Diode Laser Absorption Spectroscopy Technique Research Articles

Application of TDLAS for measuring absorption coefficients of H2S rotational absorption lines near the wavelength of 2 µm

This paper presents the experimental measurement of hydrogen sulfide absorption coefficients in the pressure range of 75–460 mbar of two rotational absorption lines of the vibrational band of H2S molecule at 021-000 transition using the tunable diode laser absorption spectroscopy technique. The obtained results are compared with the data of the HITRAN spectroscopic database.

Temperature determination of multiple gas slabs using a single absorption line

In this study, temperatures of multiple gas slabs are measured from a single absorption line using a tunable diode laser absorption spectroscopy technique. While typical laser absorption spectroscopy techniques require two or more absorption lines to determine the gas temperatures, however, this study presents the temperature determination method using an absorption line and its conditions for successful application. To generate two air slabs along with a single laser path, a gas heating apparatus composed of two chambers is used. The air is used as a test gas and a tunable diode laser absorption spectroscopy system is set up to detect an absorption line of the oxygen molecule in the air. Using the spectroscopic characteristics that the line shape and absorption strength are functions of gas properties and beam path length, temperature values of multiple gas slabs are experimentally determined by matching a measured spectral absorbance with a calculated one with known pressure and beam path length values. The determined temperatures are compared with temperature values set using thermocouples. It is observed that the temperatures of the gas slabs can be successfully measured when the pressures of the gas slabs are considerably different. The effect of gas properties and beam path length on the temperature determination is discussed. Measurement uncertainty due to random noise in the signal is investigated through a simulation approach.

Measurement of self-broadening coefficients and line intensities of ν1 + ν3 + ν4 ‒ ν4 band of acetylene in 1.5 μm spectral region: External cavity diode laser based absorption study

Acetylene is considered to be an important molecule for spectroscopic research due to its role in environmental chemistry, astrophysics, biochemistry and photochemistry of various planets. The high resolution spectroscopic studies of acetylene (12C2H2) in the 6474–6515 cm−1 spectral range have been performed using in-house developed external-cavity diode laser absorption spectroscopy technique. The study is focused on experimental measurement of self-broadening coefficients (γself) and line intensities of 27 l-doublet ro-vibrational lines belonging to the P-branch transition of ν1 + ν3 + ν4 ‒ ν4 hot band lying in the 6474–6515 cm−1 spectral range. The γself and line intensity have been retrieved by modelling of each experimentally observed rotational line with standard Voigt line profile using custom written python based computer program. The measured γself and line intensities were compared with the values present in HITRAN2020 database, showing mean difference of about -1.9% and 2.48%, respectively. In contrast to the line intensity, where rotational and symmetry dependency are readily visible, γself exhibits only rotational dependency. The l-type doubling constants of both ν1 + ν3 + ν4 and ν4 states of ν1 + ν3 + ν4 ‒ ν4 hot band have also been obtained. Detailed understanding of these molecular parameters will play an important role in modelling of planetary atmospheres.

Optimization of beam arrangement for tunable diode laser absorption tomography reconstruction based on fractional Tikhonov regularization

Beam arrangement with limited projections based on tunable diode laser absorption spectroscopy is the key to achieving a more accurate two-dimensional reconstruction of the combustion. Using fractional calculus theory, a beam optimization method based on fractional Tikhonov regularization is proposed. The beam arrangement function based on fractional Tikhonov regularization is established by extending the standard Tikhonov regularization to fractional modes. The genetic algorithm is used to analyze the calculation results of different orders in a range of (0, 1), and the beam arrangement is obtained. Using 20 laser beams to scan the characteristic absorption spectrum of H<sub>2</sub>O in the near-infrared band 7185.6 cm<sup>–1</sup>, modeling the calculations in a 10×10 element discrete tomography domain, and comparing the reconstruction results of the five beam arrangements for different Gaussian distribution models, the beam arrangement based on fractional Tikhonov regularization shows more obvious advantages. This design method proposed in this work is valuable for the theoretical study of the optimal design of two-dimensional measurement beams based on the tunable diode laser absorption spectroscopy technique, which can promote the application of this technique in the two-dimensional reconstruction of complex engine combustion and combustion efficiency improvement.

Noise Immune Absorption Profile Extraction for the TDLAS Thermometry Sensor by Using an FMCW Interferometer

Tunable diode laser absorption spectroscopy techniques usually utilize direct absorption spectroscopy (DAS) to measure absorbance profiles for temperature measurements. The DAS method is sensitive to thermal noises and fails to work in harsh application cases. A noise immune method is proposed for robust temperature measurement by using coherent detection and retroflected optical alignment. The absorption spectrum is shifted to a high-frequency band of MHz level away from ambient noises. The optical alignment suppresses thermal radiations along the laser path. Gas temperatures were measured in cases of both laboratory and porotype setup of aeroengine, i.e., the steady and the harsh states. Transition lines of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O at 7182.94 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , 7185.59 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , and 7444.35 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> were selected for performance evaluations of target molecules, i.e., of water vapor. Performance comparisons were made with the DAS method for temperature detection. In a steady-state case, standard deviations of the proposed method were below 0.9 K at SNR of 25 dB, while those of the DAS method were above 3.9 K. In an onsite harsh application case, the fluctuations of path-averaged temperatures obtained by the proposed method were also smaller. The coherent detection and retroflected alignment enable better performance in hostile cases.

Container Closure Integrity Test Using Frequency Modulation Spectroscopy Headspace Analysis with Carbon Dioxide as a Tracer Gas.

As described in USP <1207>, the container closure integrity (CCI) of a pharmaceutical package must be maintained throughout the product lifecycle to ensure sterility and stability. Current CCI test methods can be time-consuming, destructive, and lack the required sensitivity. This study presents a novel, fast, and nondestructive method for CCI testing that uses carbon dioxide as a tracer gas under effusive pressure conditions. Two types of defects were tested: laser-drilled defects located in the glass body (2, 5, and 10 μm nominal diameter) and tungsten wires inserted between the stopper and the landing seal of the vial (41, 64, and 80 μm outer diameter). During each test session, vials were placed in a pressure vessel, isolated from ambient conditions, and pressure-cycled by first pulling a vacuum and then applying an overpressure of pure carbon dioxide gas. After being exposed to 20 psig (34.7 psia) of carbon dioxide for 30 min, the overpressure was released and the vials were measured on an FMS-Carbon Dioxide Headspace Analyzer. This headspace gas analyzer utilizes a tunable diode laser absorption spectroscopy technique that employs frequency modulation to enhance measurement sensitivity. An increase of ≥1 torr in the headspace carbon dioxide content after completion of the pressure cycling procedure was intended to serve as confirmation of leak detection. All empty vials with either a 2 µm laser-drilled defect or 41 µm wire (effective defect size ∼2 µm), or greater, at the stopper-seal interface were detected by this method. Furthermore, vials filled with 1 mg/mL bovine serum albumin in phosphate-buffered saline containing a 5 μm laser-drilled defect below the liquid level or a 64 µm wire (effective defect size ∼6.1 µm), or greater, at the stopper-seal interface (defect above the liquid level) were detected. This test can be used for a wide variety of vial types and headspace compositions.

High temporal resolution measurements of ammonia emissions following different nitrogen application rates from a rice field in the Taihu Lake Region of China

Ammonia emission is one of the dominant pathways of nitrogen fertilizer loss from rice fields in China. It is difficult to measure ammonia emissions by high-frequency sampling with the chamber methods widely used in China, which is of great significance for investigating the environmental effects on the ammonia emissions. The chamber methods also can not accurately determine the ammonia emissions. In this study, the backward Lagrangian stochastic dispersion model, with ammonia concentrations continuously measured by the open-path tunable diode laser absorption spectroscopy technique, was used to determine ammonia emissions from a rice field after fertilizer application at excessive (270 kg N ha−1) and appropriate (210 kg N ha−1) rates in the Taihu Lake Region of China. High temporal resolution measurements of ammonia emissions revealed that high intraday fluctuations of ammonia emissions were significantly affected by the meteorological conditions. Multiple regression analysis showed a dominant solar radiation dependence of intraday ammonia emission cycles, especially during the rice panicle formation stage. The NH4+-N concentrations of the surface water of the rice field were found to be the decisive factor that influenced interday dynamics of ammonia emissions. Accurate quantifications of ammonia emissions indicated that the total ammonia losses under appropriate nitrogen application rate were 27.4 kg N ha−1 during the rice tillering stage and 11.2 kg N ha−1 during the panicle formation stage, which were 29.4% and 17.0% less than those under traditional excessive nitrogen application rate used by the local farmers, respectively. The ammonia loss proportions during the rice panicle formation stage were significantly lower than those of the tillering stage, which might be due to different nitrogen application rates and environmental effects during the two stages. This study indicated that the open-path tunable diode laser absorption spectroscopy technique could facilitate the investigation of high temporal resolution dynamic of ammonia emissions from farmland and the environmental influence on the ammonia emissions.

Argon metastable and resonant level densities in Ar and Ar/Cl2 discharges used for the processing of bulk niobium

A comparative analysis of two popular spectroscopy techniques is conducted in a coaxial cylindrical capacitively coupled discharge designed for the plasma processing of superconducting radio frequency (SRF) cavities. The density of the metastable and resonant levels in Ar is measured in both Ar and Ar/Cl2 discharges to properly characterize the unique discharge system and aid in the development of a cavity etching routine. The first method, deemed the “branching fraction method,” utilizes the sensitivity of photon reabsorption of radiative decay to measure the lower state (metastable and resonant) densities by taking ratios of spectral lines with a common upper level. This method has been gaining popularity as it does not require any a priori knowledge about the electron energy distribution. The second method is a tunable diode laser absorption spectroscopy technique that measures the thermal Doppler broadening of spectral lines, from which the neutral gas temperature and lower state density of the transition can be evaluated. The two methods were conducted in tandem, while external parameters that were empirically determined to be important to the etching mechanism of SRF cavities are varied. Relationships between the excited state densities and the external parameters are presented for both spectroscopy methods and conclusions about the effects of these parameters on the discharge are stated when appropriate.

Multiparameter Measurement in Ethylene Diffusion Flame Based on Time-Division Multiplexed Tunable Diode Laser Absorption Spectroscopy

An optical method for measuring soot volume fraction, flame temperature, and concentration of combustion products in a diffusion flame based on time-division multiplexed tunable diode laser absorption spectroscopy technique is presented in this article. By moving the slot burner with an electric platform, two-dimensional profile of soot volume fraction, flame temperature, and gas concentrations along the flame sheet with 1-mm spatial resolution were obtained simultaneously. The flame temperature and soot volume fraction were simultaneously on-line monitored using one distributed feedback diode laser with a center wavelength at 1397.83 nm. Tunable diode laser absorption spectroscopy based temperature measurements in the flame were carried out by two-line thermometry of H2O and compared with the thermocouple technique. The attenuation of tunable diode laser intensity caused by soot extinction and gas absorption after transmitting through the diffusion flame was detected. And the soot volume fraction was determined quantitatively with laser extinction based on Rayleigh limit assumption. Two tunable distributed feedback diode lasers with center wavelength at 2001.6 and 2302 nm were used to determine the CO2 concentration and CO concentration, respectively. The experimental results of soot volume fraction, flame temperature, and the mole fraction of combustion products by varying concentrations of CO2 added in the diffusion flame were analyzed.

Implementation of the adaptive Wiener filtering algorithm in the problem of measuring the 13CO2 content in expiratory air using the tunable diode laser absorption spectroscopy technique

This paper reports on implementing the adaptive Wiener filtering algorithm for the processing of an experimental signal obtained by the tunable diode laser absorption spectroscopy technique near the wavelength of 2 µm using a tunable diode laser. It has been shown that applying this filter may improve the content measuring accuracy in a mixture with in expiratory air.

Simultaneous In-Situ Measurement of Soot Volume Fraction, H2O Concentration, and Temperature in an Ethylene/Air Premixed Flame Using Tunable Diode Laser Absorption Spectroscopy

ABSTRACTA method and application of measuring the soot volume fraction, H2O concentration, and its temperature simultaneously in an ethylene/air premixed flame environment based on tunable diode laser absorption spectroscopy technique is presented in this article. Only one distributed feedback diode laser with a center wavelength at 1397.83 nm combined with a scanned-wavelength direct absorption spectroscopy technique was used during measurements. The attenuation of laser intensity caused by soot extinction and H2O gas absorption after transmitting through the flame region was detected and distinguished, respectively. Soot volume faction was inferred from the light extinction with Rayleigh limit assumption. The absorption signals of two H2O spectral lines were acquired during one scanning, thus both the H2O concentration and its temperature can be retrieved. Measurements have been taken in ethylene/air premixed flat flames on a standard bronze McKenna burner at different cold-gas velocities (5.9 cm/s and 4.4 cm/s) and equivalence ratios (φ = 2.3, 2.1, and 1.9). The gradient tendencies of soot volume fraction, H2O concentration, and temperature along the vertical direction and the variations in different flames were well demonstrated. The results were in good agreement with the values measured using other techniques and studies. The proposed method is proved to be suitable and accurate for measuring oot volume fraction together with gas concentration and temperature. This multipurpose measuring method with high accuracy exhibits great potential in combustion diagnostics and soot study.

Research on the NO concentration of marine diesel emission by tunable diode laser absorption spectroscopy technique

This work investigates the feasibility of TDLAS technique in the NO concentration measurement for the marine diesel engine. A system in laboratory is devised for the NO steady state measurement. An ICL laser whose center wavelength is 5263nm is chosen for the detection. With the direct detection method, a resolution about 2ppm is realized, and the relative error is less than 5%. In contrast to the direct detection method, the harmonic detection method realizes a higher resolution, and the error is better less 3%. Considering with the temperature fluctuation of the diesel emission, the absorption line strength is calculated from 300K to 500K. At the same time, the intensity of the 2f signal is measured, which is proportional to the change of the line strength. So a modification can be imported easily to avoid the error produced by the temperature fluctuation. The TDLAS proposed in this work can realize a high precision, high resolution, and transient measurement for the marine diesel emission.

Concentration and pressure measurement of water vapor in sealed glass containers based on tunable diode laser absorption spectroscopy

Water vapor in a sealed glass container processed by lyophilization is a main factor for drug metamorphism. The key to knowing whether there is a leakage occurring in the container is how to detect water concentration and pressure in the sealed container quickly and accurately. In the present paper, a strong absorption line of H2O near 1.39 m is carefully selected to avoid the interference of neighboring transitions. A distributed feedback laser semiconductor laser near 1.396 m is employed as the light source with a power of 10 mW and typical linewidth of 2 MHz by combining with tunable diode laser absorption spectroscopy technique, the concentrations and pressures of water vapor in the sealed container are successfully detected under static condition and dynamic condition. In order to isolate the interference absorption from the ambient water vapor in the air, a differential absorption technique is employed in our experiment, which makes our system simpler than routine nitrogen purging based system. During the measurement, the second harmonic signal is utilized for measuring the concentration and pressure, the concentration is retrieved by the peak value while the pressure is calculated by the full width at half maximum. For the measurement of concentration ranging from 0.2% to 12%, the linear correlation coefficient between the real values and the inversed values and the standard deviation ratio are 0.9978 and 4.81%, respectively. For the measurement of pressure, the correlation coefficient and standard deviation ratio are 0.982 and 5.6%, respectively. The minimum detection limits of the concentration and pressure are 400 ppm and 2.5 Torr, respectively. Moreover, in order to test the system for on-line applications in the pharmaceutical industry, measurements are performed in vials which are placed on a rotary stage to simulate the process of the assemble line. In particular, the amplitude of sinuous signal without absorption is used as the reference signal to validate whether the vial is in the optical path. Besides, this amplitude is also utilized to normalize the laser power. The results show that our system can handle about 300 bottles in one minute, which can meet well the requirements for rapid and real-time detections. This system can be applied directly to the medicine bottle on-line detection, and multicomponent detection can also be realized by employing two or more lasers (e.g. H2O, oxygen, etc.). In the future, we plan to build a system for detecting water vapor and oxygen simultaneously, as oxygen is another import indicator for drug metamorphism.

Simultaneous measurement of gas distribution in a premixed flame using adaptive algebraic reconstruction technique based on the absorption spectrum

Four tomography algorithms, including the algebraic reconstruction technique, simultaneous iterative reconstruction technique, the multiplicative algebraic reconstruction technique, and the adaptive algebraic reconstruction technique (AART), are compared with each other based on the tunable diode laser absorption spectroscopy technique; the determination of the relaxation parameter is discussed to improve reconstruction quality and shorten computational time as much as possible. The calculated results demonstrate that the AART algorithm can produce the reconstruction results with better quality and less computational time. In the experimental measurement, the AART algorithm with a two-line thermometry scheme is adopted to measure the spatial distribution of H2O temperature and concentration, two H2O absorption lines near 1397.8 nm are selected, the temperature and H2O concentration in the McKenna plat premixed flames are experimentally reconstructed and analyzed, and the results of integrated tomography reconstruction are consistent with the thermocouple traverse measurement.

A trace gas sensor using near infrared distributed feedback laser at 1654 nm

A cost-effective compact instrument for high-stable and sensitive detection of methane (CH4) based on the tunable diode laser absorption spectroscopy technique was experimentally demonstrated. A distributed feedback laser modulated at 1654 nm and two InGaAs photodiodes were deployed in this instrument. The laser was driven by the self-developed temperature and current controller. Spectrum measurements show that the center wavelength of the laser is stable and linearly controlled. Meanwhile, a self-developed digital lock-in amplifier was developed for the extraction of harmonics (1f, 2f) of the gas absorption signal. Experiments of gas detection were carried out to investigate the sensor performance. Firstly, the relation between gas concentration and 1f and 2f harmonics was measured and the relation curve was plotted for calibration. Then, results of accuracy test in different concentrations show that the maximum relative detection error is less than 5.6% in the range 0%–100%. Keeping the other experimental parameters constant, gas samples of 5% concentration was detected during a period of 8 h. The maximum detection error is less than 2.8%, suggesting good detection stability. The sensor can also be applied to field measurement of other gases by adopting lasers at relative wavelength.

Infrared spectroscopic and modeling studies of H2/CH4 microwave plasma gas phase from low to high pressure and power

InfraRed Tunable Diode Laser Absorption Spectroscopy technique has been implemented in a H2/CH4 Micro-Wave (MW frequency f = 2.45 GHz) plasma reactor dedicated to diamond deposition under high pressure and high power conditions. Parametric studies such as a function of MW power, pressure, and admixtures of methane have been carried out on a wide range of experimental conditions: the pressure up to 270 mbar and the MW power up to 4 kW. These conditions allow high purity Chemical Vapor Deposition diamond deposition at high growth rates. Line integrated absorption measurements have been performed in order to monitor hydrocarbon species, i.e., CH3, CH4, C2H2, C2H4, and C2H6. The densities of the stable detected species were found to vary in the range of 1012–1017 molecules cm−3, while the methyl radical CH3 (precursor of diamond growth under these conditions) measured into the plasma bulk was found up to 1014 molecules cm−3. The experimental densities have been compared to those provided by 1D-radial thermochemical model for low power and low pressure conditions (up to 100 mbar/2 kW). These densities have been axially integrated. Experimental measurements under high pressure and power conditions confirm a strong increase of the degree of dissociation of the precursor, CH4, associated to an increase of the C2H2 density, the most abundant reaction product in the plasma.

Near-Infrared Methane Detection Device Using Wavelength-Modulated Distributed Feedback Diode Laser around 1.654 µm

ABSTRACT By using a wavelength-modulated distributed feedback laser centered at 1.654 µm, a stand-alone near-infrared methane detection device was experimentally proposed based on the tunable diode laser absorption spectroscopy technique. An intelligent temperature controller, a scan and modulation module, and a cost-effective lock-in amplifier were developed to drive the distributed feedback laser and extract the second harmonic signal. Experimental results show that the relative detection error is less than 7% within the detection range of 0–106 ppm, and the limit of detection is about 11 ppm with an absorption length of 0.2 m. Long-term monitoring on two gas samples (103 ppm and 2 × 105 ppm) suggests good stability with the maximum detection errors smaller than 7% and 2.5%, respectively. Due to careful design and integration, the developed near-infrared sensor reveals competitive performances compared with our previously reported sensing devices at the mid-infrared region.

Strong seasonal disequilibrium measured between the oxygen isotope signals of leaf and soil CO2 exchange

AbstractThe oxygen isotope composition (δ18O) of atmospheric CO2 is among a very limited number of tools available to constrain estimates of the biospheric gross CO2 fluxes, photosynthesis and respiration at large scales. However, the accuracy of the partitioning strongly depends on the extent of isotopic disequilibrium between the signals carried by these two gross fluxes. Chamber‐based field measurements of total CO2 and CO18O fluxes from foliage and soil can help evaluate and refine our models of isotopic fractionation by plants and soils and validate the extent and pattern of isotopic disequilibrium within terrestrial ecosystems. Owing to sampling limitations in the past, such measurements have been very rare and covered only a few days. In this study, we coupled automated branch and soil chambers with tuneable diode laser absorption spectroscopy techniques to continuously capture the δ18O signals of foliage and soil CO2 exchange in a Pinus pinaster Aït forest in France. Over the growing season, we observed a seasonally persistent isotopic disequilibrium between the δ18O signatures of net CO2 fluxes from leaves and soils, except during rain events when the isotopic imbalance became temporarily weaker. Variations in the δ18O of CO2 exchanged between leaves, soil and the atmosphere were well explained by theory describing changes in the oxygen isotope composition of ecosystem water pools in response to changes in leaf transpiration and soil evaporation.

Combustion measurement of thermoacoustic oscillating flames by diode-laser absorption spectroscopy sensors

Diode-laser absorption spectroscopy has been applied to a swirl-stabilized turbulent combustor to detect high frequency combustion oscillation and combustion state related to combustion noise. Two diode-laser absorption spectroscopy techniques of scanned-wavelength method and fixed-wavelength method are adopted. In the scanned-wavelength method, fluctuations of temperature and H 2O mole fraction up to 1 kHz are detected. Two dominant peak frequencies of power spectra of these fluctuations, which are about 125 Hz and 140 Hz, coincide with those of pressure fluctuation in the combustor. In the case of control by secondary fuel injection, the energy at peak frequency of temperature and H 2O mole fraction decreases in accordance with noise reduction. Similar to the combustion noise, temperature fluctuation shows a minimal value at the appropriate frequency of secondary fuel injection. By analysing transmitted signals, the fixed-wavelength method provides power spectra similar to those obtained by the scanned-wavelength method. The advantage of the fixed-wavelength method is capability of detection of high frequency combustion oscillation more than 1 kHz. These results prove that the diode-laser absorption spectroscopy has great applicability as sensors for the combustion measurement of thermoacoustic oscillating flames and active control of turbulent combustion.

In-situ Measurement of Water Vapor and Oxygen Concentration in a PEMFC Channel by Tunable Diode Laser Absorption Spectroscopy Techniques

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