宽带腔增强吸收光谱技术应用于大气NO3自由基的测量

NO3自由基是夜间大气化学中最重要的氧化剂,控制着多种痕量气体成分的氧化及去除,了解NO3自由基的化学过程对研究灰霾等大气污染过程意义重大.NO3自由基浓度低、活性强,实现大气NO3自由基的高灵敏度准确测量相对困难.本文介绍了大气NO3自由基的宽带腔增强吸收光谱定量方法,采用红光LED作为宽带腔增强吸收光谱系统光源,设计低损耗且适合国内高颗粒物环境的采样气路,并通过LED光源测试确定最佳工作电流和温度;通过采用白天的大气谱作为背景光谱参与NO3自由基的光谱拟合过程,减少水汽对NO3自由基光谱反演的干扰;通过对镜片反射率和有效腔长进行标定,对系统性能进行Allan方差分析,该宽带腔增强吸收光谱系统在光谱采集时间为10 s的情况下,NO3自由基极限探测灵敏度为0.75 pptv,总测量误差约为16%.在合肥开展了实际大气NO3自由基观测,观测期间NO3自由基的浓度范围从低于探测限到23.4 pptv,NO3自由基浓度呈现夜间高、白天低的特征,符合NO3变化规律,表明该宽带腔增强吸收光谱系统能够用于实际大气NO3自由基的高灵敏度测量.     

非相干宽带腔增强吸收光谱技术应用于实际大气亚硝酸的测量

介绍了基于紫外发光二极管光源的非相干宽带腔增强吸收光谱技术, 并用于实际大气亚硝酸(HONO)和二氧化氮(NO₂)的同时测量. 分析了腔内气体的瑞利散射对测量的影响, 测试了紫外发光二极管光源的稳定性, 使用氦气和氮气的瑞利散射差异性标定了镜片反射率随波长的变化曲线, 在HONO吸收峰(368.2 nm)处镜片反射率约为0.99965. 应用Allan方差统计方法确定出测量光谱最佳采集时间为320 s, 对应的HONO和NO₂的探测限(1σ)分别为0.22 ppb 和0.45 ppb. 使用非相干宽带腔增强吸收光谱测量装置对大气HONO和NO₂进行了连续三日的实际观测, 将测量得到的HONO浓度变化与差分吸收光谱测量装置的测量结果进行对比, 线性相关系数R²为0.917.     

UV cavity enhanced absorption spectroscopy of the hydroxyl radical

We present the application of a continuous-wave ultra-violet tuneable light source for detection of the hydroxyl radical (OH) using cavity-enhanced absorption spectroscopy of the Q₁₁(2) and Q₂₁(2) absorption lines in the A²⁺(v=0)²_3/2(v=0) band at ca. 308 nm. A tuneable infra-red diode laser operating at 835 nm and either an Ar⁺ laser or a single frequency continuous-wave intracavity frequency-doubled diode laser, both operating at ca. 488 nm, were used to produce 0.1–0.5 W of tuneable radiation at ca. 308 nm by sum frequency generation in a BaB₂O₄ crystal. Cavity enhanced absorption spectroscopy was used to detect OH generated by UV photolysis of water vapour in argon, nitrogen, neon and helium at atmospheric pressure. A noise-equivalent (1) absorption sensitivity of 2.1×10^-7 cm^-1Hz^-1/2 measured over 128 scans in a time of 1.16 s was demonstrated with mirrors of reflectivity 0.9963 in a cavity of length 58.5 cm for a 2 cm^-1 scanning range at a UV power of 0.5 W. An OH detection limit (1) of 3.84×10⁹ moleculecm^-3 was estimated in argon at atmospheric pressure. OH collisional broadening in humidified N₂, Ar, Ne and He was determined at atmospheric pressure . PACS 39.30.+w; 42.55.Px; 42.62.Fi; 42.68.Ca     

Extensive characterization of the optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) technique: ringdown-time calibration of the absorption scale

The paper describes in detail an implementation of the highly sensitive optical feedback cavity enhanced absorption spectroscopic (OF-CEAS) technique. A Brewster-angle cavity has been used to enable resonant optical feedback injection. For the first time, an OF-CEAS-adapted processing allowing calibration of the cavity-enhanced spectra in absolute absorption units has been experimentally verified through the combination of ringdown-time and mode-amplitude measurements. The experimental system performance has been evaluated by measuring very weak oxygen B-band transitions near 688 nm. Absorption spectra covering the range of 1.35 cm^-1 have been averaged over the 5-s measurement time at the minimum detectable loss of 3×10^-10 /cm within the measurement dynamic range greater than four orders of magnitude. A detailed signal-to-noise ratio analysis estimates the measurement spectral precision as being 3 MHz (short time scale averaging). PACS 07.88.+y; 42.55.Px; 42.62.Fi     

高灵敏度光谱测量技术:腔振铃吸收光谱

腔振铃激光吸收光谱技术是近年快速发展的一项新颖的光谱技术,它不仅检测灵敏度高,而且结构简单,不需要高昂的光谱设备,特别适合于测量弱吸收物质,包括气体、固体、液体等稳态粒子和金属化合物、自由基、团簇等瞬态粒子.本文在介绍其基本原理的基础上,介绍了使用脉冲激光与连续激光光源的2种技术方案.     

利用离轴腔增强吸收光谱技术探测实际大气中的二氧化碳

以1.573μm窄线宽可调谐半导体激光器作光源,结合高精细度光学谐振腔的离轴腔增强吸收光谱技术,选择CO2在6 357.311 6cm-1的吸收谱线,对实际大气CO2分子进行了测量。为了得到更准确的有效方程,对谐振腔吸收程长的标定方法进行了研究,给出了一种简单、实用的标定方法。实验结果表明,高精密度光学谐振腔的有效吸收程长为～1 195.73m,测得实际大气CO2的浓度为～388.3ppm(S/N≈22),最小可探测浓度为17.65ppm。将波长调制技术与OA-CEAS技术结合后,最终将CO2分子的最小可探测浓度提高到0.36ppm(S/N≈1 064)。     

Generation of supercontinuum radiation in conventional single-mode fibre and its application to broadband absorption spectroscopy

High-pulse-energy supercontinuum radiation with a width exceeding 900 nm in the near-infrared spectral region has been generated in conventional single-mode fibre. The fibre was pumped at 1064 nm which is in the normal dispersion regime, resulting in predominantly red-shifted spectral broadening. Supercontinuum pulse energies exceeding 450 nJ were obtained. The use of conventional fibre allows for inexpensive generation of near-infrared supercontinuum radiation, featuring high pulse energies and good spatial beam quality. This supercontinuum radiation was used to acquire high-resolution (15 pm) broadband absorption spectra of H₂O, C₂H₂ and C₂H₄ in the near-infrared spectral region (1340–1700 nm), using an optical spectrum analyser for detection. H₂O spectra were also recorded at high repetition rates, by dispersing the supercontinuum pulses and detecting the transmitted signal in the time domain. A spectral resolution of 38 pm was obtained employing the dispersed supercontinuum pulses, which is comparable to the H₂O line widths at ambient conditions. PACS 07.07.Df; 42.62.Fi; 42.79.Nv; 42.81.-i     

宽带腔增强吸收光谱技术应用于痕量气体探测及气溶胶消光系数测量

基于氙灯的非相干宽带腔增强吸收光谱系统, 并将其应用于痕量气体及气溶胶消光系数的测量. 该系统的探测灵敏度通过测量NO₂在520—560 nm波长范围内的吸收得到验证, 最小可探测灵敏度为1.8× 10^-7cm^-1 (1σ, 0.12 s积分时间, 50次平均), 对应的NO₂探测极限～33 nmol/mol. 结合标准气溶胶粒子发生系统, 测量了不同浓度的单分散硫酸铵气溶胶粒子在532 nm波长处的消光系数, 得到粒径为600 nm的硫酸铵气溶胶的消光截面为1.12× 10^-8cm², 与文献报道值1.167× 10^-8cm²相一致, 验证了气溶胶测量的可行性和准确性.     

Optical feedback cavity enhanced absorption spectroscopy: effective adjustment of the feedback-phase

Optical-feedback cavity enhanced absorption spectroscopy (OF-CEAS) is a very sensitive technique for the detection of trace amounts of gaseous absorbers. The most crucial parameter in an OF-CEAS setup is the optical phase of the light fed back into the laser source, which is usually controlled by the position of a piezo driven mirror. Various approaches for the analysis of the cavity transmitted light with respect to feedback-phase are presented, and tested on simulated phase and frequency dependent cavity transmission. Finally, we present the performance of a digital signal processor based regulator—employing one of these approaches—in a real OF-CEAS experiment. The results of the simulation show that several algorithms are well suited for the task of control signal generation. They confirm also that with the presented approach, a mode by mode correction of the feedback-phase is possible. Consequently, a regulatory bandwidth of 37 Hz was achieved. This maximum control frequency was limited by the piezo system.     

Wavelength modulated cavity enhanced absorption spectroscopy of ammonia at 1994 nm

We describe the sensitive detection of ammonia by wavelength modulated cavity enhanced infrared tunable diode laser absorption spectroscopy at 1994 nm. The spectrometer can measure a fractional absorption of ∼10^-5 for an absorption pathlength of a few kilometers. The spectral resolution and sensitivity are sufficient to measure ammonia isotopomers (¹⁴NH₃, ¹⁵NH₃) in planetary atmospheres. The spectrometer is miniaturisable, so a future multiple-species version will be highly suitable for in situ planetary exploration and life-detection. PACS 42.62.Fi; 33.20.-t; 33.20.Ea     

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.     

机载腔增强吸收光谱系统应用于大气NO_2空间高时间分辨率测量

介绍了一套用于机载平台测量的非相干宽带腔增强吸收光谱(IBBCEAS)系统,并应用于实际大气NO_2空间分布的高时间分辨率观测.为满足机载测量中对时间分辨率的需求,系统采用离轴抛物面镜代替消色差透镜提高光学耦合效率;并运用Allan方差,对系统性能进行了分析.通过腔增强吸收光谱系统与长光程吸收光谱系统对实际大气NO_2的对比测试,两者线性相关系数R~2达到0.86.将IBBCEAS系统应用于机载平台,在时间分辨率为2 s的情况下,探测限达到95 ppt(1σ).通过机载观测,获得了华北地区石家庄等地上空对流层大气NO_2的廓线信息.     

Signal processing system in cavity enhanced spectroscopy

The paper presents a signal processing system used for nitrogen dioxide detection employing cavity enhanced absorption spectroscopy. In this system, the absorbing gas concentration is determined by the measurement of a decay time of a light pulse trapped in a cavity. The setup includes a resonance optical cavity, which was equipped with spherical and high reflectance mirrors, the pulsed diode laser (414 nm) and electronic signal processing system. In order to ensure registration of low-level signals and accurate decay time measurements, special preamplifier and digital signal processing circuit were developed. Theoretical analyses of main parameters of optical cavity and signal processing system were presented and especially signal-to-noise ratio was taken into consideration. Furthermore, investigation of S/N signal processing system and influence of preamplifier feedback resistance on the useful signal distortion were described. The aim of the experiment was to study potential application of cavity enhanced absorption spectroscopy for construction of fully optoelectronic NO₂ sensor which could replace, e.g., commonly used chemical detectors. Thanks to the developed signal processing system, detection limit of NO₂ sensor reaches the value of 0.2 ppb (absorption coefficient equivalent = 2.8 × 10⁻⁹ cm⁻¹).     

Theoretical detection limit of saturated absorption cavity ring-down spectroscopy (SCAR) and two-photon absorption cavity ring-down spectroscopy

Giusfredi et al. (Phys Rev Lett 104, 110801, 2010), have developed a new approach to cavity ring-down spectroscopy where a saturable sample absorption is determined simultaneously with the cavity loss, providing immunity to changes in cavity loss, thereby allowing for lower analyte detection limit. This paper presents an error analysis that provides predictions of the ultimate sensitivity limits that can be realized with this detection method. In particular, the sensitivity is strongly dependent upon the initial degree of saturation of the sample, and optimal values for this are determined both for photon detector and shot-noise-limited detection of both inhomogeneous and homogeneous broadened spectroscopic lines. Also presented are sensitivity limits expected for two-photon absorption spectroscopy determined by cavity ring-down spectroscopy.     

基于LED光源的非相干宽带腔增强吸收光谱技术探测HONO和NO2

介绍了基于近紫外发光二极管LED (中心波长约372 nm,半高宽13 nm) 光源的非相干宽带腔增强吸收光谱技术,同时用于探测痕量气体HONO和NO₂. LED出射光经准直后耦合进入长度为70 cm,由两块高反射率镜片组成的高精密光学腔内. 分别测量了氮气消光谱和氦气消光谱,通过两者瑞利散射截面的差异而引起光谱强度的变化来标定镜片反射率.在360—390 nm反演波段内,镜片反射率在390 nm处最大且为0.99962, 对应测量NO₂/HONO混合物时的最大光程约1.71 km,并利用最小二乘拟合反演出了 HONO和NO₂的浓度值.当光谱采集时间为1000 s时, HONO和NO₂的探测灵敏度(1σ) 分别为0.6 ppbv和1.9 ppbv.实验结果表明,该技术为实现大气痕量气体的高灵敏度在线监测提供了另一种可能的途径.     

Near-infrared cavity enhanced absorption spectroscopy of hot water and OH in an oven and in flames

A compact diode laser operating around 1.5 μm was used to measure cavity enhanced absorption spectra of hot water molecules and OH radicals in radiative environments under atmospheric conditions. Spectra of air were measured in an oven at temperatures ranging from 300 K to 1500 K. These spectra contained rovibrational lines from water and OH. The water spectra were compared to simulations from the HITRAN and HITEMP databases. Furthermore, spectra were recorded in the flame of a flat methane/air burner and in an oxyacetylene flame produced by a welding torch. The results show that cavity enhanced absorption spectroscopy provides a sensitive method for rapid monitoring of species in radiative environments. Received: 22 February 2001 / Revised version: 23 April 2001 / Published online: 7 June 2001     

Controlled calibration method for laser induced breakdown spectroscopy

Laser induced breakdown spectroscopy (LIBS) is a potential technique for rapid analysis of samples present in solids, gases and liquids. In the last two decades it was an object of extensive studies. Controlled calibration method used to analysis the LIBS spectra is investigated. Compared with the inner calibration and calibration-free (CF) methods, this new method overcomes "matrix effect", and demonstrates a better ability to cope with the spectra. It is used to analyze natural soil, and errors of the concentration are decreased about 5%. The result shows that the new method is feasible and accurate.     

Intracavity laser absorption spectroscopy and cavity ring-down spectroscopy in low-pressure flames

Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) have been used for measurement of the NH₂-radical spectrum near 643 nm. NH₂ was obtained in low-pressure methane/air flat flames doped with minor amounts of ammonia (as low as 0.023%). The NH₂ concentration was measured both by CRDS and ICLAS in the same conditions. This enables us to compare the practical sensitivity of the two methods. Both methods were also used for measurements in a sooting acetylene/air flame (ϕ = 2.6). The comparative advantages of the methods and their complementarities are discussed.     

New calibration method for UV-VIS photothermal deflection spectroscopy set-up

Photothermal deflection spectroscopy has emerged as a useful technique for the determination of the absorption of materials with a small absorption coefficient. The technique offers relative values of the material absorptivity and, therefore, requires a calibration procedure in order to determine the absolute values. In this work, we present a new calibration method for a photothermal deflection spectroscopy set-up working in the UV-VIS, spectral range. The method is based on the use of reference samples with different levels of absorption. The samples, consisting of single thin films of amorphous carbon on transparent substrates, are optically characterized by means of spectrophotometric measurements. The accurate characterization of the samples enables the computation of their corresponding optical absorptivity in the PDS set-up. The calibration method is cross-checked by comparison of the measurements for the different reference samples and is finally applied to the study of the absorption of dielectric films in the UV.