基于微型非共振腔的石英增强光声光谱用于氦气纯度分析的实验研究

实验中设计了一种基于微型非共振腔的石英增强光声光谱痕量气体传感器, 用来检测非纯氦气中的痕量氨气浓度. 该传感器采用的微型非共振腔只在空间上限制声波扩散以达到增强信号目的, 而不是像传统微型共振腔一样依靠共振效应. 如此的设计使探测小分子无机气体的光谱测声器尺寸远远小于共振腔的配置而有利于准直. 不同气压下的信号和噪声也被研究, 用来优化传感器性能. 在这种配置下和27.7 kPa的最优气压下, 获得的最佳氨气探测灵敏度为463 ppb (1σ , 1 s积分时间), 相应的归一化噪声等效吸收系数为4.3×10^-9cm^-1W/√Hz. 关键词： 气体传感器 石英增强光声光谱 音叉式石英晶振 类氢气体纯度分析     

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     

石英晶振用于石英增强光声光谱系统的优化实验研究

研究了音叉式石英晶振的个体尺寸、安放角度、探测部位以及外部污染对整个石英增强光声光谱系统(QEPAS)的探测灵敏度影响。测试了国内外十种不同音,结果表明顶端为楔形构造的音叉式石英晶振比规则的长方体构造的音叉拥有更高的品质因数(Q值)。在相同的测试条件下探测水的吸收线(7 306 cm-1)时获得更高的灵敏度,探测信号的强度相差高达50%。在研究音叉安放角度对探测信号影响的实验中,发现音叉的旋转角度与俯仰角度对探测信号的强度几乎没有影响,但是当光束以角度φ斜入射时,更多的噪声被带入到测量中。在正入射的情况下音叉的最佳响应位置在距离音叉底部约3.1 mm。定性研究了外部杂物污染对音叉频率的影响,发现随着污染物的附着,石英音叉的频率会呈现降低的趋势,提供了一种改变音叉式石英晶振的共振频率的方法,为石英音叉用于较低调制频率的探测提供了一种理论可能,这对于石英增强光声光谱技术用于V-T弛豫率较慢的痕量气体检测有重要的意义。     

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2.004μm的离轴石英音叉增强型光声光谱测量CO_2的研究

CO_2是大气的重要组成成分,也是现代化工业社会过多燃烧煤炭、石油和天然气的产物。一方面大量的人源排放CO_2进入大气是引发温室效应最主要因素,另一方面, CO_2是窒息性气体,在封闭环境积累过高的CO_2会导致窒息等安全问题。因此发展小型化、高灵敏度的CO_2检测技术在大气环境探测、封闭环境工作区域安全监测等方面具有重要意义和应用需求。利用近年来快速发展的小型化石英音叉谐振增强光声光谱技术,采用相对简单的离轴结构方案,开展了探测CO_2的研究。离轴石英音叉增强型光声光谱技术具有探测模块体积小、灵敏度高、抗干扰、成本低、功耗低,对激光器要求低等优点,在发展低功耗便携式气体传感器方面具有巨大的潜力。近年来,尤其是随着近红外激光器技术的逐渐成熟,为离轴石英音叉增强型光声光谱技术提供质量更好、能量更高的激励光源,使得离轴石英音叉增强型光声光谱检测技术具有更高的探测灵敏度,实现了在低浓度下对气体进行精确的检测。通过HITRAN 2012分子光谱数据库筛选出适合探测的谱线,选择2.004μm近红外分布反馈式半导体激光器作为激励光源,通过波长调制方式来激发CO_2光声信号,并采用二次谐波检测技术实现光声信号的探测。实验中通过对进样CO_2气体加湿、优化调制振幅等方式提高检测性能,实现了空气CO_2的探测。在常压下,通过配气仪配置不同浓度的CO_2样品,开展了浓度与信号的响应特性研究,获得了良好的线性响应结果。同时也开展了相同浓度CO_2样品在不同压力下的信号测量研究,并用Allan方差对系统性能进行评估。结果表明,当平均时间为1 000 s时,系统的探测极限为4×10~(-3)μL·L~(-1),在压力150 Torr时可获得最佳的测量信号,常压下系统对CO_2的最小探测灵敏度为15μL·L~(-1),相应的归一化噪声等效吸收系数为7.33×10~(-9),在150 Torr下最小探测灵敏度为6μL·L~(-1)。     

光纤倏逝波型石英增强光声光谱技术

采用块状光学准直聚焦透镜组的传统石英增强光声光谱(QEPAS)技术存在体积难以缩减,结构稳定性不佳,无法适应空间狭小、振动复杂的特殊环境等缺点.基于此,将光纤倏逝波技术与QEPAS技术相结合,提出了一种新型微纳结构光纤QEPAS痕量气体检测技术.实验中,为了提高QEPAS系统信号幅值,优化了石英音叉与激光束的空间位置、激光波长调制深度,同时对比了两种不同共振频率的石英音叉,最终采用共振频率较低的30.720 kHz石英音叉作为声波探测元件,获得的检测极限为6.25×10~(-4)(体积分数),归一化噪声等效吸收系数为4.18×10~(-7)cm~(-1).W·Hz~(-1/2).     

利用石英增强光声光谱技术在2.0μm处实现高灵敏CO_2检测的实验研究

基于石英增强光声光谱技术,以中心波长为2.0μm的窄线宽分布反馈式半导体激光器(DFB)为激励光源,采用波长调制及二次谐波解调技术通过改变激光器工作电流实现波长扫描完成了痕量CO2气体检测系统,并通过优化实验参数确定了常压下激光最佳调制深度,实现了高灵敏CO2浓度的检测。通过改变待测气体中的水汽浓度,研究了水汽对CO2气体探测结果的影响,结果显示在水汽浓度低于0.2%范围内,CO2气体光声信号随H2O浓度的上升而明显增强,当浓度高于此值后,H2O浓度的增加对CO2光声信号的增强作用几乎维持不变。数据显示,常温常压下H2O分子通过提高分子弛豫率最多可将二氧化碳R16吸收线的光声信号幅值提高约2.1倍。优化后的装置可以很好的实现大气中CO2浓度的检测。该装置获得的最小探测灵敏度为19ppm(1σ,300ms积分时间),相应的归一化噪声等效吸收系数为4.71×10-9 cm-1·W·Hz-1/2。     

光纤耦合的全固态中红外QEPAS光声探测模块

石英增强光声光谱(QEPAS)技术是近年来发展迅速的一种气体检测技术,具有灵敏度高、设备体积小、对环境噪声免疫等优点.本课题组设计了一种光纤耦合的全固态中红外QEPAS光声探测模块,并基于气体热动力学和一维声学谐振腔理论,利用COMSOL软件对探测模块的声压分布及声压级进行了研究;然后设计并加工了光机电一体化探测模块,将声学谐振腔、光声池、光纤模块和前置放大模块集成一体,使该模块具有易于准直、稳定性高、抗干扰能力强等特点.采用中心波长为2 μm的高功率中红外分布反馈式激光器,结合波长调制技术,对CO2进行了探测,结果表明,在1 s的积分时间下获得了3.7×10-3的探测极限.通过Allan方差分析发现,积分时间为1123 s时,系统的探测极限可以达到1.34×10-6.采用基于该模块的QEPAS系统可以实现对室内CO2浓度的实时监测.     

基于石英增强光声光谱的气体传感技术研究进展

基于石英增强光声光谱(quartz-enhanced photoacoustic spectroscopy, QEPAS)的气体传感技术具有系统体积小、成本低、环境适应性强等优点,是目前一种重要的光谱式痕量气体检测方法.探测灵敏度是传感器系统的重要指标,关系到能否满足实际应用,因此,本文从提高QEPAS传感系统灵敏度的角度出发,总结了常见的技术手段,包括采用高功率激发光源增大激发强度、采用与分子基频/强吸收带相匹配的激光源来增大吸收强度、采用声波共振腔增大音叉处的声波强度、采用低共振频率石英音叉提高能量积累时间、采用多光程来增大光与气体的相互作用长度等方法,并对其优缺点分别进行了阐述.针对工程应用问题,本文主要讨论了全光纤化和传感系统小型化,并以载人航天领域的应用为例进行了例证.最后,对进一步提高QEPAS传感技术灵敏度的方法进行了展望.     

光声光谱气体传感技术研究进展

光声光谱是通过光声效应把样品吸收光谱转换成声波探测,实现样品成分、浓度分析检测的一种光谱传感技术,是光谱学的一个重要分支。光声光谱除了具有吸收光谱的高选择性、高灵敏度外,还具有信号只跟样品光吸收有关,不受散射光影响,零背景, 信号与光功率成正比以及信号探测器不受光波长影响等诸多优点。在环境监测、工业过程控制与检测、医学诊断和国防危化品检测等领域得到了越来越多的应用,呈现出快速发展的趋势。除了传统的共振光声光谱技术,近年来先后出现了悬臂增强型光声光谱、石英音叉谐振增强型光声光谱、多通道光声光谱等各具特色的新技术。对光声光谱气体传感技术的研究进展进行了介绍,并分析了其应用前景和未来发展趋势。     

Quartz enhanced photoacoustic spectroscopy with a 3.38 μm antimonide distributed feedback laser

A system for gas sensing based on the quartz-enhanced photoacoustic spectroscopy technique has been developed. It makes use of a quantum well distributed feedback (DFB) laser diode emitting at 3.38 μm. This laser emits near room temperature in the continuous wave regime. A spectrophone, consisting of a quartz tuning fork and two steel microresonators were used. Second derivative wavelength modulation detection is used to perform low concentration measurements. The sensitivity and the linearity of the Quartz enhanced photoacoustic spectroscopy (QEPAS) sensor were studied. A normalized noise equivalent absorption coefficient of 4.06×10(-9) cm(-1)·W/Hz(1/2) was achieved.     

A QEPAS based methane sensor with a 2.35 μm antimonide laser

A methane sensor based on quartz-enhanced photoacoustic spectroscopy was developed. An antimonide quantum-well diode laser was used as an excitation source. The GaInAsSb/AlGaAsSb laser was fabricated by molecular beam epitaxy on GaSb substrate. This diode laser emits in the 2.35 μm range at room temperature in the continuous wave regime. A spectrophone constituted of a quartz tuning fork and two steel microresonators was used. The analysis of the sensor response with one, two or without microresonators is presented. Second derivative wavelength modulation detection was used to perform low concentrations measurements, thus we obtained a CH₄ detection limit of 1 ppmv.     

Stimulated Raman rotational photoacoustic spectroscopy using a quartz tuning fork and femtosecond excitation

Molecular alignment of linear molecules (O₂, N₂, CO₂ and CO) is measured photoacoustically in the gas phase. The rotational excitation is accomplished using a simple femtosecond stimulated Raman excitation scheme, employing two femtosecond pulses with variable delay between the pulses. Molecular alignment is determined directly by measuring the energy dumped into the gas by quartz-enhanced photoacoustic spectroscopy (QEPAS), utilizing a quartz tuning fork as a sensitive photoacoustic transducer. The experimental results demonstrate for the first time the use of a tuning fork for resonant photoacoustic detection of Raman spectra excited by femtosecond double pulses and match both simulation and literature values.     

音叉式石英晶振共振频率对QEPAS传感器性能影响的研究

以标准商用石英音叉（QTF）为测声模块的石英增强光声光谱（QEPAS）技术是近年来发展迅速的一种痕量气体检测技术。标准商用QTF拥有的小体积,高Q值,高共振频率的特性使QEPAS技术具有结构紧凑且对环境噪声免疫的特性。但传统商用QTF狭窄的振臂间距以及较高的共振频率,使其无法在光源光束质量较差或被测气体弛豫率较低的情况下被很好的应用于QEPAS系统。为克服上述难题,非标准商用QTF（f₀≠32.7 kHz）被设计制作并越来越多的被装配于QEPAS系统中。因此,QTF共振频率对QEPAS系统信噪比的影响需要被详细研究。以水汽为目标气体,采用二次谐波调制解调技术研究了QTF共振频率对基于QEPAS技术传感器性能的影响。实验结果表明,QTF共振频率的变化对QEPAS系统的输出信号及噪声均有显著影响且QTF共振频率与QEPAS系统信噪比之间存在反比关系。上述结论对QEPAS系统中非标准QTF的设计及使用均具有重要的指导价值,对该类传感器的研发及应用意义重大。     

A quartz-enhanced photoacoustic spectroscopy sensor for measurement of water vapor concentration in the air

A compact and highly linear quartz-enhanced photoacoustic spectroscopy(QEPAS)sensor for the measurement of water vapor concentration in the air is demonstrated.A cost-effective quartz tuning fork(QTF)is used as the sharp transducer to convert light energy into an electrical signal based on the piezoelectric effect,thereby removing the need for a photodetector.The short optical path featured by the proposed sensing system leads to a decreased size.Furthermore,a pair of microresonators is applied in the absorbance detection module(ADM)for QTF signal enhancement.Compared with the system without microresonators,the detected QTF signal is increased to approximately 7-fold.Using this optimized QEPAS sensor with the proper modulation frequency and depth,we measure the water vapor concentration in the air at atmospheric pressure and room temperature.The experimental result shows that the sensor has a high sensitivity of 1.058parts-per-million.     

Tuning fork enhanced interferometric photoacoustic spectroscopy: a new method for trace gas analysis

A photoacoustic trace gas sensor based on an optical read-out method of a quartz tuning fork is shown. Instead of conventional piezoelectric signal read-out, as applied in well-known quartz-enhanced photoacoustic spectroscopy (QEPAS), an interferometric read-out method for measurement of the tuning fork’s oscillation is presented. To demonstrate the potential of the optical read-out of tuning forks in photoacoustics, a comparison between the performances of a sensor with interferometric read-out and conventional QEPAS with piezoelectric read-out is reported. The two sensors show similar characteristics. The detection limit (L) for the optical read-out is determined to be L _opt=(2598±84) ppm (1σ) compared to L _elec=(2579±78) ppm (1σ) for piezoelectric read-out. In both cases the detection limit is defined by the thermal noise of the tuning fork.     

The SNR improvement for quartz-enhanced photoacoustic spectroscopy using wavelength calibration and fiber reflector

Wavelength calibration technique combined with a fiber reflector was used to improve the signal to noise ratio (SNR) of quartz-enhanced photoacoustic spectroscopy (QEPAS). A distributed feedback laser diode (DFB-LD), driven by sawtooth wave and high frequency sinusoidal wave, was used to excite the second harmonic signal of a quartz tuning fork (QTF) through laser-gas molecular interaction. Two collimators conducted the laser alignment through the spacing gap of QTF forks. Central wavelength of the DFB-LD was locked to the target gas absorption center by identifying the second harmonic signal maximum and applying calibration feedback on the driving current. The gas absorption center calibration and gas concentration measurements are conducted at a specific interval. The SNR of the photoacoustic signal was further acoustically enhanced by using a pair of on-beam acoustic resonators through increasing the photo-acoustic conversion efficient, and optically enhanced by using a fiber reflector to improve the laser power for photoacoustic signal excitation. The experimental results show that the SNR in wavelength calibration mode is 15 times higher than the conventional wavelength scanning mode and QEPAS signal with fiber reflector is 1.37 times stronger compared with that without a fiber reflector.     

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。     

Theoretical analysis of off beam quartz-enhanced photoacoustic spectroscopy sensor

Off beam quartz-enhanced photoacoustic spectroscopy (OB-QEPAS) sensors are based on a recently developed approach to off-beam photoacoustic (PA) detection which employs a quartz tuning fork (QTF) as an acoustic transducer. A microresonator (mR) with a side slit in the middle is used to enhance PA signal. This paper describes a theoretical model of an OB-QEPAS-based sensor. By deriving the acoustic impedances of the mR at two ends and the side slit in the middle in the model, we obtain a formula for numerically calculating the optimal mRs' parameters of OB-QEPAS-based sensor. We use the model to calculate the optimal mRs' lengths with respect to the resonant frequency of the QTF, acoustic velocities inside mRs, inner diameters of mRs, and acoustic conductivities of the mRs' side slits, and found out that the calculated results closely match experimental data. We also investigated the relationship between the mR selected in “on beam” QEPAS, OB-QEPAS, and an acoustic resonator (AR) excited in its first longitudinal mode used in conventional photoacoustic spectroscopy (PAS).