A trace methane gas sensor using mid-infrared quantum cascaded laser at 7.5 μm

Presented is a compact instrument developed for in situ high-stable and sensitive continuous measurement of trace gases in air, with results shown for ambient methane (CH₄) concentration. This instrument takes advantage of recent technology in thermoelectrically cooled pulsed Fabry–Perot (FP) quantum cascaded (QC) laser driving in a pulse mode operating at 7.5 μm to monitor a well-isolated spectral line near the ν4 fundamental band of CH₄. A high-quality liquid nitrogen cooled mercury cadmium telluride mid-infrared detector with time discriminating electronics is used along with a total reflection coated gold ellipsoid mirror offering 20 cm single pass optical absorption in an open-path cell to achieve stability of 5.2 × 10^?3 under experimental condition of 200 ppm measured ambient CH₄. The instrument operates continuously, and integrated software for laser control using direct absorption provides quantitative trace gas measurements without calibration. One may substitute a QC laser operating at a different wavelength to measure other gases. The instrument can be applied to field measurements of gases of environmental concern.     

采用7.5 μm中红外量子级联激光器的痕量甲烷气体检测仪

一种具有高稳定性和高敏感度的紧凑型仪器,能精确、实时、实地连续测量和显示环境空气中的痕量甲烷(CH₄)浓度。仪器采用了已集成热电制冷器激射波长为7.5μm的法布里-珀罗量子级联激光器(QCL)在室温脉冲工作模式下的最新技术,以覆盖CH₄位于ν₄附近基频特征吸收谱带。同时,采用高品质液氮制冷碲镉汞中红外探测器,配合全反射镀金椭球反射镜一同使用,在20 cm单路径开放式光路吸收气室环境下,确保被测甲烷气体浓度为200 μmol·mol-1的实验条件下保持稳定度高达5.2×10-3。此仪器所集成的软件算法通过时间鉴别电子技术实现对QCL控制,能够在无需校准的情况下,提供连续痕量甲烷气体检测。实验表明,仪器可以用于环境监测中的实地痕量气体测量,并且操作人员可以通过替换在不同波长下运行的QCL来测量其他气体。     

Sub-part-per-billion detection of nitric oxide in air using a thermoelectrically cooled mid-infrared quantum cascade laser spectrometer

Non-cryogenic, laser-absorption spectroscopy in the mid-infrared has wide applications for practical detection of trace gases in the atmosphere. We report measurements of nitric oxide in air with a detection limit less than 1 nmole/mole (<1 ppbv) using a thermoelectrically cooled quantum cascade laser operated in pulsed mode at 5.26 μm and coupled to a 210-m path length multiple-pass absorption cell at reduced pressure (50 Torr). The sensitivity of the system is enhanced by operating under pulsing conditions which reduce the laser line width to 0.010 cm^-1 (300 MHz) HWHM, and by normalizing pulse-to-pulse intensity variations with temporal gating on a single HgCdTe detector. The system is demonstrated by detecting nitric oxide in outside air and comparing results to a conventional tunable diode laser spectrometer sampling from a common inlet. A detection precision of 0.12 ppb Hz^-1/2 is achieved with a liquid-nitrogen-cooled detector. This detection precision corresponds to an absorbance precision of 1×10^-5 Hz^-1/2 or an absorbance precision per unit path length of 5×10^-10 cm^-1 Hz^-1/2. A precision of 0.3 ppb Hz^-1/2 is obtained using a thermoelectrically cooled detector, which allows continuous unattended operation over extended time periods with a totally cryogen-free instrument. Received: 1 May 2002 / Revised version: 6 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-978/663-4918, E-mail: ddn@aerodyne.com     

High precision and continuous field measurements of δ 13C and δ 18O in carbon dioxide with a cryogen-free QCLAS

The present paper describes a compact and cryogen-free, quantum cascade laser based absorption spectrometer (QCLAS) designed for in situ, continuous and high precision isotope ratio measurements of atmospheric CO₂. The mobile instrument incorporates several new features including a novel astigmatic multi-pass cell assembly, a quasi-room temperature quantum cascade laser, thermoelectrically cooled detectors as well as a new retrieval approach. The combination of these features now makes it possible to measure isotope ratios of ambient CO₂ with a precision of 0.03 and 0.05‰ for δ¹³C and δ¹⁸O, respectively, using a 100 s integration time. A robust and optimized calibration procedure was developed to bring the retrieved isotope ratios on an absolute scale. This assures an accuracy better than 0.1‰ under laboratory conditions. The instrument performance was also assessed in a field campaign in which the spectrometer operated autonomously and provided mixing ratio values for the main three CO₂ isotopologues at one second time resolution. An accuracy of 0.2‰ was routinely obtained for both isotope ratios during the entire period. The results were in excellent agreement with the standard laboratory-based isotope ratio mass spectrometer measurements made on field-collected flask samples. A few illustrative examples are used to depict the potential of this optical method in atmosphere–biosphere research. PACS  07.57.Ty; 42.62.Fi; 92.60.Kc     

Frequency modulation spectroscopy by means of quantum-cascade lasers

In this paper we investigate the performance of quantum cascade (QC) lasers for high frequency modulation spectroscopy, particularly using frequency modulation (FM) and two-tone (2T) techniques. The coupling of the rf signal to the QC laser through the cryostat is studied in detail as well as the noise contributions of both the detector and the laser source to the final spectra. The experimental traces are obtained by spectroscopy on low-pressure N₂O and CH₄ gases at 8.0 μm and 7.3 μm wavelength, respectively, and reproduce the line profiles predicted by theory. As a preliminary result, an enhancement of a factor six is measured with respect to direct absorption line recording. PACS 42.62.Fi; 42.72.A1; 07.88.+y     

Shock tube study of the interaction between ammonia and nitric oxide at high temperatures using laser absorption spectroscopy

The interaction between ammonia (NH₃) and nitric oxide (NO) at high temperatures is studied in this work using a shock tube combined with laser absorption diagnostics. The system simultaneously measured the NH₃ and NO time-histories during the reaction processes of the shock-heated NH₃/NO/CO/Ar mixtures (NH₃:NO ≈ 0.9:1.0 and 1.4:1.0). The absorption cross-sections of NH₃ near 1122.10 cm^–1 and NO at 1900.52 cm^–1 (characterized in this study) were used for measuring NH₃ and NO time-histories with the temperature measured by two CO absorption lines. The measured NH₃ and NO time-histories at 1614–1968 K and 2.4–2.8 atm were compared with predictions of seven recent kinetics models. The predictions that based on different mechanisms are very different and the measured profiles are within the range of the predictions. The Glarborg, NUI Galway Syngas-NOx, and Mathieu mechanisms give the closest predictions to the measurements. Kinetics analyses indicate that the NH₃ and NO consumption rates are extremely sensitive to the rate constants and branching ratio of NH₂ + NO = N₂ + H₂O and NH₂ + NO = NNH + OH, which are more reliably represented in the Glarborg and NUI Galway Syngas-NOx mechanisms. The performances of Glarborg mechanisms at lower initial temperatures can be apparently improved by revising the rate constants and branching ratio of NH₂ + NO = N₂ + H₂O and NH₂ + NO = NNH + OH. These two reactions are also the primary pathways for NO reduction and NH₃ is mainly consumed via NH₃ + OH = NH₂ + H₂O and NH₃ + H = NH₂ + H₂. Trace amounts of NO₂ and N₂O impurities decompose to form O radical followed by the generation of OH radical via H-abstraction reactions, which significantly affects the predictions of NH₃ and NO according to kinetics analyses.     

Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity

Cavity ringdown (CRD) absorption spectroscopy enables spectroscopic sensing of gases with a high sensitivity and accuracy. Instrumental improvements result in a new high-performance continuous-wave (cw) CRD spectrometer using a rapidly-swept cavity of simple design. It employs efficient data-acquisition procedures, high-reflectivity mirrors, a low-adsorption flow cell, and various compact fibre-optical components in a single-ended transmitter-receiver configuration suitable for remote sensing. Baseline noise levels in our latest cw-CRD experiments yield a competitive noise-equivalent absorption limit of ∼5×10^-10 cm^-1Hz^-1/2, independent of whatever molecules are to be detected. Measurements in the near-infrared wavelength range of 1.51–1.56 μm yield sub-ppmv (i.e., ppbv or better) sensitivity in the gas phase for several representative molecules (notably CO₂, CO, H₂O, NH₃, C₂H₂, and other hydrocarbons). By measuring spectroscopic features in the 1.525 μm band of C₂H₂ gas, we realise detection limits of 19 nTorr (2.5×10^-11 atm) of neat C₂H₂ (Doppler-limited at low pressure) and 0.37 ppbv of C₂H₂ in air (pressure-broadened at 1 atm). Our cw-CRD spectrometer is a high-performance sensor in a relatively simple, low-cost, compact instrument that is amenable to chemical analysis of trace gases in medicine, agriculture, industry, and the environment. PACS 07.07.Df; 07.57.Ty; 42.62.Fi     

星载大气痕量气体差分吸收光谱仪光谱定标技术研究

星载大气痕量气体差分吸收光谱仪用于遥感监测痕量气体的全球分布。该载荷探测地球大气或表面反射、散射的紫外/可见光辐射,利用差分吸收光谱算法来解析痕量污染气体成分的分布和变化。光谱定标是仪器遥感数据定量化的前提和基础,定标的精度直接决定了仪器研制和应用水平的高低。针对星载大气痕量气体差分吸收光谱仪视场大、波长宽、空间分辨率和光谱分辨率高的特点,提出了相应的光谱定标方法,建立了定标装置,通过寻峰和回归分析计算光谱定标方程,实现了对载荷的全视场光谱定标工作。并利用太阳光的夫琅禾费线对定标精度进行了检验。     

Study of gas detection based on integrated cavity output spectroscopy

A trace gases detection system based on integrated cavity output spectroscopy (ICOS) was developed, where a NIR tunable diode laser (TDL) was used as light source, an optical cavity composed by two plan-concave mirrors with reflection near 99.7% was used as the absorption cell. Trace water vapour (H₂O), carbon dioxide (CO₂), methane (CH₄), carbon monoxide (CO) and mixture of CO₂ and CO were tested by ICOS based on the characteristics absorption. The wavelength calibration, cavity transmission characteristics, quantitative measurement ability and sensitivity of the TDL-ICOS were also studied, and a evaluated minimum detectable sensitivity of 1.15 × 10^?7 cm^?1 was obtained when the system was used to CH₄ detection. The experiment results show that TDL-ICOS is expected to be a reliable and promising system for the detection of trace gases since it has some advantages such as real-time monitoring, simple device, easy operation, high sensitivity, good stability and quantitative ability.     

Investigation on infrared laser absorption spectroscopy measurement of acetylene trace quantities

A laser-based infrared spectrometer was developed for use in high-resolution spectroscopic analysis of trace gases in the atmosphere. Continuous wave, broadly tunable coherent infrared radiation was generated from 8 to 19 μm in a gallium selenide crystal by laser difference-frequency mixing. Measurements of acetylene trace concentrations using laser absorption spectroscopy are reported. The measurements have been performed by using the P(21), Q(11), and R(9) lines of the ν₅ band, respectively, in order to investigate optimal detection conditions: a trade-off choice between higher line absorption strength for sensitive detection and better spectral discrimination from lines overlapping for open path trace-gas monitoring applications. Minimum detectable concentrations are compared under different experimental conditions. The ν₅ R(9) line seems to be close to optimum in terms of absorption strength and freedom from spectral interference for spectroscopic detection of acetylene trace concentration at atmospheric pressure.     

Telecom-grade fiber laser-based difference-frequency generation and ppb-level detection of benzene vapor in air around 3 μm

We report on the development of a field deployable compact laser instrument tunable over ∼232 cm⁻¹ from 3.16 to 3.41 μm (2932.5–3164.5 cm⁻¹) for chemical species monitoring at the ppb-level. The laser instrument is based on widely tunable continuous-wave difference-frequency generation (DFG), pumped by two telecom-grade fiber lasers. DFG power of ∼0.3 mW near 3.3 μm with a spectral purity of ∼3.3 MHz was achieved by using moderate pumping powers: 408 mW at 1062 nm and 636 mW at 1570 nm. Spectroscopic performance of the developed DFG-based instrument was evaluated with direct absorption spectra of ethylene at 3.23 μm (∼3094.31 cm⁻¹). Absorption spectra of vapor-phase benzene near 3.28 μm (∼3043.82 cm⁻¹) were recorded with Doppler-limited resolution. Line intensities of the most intense absorption lines of the ν ₁₂ band near 3043.8 cm⁻¹ were determined to support development of sensitive mid-infrared trace gas detection of benzene vapor in the atmosphere. Detection of benzene vapor in air at different concentration levels has been performed for the first time using multi-pass cell enhanced direct absorption spectroscopy at ∼3.28 μm with a minimum detectable concentration of 50 ppb (1σ).     

Intracavity laser spectroscopy of CH<Subscript>4</Subscript> and NH<Subscript>3</Subscript> gases by using a pulse-periodic Cr<Superscript>2+</Superscript>:ZnSe laser

The results of studying the characteristics of an intracavity laser spectrometer based on a polycrystalline Cr²⁺:ZnSe laser operating in a pulse-periodic regime with the pulse-repetition rate of 3 kHz and pulse duration of ∼50 ns are presented. Intracavity spectra of absorption of NH₃ and CH₄ gases in the vicinity of 2.35 μm are measured. The estimate of the spectrometer sensitivity is provided.     

The influence of wavelength in extinction measurements and beam steering in laser-induced incandescence measurements in sooting flames

The accuracy of laser-induced incandescence (LII) measurements is significantly influenced by the calibration process and the laser profile degradation due to beam steering. Additionally, the wavelength used for extinction measurements, needed for LII calibration, is critical and should be kept as high as possible in order to avoid light absorption by molecular species in the flame. The influence of beam steering on the LII measurement was studied in turbulent sooting C₂H₄/air flames at different pressures. While inhomogeneities in the laser profile become smoothed out in time-averaged measurements, especially at higher pressure, the corresponding single-shot beam profiles reveal an increasing effect of beam steering. In the current configuration it was observed that the resulting local laser fluence remains within certain limits (30% to 200%) of the original value. A sufficiently high incident laser fluence can thus prevent the local fluence from dropping below the LII threshold value of approximately 0.3 J/cm² at the cost of increased soot surface vaporization. A spatial resolution in the dimension of the sheet thickness of below 1 mm cannot be guaranteed at increased pressure of 9 bars due to beam steering. A feasibility study in a combustor at technical conditions demonstrates the influence of both effects beam steering and choice of calibration wavelength and led to the conclusion that, however, a shot-to-shot calibration of LII with simultaneously measured extinction can be realized.     

Quantum cascade laser-based spectrometer for high sensitive measurements of trace gases in air

A quantum cascade(QC)laser-based spectrometer is developed to measure trace gases in air.The proposed spectrometer is tested for N2O,and the results presented in this letter.This system takes advantage of recent technology in QC lasers by utilizing intra-pulse scan spectroscopy,which allows high sensitive measurement.Without calibration gases,the gas concentration can be calculated with scan integration and the corresponding values from the HITRAN04 database.By analyzing the Allan variance,a detection limit of 2 ppb is obtained.Continuous measurement of N2O sampled from ambient air shows the applicability of the proposed system for the field measurements of gases of environmental concern.     

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.     

Absorption line strengths of <Superscript>15</Superscript>NH<Subscript>3</Subscript> in the near infrared spectral region

We present initial results of an investigation of the near infrared absorption spectrum of ¹⁵NH₃ between 6468 and 6692 cm⁻¹. A widely tunable external cavity diode laser is used in a direct absorption setup to determine the line positions and line strengths of several lines in that spectral range. Line data measurements on a ¹⁴NH₃ sample are used for validation of the setup by comparison of the results with available literature data. The presented overview measurements on absorption lines of ¹⁵NH₃ have been performed to serve as a starting point for candidate line selection for prospective isotopic ratio measurements of ¹⁴NH₃ and ¹⁵NH₃.     

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.     

Wavelength stabilization of an Ar+ laser with an external129I2 absorption cell

Wavelength stabilization of the green Ar⁺ laser line at 514.5 nm by¹²⁹I₂ absorption was found to be more favourable than by¹²⁷I₂ absorption because the maximum of¹²⁹I₂ absorption is closer to the center of the 514.5 nm gain curve. A simple method for stabilizing a short air cooled laser with only one servo loop is given, yielding a stability of 10⁻⁸ λ.     

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.     

Evaluation of ammonia absorption coefficients by photoacoustic spectroscopy for detection of ammonia levels in human breath

Photoacoustic spectroscopy represents a powerful technique for measuring extremely low absorptions independent of the path length and offers a degree of parameter control that cannot be attained by other methods. We report precise measurements of the ammonia absorption coefficients at the CO₂ laser wavelengths by using a photoacoustic (PA) cell in an extracavity configuration and we compare our results with other values reported in the literature. Ammonia presents a clear fingerprint spectrum and high absorption strengths in the CO₂ wavelengths region. Because more than 250 molecular gases of environmental concern for atmospheric, industrial, medical, military, and scientific spheres exhibit strong absorption bands in the region 9.2–10.8 μm, we have chosen a frequency tunable CO₂ laser. In the present work, ammonia absorption coefficients were measured at both branches of the CO₂ laser lines by using a calibrated mixture of 10 ppm NH₃ in N₂. We found the maximum absorption in the 9 μm region, at 9R(30) line of the CO₂ laser. One of the applications based on the ammonia absorption coefficients is used to measure the ammonia levels in exhaled human breath. This can be used to determine the exact time necessary at every session for an optimal degree of dialysis at patients with end-stage renal disease.