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     

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。     

干涉型全光学光声光谱气体传感技术研究进展

光声光谱技术作为一种超高灵敏度的气体检测技术,声波传感器作为核心部件直接影响着系统的体积和检测极限。传统光声光谱技术使用电容式麦克风作为声波探测单元,但该器件的电学特性易受到高温环境和电磁干扰影响。在全光学光声光谱系统中,利用光学声波传感器对光声信号进行探测,避免了电子探测元件的使用,具有环境适应性强、灵敏度高等优点,且系统中全光学的设计可以极大地减小光声传感单元的体积。综述了基于干涉型光学声波传感器的全光学光声光谱气体传感技术的研究进展,并展望了其未来的发展方向。     

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。     

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.     

石英增强光声光谱技术研究进展

石英增强光声光谱(QEPAS)技术是一种新颖的气体探测技术,具有体积小、灵敏度高等优点,是痕量气体检测技术的研究热点.本文对QEPAS技术的基本原理、发展历史及发展现状进行了综述,并对多种不同结构的QEPAS系统发展情况进行了介绍,最后对该技术的发展前景进行了展望.     

基于7.6μm量子级联激光的光声光谱探测N_2O气体

近年来,气候变化对地球的生态环境产生严重影响,而大气温室气体在气候变化中具有重要的作用.一氧化二氮(N_2O)作为一种重要的温室气体,其浓度变化对大气环境产生重要影响,因此对其浓度的探测在大气环境研究中具有重要意义.本文开展了基于中国自主研发的7.6μm中红外量子级联激光的共振型光声光谱探测N_2O的研究,建立了N_2O光声光谱传感实验系统.此系统在传统的光声光谱探测的基础上优化改进,采用双光束增强的方式,增加了有效光功率,进一步提高了系统的探测灵敏度.探测系统以1307.66 cm~(-1)处的N_2O吸收谱线作为探测对象,结合波长调制技术对N_2O气体进行探测研究.通过对一定浓度的N_2O气体在不同调制频率和调制振幅的光声信号的探测,确定了系统的最佳调制频率和调制振幅分别为800 Hz和90 mV.在最优实验条件下对不同浓度的N_2O气体进行了测量,获得了系统的信号浓度定标曲线.实验表明,在锁相积分时间为30 ms时,系统的浓度探测极限为150×10~(-9).通过100次平均后,系统噪声进一步降低,实现了大气N_2O的探测,浓度探测极限达到了37×10~(-9).     

基于功率增强型QEPAS技术的二氧化碳气体高灵敏检测研究

二氧化碳(CO₂)是环境大气以及燃烧废气的主要成分,同时也是重要的化工原料,对其浓度进行高灵敏度检测在物理、生物、化学等众多学科中均有重要的应用。传统检测方法已经无法满足国防科研、能源化工、医疗诊断等科技前沿领域中对CO₂浓度检测的需求。石英增强光声光谱(QEPAS)技术是近年来发展迅速的一种激光检测技术,具有高分辨率、小体积、对环境噪声免疫等优点。基于QEPAS技术探测灵敏度与激励光功率成正比的特性,以中心波长为1 572 nm的窄线宽分布反馈式半导体激光器为激励光源,将掺饵光纤放大器(EDFA)与QEPAS技术联用,提出了功率增强型QEPAS技术,实现了光声信号的大幅度提高。此外,通过波长调制技术、谐波解调技术以及电调制相消技术的使用,成功将装置的整体噪声压制在音叉式石英晶振的理论热噪声水平。激光波长调制深度对装置信号幅度的影响也通过实验在一个标准大气压下进行了研究。结果显示,对6 361.25 cm-1处CO₂气体吸收线,当激光功率为1 495 mW,调制深度为0.33 cm-1,系统探测带宽为0.833 Hz时,功率增强型QEPAS装置对CO₂的探测灵敏度为3.5 ppm,归一化等效吸收系数为1.01×10-8 W·cm-1·Hz-1/2。     

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 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.     

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.     

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.     

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.     

Theoretical analysis of a quartz-enhanced photoacoustic spectroscopy sensor

Quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors are based on a recent approach to photoacoustic detection which employs a quartz tuning fork as an acoustic transducer. These sensors enable detection of trace gases for air quality monitoring, industrial process control, and medical diagnostics. To detect a trace gas, modulated laser radiation is directed between the tines of a tuning fork. The optical energy absorbed by the gas results in a periodic thermal expansion which gives rise to a weak acoustic pressure wave. This pressure wave excites a resonant vibration of the tuning fork thereby generating an electrical signal via the piezoelectric effect. This paper describes a theoretical model of a QEPAS sensor. By deriving analytical solutions for the partial differential equations in the model, we obtain a formula for the piezoelectric current in terms of the optical, mechanical, and electrical parameters of the system. We use the model to calculate the optimal position of the laser beam with respect to the tuning fork and the phase of the piezoelectric current. We also show that a QEPAS transducer with a particular 32.8 kHz tuning fork is 2–3 times as sensitive as one with a 4.25 kHz tuning fork. These simulation results closely match experimental data.     

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).     

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.     

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).