Tunable Diode Laser Spectroscopy Research Articles

Perovskite photodetector-based laser absorption spectroscopy for gas detection

A gas detection method based on CH3NH3PbI3 (MAPbI3) and poly (3,4-ethylenedioxythiophene): poly (4-styrene sulfonate) (PEDOT:PSS) composite photodetectors (PDs) is proposed. The operation of the PD primarily relies on the photoelectric effect within the visible light band. Our study involves constructing a gas detection system based on tunable diode laser spectroscopy (TDLAS) and MAPbI3/PEDOT:PSS PD, and O2 was selected as the target analyte. The system has achieved a minimum detection limit (MDL) of 0.12% and a normalized noise equivalent absorption coefficient (NNEA) of 8.83 × 10−11 cm-1⋅W⋅Hz-1/2. Furthermore, the Allan deviation analysis results indicate that the system can obtain sensitivity levels as low as 0.058% over an averaging time of 328 seconds. This marks the first use of MAPbI3/PEDOT:PSS PD in gas detection based on TDLAS. Despite the detector's performance leaves much to be desired, this innovation offers a new approach to developing spectral based gas detection system.

Quantification of Elevated Hydrogen Cyanide (HCN) Concentration Typical in a Residential Fire Environment Using Mid-IR Tunable Diode Laser.

A versatile portable tunable diode laser based measurement system for measuring elevated concentrations of hydrogen cyanide (HCN) in a time-resolved manner is developed for application in the fire environment. The direct absorption tunable diode laser spectroscopy (DA-TDLAS) technique is employed using the R11 absorption line centered at 3345.3 cm-1 (2989.27 nm) in the fundamental C-H stretching band (ν1) of the HCN absorption spectrum. The measurement system is validated using calibration gas of known HCN concentration and the relative uncertainty in measurement of HCN concentration is 4.1% at 1500 ppm. HCN concentration is measured with a sampling frequency of 1 Hz, in gas sampled from 1.5 m, 0.9 m, and 0.3 m heights in the Fireground Exposure Simulator (FES) prop at the University of Illinois Fire Service Institute, Champaign, Illinois. The immediately dangerous to life and health (IDLH) concentration of 50 parts per million (ppm) is exceeded at all the three sampling heights. A maximum concentration of 295 ppm is measured at the 1.5 m height. The HCN measurement system, expanded to measure HCN simultaneously from two sampling locations, is then deployed in two full-scale experiments designed to simulate a realistic residential fire environment at the Delaware County Emergency Services Training Center, Sharon Hill, Pennsylvania.

Sine-Cosine Techniques for Detecting Fast Optical Frequency Variations With Ultra-High Resolution

We introduce the concept of sine-cosine optical frequency detection (OFD) for measuring fast optical frequency variations with the ultra-high spectral resolution for applications requiring rapidly tunable or low frequency-jitter lasers, including coherent detection based distributed sensing and communication systems and tunable diode laser spectroscopy (TDLS) systems, for which conventional spectral measurement techniques cannot meet the speed and spectral resolution requirements. The concept relies on representing the frequency-dependent differential phase of the laser light passing through two different optical paths as sine and cosine functions to calculate the optical frequency, similar to the sine-cosine encoder concept commonly used for obtaining the rotation information of a motor, which can simultaneously achieve both high spectral resolution and wide frequency range without mutual constraints. We present multiple schemes, some are seemingly very different, for implementing the concept and demonstrate that all frequency-related parameters of a tunable light source, including frequency tuning rate, nonlinearity, ripple, range, duty cycle, repeatability, and power swing can be precisely characterized. In addition, the frequency jitter of a fixed narrow line-width laser can also be accurately measured. Ultra-fast optical frequency sweep with a rate in excess of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> nm/s and a resolution down to MHz is demonstrated with a differential delay of only 0.8 mm, suggesting that the concept is feasible to be implemented with photonic integrated circuit (PIC). Our work is attractive for applications such as coherent distributed sensing (OCT, OFDR and FMCW lidar), coherent detection, FBG sensing interrogation and TDLS, especially considering that some schemes can be integrated on PIC chips.

On the role of helium metastable (23S1) measured by time resolved tunable diode laser spectroscopy in high current magnetron discharge

Tunable diode laser absorption spectroscopy is applied to measure the density and gas temperature of helium metastable (23S1) in the ionization region of magnetron discharge operated at very high current density (), which is achievable in helium at 10 Pa with molybdenum target. Those measurements were combined to electrical characterization of the current and the voltage at the magnetron cathode. The discharge current waveform is anti-correlated with the time-evolution of helium metastable () density showing that the main depletion process of is its ionization. Counterintuitively, the higher the gas temperature the lower the discharge peak current. Gas rarefaction and gas recycling are discussed as major contributors for this phenomenon in very high current glow discharge.

Phase-locked absorption tomography for retrieving 5 kHz time-resolved tracer profiles in solid fuel combustion

Turbulent mixing processes and species transport govern many important properties like stability and pollutant formation of reactive systems. We propose spectrally resolved absorption tomography as a viable tool to quantitatively assess species transport by imaging tracer pulses at high time resolution. Measurements on a gas-assisted solid-fuel combustor operating in air and oxy-fuel atmospheres are presented. The measurements are conducted using a scanning tomographic imaging system that has previously been validated for gas temperature measurements. As the scanning system itself is not able to achieve the necessary time resolution, a phase-locked tomographic scheme is applied, i.e. each measurement beam is acquired separately for several tracer pulses and phase-averaged. Hence, the full 5 kHz time resolution of the underlying tunable diode laser spectroscopy system can be utilized, granting insight into species transport in the recirculation zones as well as the flame brush. Despite a possible influence of the tracer injections on the inner recirculation zone, it is shown that qualitative differences between methane combustion and gas-assisted solid-fuel operating conditions can be detected, and influences of devolatilization as well as volatile combustion become visible.

Tomographic imaging of carbon dioxide in the exhaust plume of large commercial aero-engines.

We report here the first implementation of chemically specific imaging in the exhaust plume of a gas turbine typical of those used for propulsion in commercial aircraft. The method used is chemical species tomography (CST) and the target species is CO2, absorbing in the near-infrared at 1999.4 nm. A total of 126 beams propagate transverse to the plume axis, along 7 m paths in a coplanar geometry, to probe a central region of diameter ≈1.5m. The CO2 absorption spectrum is measured using tunable diode laser spectroscopy with wavelength modulation, using the second harmonic to first harmonic (2f/1f) ratio method. The engine is operated over the full range of thrust, while data are recorded in a quasi-simultaneous mode at frame rates of 1.25 and 0.3125 Hz. Various data inversion methodologies are considered and presented for image reconstruction. At all thrust levels a persistent ring structure of high CO2 concentration is observed in the central region of the measurement plane, with a raised region in the middle of the plume assumed to be due to the engine's boat tail. With its potential to target various exhaust species, the CST method outlined here offers a new approach to turbine combustion research, turbine engine development, and aviation fuel research and development.

Mid‐infrared dissolved CO2 measurement system using the ultra‐compact multipass cell

AbstractTo measure natural gas hydrate, a mid‐infrared dissolved CO2 measurement system is designed using the tunable diode laser spectroscopy with ultra‐compact multipass cell. The QCL laser of 4319 nm has a scanning range from 2315.05 to 2315.40 cm−1, which contains 12CO2 and 13CO2 for isotope measurement and wide range of concentration detection. The optimized 3f + 1f wavelength modulation method reduces the limit of detection to 7.1 ppbv. An ultra‐compact multi‐pass cell with effective optical path length of 6.3 m and small volume of 15 ml is designed to achieve the system response time of 40 s. The dissolved CO2 sensing system has been used in deep sea natural gas hydrate detection.

가상센서를 이용한 제철소 가열로의 연소가스 분석기 고장 감시 및 백업 기술개발

Combustion optimization, determining the optimal combustion air flow by measuring O2 and CO concentrations of exhaust gas, is essential for reducing greenhouse gas emissions in steel plants. Recently, the tunable diode laser spectroscopy(TDLS) analyzer is used to measure O2 and CO concentrations. However it is expensive and sometimes out of order even less than the sampling type analyzer. Virtual sensors were developed to predict O2 and CO concentrations of the exhaust gas and used for detecting analyzer faults and backing them up. Four machine learning techniques, PLS, ANN, A-ANN, and XGBoost, were compared and evaluated for virtual sensor modeling. Among them, XGBoost had the highest coefficient of determination (R²) of 80% or more and relative accuracy (RA) of 89% or more. Additional machine learning was performed to minimize errors caused by faults of the measurement sensors and increase the reliability of the virtual sensors. PCA and A-ANN were applied to 40 measurement sensors to build prediction models, and A-ANN had higher R² and RA of more than 90%. The study found that virtual sensors helped maintain combustion optimization without interruption from fault TDLS readings.

Landfill Emissions of Methane Inferred from Unmanned Aerial Vehicle and Mobile Ground Measurements

Municipal solid waste landfills are significant sources of atmospheric methane, the second most important greenhouse gas after carbon dioxide. Large emissions of methane from landfills contribute not only to global climate change, but also to local ozone formation due to the enhancement of radical chain lengths in atmospheric reactions of volatile organic compounds and nitrogen oxides. Several advanced techniques were deployed to measure methane emissions from two landfills in the Southeast Michigan ozone nonattainment area during the Michigan–Ontario Ozone Source Experiment (MOOSE). These techniques included mobile infrared cavity ringdown spectrometry, drone-mounted meteorological sensors and tunable diode laser spectrometry, estimation of total landfill emissions of methane based on flux plane measurements, and Gaussian plume inverse modeling of distributed methane emissions in the presence of complex landfill terrain. The total methane emissions measured at the two landfills were of the order of 500 kg/h, with an uncertainty of around 50%. The results indicate that both landfill active faces and leaking gas collection systems are important sources of methane emissions.

Determining the most suitable spectral range for TDLS – a quantitative approach

We present a mathematical method which allows determination of an optimal spectral range for gas mixture analysis based on theoretical absorption spectra. The resulting center wavelength is particularly suited for tunable diode laser spectroscopy (TDLS). The procedure contains several steps of numerical calculations which can easily be implemented in almost any programming language. We apply our method to three exemplary mixtures of hydrocarbons and present and validate the individual results.

Study of water vapor line parameters in the region around 1.1 µm by tunable diode laser spectroscopy

AbstractTo analyze the absolute line intensities, as well as the coefficients of air‐ and self‐broadening of water vapor, 31 transition lines were measured using an external‐cavity diode laser (ECDL) in the range of 9090–9750 cm−1. A multispectrum fitting (MSF) program was used in conjunction with a quadratic speed‐dependent Voigt profile (qSDVP) and a Voigt profile (VP) to extract the line intensities and self‐ and air‐broadening coefficients. Values at room temperature (296 K) were then compared with the HITRAN2016 and HITRAN2020 database. The analysis results show that the line intensities and air‐broadening coefficients are more consistent with the database while also verifying that the self‐broadening coefficients obtained based on qSDVP are larger than those based on VP.

Compact carbon dioxide sensor using tunable diode laser absorption spectroscopy for capnographic monitoring

In order to realize real-time capnographic monitoring, a compact carbon dioxide sensor based on tunable diode laser spectroscopy technology was developed by means of a 2004 nm distributed feedback laser and a circular gas cell. The toroidal cell can achieve an optical path length of 3.49 m in a volume of 38 ml, which enables the tunable diode laser spectroscopy carbon dioxide sensor to achieve a detection limit of 175 ppm at a measurement rate of 12.5 Hz. The actual exhaled carbon dioxide gas detection experiments were implemented, including the breath analysis of a normal subject under different conditions and another subject with pneumothorax. It indicates that the sensor has good effectiveness and practicability for carbon dioxide metabolic monitoring and related disease diagnosis.

High and flat spectral responsivity of quartz tuning fork used as infrared photodetector in tunable diode laser spectroscopy

Infrared laser technology over the last decades has led to an increasing demand for optical detectors with high sensitivity and a wide operative spectral range suitable for spectroscopic applications. In this work, we report on the performance of a custom quartz tuning fork used as a sensitive and broadband infrared photodetector for absorption spectroscopy. The photodetection process is based on light impacting on the tuning fork and creating a local temperature increase that generates a strain field. This light-induced, thermoelastic conversion produces an electrical signal proportional to the absorbed light intensity due to quartz piezoelectricity. A finite-element-method analysis was used to relate the energy release with the induced thermal distribution. To efficiently exploit the photo-induced thermoelastic effects in the low-absorbance spectral region of quartz also, chromium/gold layers, acting as opaque surface, have been deposited on the quartz surface. To demonstrate the flat response as photodetectors, a custom tuning fork, having a fundamental resonance frequency of 9.78 kHz and quality factor of 11 500 at atmospheric pressure, was employed as photodetector in a tunable diode laser absorption spectroscopy setup and tested with five different lasers with emission wavelength in the 1.65–10.34 μm range. A spectrally flat responsivity of ∼2.2 kV/W was demonstrated, corresponding to a noise-equivalent power of 1.5 nW/√Hz, without employing any thermoelectrical cooling systems. Finally, a heterodyne detection scheme was implemented in the tunable diode laser absorption spectroscopy setup to retrieve the resonance properties of the quartz tuning fork together with the gas concentration in a single, fast measurement.

Optical diagnostics in a detonation-driven direct-connected circular combustor fueled with hydrogen for mach 10 scramjet

Various optical methods are applied to study flame characteristics of a circular-cross-section combustor for a Mach 10 scramjet. Experiments are performed on a direct-connected test facility fueled by hydrogen and driven using hydrogen/oxygen detonation. The circular-cross-section combustor is advantageous in solving thermal inhomogeneity, corner boundary layer and aircraft integration problems. However, it is difficult to use traditional optical measurement methods, which require large-sized measurement windows. In this study, tunable diode laser spectroscopy and a multi-view imaging system are used with small windows. High repetition rate measurements allow the effective time of facility, which is approximately 5 ms in this case, to be obtained. The dynamic flame characteristics are diagnosed upstream and downstream of the cavity with/without strut injection. When combined with proper orthogonal decomposition to obtain the flame pulsation state during the effective time period, the method is expected to be effective for engine performance evaluation and numerical simulation verification.

Ultra-Low Sampled and High Precision TDLAS Thermometry Via Artificial Neural Network

Water vapor temperatures in a heating device were measured by ultra-low sampled and high-precision tunable diode laser spectroscopy (TDLAS) Bichromatic distributed feedback (DFB) lasers were used as light sources. Sampled data at an ultra-low rate was collected after laser passing through a low-pass filter and served as the inputs of an artificial neural network. Classical direct absorption spectroscopy using the line-shape fitting method provided the training dataset, i.e., the integrated absorbances. The proposed method required an ultra-low sampling rate, i.e., only 1/50 of the classical method, but its calculation speed was nearly 14,000 times faster. Also, the proposed method yielded satisfying estimates at temperatures uncovered by the training dataset and was insensitive to random noises.

CO2 diffusion in tobacco: a link between mesophyll conductance and leaf anatomy.

The partial pressure of CO2 at the sites of carboxylation within chloroplasts depends on the conductance to CO2 diffusion from intercellular airspace to the sites of carboxylation, termed mesophyll conductance (gm). We investigated how gm varies with leaf age and through a tobacco (Nicotiana tabacum) canopy by combining gas exchange and carbon isotope measurements using tunable diode laser spectroscopy. We combined these measurements with the anatomical characterization of leaves. CO2 assimilation rate, A, and gm decreased as leaves aged and moved lower in the canopy and were linearly correlated. This was accompanied by large anatomical changes including an increase in leaf thickness. Chloroplast surface area exposed to the intercellular airspace per unit leaf area (Sc) also decreased lower in the canopy. Older leaves had thicker mesophyll cell walls and gm was inversely proportional to cell wall thickness. We conclude that reduced gm of older leaves lower in the canopy was associated with a reduction in Sc and a thickening of mesophyll cell walls.

Fibre-coupled, multiplexed methane detection using range-resolved interferometry

We describe the first use of range-resolved interferometric signal processing for measurement of spectral transmission. This was applied to gas sensing using tunable diode laser spectroscopy, allowing the simultaneous and independent measurement of methane concentrations in multiple gas cells. The system uses a single injection-current modulated diode laser and a single photodetector. For three gas cells, we show the ability of the system to measure methane at noise equivalent concentrations of less than 200 ppm for a 0.5 s measurement period and a potential noise equivalent concentration (1σ) of <20 ppm with 150 s averaging time. We further show that cross-talk between cells is below the experimental uncertainty for the system.

Intensity Modulation-Normalized Calibration-Free 1f and 2f Wavelength Modulation Spectroscopy

This paper reports the development and validation of a field-deployable quantum cascade laser-based tunable diode laser spectroscopy (TDLS) sensor for in situ measurement of atmospheric CO and CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> using a new calibration-free 1f and 2f wavelength modulation spectroscopy (WMS) method. The 1f WMS and 2f WMS signals are normalized by the linear and non-linear intensity modulation components respectivelyto make the signals immune to laser intensity variations. The advantage of this method is that the normalized signals appear on a background that is independent of the laser parameters. The error in mole fraction extraction is 0.2% and 0.98% for CO and CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> respectively. The system has a single-pass detection limit of 3 ppb for CO (using 1f WMS and 4.05 m path length) and 45 ppb for CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (using 2f WMS and 20 cm path length) for an optimum integration time of 17 s and 69 s respectively. These values correspond to normalized detection limits of 12 ppb-m for CO and 9 ppb-m for CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . The mean mole fraction of CO and CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> measured in Gandhinagar, India from 19-25 July 2019 were found to be 477 ± 32 ppb and 486 ± 29 ppm respectively. These measurements demonstrate that this WMS algorithm and the sensor system are suitable for accurate long-term monitoring of greenhouse gases and air pollutants.

Water content of liquid H2S in equilibrium with the hydrate phase

Acid gas injection containing H2S is one strategy for managing atmospheric sulfur emissions where sulfur recovery may not be feasible or economical in some locations. Apart from its toxicity and corrosivity in wet environments, H2S forms a high temperature hydrate in the presence of water, which can cause issues such as plugging of transport pipelines or injection wellheads. Before transportation to injection, liquid water is removed from an injectate within the interstage suction scrubbers before each compression cycle and/or by further dehydration. Water content data are important in order to determine and define the dehydration requirements of acid gas injectates. Prior to this work, there were no water content data for H2S at hydrate forming conditions because of the difficulties in measuring low concentration of water above hydrates and high H2S toxicity. In this study, we report the water content of pure H2S in the hydrate forming regions from p = 4.178 to 20.572 MPa and T = 247.65 to 298.28 K using tunable diode laser spectroscopy. The measured data show a relative water content difference of ca. 2.5% from the model reported in this work using the Sloan et al. (1976), and van der Waal & Platteeuw (1959) equations for the hydrate phase, and the fluid phase model of Bernard et al. (2012).

Time-Resolved Studies of Bioluminescence From Photobacterium Leiognathi and Rapid Antimicrobial Susceptibility Testing on E. Coli Using Tunable Diode Laser Spectroscopy

We demonstrate that tunable diode laser spectroscopy (TDLS) can be used as a reliable, convenient and versatile investigative technique for non-invasive, high-sensitivity and time-resolved studies of the expression of various traits by bacterial strains. We use a 2004 nm vertical cavity surface emitting laser and a 4320 nm mid-infrared quantum cascade laser to show that the bioluminescence and simultaneous CO2 emission from Photobacterium leiognathi are correlated. The bioluminescence peaks when the growth rate is maximum. This supports the hypothesis that bioluminescence is a cell density-dependent phenomenon. We also show that TDLS can be used for ultra-rapid antimicrobial susceptibility testing without any need of sample preparation. The mole fraction of CO2 emitted during the growth of E. coli was recorded for different standardized doses of ampicillin to show the drastic suppression of growth as the antibiotic dose was increased to the recommended minimum inhibitory drug concentration. Growth suppression was detectable within 2 h, which is much shorter than the time required by conventional methods. We conclude that TDLS is a potent technique for validation and refinement of mathematical models of microbial growth that are necessary to better understand the dynamics of the expression of various traits at specific stages of growth. It is also important for rapid antimicrobial susceptibility testing.