A cavity ring-down analyzer for measuring atmospheric levels of methane, carbon dioxide, and water vapor

Researchers investigating global climate change need measurements of greenhouse gases with extreme precision and accuracy to enable the development and benchmarking of better climate models. Existing atmospheric monitors based on non-dispersive infrared (NDIR) sensors have known problems – they are non-linear, sensitive to water vapor concentration, and susceptible to drift. Many cannot easily be simultaneously calibrated across different sites to the level of accuracy required for use in atmospheric studies. We present results from field trials by Pennsylvania State University and the National Oceanographic and Atmospheric Administration (NOAA) of a newly available analyzer, based on cavity ring-down spectroscopy (CRDS), capable of measuring the concentrations of carbon dioxide (CO₂) and water vapor (H₂O). In addition, we present data from a new analyzer which measures CO₂, methane (CH₄), and H₂O. PACS  07.88.+y     

An IH-QCL based gas sensor for simultaneous detection of methane and acetylene

Extended wavelength tuning of an IH-QCL (integrated heater quantum cascade laser) is exploited for simultaneous detection of methane and acetylene using direct absorption spectroscopy. The integrated heater, placed within few microns of the laser active region, enables wider wavelength tuning than would be possible with a conventional DFB (distributed feedback) QCL. In this work, the laser current and heater resistor current are modulated simultaneously at 25?kHz to tune the laser over 1279.6–1280.1 cm^?1, covering absorption transitions of methane and acetylene. The laser is characterized extensively to understand the dependence of wavelength tuning on modulation frequency, modulation amplitude and phase difference between laser/heater modulation. Thereafter, the designed sensor is validated in both room-temperature static cell experiments and non-reactive high-temperature-measurements in methane-acetylene-argon gas mixtures in the shock tube. Finally, the sensor is applied for simultaneous detection of methane and acetylene during the high-temperature pyrolysis of iso-octane behind reflected shock waves.     

Quantum cascade laser-based sensor for detection of exhaled and biogenic nitric oxide

A real-time, in situ CO sensor using 2.3 μm DFB diode laser absorption, with calibration-free wavelength-modulation-spectroscopy, was demonstrated for continuous monitoring in the boiler exhaust of a pulverized-coal-fired power plant up to temperatures of 700 K. The sensor was similar to a design demonstrated earlier in laboratory conditions, now refined to accommodate the harsh conditions of utility boilers. Measurements were performed across a 3 m path in the particulate-laden economizer exhaust of the coal-fired boiler. A 0.6 ppm detection limit with 1 s averaging was estimated from the results of a continuous 7-h-long measurement with varied excess air levels. The measured CO concentration exhibited expected inverse trends with the excess O₂ concentration, which was varied between 1 and 3 %. Measured CO concentrations ranged between 6 and 200 ppm; evaluation of the data suggested a dynamic range from 6 to 10,000 ppm based on a minimum signal-to-noise ratio of ten and maximum absorbance of one. This field demonstration of a 2.3 μm laser absorption sensor for CO showed great potential for real-time combustion exhaust monitoring and control of practical combustion systems.     

基于中红外分布反馈量子级联激光器的光声光谱技术用于痕量甲烷气体检测

由于工业监控和环境检测的需要,甲烷气体检测日益得到人们的关注。研究了基于中红外分布反馈量子级联激光器(DFB-QCL)的光声光谱技术,并应用于痕量甲烷的检测。自主研发的DFB-QCL室温工作时的激射波长在7.6μm附近,覆盖了甲烷的特征吸收谱线1 316.83cm-1。待测甲烷气体充入亥姆霍兹光声谐振腔中,DFB-QCL的工作频率为234Hz、室温脉冲工作时峰值功率为80mW。中红外光经过甲烷吸收后,产生的声波信号经麦克风检测,由锁相放大器对信号进行采集并输入计算机进行处理。按信噪比为1计算,得到甲烷的探测极限为189nmol.mol-1。     

Modified grating-based external cavity diode laser for simultaneous dual-wavelengths operation

We have reported a modified V-shaped external cavity, which is constructed around a commercial diode laser operating at a center wavelength of λ=785 nm by adding a new coated glass plate with about 50% reflectivity to the cavity. This allows simultaneous dual-wavelengths operation in the vicinity of Δν_min=0.18 THz to Δν_max=0.22 THz, which can be used as laser source for terahertz generation either for semiconductor devices or nonlinear schemes.     

Nanoscale microcavity sensor for single particle detection

Recently we demonstrated a biosensor based on a two-dimensional photonic crystal microcavity for detection of proteins. We present a theoretical and experimental study of a modified structure for single particle detection. With an active sensing volume of approximately 0.15 microm(3), the device is capable of detecting approximately 1 fg of matter. Its performance is tested with latex spheres with sizes that fall in the size range of a variety of viruses.     

Frequency-stabilized cavity ring-down spectrometer for high-sensitivity measurements of water vapor concentration

We present a portable spectrometer that uses the frequency-stabilized cavity ring-down spectroscopy technique capable of high-precision measurements of trace water vapor concentration. Measuring one of the strongest rovibrational transitions in the ν₁+ν₃ water vapor combination band near ˜ν=7181.156 cm^-1, we compare spectroscopic and thermodynamic determinations of trace water vapor in N₂, and find systematic differences attributable to water vapor background effects and/or uncertainties in line intensities. We also compare the frequency-stabilized ring-down method with other cavity ring-down approaches that are based on unstabilized probe lasers and unstabilized ring-down cavities. We show that for the determination of water vapor concentration, the frequency-stabilized cavity ring-down method has the minimum measurement uncertainty of these techniques. The minimum noise-equivalent absorption coefficient of the spectrometer was 1.2×10^-10 cm^-1 Hz^-1/2, which further corresponds to a minimum detectable water vapor mole fraction equal to 0.7×10^-9 for an absorption spectrum of 10 minutes duration. PACS 33.20.-t; 33.70.Jg; 33.70.Fd; 42.62.Fi     

Compact QEPAS sensor for trace methane and ammonia detection in impure hydrogen

A compact two-gas sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) was developed for trace methane and ammonia quantification in impure hydrogen. The sensor is equipped with a micro-resonator to confine the sound wave and enhance QEPAS signal. The normalized noise-equivalent absorption coefficients (1σ) of 2.45×10^?8 cm^?1?W/ $\sqrt{}$ Hz and 9.1×10^?9 cm^?1?W/ $\sqrt{}$ Hz for CH₄ detection at 200 Torr and NH₃ detection at 50 Torr were demonstrated with the QEPAS sensor configuration, respectively. The influence of water vapor on the CH₄ channel was also investigated.     

Laser probing of atmospheric water vapor by a resonance method

The absorption of laser radiation at a wavelength of 694.38 nm is estimated. Various spectral widths of the pulses probing the atmosphere are considered. Questions related to the interpretation of the data obtained from laser probing of the water vapor concentration along the horizontal and vertical directions in the atmosphere are discussed. The effect of a shift of the center of the emission line with respect to the center of the absorption line on the error of the reconstructed profile of the absorbing gas is estimated. The possible use of the shape of the water vapor absorption line for probing purposes is studied.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 62–70, January, 1977.     

Handheld, battery-powered near-IR TDL sensor for stand-off detection of gas and vapor plumes

A handheld, battery-powered tunable-diode-laser sensor platform is described. The sensor is based on frequency modulation (FM) spectroscopy using near-IR diode lasers and passive topographic backscatter from common environmental targets such as buildings, ground and foliage. A specific application to the detection of methane using a 1.65-micron diode laser is described in detail, showing a detection sensitivity sufficient to identify typical leaks from buried residential natural gas service lines at stand-off distances up to 30 m. Signal and noise sources are analyzed in detail, along with laboratory and field-test data, including known service leaks. Received: 15 April 2002 / Revised version: 4 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-978/689-3232, E-mail: allen@psicorp.com     

Optofluidic Raman sensor for simultaneous detection of the toxicity and quality of alcoholic beverages

We report a chemometric prediction of the toxicity and quality of liquor using an optofluidic sensor based upon Waveguide Confined Raman Spectroscopy (WCRS). The WCRS sensor was used to record the Raman spectra, each obtained from a 20 µl sample of a given alcoholic beverage with and acquisition time of 20 s. This was used to predict, simultaneously, both the methanol concentration (toxicity) and ethanol concentration (quality), with an accuracy of 0.1% and 0.7% by volume, respectively, using a Partial Least Squares‐based chemometric model. The model sensor is shown to be capable of identifying toxic liquors, based on the test performed on different types of liquor samples. Copyright © 2013 John Wiley & Sons, Ltd.     

Simultaneous remote monitoring of atmospheric methane and water vapor using an integrated path DIAL instrument based on a widely tunable optical parametric source

We report on the remote sensing capability of an integrated path differential absorption lidar (IPDIAL) instrument, for multi-species gas detection and monitoring in the 3.3–3.7 µm range. This instrument is based on an optical parametric source composed of a master oscillator-power amplifier scheme—whose core building block is a nested cavity optical parametric oscillator—emitting up to 10 µJ at 3.3 µm. Optical pumping is realized with an innovative single-frequency, 2-kHz repetition rate, nanosecond microchip laser, amplified up to 200 µJ per pulse in a single-crystal fiber amplifier. Simultaneous monitoring of mean atmospheric water vapor and methane concentrations was performed over several days by use of a topographic target, and water vapor concentration measurements show good agreement compared with an in situ hygrometer measurement. Performances of the IPDIAL instrument are assessed in terms of concentration measurement uncertainties and maximum remote achievable range.     

On the anomalies in submillimeter absorption spectrum of atmospheric water vapor

Using a tunable source of monochromatic radiation (BWO) and a pneumatic detector, the field and laboratory investigations have been performed to obtain spectral distribution of atmospheric water vapor absorption coefficient in transparent windows centered at the wavelengths of 0.88 and 0.73 mm.The measurements have not found any spectral features mentioned repeatedly in the literature, besides the usually observed excess of the measured absorption above the calculated one for water vapor monomers.     

Single-QCL-based absorption sensor for simultaneous trace-gas detection of CH4 and N2O

A quantum cascade laser (QCL)-based absorption sensor for the simultaneous dual-species monitoring of CH₄ and N₂O was developed using a novel compact multipass gas cell (MGC). This sensor uses a thermoelectrically cooled, continuous wave, distributed feedback QCL operating at ~7.8 µm. The QCL wavelength was scanned over two neighboring CH₄ (1275.04 cm^?1) and N₂O (1274.61 cm^?1) lines at a 1 Hz repetition rate. Wavelength modulation spectroscopy (f = 10 kHz) with second harmonic (2f) detection was performed to enhance the signal-to-noise ratio. An ultra-compact MGC (16.9 cm long and a 225 ml sampling volume) was utilized to achieve an effective optical path length of 57.6 m. With such a sensor configuration, a detection limit of 5.9 ppb for CH₄ and 2.6 ppb for N₂O was achieved, respectively, at 1-s averaging time.     

Combustion of a single magnesium particle in water vapor

The combustion of magnesium particles in water vapor is of interest for underwater propulsion and hydrogen production. In this work, the combustion process of a single magnesium particle in water vapor is studied both experimentally and theoretically. Combustion experiments are conducted in a combustor filled with motionless water vapor. Condensation of gas-phase magnesia on the particle surface is confirmed and gas-phase combustion flame characteristics are observed. With the help of an optical filter and a neutral optical attenuator, flame structures are captured and determined. Flame temperature profiles are measured by an infrared thermometer. Combustion residue is a porous oxide shell of disordered magnesia crystal, which may impose a certain influence on the diffusivity of gas phases. A simplified one-dimensional, spherically symmetric, quasi-steady combustion model is then developed. In this model, the condensation of gas-phase magnesia on the particle surface and its influence on the combustion process are included, and the Stefan problem on the particle surface is also taken into consideration. With the combustion model, the parameters of flame temperature, flame diameter, and the burning time of the particle are solved analytically under the experimental conditions. A reasonable agreement between the experimental and modeling results is demonstrated, and several features to improve the model are identified.     

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.     

Displacement-noise-free gravitational-wave detection with a single Fabry-Perot cavity: A toy model

We propose a detuned Fabry-Perot cavity, pumped through both the mirrors, as a toy model of the gravitational-wave (GW) detector partially free from displacement noise of the test masses. It is demonstrated that the noise of cavity mirrors can be eliminated, but the one of lasers and detectors cannot. The isolation of the GW signal from displacement noise of the mirrors is achieved in a proper linear combination of the cavity output signals. The construction of such a linear combination is possible due to the difference between the reflected and transmitted output signals of detuned cavity. We demonstrate that in low-frequency region the obtained displacement-noise-free response signal is much stronger than the -limited sensitivity of displacement-noise-free interferometers recently proposed by S. Kawamura and Y. Chen. However, the loss of the resonant gain in the noise cancelation procedure results is the sensitivity limitation of our toy model by displacement noise of lasers and detectors.