Tunable Diode Laser Absorption Spectroscopy Research Articles

Simultaneous measurement of NH3 and NO by mid-infrared tunable diode laser absorption spectroscopy based on machine-learning algorithms

Accurate measurements of NH3 and NO are essential for controlling NOx emissions from coal-fired power plants, reducing ammonia slip, and improving the efficiency of selective catalytic reduction (SCR) operation. We propose a method for measuring NH3 and NO concentrations based on machine learning algorithms to address the challenges posed by overlapping interference of CO2 and H2O spectral lines in flue gas. Our approach introduces a novel method for acquiring mixture spectra for the model. Initially, we measure the spectra of individual components under different concentrations and temperatures. Subsequently, mixed spectral samples are generated by combining the measured spectra of the individual components. This approach simplifies the spectral measurement process while preserving accuracy. The particle swarm optimization support vector machine (PSO-SVM) algorithm is leveraged, providing a reliable foundation for the continuous and synchronous measurement of NH3 and NO. Upon testing, the PSO-SVM demonstrates average relative errors of 4.0 % and 0.8 % for NH3 and NO concentrations, respectively. The corresponding measurement precision is 0.05 ppm for NH3 and 0.42 ppm for NO, better than the conventional integral absorbance method. The minimum detection limit (MDL) for NH3 is 16.1 ppb at an average time of 50 s, while for NO, it is 38.4 ppb at an average time of 71 s. The methodology of this paper is expected to play an important role in reducing the influence of interfering components and improving the accuracy of field measurements.

A compact single-ended optical sensor for temperature measurements via laser absorption spectroscopy

The tunable diode laser absorption spectroscopy (TDLAS) has been employed in various fields successfully. However, most current TDLAS sensors have low spatial resolution, which is the inherent disadvantage of TDLAS techniques. To address this issue, a compact single-ended optical sensor based on the laser absorption spectroscopy is developed in this manuscript. The laser is delivered to the region of interest by a quartz rod, providing the sensor with millimeter-level spatial resolution. And the design of probe makes the singled-end sensor convenient and capable for measurements in diverse situations. In order to validate the feasibility of this structure, temperature of the flame produced with a McKenna burner is measured via absorption of H2O 7185.59 cm−1 and 7444.35 cm−1 transitions. Applying the scanned-wavelength modulation spectroscopy with second-harmonic detection and first-harmonic normalization (WMS-2f/1f) strategy, an enhancement of the signal-to-noise ratio over the short absorption path can be obtained. The temperature obtained by the single-ended sensor agrees well with that measured by a B-type thermocouple across different radial positions and heights, regardless of variations in equivalence ratios, confirming the accuracy of the sensor. In addition, a CFD simulation is performed to investigate the nonuniformity in the measurement region and the results show that the nonuniformity can be negligible.

Multi-View Synthesis of Sparse Projection of Absorption Spectra Based on Joint GRU and U-Net

Tunable diode laser absorption spectroscopy (TDLAS) technology, combined with chromatographic imaging algorithms, is commonly used for two-dimensional temperature and concentration measurements in combustion fields. However, obtaining critical temperature information from limited detection data is a challenging task in practical engineering applications due to the difficulty of deploying sufficient detection equipment and the lack of sufficient data to invert temperature and other distributions in the combustion field. Therefore, we propose a sparse projection multi-view synthesis model based on U-Net that incorporates the sequence learning properties of gated recurrent unit (GRU) and the generalization ability of residual networks, called GMResUNet. The datasets used for training all contain projection data with different degrees of sparsity. This study shows that the synthesized full projection data had an average relative error of 0.35%, a PSNR of 40.726, and a SSIM of 0.997 at a projection angle of 4. At projection angles of 2, 8, and 16, the average relative errors of the synthesized full projection data were 0.96%, 0.19%, and 0.18%, respectively. The temperature field reconstruction was performed separately for sparse and synthetic projections, showing that the application of the model can significantly improve the reconstruction accuracy of the temperature field of high-energy combustion.

Integrated-path multi-gas sensor using near-infrared diode lasers: An alternative to vehicle-driven point gas analyzer

Sensors for quantitative monitoring of greenhouse gases from high-emission facilities, such as wastewater treatment and industrial plants, are becoming essential to enforce regulatory compliance in climate action initiatives. A predominant sensing technique is the tracer-gas dispersion method (TDM) using closed-path gas analyzers typically installed in a vehicle to measure concentration as the gas plume is transected. Here, we validate for the first time the use of integrated-path gas sensors in TDM, measuring instantaneously the concentration along a predetermined laser beam path, thereby facilitating a stationary setup with orders of magnitude higher update rate and without the need for vehicles or road access. Our approach relies on a sensitive, integrated-path multi-gas sensor for real-time emission quantification using tunable diode laser absorption spectroscopy. An optical sensor head with spectral coverage from 1.52 μm to 1.65 μm is demonstrated, supporting multiplexed gas sensing in a modular fashion using low-power fiber-coupled diode lasers. The sensor measures real-time integrated-path concentrations of CO2 and CH4, and the tracer-gas C2H2 at 130 Hz with 0.5 ppm, 3 ppb, and 2 ppb sensitivity, respectively, at 1 s averaging. The integrated-path sensor is systematically benchmarked for absolute accuracy against a state-of-the-art point sensor, revealing an excellent match over 16 hours. Furthermore, the integrated-path sensor is deployed outdoors for emission quantification using TDM. A comparison to the gold-standard point sensor reveals identical results, demonstrating the integrated-path sensor as a potent substitute for vehicle-driven point sensors.

Fiber optic photoacoustic gas sensor enhanced by multi-pass cell with overlapping phantom spots

A neotype fiber optic photoacoustic gas sensor with overlapping phantom spots based multi-pass is proposed, which greatly improves the reflection times and the length of gas absorption path. Moreover, the detection accuracy of photoacoustic spectroscopy (PAS) system is independent of the optical interference effect caused by overlapping spots, which is different from tunable diode laser absorption spectroscopy (TDLAS) technology. A mathematical model including light beam motion space, spot size and overlap discrimination of multi-pass structure based on overlapping phantom spots is established. The incident light is reflected up to dozens of times in the resonator with a diameter of only 8 mm. The emergent light does not need to converge on a photodetector, which reduces the difficulty of adjusting the optical path and promotes the realization of overlapping phantom spots. The experimental results show that the amplitude of photoacoustic signal is improved by about 40 times compared with the single spot structure. Appling high-sensitivity optical fiber cantilever to detect the sound pressure signal. The detection limit of C2H2 gas reaches 8.2 ppt. The normalized noise equivalent absorption (NNEA) coefficient of the gas sensor is achieved to be 9.3×10−11 cm−1WHz−1/2.

Ar(1s5) density in a co-axial argon plasma jet with N2–O2 shielding

Atmospheric-pressure microplasma jets (µAPPJs) are versatile sources of reactive species with diverse applications. However, understanding the plasma chemistry in these jets is challenging due to plasma-flow interactions in heterogeneous gas mixtures. Spatial metastable density profiles help to understand these physical and chemical mechanisms. This work focuses on controlling the shielding gas around a µAPPJ. We use a dielectric barrier discharge co-axial reactor where a co-flow shields the pure argon jet with different N2–O2 gas mixtures. A voltage pulse (4 kV, 1 µs, 20 kHz) generates a first discharge at the pulse’s rising edge and a second discharge at the falling edge. Tunable diode laser absorption spectroscopy measures the local Ar(1s5) density. A pure N2 (100%N2–0%O2) co-flow leads to less reproducible and lower peak Ar(1s5) density ( 5.8×1013cm−3 ). Increasing the O2 admixture in the co-flow yields narrower Ar(1s5) absorbance profiles and increases the Ar(1s5) density ( 6.9×1013 to 9.1×1013cm−3 ). The position of the peak density is closer to the reactor for higher O2 fractions. Absence of N2 results in comparable Ar(1s5) densities between the first and second discharges (maxima of 9.1×1013 and 9.3×1013cm−3 , respectively). Local Ar(1s5) density profiles from pure N2 to pure O2 shielding provide insights into physical and chemical processes. The spatially-resolved data may contribute to optimising argon µAPPJ reactors across the various applications and to validate numerical models.

Beam-steering-effect immune laser absorption spectrum extraction for precise and robust temperature measurement

Tunable diode laser absorption spectroscopy is widely used for temperature measurement in combustion monitoring due to its merits of rapidity, non-contact capability, and high precision. In actual applications, the beam-steering effect will cause time-varying distortion in the laser intensity signal, resulting in a low signal-to-noise ratio and inaccurate extraction of the absorption spectrum. This work proposes a cost-effective distortion detection scheme and adaptive distortion filter and compensation algorithm for the extraction of the beam-steering effect immune laser absorption spectrum. Turbulent flame monitoring experiment results prove the proposed method can improve the signal-to-noise ratio of the raw transmitted absorptive laser signal by 35 dB after the beam-steering distortion has been compensated. When compared with the temperature results calculated from the distorted absorptive laser signal, the results from the distortion-compensated absorptive laser signal show a significant improvement in both robustness and precision. The experimental results demonstrate that the successful temperature fitting rate has increased from 93.7 % to 100 % (over 1000 repetitions), and the average standard deviations of temperature have decreased from 296.6 K to 61.8 K. The proposed method is cost-effective and compact, which will potentially play an important role in dynamic combustion monitoring and diagnosis.

Particles influence on the direct absorption spectroscopy of TDLAS

Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology is widely used for monitoring the gas pollutants. But in actual measurement environments, there are usually particles, which will affect the TDLAS direct absorption spectroscopy. In this paper, the influence of particles on the direct absorption spectroscopy of TDLAS was analyzed by using TDLAS-DAS technology combined with particle extinction method. According to analysis, it was found that the presence of particles does not affect the gas characteristic absorption spectral line strength and the gas absorption spectroscopy broadening, but it does affect the output laser intensity and absorbance received by the detector. The transmittance of particles can be obtained through the slope of the output laser intensity, which can be used to correct the absorbance to achieve accurate measurement of gas concentration. This paper analyzed the effects of particle size, incident wavelength, and composition on particle extinction coefficient and obtained the extinction coefficient of soot. If the laser wavelength is 1.53 μm and the particle size of soot is 45 μm, the extinction coefficient of soot is 2.1016. The magnitude of laser intensity attenuation at different particle concentrations can be obtained based on the extinction coefficient of particulate matter. When the concentration of soot is greater than 26290 mg/m3 and the optical path is 100 cm, the transmittance is less than 0.1 and the laser intensity attenuation reach over 90 %, which may affect the detection of gas absorption spectroscopy information and the accuracy of measurement. On this basis, this paper established the relationship between gas concentration and corrected absorbance to invert the gas concentration in the presence of particulate matter when the concentration of soot is less than 26290 mg/m3. The inversion gas concentration error obtained by the absorbance peak calibration method is within 3 %, and the inversion gas concentration error obtained by the absorbance integration method is within 1 %. From the inversion results, it can be seen that the accurate measurement of gas concentration in the presence of particulate matter can be achieved by extracting the slope of the output laser intensity to correct absorbance.

Research on etalon‐free CO2 measurement based on direct absorption signal fitting

AbstractThe emission of carbon dioxide (CO2) by thermal power plants is the main factor leading to the greenhouse effect. The measurement of CO2 is of great significance to carbon emission data statistics. In this paper, a measurement system based on tunable diode laser absorption spectroscopy (TDLAS) was built to measure the spectrum of CO2 near 1579 nm. To simplify the experimental system, a direct absorption signal fitting method is proposed to replace etalons for time‐frequency conversion. The method was applied to the measurement of four different concentrations of CO2 in the range of 7%–16%, and the measurement results were compared with the multipeak method, another etalon‐free method. The results show that the mean relative error of the measurement results is 4.23%, which is 1.76% lower than the measurement results of the multipeak method. The experimental research provides a feasible idea to simplify the CO2 measurement system based on TDLAS.

Methane concentration inversion under multiple conditions using feature fusion residual network

In the realm of methane concentration measurement using tunable diode laser absorption spectroscopy (TDLAS), relying on a single second harmonic signal peak often leads to information loss. Furthermore, second harmonic peaks exhibit conflicts when subjected to different parameters, thereby posing challenges for accurate methane concentration measurement. This paper presents a novel approach employing a multi-scale feature fusion residual network (MResNet) to measure methane concentration under various operational conditions. To begin, we established a hardware experimental platform based on TDLAS to collect direct absorption spectrum data, and second harmonic data with varying parameters. Then we conducted a comparative analysis between MResNet and conventional models. The results of our experiments demonstrate the effectiveness of MResNet in predicting methane concentration, with a decrease in MAE by 39% and MSE by 70%. Notably, the coefficient of determination for methane concentration prediction using MResNet both reached 99.92% when applied to both direct absorption spectrum and second harmonic data. This capability empowers accurate methane concentration measurement across a wide range of operating conditions.

Design of ammonia leakage online monitoring system based on tunable diode laser absorption spectroscopy

A real-time online ammonia volume fraction monitoring system based on tunable diode laser absorption spectroscopy (TDLAS) is designed to meet the demand for ammonia gas leakage monitoring in ammonia-fueled ships. The system uses a 1, 512 nm distributed-feedback (DFB) laser as the light source and a 5 m high-temperature Boss-Gas-Cell as the absorption cell. By applying a 4 Hz scan signal and a 31.4 kHz modulation signal to the laser current, the effective absorption signal of the gas is transferred to the high-frequency band, then the second harmonic (2f) signal about gas volume fraction is extracted by a digital lock-in amplifier from the gas absorption signal, and ammonia volume fraction can be calculated from the amplitude of 2f signal. The test results show that the ammonia detection accuracy in the range of 0-500 ppm can reach ±1% FS, the response time is less than 30 s, and the system measurement repeatability is less than 1%, which meets the needs of ammonia leakage monitoring in ammonia-fueled vessels.

Circuit design of a high scanning frequence and fast response laser methane remote sensor

To solve the problem of slow detection of laser methane sensors based on tunable diode laser absorption spectroscopy (TDLAS), a methane detection system with a high detection rate was proposed. Focusing on the hardware circuit design of the system which consists of a laser driver circuit, laser temperature control circuit, optical amplifier and filter circuit and main control circuit based on FPGA+ARM, this circuit is designed to achieve a laser scanning frequency of 50 kHz. A voltage-controlled current source is designed to drive the laser diode (LD). A TEC controller is designed to keep the operating temperature of the laser diode stable. Since the photocurrent can be very weak, to ensure the quality of signals from the photodiode (PD), a signal collection circuit consisting of the transimpedance amplifier, gain-controlled amplifier and low-pass filter was designed. An experiment is carried out to test system measurement capabilities. The results show the circuit can reach a response time of 2.5 ms while ensuring a measurement error of less than 4.3%.

Velocity measurements in particle-laden high-enthalpy flow using non-intrusive techniques

This research aims at analysing the particle-laden flow of the hypersonic high-enthalpy wind tunnel L2K, situated in Köln at the German Aerospace Center (DLR). In the L2K wind tunnel, Martian atmosphere can be created, and the facility can simulate heat load conditions encountered during atmospheric entry of Martian missions. In the tests, a simplified Martian atmosphere (97% CO2 and 3% N2) was used. The high-enthalpy flow was loaded with micrometric particles of magnesium oxide. The particles’ mean velocity was measured with a 2D–2C particle image velocimetry (PIV) system, in the region right downstream the nozzle expansion of the wind tunnel. The work proves the possibility of creating a high-enthalpy particle-laden flow for thermal protection systems (TPS) testing with simulated Martian atmosphere. Average particle velocities of around 2000 m/s are measured and compared with the numerical simulation of the wind tunnel’s particle-free flow, and with the flow velocity measured with tunable diode laser absorption spectroscopy (TDLAS). The study also highlights some unexpected results and features of the high-enthalpy particle-laden flow and proposes some theories for the causes of such effects, which include agglomeration due to melting, and gravitational effect.

Measurement of Light-Duty Vehicle Exhaust Emissions with Light Absorption Spectrometers

Light-duty vehicle emission regulations worldwide set limits for the following gaseous pollutants: carbon monoxide (CO), nitric oxides (NOX), hydrocarbons (HCs), and/or non-methane hydrocarbons (NMHCs). Carbon dioxide (CO2) is indirectly limited by fleet CO2 or fuel consumption targets. Measurements are carried out at the dilution tunnel with “standard” laboratory-grade instruments following well-defined principles of operation: non-dispersive infrared (NDIR) analyzers for CO and CO2, flame ionization detectors (FIDs) for hydrocarbons, and chemiluminescence analyzers (CLAs) or non-dispersive ultraviolet detectors (NDUVs) for NOX. In the United States in 2012 and in China in 2020, with Stage 6, nitrous oxide (N2O) was also included. Brazil is phasing in NH3 in its regulation. Alternative instruments that can measure some or all these pollutants include Fourier transform infrared (FTIR)- and laser absorption spectroscopy (LAS)-based instruments. In the second category, quantum cascade laser (QCL) spectroscopy in the mid-infrared area or laser diode spectroscopy (LDS) in the near-infrared area, such as tunable diode laser absorption spectroscopy (TDLAS), are included. According to current regulations and technical specifications, NH3 is the only component that has to be measured at the tailpipe to avoid ammonia losses due to its hydrophilic properties and adsorption on the transfer lines. There are not many studies that have evaluated such instruments, in particular those for “non-regulated” worldwide pollutants. For this reason, we compared laboratory-grade “standard” analyzers with FTIR- and TDLAS-based instruments measuring NH3. One diesel and two gasoline vehicles at different ambient temperatures and with different test cycles produced emissions in a wide range. In general, the agreement among the instruments was very good (in most cases, within ±10%), confirming their suitability for the measurement of pollutants.

Characterization of a co-planar dielectric barrier discharge design as a plasma source for trace element detection by atomic spectrometry

Dielectric barrier discharges (DBD) are used in analytical chemistry for the dissociation of molecules, which can then be detected with methods like atomic absorption spectroscopy or emission spectroscopy. However, a drawback of this approach is that the analyte can be deposited on the reactor walls, lowering the density available for element detection. In this work, we propose a novel design of a DBD for element detection which consists of a T-shaped quartz glass body with line-shaped co-planar electrodes. This co-planar configuration was selected to force the discharge towards the glass surface, thus increasing the release of deposited analyte from the reactor walls. This was verified by temporally and spatially resolved optical emission spectroscopy showing that the plasma propagates in a bent structure, close to the wall between the electrodes. The optical results also point towards the importance of helium metastable states in the dissociation of molecules. Thus, spatially resolved metastable densities were measured by tunable diode laser absorption spectroscopy. The metastable density was in the order of 4.5×1010cm−3, and the density profile correlated with the bent structure observed in the emission measurements.

Real-Time Measurement of CH4 in Human Breath Using a Compact CH4/CO2 Sensor.

The presence of an elevated amount of methane (CH4) in exhaled breath can be used as a non-invasive tool to monitor certain health conditions. A compact, inexpensive and transportable CH4 sensor is thus very interesting for this purpose. In addition, if the sensor is also able to simultaneously measure carbon dioxide (CO2), one can extract the end-tidal concentration of exhaled CH4. Here, we report on such a sensor based on a commercial detection module using tunable diode laser absorption spectroscopy. It was found that the measured CH4/CO2 values exhibit a strong interference with water vapor. Therefore, correction functions were experimentally identified and validated for both CO2 and CH4. A custom-built breath sampler was developed and tested with the sensor for real-time measurements of CH4 and CO2 in exhaled breath. As a result, the breath sensor demonstrated the capability of accurately measuring the exhaled CH4 and CO2 profiles in real-time. We obtained minimum detection limits of ~80 ppbv for CH4 and ~700 ppmv for CO2 in 1.5 s measurement time.

CO2 Measurement under Different Pressure and Vibration Conditions Using Tunable Diode Laser Absorption Spectroscopy

The greenhouse effect resulting from fuel combustion has drawn growing attention, and CO2 emissions from fossil fuel power stations are one of the main sources of greenhouse gases. It is crucial to monitor the concentration of CO2 in the flue gas ducts of these stations. However, pressure and vibration caused by the combustion of boilers make the measurement of CO2 in flue gas ducts extremely challenging. In this study, tunable diode laser absorption spectroscopy (TDLAS) combined with Wave Modulation Spectroscopy (WMS) was employed to measure the concentration of CO2 under different pressure and vibration conditions in the laboratory. The absorption line of CO2 at the wavenumber 6357.38 cm−1 was recorded under varying pressure conditions ranging from 0 to 1.4 atm, acceleration conditions ranging from 0 to 7.7 m/s2, and a combination of both. Firstly, a negative linear correlation was found between the pressure and the amplitude of the second harmonic, with a maximum relative error of 4.645% observed at a pressure of 1.4 atm. Secondly, the maximum acceleration that the system can withstand was determined to be 7.3 m/s2, as it was not possible to provide a sufficiently low fitting error at higher accelerations. For the combined effects of the pressure and vibration, a dramatic increase in the relative error of amplitude can be observed within the acceleration range of 5.0~6.0 m/s2 while under the pressure conditions of 0.6 atm, 1.0 atm, and 1.4 atm. Moreover, the maximum endurable acceleration decreases with the increase in pressure, which infers that effective measurements can be achieved when the acceleration is below 5 m/s2 within the pressure range of 0~1.4 atm. This suggests that TDLAS combined with WMS possesses a potential for online measuring of CO2 concentrations in flue gas ducts within a certain acceleration range. This work can provide some insights for stable gas detection using TDLAS under varied pressure and vibration conditions.

Hard-Target Reflection Laser Spectroscopy of Carbon Monoxide Gas Concentration for the Early Detection of Spontaneous Combustion of Coal.

We report on the hard-target reflection spectroscopy of carbon monoxide (CO) gas based on the technique of infrared tunable diode laser absorption spectroscopy aiming at developing a low-cost yet sensitive sensor for the early detection of spontaneous coal combustion. A narrow-band distributed feedback laser emitting around 2333.7 nm is used to monitor CO gas molecules contained in a 5 cm gas cell. The light diffusely backscattered from the surface of a lump of coal placed at the end of a 50 cm light path is detected with a photodiode in the coaxial transmitter/receiver setup. From the variation of the detected signal profile with the CO partial pressure in the cell, the detection limit of the current system is estimated to be about 30 parts per million per meter (ppm·m), which meets the sensitivity required for monitoring the self-heating of coal in mines, silos, or stockpiles.

Research on Multi-point Measurement System of Mine Gas Based on TDLA

Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology combines the advantages of wavelength modulation technology and direct absorption spectroscopy technology, and the system achieves highly sensitive and full-range detection of methane gas concentrations. In the system, only the pure optical methane probe is positioned under the mine, which establishes connectivity with the host computer on the surface through the optical fiber. This setup enables the simultaneous detection of 16 channels of methane gas through time-sharing multiplexing technology. Experimental results demonstrate a measurement error of less than ±6.0×10−4 at low concentrations (0∼1%) and within ±6% of the true value at high concentrations under normal temperature and pressure conditions. The full-range linear fit attains 0.9998. According to the one-fold standard deviation, the minimum detection lower limit of the system stands at 68.36 ppm and the measured gas concentrations across different channels of the methane probes remain consistent inhe same environment. This technology provides a new solution for continuous, accurate, and multi-point monitoring of mine gas.

Measurement of binary diffusion at elevated Knudsen numbers using laser absorption spectroscopy

Mass diffusion coefficients of gas mixtures have been measured for more than 100 years. However, the experimental data for the mass diffusion coefficient of gas mixtures in the rarefied gas regimes at Knudsen numbers (Kn) above 0.01 are few and remain uncertain due to the inherent precision limitations of the available state-of-the-art measurement techniques. The increased frequency of gas-wall collision, wall-friction, and surface-diffusion over the wall surface at Kn > 0.01 increases the uncertainty of the diffusive mass transport processes for internal gas flow in microcapillaries. Due to the growing interest in microfluidic applications at rarefied gas conditions, accurate diffusion coefficient measurements are needed to inform theoretical predictions and empirical relations in rarefied gas regimes. Thus, this article introduces a new experiment methodology consisting of a two-bulb (TB) diffusion configuration accompanied by a tunable diode laser absorption spectroscopy (TDLAS) detection technique that uses the measured time history of path-integrated absorbance to provide a non-intrusive, species-specific, in situ measurement of mass diffusion for a He–CO2 binary gas mixture at Kn > 0.01. To demonstrate the TB-TDLAS method's capability, the effective diffusion coefficient for a He–CO2 binary gas mixture was measured in the transition gas regime at Knudsen numbers relative to the tube radius in the range 0.1 < Kn < 5.4, and the results are compared against the Bosanquet empirical relation.