基于光纤光声传感的多组分痕量气体检测技术

为进一步提升多组分痕量气体检测灵敏度,设计了一套光纤光声传感系统。系统主要集成了2个近红外DFB激光器、近红外宽带光源、高速光谱模块、现场可编程逻辑门阵列信号采集与处理电路,具有激光调制控制、光声信号解调和数字锁相放大等功能。利用声学共振腔和干涉型光纤声波传感器对光声信号进行激发增强和探测增强,实现了乙炔和甲烷气体的高灵敏度检测。光纤声波传感器中以微机电系统悬臂梁作为声学敏感元件,设计了光纤法布里-珀罗干涉结构,将悬臂梁偏转位移转换为F-P腔长的变化。采用高分辨率光谱解调技术,实现了基于光纤F-P传感器的超高灵敏度光声信号检测。系统对乙炔和甲烷的检测极限分别达到2×10^-9和3×10^-9,归一化噪声等效吸收系数为8×10^-10cm^-1W Hz^-1/2。     

全光型石英增强光声光谱

设计并演示了一种全光型石英增强光声光谱技术, 该技术在传统的石英增强光声光谱系统中增加了另一束探测光束, 把与气体浓度成正比的石英晶振振臂的振动幅值转化为探测光束的强度变化, 实现了探测气体处无电子元件的全光学系统. 如此的设计使该系统具有较强的抗电磁干扰能力和非常小的传感头体积, 能够用于探测空间受限或探测环境恶劣的情况下, 并实现远距离探测. 在这种配置下, 探测大气压下的水汽, 获得的噪声等效吸收系数为1.13×10^-6 cm^-1W/√Hz. 进一步讨论了优化系统和提升其探测灵敏度的途径. 关键词： 石英增强光声光谱 音叉式石英晶振 气体传感     

基于椭圆腔共振的石英增强光声光谱理论研究

石英增强光声光谱技术作为一种新型的光学检测技术,已被广泛应用于痕量气体检测场合.其中声波共振增强性能是决定检测灵敏度的重要因素.为提高光声光谱检测系统的信噪比和检测极限,提出一种新型的椭圆腔共振石英增强光声光谱检测方法,建立了其声学特征模型并利用有限元分析方法对光声腔内部声学特性进行仿真研究.研究结果表明,椭圆腔的特征模态在(2,1)模态下长轴两端声压达到最大值.通过对椭圆腔的尺寸和形状进行优化,建立实验装置,得到目标气体硫化氢检测极限为6.3 ppm(parts per million),相应的归一化噪声等效吸收系数为2.02×10~(-9)cm~(-1)W/Hz~(1/2).     

珐珀解调的石英增强光声光谱气体探测系统

提出一种珐珀解调,适用于开放环境的全光式石英增强光声光谱气体探测系统。基于石英增强光声光谱系统,采用法珀干涉解调代替传统的电解调方式,通过拾取石英音叉的叉指侧面与光纤端面之间形成的法珀腔的腔长变化解调得到被测气体的光声光谱信号。构建了实验系统,在开放环境中完成了对空气中水蒸气的探测实验,得到其归一化噪声等效吸收系数为2.80×10-7 cm-1.W.Hz-1/2。结果表明,该探测系统的探测灵敏度是传统石英增强光声光谱探测系统的2.6倍。该系统具有极强的抗电磁干扰能力、能够用于易燃易爆气体检测、适用于高温、高湿度等恶劣环境并实现远距离多点、组网探测。     

基于泛频振动的石英增强光声光谱测声器优化设计

为了进一步提高基于泛频振动的石英增强光声光谱测声器探测灵敏度,在一次泛频振动模式下采用一个比商用标准音叉外形尺寸大5倍的定制大音叉,并对其性能进行优化.通过理论和实验研究得出了音叉与激光的最佳作用位置,发现音叉的一次泛频振动有两个波腹点,且在距离音叉根部8 mm处,音叉振臂的振动幅度最大.微型声音谐振腔由三种不同内径的不锈钢毛细管加工而成,与音叉组成共轴配置石英增强光声光谱光谱测声器,用来进一步增强信号幅值.在最佳微型声音谐振腔配置下,获得了30倍的信号增益因子,有效提高了石英增强光声光谱光谱测声器的探测灵敏度.     

基于QEPAS技术的乙炔微量气体高灵敏度检测研究

石英增强光声光谱(QEPAS)是近年来发展起来的一种痕量气体探测技术,具有系统体积小、价格低廉、探测灵敏度高等优点。乙炔(C₂H₂)是一种化学性质活泼的有毒气体,对它进行高灵敏度检测在变压器故障诊断、环境监测等领域有着重要的意义,基于此,采用QEPAS技术对C₂H₂微量气体展开高灵敏度检测研究。采用输出波长为1.53 μm的连续波分布反馈半导体激光器作为激发光源。为了提高信噪比和简化数据处理过程,QEPAS传感器系统采用波长调制和2次谐波探测技术。为了提高QEPAS系统信号幅值,相比于常见的共振频率为32.768 kHz的石英音叉,采用了共振频率较低的30.72 kHz石英音叉作为声波传导器,同时还优化了石英音叉与激光束的空间位置、激光波长调制深度,并添加了声波微共振腔,选择的微共振腔长度为4 mm、内径为0.5 mm,最终获得了2.7 ppm的优异检测极限,归一化噪声等效吸收系数为1.3×10-8 cm-1·W·Hz-1/2。     

基于石英增强光声光谱的HCl痕量气体高灵敏度探测研究

HCl是一种有毒有害气体,对其高灵敏度探测具有非常重要的意义,然而到目前为止,采用激光光谱的手段对其探测的研究报道很少。石英增强光声光谱(QEPAS)是近年来发展起来的一种痕量气体探测技术,具有系统体积小、价格低廉、探测灵敏度高等优点。以5 000 ppm HCl∶N₂混合气作为待测目标,采用输出波长为1 742.38 nm的分布反馈连续波单纵模半导体激光器,开展对基于QEPAS技术的HCl高灵敏度探测研究。为了提高信噪比和简化数据处理过程,QEPAS传感器系统采用波长调制和2次谐波探测技术。研究中,首先对声波探测系统中微共振腔强声波增强特性进行了讨论,选择了“共轴”形式的声波微共振腔,并对其尺寸进行了优化,选择的微共振管长度为4 mm、内径为0.5 mm。实验中研究了激光波长调制深度对QEPAS系统产生的信号幅度的影响,当QEPAS系统积分时间为1 s、激光波长调制深度为0.23 cm-1时,获得了815 ppb的优异检测极限,归一化噪声等效吸收系数为7.41×10-9 cm-1·W·Hz-1/2。在后续的实验中,可在待测HCl气体中加入水汽分子,提高HCl分子的热弛豫速率,进一步提高HCl-QEPAS传感器系统的信号强度。     

基于电学调制相消法和高功率蓝光LD的离轴石英增强光声光谱NO2传感器设计和优化

使用中心波长为450 nm的高功率多模蓝光激光管(LD)作为激励光源, 结合电学调制相消法和离轴石英增强光声光谱(QEPAS)配置, 设计了一款高灵敏二氧化氮传感器. 电学调制相消法使离轴QEPAS传感器的背景噪声降低至1/269, 在标准大气压和1 s积分时间下, 获得的探测灵敏度为4.5 ppb, 对应的归一化噪声等效吸收系数(1σ )为2.2×10^-8 cm^-1·W/Hz^1/2. 延长积分时间到46 s, 灵敏度能够进一步下降到0.34 ppb. 气体流速对该传感器的影响也被研究.     

基于共振型高灵敏度光声光谱技术探测痕量乙炔气体浓度

乙炔气体作为判断变压器运行状态的一种故障气体,其浓度的高低反映了变压器的运行状况,因此对其浓度的探测在变压器的维护中具有重要意义。为了准确探测变压器运行过程中产生的乙炔气体浓度,为变压器的维护提供技术参数,针对基于DFB激光器的共振型光声光谱技术痕量乙炔气体检测技术开展研究,对传统的光声光谱探测系统进行改进。根据光声光谱技术的理论可知,光声信号的强度与入射激光的功率成正比,所以在光声池的出射窗口采用一个平面反射镜将红外光再次反射到光声池中以增加入射光功率,增强光声信号强度,进一步提高了光声系统的探测灵敏度。通过一定浓度的乙炔气体在不同调制频率和不同调制深度下光声信号强度的变化,确定光声探测系统的最佳调制频率和最佳调制深度为767 Hz和0.3 mV。利用不同浓度乙炔气体对系统进行标定,然后采用最小二乘法对光声信号与气体浓度进行拟合,二者具有很好的线性度。通过Allan方差计算可知,系统在平均时间达到200 s时,能够达到最低探测极限浓度。实验表明,在一个大气压下,积分时间为10 ms时,改进后的共振型光声光谱探测系统对乙炔气体的最低探测极限浓度达到了0.3 μL·L-1。还将小波去噪技术引入到低浓度下乙炔气体的光声信号处理中,有效消除了低浓度气体光声信号中的噪声,提高了信噪比。设计的共振型光声光谱探测系统操作简单,最低探测浓度符合国标中对变压器维护过程中对乙炔气体的探测需求,在变压器维护领域具有广阔的应用前景。     

基于CEEMDAN的光声光谱CO检测技术

为实现高灵敏度痕量CO气体探测,设计并实现了一种基于光声光谱的CO气体传感器,采用近红外分布反馈式激光器和掺铒光纤放大器作为激励光源系统,结合双通道差分光声池降低系统噪声。用自适应噪声的完备经验模态分解算法结合Savitxky-Golay滤波器对采集的非线性光声信号进行降噪,通过评估降噪后分量与原始信号的相关系数选取有效模态并重构。实验结果表明,室温和常压条件下,积分时间为100 ms,经算法处理,该系统CO检测信噪比提高至原来的4.6倍,最低检测极限降低到2.6×10^-6,且该传感器对气体浓度具有良好的线性响应,验证了该算法在提高光声光谱系统检测性能方面的可行性与有效性。     

First observation of spin flip in dμ-atoms via formation and back decay of ddμ molecules

We present the results of precision measurements of dμ spin-flip rates in pure D₂, in pure HD gas, and in the non-equilibrium and equilibrium H₂+D₂ gas mixtures. The experiments were performed at PSI in 1994-1996 using the high pressure ionization chamber from Gatchina as a 100% efficient detector of the charged fusion products. The measurements have been carried out in the temperature range 28-300 K, at gas density of 5% of LHD. In pure deuterium, the measured temperature dependence of the spin-flip rate allowed us to separate the two components (resonant and non-resonant) of the spin-flip processes. From the experiments in H/D mixtures we have measured the non-resonant spin-flip rate in collisions of dμ-atoms with HD molecules. Comparison with the theory revealed considerable disagreement in the case of non-resonant spin-flip in collisions of the dμ-atoms with D₂ molecules. This revised version was published online in August 2006 with corrections to the Cover Date.     

13CO2/12CO2 isotopic ratio measurements using a difference frequency-based sensor operating at 4.35 micrometers

A portable modular gas sensor for measuring the ¹³C/¹²C isotopic ratio in CO₂ with a precision of 0.8‰(±1σ) was developed for volcanic gas emission studies. This sensor employed a difference frequency generation (DFG)-based spectroscopic source operating at 4.35 μm (∼2300 cm^-1) in combination with a dual-chamber gas absorption cell. Direct absorption spectroscopy using this specially designed cell permitted rapid comparisons of isotopic ratios of a gas sample and a reference standard for appropriately selected CO₂ absorption lines. Special attention was given to minimizing undesirable precision degrading effects, in particular temperature and pressure fluctuations. Received: 16 April 2002 / Revised version: 28 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-713/5245237, E-mail: fkt@rice.edu     

Cantilever enhanced photoacoustic detection of carbon dioxide using a tunable diode laser source

Recently introduced cantilever enhanced photoacoustic sensing has been applied to the tunable diode laser-based trace gas detection. The pressure variations due to the photoacoustic signal are detected with a miniature silicon cantilever, whose displacement is measured with a compact Michelson type laser interferometer. The system has been used to detect carbon dioxide (CO₂) at 1572 nm with a distributed feedback diode laser. With a new photoacoustic cell, that was optimized for the laser sources, a normalized noise equivalent sensitivity of 1.7×10^-10 cm^-1W/ at atmospheric pressure was realized. The result obtained in the non-resonant operation mode is at least 10 times better than in previous reports. The future improvements of the technique are also discussed. PACS 07.07.Df; 42.55.Px; 82.80.Kq     

温室气体及碳同位素比值红外光谱反演精度的影响因素研究

由于傅里叶变换红外光谱(FTIR)技术在定量反演中受到气体温度、压强等气体特性以及水汽交叉吸收的影响, 使其在温室气体及碳同位素比值高精度检测领域的应用受到限制. 本文首先研究了气体特性与水汽吸收敏感性修正方法; 然后,结合实验研究中建立的敏感性修正函数, 对标准气体实测数据进行了气体特性敏感性修正, 修正后,五种测量组分的精密度均有明显提高, 其标准偏差降低倍数分布在1.80到3.38之间. 研究结果对于FTIR技术在大气本底温室气体及碳同位素比值高精度监测领域的应用具有重要意义.     

Trace gas sensor based on quartz tuning fork enhanced laser photoacoustic spectroscopy

A compact photoacoustic gas sensor based on a quartz tuning fork and fiber-coupled distributed feedback (DFB) diode laser for detection of trace gas at atmospheric pressure has been developed. The sensor performance was evaluated by detection of water vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient of 1.68×10⁻⁸ cm⁻¹ W/Hz^1/2 was achieved. Influence of different acoustic microresonators and sample pressure on the sensor performance, and the characterization of the sensor response time were investigated. Approaches to improve the current sensor performance are discussed.     

Magnetic Resonance Imaging of Gases: A Single-Point Ramped Imaging with T1 Enhancement (SPRITE) Study

A pure phase-encoding MRI technique, single-point ramped imaging withT₁enhancement, SPRITE, is introduced for the purpose of gas phase imaging. The technique utilizes broadband RF pulses and stepped phase encode gradients to produce images, substantially free of artifacts, which are sensitive to the gasT₁andT^*:₂relaxation times. Images may be acquired from gas phase species with transverse relaxation times substantially less than 1 ms. Methane gas images,¹H, were acquired in a phantom study. Sulfur hexafluoride,¹⁹F, images were acquired from a gas-filled porous coral sample. High porosity regions of the coral are observed in both the MRI image and an X-ray image. Sensitivity and resolution effects due to signal modulation during the time-efficient acquisition are discussed. A method to increase the image sensitivity is discussed, and the predicted improvement is shown through 1D images of the methane gas phantom.     

The light-enhanced NO2 sensing properties of porous silicon gas sensors at room temperature

The NO2 gas sensing behavior of porous silicon（PS） is studied at room temperature with and without ultraviolet（UV） light radiation.The PS layer is fabricated by electrochemical etching in an HF-based solution on a p ＋-type silicon substrate.Then,Pt electrodes are deposited on the surface of the PS to obtain the PS gas sensor.The NO2 sensing properties of the PS with different porosities are investigated under UV light radiation at room temperature.The measurement results show that the PS gas sensor has a much higher response sensitivity and faster response-recovery characteristics than NO2 under the illumination.The sensitivity of the PS sample with the largest porosity to 1 ppm NO2 is 9.9 with UV light radiation,while it is 2.4 without UV light radiation.We find that the ability to absorb UV light is enhanced with the increase in porosity.The PS sample with the highest porosity has a larger change than the other samples.Therefore,the effect of UV radiation on the NO2 sensing properties of PS is closely related to the porosity.     

Gas-phase photoacoustic sensor at 8.41 μm using quartz tuning forks and amplitude-modulated quantum cascade lasers

We demonstrate the performance of a novel infrared photoacoustic laser absorbance sensor for gas-phase species using an amplitude-modulated quantum cascade (QC) laser and a quartz tuning fork microphone. The photoacoustic signal was generated by focusing 5.3 mW of a Fabry–Pérot QC laser operating at 8.41 μm between the tines of a quartz tuning fork which served as a transducer for the transient acoustic pressure wave. The sensitivity of this sensor was calibrated using the infrared absorber Freon 134a by performing a simultaneous absorption measurement using a 31-cm absorption cell. The power and bandwidth normalized noise equivalent absorption sensitivity (NEAS) of this sensor was determined to be D=2.0×10^-8 W cm^-1/Hz^1/2. A corresponding theoretical analysis of the instrument sensitivity is presented and is capable of quantitatively reproducing the experimental NEAS, indicating that the fundamental sensitivity of this technique is limited by the noise floor of the tuning fork itself. PACS 43.60.Vx; 43.58.Wc; 43.58.Hp; 84.40.Xb     

A simple fibre Fabry-Pérot sensor

A simple fibre Fabry-Pérot sensor is described in which the uncoated cleaved ends of a lenght of single mode fibre are employed as the mirror surfaces to produce high visibility near perfect cosine-squared fringes in the back-reflected light. The experimentally observed signal-to-noise performance of the interferometer illuminated by a HeNe gas laser and a semiconductor diode laser is reported. In an accelerometer configuration the device displayed a below resonance sensitivity of ≥200 rad g^-1.     

Photoacoustic gas sensing employing fundamental and frequency-doubled radiation of a continuously tunable high-pressure CO2 laser

We present the first photoacoustic spectrometer for gas sensing employing both the fundamental and the frequency-doubled radiation of a continuously tunable high-pressure CO₂ laser with room temperature operation. A quasi-phase-matched diffusion-bonded GaAs crystal is used in the system for second-harmonic generation. A pulsed photoacoustic detection scheme with a non-resonant cell, equipped with an 80-microphone array, is employed. The wide continuous tuning range in the fundamental (9.2–10.7 μm) and the frequency-doubled (4.6–5.35 μm) regimes, together with the narrow linewidth of 540 MHz (0.018 cm^-1) for the 10-μm region and of 1050 MHz (0.0315 cm^-1) for the 5-μm region, allow the measurement of gas mixtures, individual species and isotope discrimination. This is illustrated with measurements on NO and CO₂. The measured isotope ratio ¹⁵ NO/¹⁴ NO=(3.58±0.55)×10^-3 agrees well with the literature (3.700×10^-3) and demonstrates the good selectivity of the system. Received: 30 April 2002 / Revised version: 10 June 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +41-1/633-1077, E-mail: sigrist@iqe.phys.ethz.ch