Wide dynamic detection range of methane gas based on enhanced cavity absorption spectroscopy

Integrated cavity output spectroscopy(ICOS) is an effective technique in trace gase detection.The strong absorption due to the long optical path of this method makes it challenging in the application scenes that have large gas concentration fluctuation,especially when the gas concentration is high.In this paper,we demonstrate an extension of the dynamic range of ICOS by using a detuned laser combined with an off-axis integrating cavity.With this,we improve the upper limit of the dynamic detection range from 0.1%(1000 ppm) to 20% of the gas concentration.This method provides a way of using ICOS in the applications with unpredictable gas concentrations such as gas leak detection,ocean acidification,carbon sequestration,etc.

High-sensitivity methane monitoring based on quasi-fundamental mode matched continuous-wave cavity ring-down spectroscopy

Continuous-wave cavity ring-down spectroscopy(CW-CRDS)is an important technical means to monitor greenhouse gases in atmospheric environment.In this paper,a CW-CRDS system is built to meet the needs of atmospheric methane monitoring.The problem of mode matching is explained from the perspective of transverse mode and longitudinal mode,and the influence of laser injection efficiency on measurement precision is further analyzed.The results of cavity ring-down time measurement show that the measurement precision is higher when the laser is coupled with the fundamental mode.In the experiment,DFB laser is used to calibrate the system with standard methane concentration,and the measurement residual is less than±4×10^(-4)μs^(-1).The methane concentration in the air is monitored in real time for two days.The results show the consistency of the concentration changes over the two days,which further demonstrates the reliability of the system for the measurement of trace methane.By analyzing the influence of mode matching,it not only assists the adjustment of the optical path,but also further improves the sensitivity of the system measurement.

Measurement of OH Radicals in Dielectric Barrier Discharge Plasmas by Cavity Ring-Down Spectroscopy

Near-infrared continuous wave cavity ring-down spectroscopy was applied to mea- sure the OH radicals in dielectric barrier discharge plasmas, which play an important role in combustion systems, atmospheric chemistry and the removal of air pollutants by non-thermal plasmas. The P-branches of OH X2YIi （vI ： 2 ＋-- it 0） bands were used for number density measurements. The OH number density and plasma temperature were determined for different applied voltages, gas pressures and concentrations of both oxygen and water. The temporal evolu- tion of the OH number density was obtained by using the ＂time window＂ method, which was used to extract individual ring-down times at different times in a half period of the sine wave applied voltage in dielectric barrier discharge plasmas.

Cavity Ring-Down Spectroscopy Measurements of Ambient NO3 and N2O5

NO3 and N2O5 are important participants in nocturnal atmospheric chemical processes,and their concentrations are of great significance in the study of the mechanism of nocturnal atmospheric chemical reactions.A two-channel diode laser based cavity ring-down spectroscopy(CRDS)instrument was developed to monitor the concentrations of NO3 and N2O5 in the atmosphere.The effective absorption length ratio and the total loss coefficient of the instrument were calibrated using laboratory standard samples.The effective absorption cross section of NO3 at 662 nm was derived.A detection sensitivity of 1.1 pptv NO3 in air was obtained at a time resolution of 1 s.N2O5 was converted to NO3 and detected online in the second CRDS channel.The instrument was used to monitor the concentrations of NO3 and N2O5 in the atmosphere of winter in Hefei in real time.By comparing the concentration changes of pollutants such as nitrogen oxides,ozone,PM2:5 in a rapid air cleaning process,the factors affecting the concentrations of NO3 and N2O5 in the atmosphere were discussed.

Detection of Radiocarbon Dioxide with Double-Resonance Absorption Spectroscopy

Fast and accurate quantitative detection of ^(14)CO_(2) has impor-tant applications in many elds.The optical detection method based on the sensitive cavity ring-down spectroscopy technol-ogy has great potential.But currently it has diffculties of insuffcient sensitivity and susceptibility to absorption of other isotopes/impurity molecules.We propose a stepped double-resonance spectroscopy method to excite ^(14)CO_(2) molecules to an intermediate vibrationally excited state,and use cavity ring-down spectroscopy to probe them.The two-photon process signi cantly improves the selectivity of detection.We derive the quantitative measurement capability of double-resonance absorption spectroscopy.The simulation results show that the double-resonance spectroscopy measurement is Doppler-free,thereby reducing the e ect of other molecular absorption.It is expected that this method can achieve high-selectivity detection of ^(14)CO_(2) at the sub-ppt level.

Analysis of the influence of various effects on frequency shifts of the acetylene saturated absorption lines

Frequency shifts of the acetylene saturated absorption lines at 1.5μm with temperature, gas pressure and laser power have been investigated in detail. The second-order Doppler effect, the recoil effect, the Zeeman effect, the pressure shift and the power shift are taken into consideration. The magnitudes of those shifts caused by various effects are evaluated. In order to reproduce the stability of 5.7 × 10^-14 obtained by Edwards, all necessary conditions are given. The results show that when there is a larger external magnetic field, the Zeeman shift could not be neglected, so that the shield should be employed. And the design of a long cavity is advantageous to reduce the influence of the second-order Doppler effect. The results also show that at least 4-2.5℃ temperature control for cavity can effectively prevent several effects and improve the frequency stability.

Optical Frequency Comb-Based Cavity-Enhanced Fourier-Transform Spectroscopy:Application to Collisional Line-Shape Study

Direct-comb spectroscopy techniques uses optical frequency combs(OFCs)as spectroscopic light source.They deliver high sensitivity,high frequency resolution and precision in a broad spectral range.Due to these features,the field has burgeoned in recent years.In this work we constructed an OFC-based cavity-enhanced Fourier-transform spectrometer in the nearinfrared region and used it for a line-shape study of rovibrational transitions of CO perturbed by Ar.The highly sensitive measurements spanned the wavenumber range from 6270 cm^-1 to 6410 cm^-1,which covered both P and R branch of the second overtone band of CO.The spectrometer delivers high-resolution surpassing the Fourier-transform resolution limit determined by interferogram length,successfully removing ringing and broadening effects caused by instrumental line shape function.The instrumental-line-shape-free method and high signal-to-noise ratio in the measurement allowed us to observe collisional effects beyond those described by the Voigt profile.We retrieved collisional line-shape parameters by fitting the speed-dependent Voigt profile and found good agreement with the values given by precise cavity ring-down spectroscopy measurements that used a continuous-wave laser referenced to a stabilized OFC.The results demonstrate that OFC-based cavity-enhanced Fouriertransform spectroscopy is a strong tool for accurate line-shape studies that will be crucial for future spectral databases.

^(12)C^(16)O_(2)分子位于1.57μm的饱和吸收光谱

本文展示了基于光梳锁频-光腔衰荡光谱装置测量的^(12)C^(16)O_(2)分子位于1.57μm附近的30012←00001振动带中转动量子数最高为68的共计37根振转跃迁的饱和吸收光谱,其频率确定至kHz水平;文献和CDSD2019数据库中的多普勒光谱确定的30012振动态的振转能级与本工作确定的数值存在超过1 MHz的偏差.

基于TDLAS的近红外甲烷高灵敏检测技术

为增强甲烷气体检测技术的气体吸收率,提高检测灵敏度,利用可调谐二极管激光吸收光谱技术,采用中心波长为1 653.7 nm的分布反馈激光器作为光源,研制了有效光程为14.5 m的Herriott型气体吸收池,并采用波长调制光谱法进行甲烷气体浓度检测。结果表明,二次谐波峰值信号与甲烷气体浓度成较强的线性关系,线性度为0.998 52,检测下限为4.82 ppm;初始积分时间为1 s时的Allan方差为6.37 ppm;积分时间到112 s时,Allan方差为427 ppb,检测灵敏度为4.27×10^(-7)。

基于不同宽带非相干光源的甲烷浓度测量研究

目的:研究非相干宽带腔增强吸收光谱(IBBCEAS)技术中所应用的不同类型宽带光源的性能差异对系统输出信号的影响。方法:搭建了直径2.54 cm,长30 cm的气池,通入体积分数为99.99%的甲烷气体,分别采用非相干宽带卤素光源和超连续谱光源入射气池,通过光纤准直器耦合出射光进入近红外多模光纤,光纤接入近红外光谱仪测量吸收光谱。结果:两种光源的光学密度谱线对比显示超连续谱光源的光学噪声远大于卤素光源,其主要原因为超连续谱光源内部温度控制欠稳定导致输出功率在频域上出现较大波动造成的基线偏移。结论:相较于卤素光源,超连续谱光源具有光功率高,空间相干性好等诸多优点,但由于温度控制原因所导致的光功率输出不够稳定,导致该光源在IBBCEAS系统中的应用受到一定限制。

基于高灵敏激光吸收光谱技术的稳定气态同位素测量及其应用(特邀)

稳定同位素测量技术已经在地球化学、地球物理、农业、生物、临床医学和生态科学等领域得到了众多应用。相对于传统的稳定同位素分析方法,基于激光吸收光谱技术的同位素分析技术,作为一种较新的同位素丰度测量方法,具有选择性好、精度高、体积小、无需样品预处理、可以实时原位同时测量气体浓度及同位素丰度等众多优点,得到了极大的关注和使用。本文主要以目前同位素测量的可调谐半导体激光吸收光谱技术、积分腔吸收光谱技术、腔衰荡吸收光谱技术三种激光吸收光谱方法为例,阐述了其基本原理、谱线选择、温压影响因素及其控制、系统组成结构以及部分应用测试结果。通过对测量系统的压力与温度的稳定控制的前提下,选取了合适的同位素测量谱线对,实现了大气CO_(2)气体的13C测量精度为0.3‰,煤层气CH4气体的13CH4测量精度为1.25‰,冰川水H_(2)O中18O、17O和2H的测量精度分别为0.3‰、0.2‰和0.5‰,以及呼吸气体中的13CO_(2)判识“金标准”。通过分析验证了激光吸收光谱技术在同位素测量方面的可行性和可靠性,也充分说明了基于激光吸收光谱技术的测量方法具有非常好的技术优势,将是光谱研究领域关注的重点内容,并在后续的科学研究中占据举足轻重的作用。

中红外波长调制离轴积分腔输出光谱技术应用于OH自由基高灵敏度探测研究

将波长调制技术与离轴积分腔技术相结合,建立了波长调制离轴积分腔输出光谱实验装置,增加吸收光程,避免低频的1/f噪声和与波长无关的背景功率的影响,将其应用于OH自由基探测研究。探测激光器选择2.8μm中红外室温型连续波分布反馈式二极管激光器,OH自由基选择3568.52 cm^(-1)处Q(1.5e)跃迁谱线开展光谱探测,在512 m有效吸收光程和100 s采样时间下,实现了OH自由基1.2×10^(8)molecule/cm3的探测极限。实验研究发现,在弱透过光强下,激光器放大的自发辐射对吸收系数测量影响大,本实验装置下造成了约70倍的吸收低估,需要有效避免。

CRDS-CARS-PLIF技术精确定量测量火焰OH浓度实验研究

在预混甲烷/空气燃烧的平面火焰炉上,采用脉冲式光腔衰荡光谱技术(cavity ring-down spectroscopy,CRDS)实现了对OH分子浓度的定量测量。根据光腔衰荡吸收光谱理论,选取OH的A2Σ+-X2Π(0,0)电子跃迁带中的P1(2)吸收谱线构搭建了一套激光波长在308.6 nm的脉冲CRDS实验装置。脉冲CRDS装置中的衰荡光腔是由一对反射率为99.7%的高反射镜组成且其衰荡腔的腔长为270 cm,并测量空腔(光腔中无火焰)的衰荡时间为2.33μs。通过理论分析影响浓度精确测量的实验参数,分别采用平面激光诱导荧光(planar laser induced fluorescence,PLIF)、相干反斯托克斯拉曼散射(coherent anti-stokes Raman scattering,CARS)和脉冲CRDS三种技术精确测量OH的有效吸收长度、高温火焰的温度和有效的光腔衰荡时间。当在平面火焰炉上燃烧预混的甲烷(1.1 L·min^(-1))和空气(15 L·min^(-1))且在距离炉面高度为6 mm时,采用PLIF技术测量的有效吸收长度比直接选用燃烧器炉面直径作为吸收长度的精度提高7.1%,室温下利用CARS技术测量的温度要比热电偶测量的温度精度提高45%,衰荡光腔内有火焰且选用非OH吸收波长时测得的光腔衰荡时间要比采用空腔时测得的光腔衰荡时间精度提高21.6%。因此,通过以上多种测量技术相结合的方式精准测量各实验参量,最后得到OH分子数密度在距离炉面高度为6 mm时达到最大值(3.59×10^(13)molecules·cm^(-3))且OH浓度精度要比于未修正的OH浓度提高了35.6%。另外,在不同当量比下(Φ=0.7~1.1),OH粒子数密度都会随着距离炉面高度的增加而减少,通过曲线拟合发现OH浓度随着距离炉面高度的增加呈e指数衰减。在同一燃烧高度的富氧燃烧状态下,OH浓度随着当量比的增加而增加;当甲烷流量保持恒定时,富氧燃烧状态下的OH浓度要高于低氧燃烧状态下的OH浓度。在燃烧场中,采用这种多光谱技术相结合(CRDS-CARS-PLIF)的精准测量方式不仅能够实现对OH浓度精准的定量测量提高了测量精度,还可为定量测量其他燃烧产物分子的浓度提供技术支撑,对研究燃烧化学反应起着至关重要的作用。

增强吸收光谱技术的研究进展及展望

光程吸收光谱技术是吸收光谱技术发展中的一个重要分支,近年来基于不同光源技术、吸收腔技术、探测方式的光程吸收光谱技术大量涌现。随着对探测灵敏度和吸收光程长度需求的提高,出现了基于增强吸收原理的光程吸收光谱技术,包括:积分腔光谱(ICOS)、腔增强吸收光谱(CEAS)和腔衰荡光谱(CRDS)。增强吸收光谱技术具有高光谱分辨率、高灵敏度、快速响应、便携等优势,但至今缺乏统一的概念和明确的分类依据。本文梳理了吸收光谱技术的发展历程,明确了多光程吸收光谱技术的概念。依据吸收腔内是否发生谐振吸收,提出了基于谐振原理的吸收光谱技术这一概念,分析总结了谐振吸收光谱技术的研究现状,并对这些技术在各领域的应用进行概述。最后,对谐振吸收光谱技术中关键技术的未来发展进行了展望。

腔增强吸收光谱技术研究与应用进展(特邀)

腔增强吸收光谱技术(CEAS)具有测量精度高、响应时间快、空间占用少等优势,在痕量气体检测与精密光谱测量中发挥了重要作用。本文结合计算与实验结果,阐明了腔增强技术对信号增强的共性原理及实验测量吸收系数的方法,进而介绍了几种具有代表性的腔增强吸收光谱技术,包括:相干光CEAS技术、非相干光CEAS技术及光梳相干宽带CEAS技术等。以此为基础对腔增强吸收光谱技术在大气污染检测、生物医疗传感、化学反应动力学诊断等方面的应用进行综述。最后对腔增强吸收光谱技术的未来发展进行了展望。

腔增强/衰荡光谱应用于气溶胶消光检测研究进展

腔增强/衰荡吸收光谱技术,是目前应用最广泛的气溶胶光学特性原位测量方法之一,由于其原位、实时特性,测量过程中气溶胶状态不会发生改变,测量具有代表性,近些年来已经开始作为各种仪器综合比对实验中使用的参考标准。在经过近30年的发展后,相关技术发展日益成熟。对腔增强/衰荡吸收光谱技术的发展及其在气溶胶光学特性测量方面的应用研究做了简要的回顾。

光纤腔衰荡光谱技术及其最新进展

阐述了腔衰荡光谱技术的基本原理及应用;从结构设计的角度考虑,归纳出光纤衰荡腔的三种基本结构,并对光纤腔衰荡光谱技术及其最新进展进行了评述;报道了我们采用脉冲光纤环衰荡光谱技术进行液体检测取得的最新进展;在总结光纤腔衰荡光谱技术优点的基础上,展望了其发展前景。

基于LED的非相干宽带腔增强吸收光谱技术

介绍了基于可见光波段高功率LED作为光源的高灵敏度宽带腔增强吸收光谱技术,该系统的探测灵敏度通过测量NO2在472.3～479.3 nm范围内的吸收得到验证。将中心波长为457 nm的高功率LED发出的宽带非相干光耦合进入92.5 cm长、由两片高反射率透镜组成的高精度光学谐振腔内,使用CCD光谱仪(HR2000)测量透过光学腔的光强信号。腔镜在472.3～479.3 nm波长范围内的反射率通过O2-O2聚合物的吸收确定,实验测量了一系列低浓度NO2气体样品,采用差分光谱拟合技术在80 s的平均时间内NO2浓度反演的统计不确定性约为3.1 ppb(ng.mL-1)。

基于高精细度光腔锁频激光的分子吸收光谱测量

利用高精细度光腔锁定激光频率,实现了对分子吸收光谱的高精度测量.光腔采用低热膨胀系数的殷钢结构设计和温度控制,实现了腔长度的稳定;通过将激光频率锁定在光腔纵模上,实现了高频率精度和高灵敏度的光腔衰荡光谱测量.利用该装置示范性地测量了二氧化碳分子在6470.42 cm^(-1)附近的光腔衰荡光谱和色散光谱,得到了高精度的谱线参数,并和数据库谱线参数进行了对比.

高重复频率实时采集的光腔衰荡光谱

建立了一套脉冲激光衰荡光谱实验信号采集与处理系统。它以ＧＰＩＢ为控制和数据传输的界面。采用比国际上同类实验更合理严格的方式采集处理数据。实现了高重复频率。实时的波形信号传输。显示和处理。利用此装置测量了Ｈ２Ｏ从６２０～６６５ｎｍ的振动泛频光谱。实验结果表明。该装置检测灵敏度达到５×１０－８ｃｍ－１。