Sub-ppb NO2 detection by optical feedback cavity-enhanced absorption spectroscopy with a blue diode laser

We report preliminary results on the first application of optical feedback cavity-enhanced absorption spectroscopy with a blue (411 nm) extended cavity diode laser (ECDL) for NO₂ detection. While this technique was originally developed to operate with distributed feedback diode lasers in the near infrared, it is here extended to ECDLs and applied to the blue spectral region. With a simple and compact optical setup, we demonstrate from the baseline noise a minimum detectable NO₂ concentration of 6×10⁹ molecules/cm³ for a single laser scan (70 ms), which extrapolated under atmospheric conditions corresponds to 200 pptv. Signal averaging should allow further lowering of this limit. Observed absorption spectra display more structure than previous spectra obtained at lower resolution by Fourier-transform spectroscopy at the same wavelength. PACS 07.88.+y; 42.55.Px; 42.62.Fi     

Applications of quantum cascade lasers for sensitive trace gas measurements of CO, CH4, N2O and HCHO

We describe here a sensitive quantum cascade laser absorption spectrometer (QCLAS) employed for aircraft based measurements during the GABRIEL 2005 and HOOVER 2006 and 2007 campaigns. This 3-channel instrument measures CO, HCHO, CH₄ and N₂O using a 64-m path double corner cube White cell. Performance of the instrument was examined for the four species and precisions for CO, N₂O and CH₄ were measured in the field to be 0.5, 0.5 and 0.7% respectively (2σ). The 1σ detection limit for HCHO was ∼500 pptv for a 2 s average, while signal averaging of the HCHO over a 2 min time interval resulted in a 150 pptv detection limit with a duty cycle of 60%. PACS  82.80.Gk; 07.88.+y; 42.62.-b; 92.60.H-     

CW DFB RT diode laser-based sensor for trace-gas detection of ethane using a novel compact multipass gas absorption cell

The development of a continuous wave, thermoelectrically cooled (TEC), distributed feedback diode laser-based spectroscopic trace-gas sensor for ultra-sensitive and selective ethane (C₂H₆) concentration measurements is reported. The sensor platform used tunable diode laser absorption spectroscopy (TDLAS) and wavelength modulation spectroscopy as the detection technique. TDLAS was performed using an ultra-compact 57.6 m effective optical path length innovative spherical multipass cell capable of 459 passes between two mirrors separated by 12.5 cm and optimized for the 2.5–4 μm range TEC mercury–cadmium–telluride detector. For an interference-free C₂H₆ absorption line located at 2,976.8 cm^?1, a 1σ minimum detection limit of 740 pptv with a 1 s lock-in amplifier time constant was achieved.     

Incoherent broad-band cavity-enhanced absorption spectroscopy for simultaneous trace measurements of NO2 and NO3 with a LED source

In the past decade, due to a growing awareness of the importance of air quality and air pollution control, many diagnostic tools and techniques have been developed to detect and quantify the concentration of pollutants such as NO _x , SO _x , CO, and CO₂. We present here an Incoherent Broad-Band Cavity-Enhanced Spectroscopy (IBB-CEAS) set-up which uses a LED emitting around 625 nm for the simultaneous detection of NO₂ and NO₃. The LED light transmitted through a high-finesse optical cavity filled with a gas sample is detected by a low resolution spectrometer. After calibration of the spectrometer with a NO₂ reference sample, a linear multicomponent fit analysis of the absorption spectra allows for simultaneous measurements of NO₂ and NO₃ concentrations in a flow of ambient air. The optimal averaging time is found to be on the order of 400 s and appears to be limited by the drift of the spectrometer. At this averaging time the smallest detectable absorption is 2×10⁻¹⁰ cm⁻¹, which corresponds to detection limits of 600 pptv for NO₂ and 2 pptv for NO₃. This compact and low cost instrument is a promising diagnostic tool for air quality control in urban environments.     

Continuous wave,distributed feedback diode laser based sensor for trace-gas detection of ethane

The development of a continuous wave (CW), thermoelectrically cooled (TEC), distributed feedback (DFB) laser diode based spectroscopic trace-gas sensor for ultra-sensitive and selective ethane (C₂H₆) concentration measurements is reported. The sensor platform used tunable diode laser absorption spectroscopy (TDLAS) based on a 2f wavelength modulation (WM) detection technique. TDLAS was performed with a 100 m optical path length astigmatic Herriott cell. For an interference free C₂H₆ absorption line located at 2976.8 cm⁻¹ a 1σ minimum detection limit of 240 pptv (part per trillion by volume) with a 1 second lock-in amplifier time constant was achieved. In addition, reliable and long-term sensor performance was obtained when operating the sensor in an absorption line locked mode.     

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.     

Cavity-enhanced absorption spectroscopy with a red LED source for NOx trace analysis

Incoherent broad-band cavity-enhanced absorption spectroscopy (IBB-CEAS) based on arc lamps has been around for a few years, but only two reports exist using light-emitting diodes (LEDs). We present a setup based on a 643-nm LED which is of interest for the simultaneous detection of NO₃ and NO₂. The latter is chosen for testing as it is stable and available in calibrated diluted samples. A detection limit in the ppbv range is obtained with 2-min averaging (5×10^-9 /cm rms baseline noise level), comparable to the best performance of chemiluminescence devices used for pollution monitoring. At 1-s acquisition time, the detection limit is below 10 ppbv. Extrapolation to NO₃ yields a detection limit of a few pptv for a few minutes averaging. We also test the retrieval of absolute sample absorption (and concentration) using the cavity mirror reflectivity obtained with a commercial spectrophotometer, and we conclude that a calibration based on a reference sample of known concentration is preferable for accurate concentration measurements with IBB-CEAS. Finally, we present a rigorous frequency-domain derivation of cavity transmission as a function of wavelength for a broad-band spectrally smooth source, which complements the time-domain derivation by Fiedler et al. This derivation exposes an issue with multiple transverse mode excitation inherent to this technique, which may result in slightly distorted spectral profiles. PACS 07.88.+y; 07.60.Rd; 42.60.Da     

A high sensitivity spectrometer with tunable diode laser for ambient methane monitoring

Tunable diode laser absorption spectroscopy (TDLAS) has been widely employed in atmospheric trace gases detection. In the measurement of these trace gases, harmonic detection combined with a multi-pass white cell could remarkably enhance the detection sensitivity. In this paper, a portable TDLAS system built specifically for long time monitoring methane in the atmosphere is introduced. The detection limit is below 100 ppb that is enough for the monitoring of ambient methane, and the long time monitoring results obtained in Beijing are given, which is well coincident with that of the Fourier transform infrared (FTIR) spectroscopy.     

Influence of laser intensity in second-harmonic detection with tunable diode laser multi-pass absorption spectroscopy

Tunable diode laser absorption spectroscopy （TDLAS） has been widely employed in atmospheric trace gases detection. The ratio of the second-harmonic signal to the intensity of laser beam incident to the multi-pass cell is proved to be proportional to the product of the path length and the gas concentration under any condition. A new calibration method based on this relation in TDLAS system for the measurement of trace gas concentration is proposed for the first time. The detection limit and the sensitivity of the system are below 110 and 31ppbv （parts-per-billion in volume）, respectively.     

Optical-feedback cavity-enhanced absorption spectroscopy with a quantum-cascade laser yields the lowest formaldehyde detection limit

We report on the first application of Optical Feedback-Cavity Enhanced Absorption Spectroscopy to formaldehyde trace gas analysis at mid-infrared wavelengths. A continuous-wave room-temperature, distributed-feedback quantum cascade laser emitting around 1,769 cm^?1 has been successfully coupled to an optical cavity with finesse 10,000 in an OF-CEAS spectrometer operating on the ν₂ fundamental absorption band of formaldehyde. This compact setup (easily transportable) is able to monitor H₂CO at ambient concentrations within few seconds, presently limited by the sample exchange rate. The minimum detectable absorption is 1.6 × 10^?9 cm^?1 for a single laser scan (100 ms, 100 data points), with a detectable H₂CO mixing ratio of 60 pptv at 10 Hz. The corresponding detection limit at 1 Hz is 5 × 10^?10 cm^?1, with a normalized figure of merit of 5 × 10^?11cm $^{-1}/\sqrt{\rm Hz}$ (100 data points recorded in each spectrum taken at 10 Hz rate). A preliminary Allan variance analysis shows white noise averaging down to a minimum detection limit of 5 pptv at an optimal integration time of 10 s, which is significantly better than previous results based on multi-pass or cavity-enhanced tunable QCL absorption spectroscopy.     

近红外波段CO高灵敏检测的稳定性研究

本文采用中心波长1566.64 nm的DFB激光器, 结合光程长度为56.7 m 的多次反射池, 对不同浓度的CO气体进行了长时间测量, 分析了系统的稳定性和线性度. 通过计算Allan方差, 预测了在积分时间为30 s时, 系统检测极限为0.25 ppmv, 基本上实现了在近红外波段CO的高灵敏检测. 关键词： 可调谐半导体激光吸收光谱 多次反射池 CO检测 Allan方差     

光腔衰荡光谱法测量大气中的NO3和N2O5

本文应用基于二极管激光器的双路光腔衰荡光谱技术，分别对大气中NO₃和N₂O₅浓度进行监测. 通过使用实验室标准样校正有效吸收腔长比R_L和系统的总损耗系数?，并获得了NO₃有效吸收截面. 该装置在时间分辨率为1 s时，对NO₃的测量灵敏度达到1.1 pptv，N₂O₅被在线转换成NO₃，从而被另一路光腔衰荡光谱装置探测. 利用该装置，对合肥市区冬季夜间大气中的NO₃，N₂O₅浓度进行了实时监测. 通过对比一次大气快速清洁过程中氮氧化物、臭氧、PM_2.5等组分的浓度变化，讨论了大气环境下可能影响NO₃及N₂O₅浓度的因素.     

Gain versus tuning issues to Q-switch with Yb<Superscript>3+</Superscript>:LSO and amplify broad-bandwidth pulses

We report on the development of Incoherent Broadband Cavity Enhanced Absorption Spectroscopy (IBBCEAS) using a blue light emitting diode (LED) for the detection of NO₂ in laboratory ambient air. Absorption of the oxygen collisional pair in the atmosphere was also detected in the same spectral range. The mirror reflectivity was determined using a standard gas sample mixture of NO₂, and calibrated with the help of the absorption spectrum of the oxygen collisional pair in pure oxygen at atmospheric pressure. Optimization of the experimental parameters was investigated and is discussed in detail. For the first time in IBBCEAS involving broadband absorption spectra, averaging time for signal-to-noise ratio enhancement has been optimized using Allan variance plot. 18.1 ppbv NO₂ in laboratory ambient air has been retrieved from the absorption spectra using differential fitting method over a 40 nm spectral region centered at 470 nm. A minimum detection sensitivity of about 2.2 ppbv (1σ) for NO₂ at atmospheric pressure has been achieved using the optimal averaging time of 100 s by means of a high finesse optical cavity formed with two moderate reflectivity (∼99.55%) mirrors. No purging of the cavity mirrors by high purity He or N₂ gas streams was necessary to prevent contamination of the mirror faces for the in situ measurements.     

基于可调谐激光吸收光谱的大气甲烷监测仪

可调谐二极管激光吸收光谱(TDLAS)技术利用二极管激光器波长调谐特性,获得被测气体的特征吸收光谱范围内的吸收光谱,从而对污染气体进行定性或者定量分析,这种方法不仅精度较高,选择性强而且响应速度快,已经被用于大气痕量气体监测以及工业控制。在对空气中的痕量气体进行检测中,由于气体浓度较低,需要和长吸收光程技术相结合。将可调谐二极管激光吸收光谱与经过108次反射后达到27 m光程的多次反射池相结合研制了用于地面环境空气中甲烷含量监测的便携式吸收光谱仪,并结合了用于微弱信号检测的二次谐波检测技术,从而达到了体积分数低于1×10-7的检测限,并利用不同体积分数的甲烷气体对系统进行了测试,得到了很好的测试结果。     

1.653 μm处甲烷分子谐波探测技术研究

谐波探测被广泛应用于激光光谱技术中,利用它可以提高探测灵敏度。利用1.653 μm的分布反馈式(DFB)二极管激光器作为光源,建立了一套可调谐半导体激光吸收光谱(TDLAS)甲烷探测装置。该装置利用由2块圆形柱面镜构成的光学多通池增加吸收光程,提高探测灵敏度。吸收池基长为15 cm,在112次反射情况下,有效吸收光程达到16.8 m,实现甲烷0.60×10-6(2 s采样时间)的探测极限,可应用于实际大气甲烷的痕量探测。     

Advances in hardware, system diagnostics software, and acquisition procedures for high performance airborne tunable diode laser measurements of formaldehyde

A tunable diode laser absorption spectroscopy (TDLAS) instrument was deployed onboard a DC-8 aircraft as part of the International Chemical Transport Experiment – North America (INTEX-NA) during the summer of 2004 to quantify atmospheric formaldehyde (CH₂O) concentrations. A number of improvements, both software and hardware, are discussed and include the laser tuning waveform, spectral wavelength centering, and optical stabilization. In addition, the impact of perturbations to the instrument in flight is reviewed and a number of advanced TDLASdata-acquisition and processing concepts are introduced to identify the presence of optical perturbations in flight to objectively eliminate such perturbed data, assess the validity of the fitting routine in the presence of perturbed data, provide various diagnostic measures to elucidate system behavior, and assess the efficacy of various opto-mechanical improvements implemented to reduce the magnitude of such perturbations. The concepts specific to our TDLASmeasurements of CH₂O should have broader and more universal applicability to measurement of other trace gases and possibly other methods of detection.     

调谐半导体激光吸收光谱自平衡检测方法研究

可调谐半导体激光吸收光谱技术(TDLAS)是利用半导体激光器的波长调谐特性,扫描待测气体特征吸收线,从而获得待测气体的浓度信息。基于可调谐半导体激光吸收光谱的自平衡检测方法能够有效地消除激光器光强波动等共模噪声和其他同性干扰的影响。实验表明自平衡检测方法可以获得较理想的结果,检测限低于体积比1.2×10-6,与直接吸收光谱法相比降低了一个数量级。自平衡检测电路简单,自带的电子增益补偿机制能够自动进行平衡探测,该方法不用加信号调制和锁相放大器,直接探测待测气体的吸收光谱,从而降低成本,减小系统装置体积,易于集成为便携式痕量气体检测仪。     

Measuring δ13C of atmospheric air with non-dispersive infrared spectroscopy

The potential use of non-dispersive infrared spectroscopy for measuring δ¹³C in air is demonstrated. This technique has already been successfully established for breath test analyses in medical diagnostics, where the CO₂ concentration ranges from 1 to 5 vol.% in the exhaled breath of vertebrates. For breath tests, the sensitivity and accuracy has been improved to reach a standard deviation of 0.2 ‰ (delta-value). Further adjustments were necessary to improve the sensitivity of the instrument at concentration levels typical of atmospheric air. The long-term stability is given by a standard deviation of 0.35 ‰ for CO₂ concentrations of about 400 ppm with signal averaging over 60 s.     

可调谐半导体激光吸收光谱技术检测痕量乙烯气体的系统研制

可调谐半导体激光吸收光谱技术(TDLAS)利用激光器的窄线宽和波长调谐特性,使其扫描被测气体的单个吸收峰,实现痕量气体的高分辨率、高灵敏度快速检测。通过分析近红外波段的乙烯吸收谱线特性,选取1 626.8 nm附近的吸收峰作为检测谱线,研制了基于white池结构的TDLAS检测系统,结合波长调制和二次谐波检测,对体积分数为20～1 200 ppmv的乙烯气体进行了测量,推算该系统的检测下限约为10 ppmv。     

可调谐二极管激光吸收光谱法监测大气痕量气体中的浓度标定方法研究

可调谐二极管激光吸收光谱(TDLAS)技术是利用二极管激光器波长调谐特性,获得被测气体的特征吸收光谱范围内的吸收光谱,从而对污染气体进行定性或者定量分析。在空气痕量气体检测中,一般需要和长光程吸收池相结合使用。可调谐二极管激光吸收光谱法就是在可调谐二极管激光器与长光程吸收池技术相结合的基础上发展起来的一种新的痕量气体检测方法。这种方法不仅精度较高,选择性强而且响应速度快。已经广泛用于大气中多种痕量气体的检测以及地面的痕量气体和气体泄漏的检测。在大部分痕量气体检测仪器中需要精确地对检测气体进行在线的浓度标定,文章介绍了一个简单而精确的浓度标定方法,从理论上进行了分析,然后通过实验证明了这种方法的可行性。