐振腔长、内腔半径以及调制频率的关系, 据此设计了光声池几何结构. 实验表明该系统所测得的光声信号与气体浓度有很好的线性关系, 并且对NO, NO₂气体极限检测灵敏度分别达到4.01 和1.07 μL. 通过调节激光发射波长和选取滤波片, 该系统还可用于其他微量气体的浓度检测. 关键词： 大气污染 光声光谱 气体检测     

基于可调谐激光吸收光谱技术的脱硝过程中微量逃逸氨气检测实验研究

为了对电厂脱硝过程中逃逸的微量氨气进行在线检测,实验室采用可调谐激光吸收光谱技术对常温常压下以及不同温度下的低浓度氨气进行了测量试验,其中电厂逃逸氨气检测处温度约为650 K。通过分析近红外波段的氨气吸收谱线,并考虑实际测量环境H₂O和CO₂等浓度很大的气体吸收谱线的干扰,实验选取2.25 μm附近的ν₂+ν₃谱线作为浓度检测谱线。为了验证所选谱线对低浓度NH₃的测量能力,实验对H₂O,CO₂和NH₃的吸收谱线进行模拟,发现低浓度NH₃受较大浓度的H₂O和CO₂谱线的干扰较小,尤其是CO₂谱线的干扰可以忽略不计,且2.25 μm处谱线强度远远大于通讯波段1.53 μm处的谱线。基于新型Herriott池以及高温管式炉,结合可调谐激光吸收光谱中的直接吸收技术和波长调制技术,实现了对不同温度下超低浓度NH₃的高分辨率快速检测。常温常压下其线型函数可以利用洛伦兹线型来近似描述,直接吸收测量技术可以使探测极限降低到0.225×10-6。通过采用简单降噪处理技术如多次平均、简单小波分析等,得到不同温度下的谐波信号与浓度具有良好的线性关系,为采用可调谐激光吸收光谱技术进行现场低浓度逃逸氨气检测提供了很好的依据。     

Measurements of air and noble-gas broadening and shift coefficients of the methane R3 triplet of the 2ν3 band

By using a tunable diode laser spectrometer with one absorption White cell for low pressure and one photoacoustic cell for high pressure, line shape parameters of the R3 triplet of the 2ν₃ band of methane at 1.65 μm were measured. The absorption line was recorded by using the wavelength modulation spectroscopy technique with first harmonic detection. The broadening and shift coefficients were obtained by fitting the first harmonic absorption signal while varying the pressure of different perturbing gases: air and noble gases (helium, neon, argon, krypton and xenon). We present here the results for the R3 triplet. The observed shift and broadening coefficients behaviors are discussed. Received: 17 November 2000 / Revised version: 19 February 2001 / Published online: 27 April 2001