Calibration-free Wavelength Modulation Spectroscopy Research Articles

Ultra-low concentration detection of dissolved C2H2 in oil using fixed-frequency RF-assisted calibration-free wavelength modulation spectroscopy

The monitoring sensitivity of dissolved C2H2 in transformer oil is crucial to ensuring the safe operation of electric power systems. A highly sensitive sensor system based on fixed-frequency radio frequency assisted calibration-free wavelength modulation spectroscopy (FF-RF-WMS-2f/1f) for dissolved C2H2 in transformer oil was developed and evaluated. Lowering the headspace sampling pressure to 0.1 atm effectively increased the outgassing ratio and detection signal of dissolved C2H2, while also reducing background interference. By employing a novel fixed-frequency RF-assisted modulation technique, we effectively suppressed the amplitude and influence of optical fringes and improved the system’s linear fitting R-squared value for gas-phase concentration from 0.9997 to 0.9999. Furthermore, the minimum detection limit (MDL), as calculated by Allan deviation, was reduced from 8.58 ppb to 4.36 ppb, while the optimal integration time increased slightly from 62 s to 79 s. The equivalent MDL for dissolved C2H2 reached 0.9 ppb. The comparative experiments showed that the error was less than 0.2 ppm or 8 %, the relative standard deviations (RSDs) of continuous measurement were less than 3 %, and the RSD at ultra-low concentration was less than 8 %. The experiments verified the high sensitivity and accuracy of the sensor system, making it suitable for ultra-low concentration dissolved C2H2 detection in transformer insulating oil.

Concentration Retrieval in a Calibration-Free Wavelength Modulation Spectroscopy System Using Particle Swarm Optimization Algorithm.

This paper develops a concentration retrieval technique based on the particle swarm optimization (PSO) algorithm, which is used for a calibration-free wavelength modulation spectroscopy system. As compared with the commonly used technique based on the Levenberg-Marquardt (LM) algorithm, the PSO-based method is less dependent on the pre-characterization of the laser tuning parameters. We analyzed the key parameters affecting the performance of the PSO-based technique and determined their optimal parameter values through testing. Furthermore, we conducted a comparative analysis of the efficacy of two techniques in detecting C2H2 concentration. The results showed that the PSO-based concentration retrieval technique is about 63 times faster than the LM-based one in achieving the same accuracy. Within 5 s, the PSO-based technique can produce findings that are generally consistent with the values anticipated.

Online tomography algorithm based on laser absorption spectroscopy

Conventional calibration-free wavelength modulation spectroscopy generally requires complex absorption spectrum simulations in combination with spectral databases and laser modulation parameters, placing high demands on the accuracy of a priori spectral parameters and hardware parameters. Meanwhile, inappropriate initial values can increase the computation time and even lead to local optimal solutions. In order to improve the computational efficiency, a rapid calibration-free wavelength modulation spectroscopy to obtain the integrated absorbance is presented in this work. First, this method is computationally efficient, requiring only algebraic calculations by using the 2nd, 4th, and 6th harmonic center peak height parameters to obtain the integrated absorbance, eliminating the need for computationally intensive harmonic fitting calculations. Secondly, this method has low dependence on the spectral database, requiring only line intensity and low-state energy level spectral parameters. Finally, this method is highly adaptable and does not require scanning the complete absorption spectral line shape, which solves the problem of incomplete harmonic signals caused by the conventional method at high temperature and high pressure due to the broadening of the absorption spectral line. This method has previously been used only for line-of-sight measurements at low-frequency experimental signals in stable environments, and for calculating the integrated absorbance at average temperature, concentration and pressure states. In this work, the method is applied to non-uniform complex combustion field tomography and combined with the proposed tomographic system to achieve online reconstructing temperature and concentration distributions. The accuracy and computational efficiency of the method in obtaining the integrated absorbance are verified by numerical simulations and experiments on the butane burner flame. The results show that the presented method is consistent with the reconstructed distribution compared with the conventional wavelength modulation method, with a maximum relative deviation of only 0.94% from the measurement and 3.5% from the thermocouple measurement, verifying the accuracy of the method. The computational efficiencies of the two methods for obtaining the integrated absorbance are analyzed. The average calculation time per path is 0.15 s for the present method and 21.10 s for the conventional method. The calculation efficiency of the present method is at least two orders of magnitude higher than that of the conventional method, which provides a fast and reliable research method and technical means to realize the industrial-grade online reconstruction of temperature and concentration distribution of combustion fields.

Principle and Performance Analysis of the Levenberg–Marquardt Algorithm in WMS Spectral Line Fitting

Calibration-free wavelength modulation spectroscopy (WMS) is an efficient technique for trace gas monitoring. It is widely used due to its resistance to light intensity fluctuations, strong suppression of low-frequency noise, fast response time, and excellent environmental adaptability. The calibration-free WMS often employs the Levenberg–Marquardt algorithm for spectral fitting to retrieve gas characteristics. However, to the best of our knowledge, an analysis of the main factors affecting the operational effectiveness of the Levenberg–Marquardt algorithm in calibration-free WMS has merely been reported. In this paper, we have systematically analyzed the Levenberg–Marquardt algorithm’s operating mechanism in WMS-2f/1f. The results show that the number of parameters and the estimation errors of the initial parameters are the main factors limiting the retrieval accuracy of the algorithm, which provides some important guidelines for the subsequent optimization of the spectral fitting scheme.

Mid-infrared fiber-coupled laser absorption sensor for simultaneous NH3 and NO monitoring in flue gases

We report the development of a mid-infrared fiber-coupled laser absorption sensor for in situ and simultaneous detection of NH3 and NO, which is suitable for flue gas monitoring at the SCR (selective catalytic reduction) exhaust. Two quantum cascade lasers (QCLs) are used to detect the optimal absorption lines of NH3 and NO centered at 1103.45 cm−1 and 1929.03 cm−1, respectively. The two QCLs are coupled into a single hollow-core fiber and delivered to an open-path, single-ended optical probe for in situ gas detection. The gas concentration is determined by an improved calibration-free wavelength modulation spectroscopy (CF-WMS) technique with a reduced line model. A series of experiments are conducted to evaluate the sensor performance at varied temperatures from 296 to 573 K and the pressure of 1 atm. Such a dual-species sensor realizes a minimum detection limit of 30 ppb at 100 s average time for NO and 14 ppb at 70 s for NH3, respectively. The uncertainty of NH3 and NO detection is determined to be less than 5 %. By evaluating the WMS spectra of other species, the current sensor shows negligible spectral interferences under typical flue gas conditions.

Standoff sub-ppb level measurement of atmospheric ammonia with calibration-free wavelength modulation spectroscopy

Ammonia (NH3) plays a significant role in the formation of atmospheric particulate matter, and influences on environmental and public health as well as climate change. Thus, it is important to sensitive measurement of atmospheric NH3. In the present work, a sub-ppb level standoff open-path NH3 sensor was developed for on line, sensitive measurement of atmospheric NH3. A 9.06 μm distributed feedback quantum cascade laser was employed to probe the ammonia absorption lines located on fundamental rotational-vibrational absorption band and calibration-free wavelength modulation spectroscopy technique was employed to retrieve NH3 concentration directly. The standoff open-path NH3 sensor performance was investigated in laboratory corridor with 80 m open path length (Hefei, China) and a minimum detection limit of 0.46 ppb (3σ) was obtained. Finally, field campaign measurement was carried out in a winter wheat farmland (Changshu, China). Field measurement shown that the concentration of NH3 varies from 7 ppb to 30 ppb with an average of 14 ppb. The developed standoff sensor has high potential to be a robust tool for monitoring atmospheric NH3 or study of regional ammonia emissions in farmland or feedlot scale.

Mid-infrared absorption spectroscopic sensor for simultaneous and in-situ measurements of ammonia, water and temperature

A mid-infrared laser-based absorption sensor is developed for in-situ and simultaneous detection of ammonia, water and temperature for selective catalytic reduction (SCR) exhaust monitoring. The sensor used a quantum cascade laser (QCL) to target the NH3 line at 1103.45 cm−1 and a distributed-feedback (DFB) laser for H2O concentration and temperature by exploiting the line pair at 4029.52 cm−1 and 4029.78 cm−1. The time-division multiplexing (TDM) strategy was implemented to fulfill the H2O and temperature measurement with direct absorption spectroscopy (DAS) and the absolute concentration measurement of NH3 by calibration-free wavelength modulation spectroscopy (CF-WMS). All the experiments were conducted in a test chamber with a controlled temperature range of 296–673 K and pressure of 1 atm. The NH3 sensor was validated to achieve the detection limit of 7 ppb at the integration time of 100 s. The temperature measurement was also verified to show an uncertainty less than 3.5 %. We further performed the multi-parameter (NH3, H2O and temperature) measurement under the static condition and gas flowing condition, respectively. Our sensor successfully captured the time-varying species concentrations and temperature with a time resolution of 2.3 s, which is promising for SCR monitoring in terms of accuracy, sensitivity, and time response.

In Situ Measurement of NO, NO2, and H2O in Combustion Gases Based on Near/Mid-Infrared Laser Absorption Spectroscopy.

In this study, a strategy was developed for in situ, non-intrusive, and quantitative measurement of the oxides of nitrogen (NO and NO2) to describe emission characteristics in gas turbines. The linear calibration-free wavelength modulation spectroscopy (LCF-WMS) approach combined with the temperature profile-fitting strategy was utilized for trace NO and NO2 concentration detection with broad spectral interference from gaseous water (H2O). Transition lines near 1308 nm, 5238 nm, and 6250 nm were selected to investigate the H2O, NO, and NO2 generated from combustion. Experiments were performed under different equivalence ratios in a combustion exhaust tube, which was heated at 450–700 K, with an effective optical length of 1.57 m. Ultra-low NOx emissions were captured by optical measurements under different equivalence ratios. The mole fractions of H2O were in agreement with the theoretical values calculated using Chemkin. Herein, the uncertainty of the TDLAS measurements and the limitation of improving the relative precision are discussed in detail. The proposed strategy proved to be a promising combustion diagnostic technique for the quantitative measurement of low-absorbance trace NO and NO2 with strong H2O interference in real combustion gases.

Y-Net: a dual-branch deep learning network for nonlinear absorption tomography with wavelength modulation spectroscopy.

This paper demonstrates a new method for solving nonlinear tomographic problems, combining calibration-free wavelength modulation spectroscopy (CF-WMS) with a dual-branch deep learning network (Y-Net). The principle of CF-WMS, as well as the architecture, training and performance of Y-Net have been investigated. 20000 samples are randomly generated, with each temperature or H2O concentration phantom featuring three randomly positioned Gaussian distributions. Non-uniformity coefficient (NUC) method provides quantitative characterizations of the non-uniformity (i.e., the complexity) of the reconstructed fields. Four projections, each with 24 parallel beams are assumed. The average reconstruction errors of temperature and H2O concentration for the testing dataset with 2000 samples are 1.55% and 2.47%, with standard deviations of 0.46% and 0.75%, respectively. The reconstruction errors for both temperature and species concentration distributions increase almost linearly with increasing NUC from 0.02 to 0.20. The proposed Y-Net shows great advantages over the state-of-the-art simulated annealing algorithm, such as better noise immunity and higher computational efficiency. This is the first time, to the best of our knowledge, that a dual-branch deep learning network (Y-Net) has been applied to WMS-based nonlinear tomography and it opens up opportunities for real-time, in situ monitoring of practical combustion environments.

Calibration-Free Wavelength Modulation Spectroscopy Method Based on the Height-Width Characteristic of Second Harmonic

Calibration-Free Wavelength Modulation Spectroscopy Method Based on the Height-Width Characteristic of Second Harmonic

Sensitivity and Accuracy Enhanced Wavelength Modulation Spectroscopy Based on PSD Analysis

This letter proposes a novel method to implement calibration-free wavelength modulation spectroscopy (CF-WMS) with enhanced sensing sensitivity and accuracy. Compared with traditional 1f-normalized CF-WMS implementation, the proposed method is derived, for the first time, from the power spectral density (PSD) spectrum of transmitted laser. This novel derivation enables estimation of noise power, thus enhancing of sensing sensitivity and retrieval accuracy of gas parameters, particularly in noisy environment. The proposed method is validated by both numerical simulation and condition-controlled experiment, indicating improvement of the detection limit of gas parameters by a factor of 2.64 compared to the traditional method.

Calibration-free wavelength modulation spectroscopy based on even-order harmonics.

This paper proposes a novel and rapid calibration-free wavelength modulation spectroscopy algorithm based on even-order harmonics. The proposed algorithm, analytically deduced from Voigt line-shape function, only involves simple algebraic operations to describe the actual gas absorption spectra, thus eliminating the time-consuming simulations and line-shape fitting procedures adopted in traditional algorithms. Instead of acquiring the entirely scanned absorption line-shape, the proposed technique only requires extraction of the peak values of the harmonics. This characteristic significantly benefits gas diagnosis at elevated pressure and/or temperature, in which the entirely scanned absorption is very difficult to be obtained due to the broadened line-shapes. The proposed algorithm is validated by both numerical simulation and condition-controlled experiment, indicating millisecond-level calculation of gas parameters with the relative error less than 4% in the experiments.

Soil respiration analysis using a mid-infrared quantum cascade laser and calibration-free WMS-based dual-gas sensor.

A high response and sensitive dual-gas sensor based on calibration-free wavelength modulation spectroscopy (CF-WMS) has been developed for the simultaneous detection of carbon monoxide (CO) and nitrous oxide (N2O) to eliminate the detection errors caused by light intensity variations. A multi-pass cell (MPC) was employed to lengthen the optical path to improve the precision of the sensing system by combining with a 4.56 μm mid-infrared quantum cascade laser (MIR-QCL). Meanwhile, a LabVIEW-based bi-molecular iterative fitting algorithm was used to infer the respective abundances of each species. The performance of the completed system was accurately evaluated with precisions of 3.4 ppb for CO and 3.8 ppb for N2O at a 1 s averaging time, which could be improved to 0.48 ppb for CO and 0.53 ppb for N2O at an averaging time of 154 s and 278 s, respectively. The grassland soil respiration analysis of CO and N2O was performed under different moisture conditions, which indicated that dried soil samples appeared to be a significant source of CO, while the sinks of CO and the sources of N2O occurred in the moist soil samples. The maximum exchange rates of the two gases were exhibited in moderate moisture soil samples rather than in the over-wet or arid soil samples. Moreover, a possible positive relationship between the sinks of CO and sources of N2O was established to illustrate the correlation of the two species in soil respiration.

Real-Time Monitoring of 13C- and 18O-Isotopes of Human Breath CO2 Using a Mid-Infrared Hollow Waveguide Gas Sensor.

Real-time measuring of CO2 isotopes (13CO2, 12CO2, and 18OC16O) in exhaled breath using a mid-infrared hollow waveguide gas sensor incorporating a 2.73 μm distributed feedback laser was proposed and demonstrated for the first time based on calibration-free wavelength modulation spectroscopy. The measurement precisions for δ13C and δ18O were, respectively, 0.26 and 0.57‰ for an integration time of 131 s by Allan variance analysis. These measurement precisions achieved in the present work were at least 3.5 times better than those reported using direct absorption spectroscopy and 1.3 times better than those obtained by calibration-needed wavelength modulation absorption spectroscopy. Continuous measurement of three isotopes in the breathing cycle was performed. Alveolar gas from the expirogram was identified, and the 13C/12C and 18O/16O ratios were found to be almost constant during the alveolar plateau, which enables optimization of breath sampling and provides accurate information on metabolic processes. The 13C/12C and 18O/16O isotope ratios at the alveolar plateau of five breath cycles were averaged, yielding δ13C and δ18O values of (-24.3 ± 3.4) and (-30.7 ± 2.6) ‰, respectively. This study demonstrates the feasibility of real-time analysis of 13C- and 18O-isotopes of human breath CO2 in clinical applications and shows its potential for diagnosing respiratory-related diseases with high sensitivity, selectivity, and specificity.

Absolute noninvasive measurement of CO2 mole fraction emitted by E. coli and S. aureus using calibration-free 2f WMS applied to a 2004 nm VCSEL.

We report the first demonstration, to the best of our knowledge, of accurate real-time noninvasive measurement of the absolute cumulative mole fraction of metabolic carbon dioxide emitted by Escherichia coli and Staphylococcus aureus over a period of several hours of their life cycles using a recently developed calibration-free wavelength modulation spectroscopy technique. A 1mW vertical-cavity surface-emitting laser is used to interrogate a single rotational vibrational absorption line of carbon dioxide at 2003.5nm. The measurements are immune to laser intensity fluctuations and variable optical coupling that is inevitable in such free-space coupled experiments that run over 10-18h. The cumulative carbon dioxide mole fraction follows the characteristic modified Gompertz model that is typical of bacterial growth in batch cultures. The characteristic growth parameters are extracted from this curve. The technique can be readily extended to study multiple volatile organic compounds that bacteria are known to emit.

Calibration-free wavelength modulation spectroscopy for gas concentration measurements using a quantum cascade laser

We report the development of an accurate calibration-free wavelength-scanned wavelength modulation spectroscopy system based on the temporal wavelength response of a current-modulated quantum cascade laser (QCL) for gas concentration detections. Accurate measurements and determination of the QCL output intensity and wavelength response to current modulation enabled calculations of 1f-normalized 2f signal to obtain spectroscopic information with and without gas absorption in the beam path. The gas concentration was retrieved by fitting a simulation spectrum based on spectral line parameters to the background-subtracted 1f-normalized 2f signal based on measurements. In this paper, we demonstrate the performance of the developed system for the CH4 detection by applying an infrared QCL (at ~7.84 µm or ~1275 cm−1) to probe its two infrared transition lines at 1275.042 cm−1 and 1275.387 cm−1. The experimental results indicated very good agreements between measurements and modeling, for integrated absorbance ranging from 0.0057 cm−1 to 0.11 cm−1 (or absorbance ranging from ~0.029 to ~0.57). The extracted integrated absorbance was highly linear (R = 0.99996) to the gas sample concentration. Deviations between the nominal sample gas concentrations and the extracted gas concentrations calculated based on HITRAN spectroscopic parameters were within 3.5%.

Calibration-free wavelength modulation spectroscopy: symmetry approach and residual amplitude modulation normalization

This paper offers a simple, practical strategy to implement wavelength modulation spectroscopy (WMS) with a tunable diode laser. It eliminates the need to pre-characterize the laser intensity parameters or make any design changes to a conventional WMS system. Consequently, sensitivity and signal strength remain the same as what can be obtained from a traditional WMS setup at low modulation amplitude. Like previously proposed calibration-free approaches, this new method also yields an absolute absorption line shape function. To recover residual amplitude modulation (RAM) contributions present in the first and second harmonic signals of WMS, we exploited their even or odd symmetric nature. We then used these isolated RAM signals to estimate the absolute line shape function, thus removing the impact of optical intensity fluctuations on measurement. We have also discussed uncertainties and noises associated with the estimated absolute line shape function and the applicability of this new method to detect several gases in the near infrared region. We used measurements of the 1650.96 nm absorption line for 1% and 8% methane concentration in the 60-100 kPa pressure range to validate the efficacy of this new RAM recovery technique and demonstrated a calibration-free system. Because this approach has minimal dependency on diode laser operating conditions, it is more robust and suitable for harsh industrial environments.

Calibration-free wavelength-modulation spectroscopy based on a swiftly determined wavelength-modulation frequency response function of a DFB laser.

A methodology for calibration-free wavelength modulation spectroscopy (CF-WMS) that is based upon an extensive empirical description of the wavelength-modulation frequency response (WMFR) of DFB laser is presented. An assessment of the WMFR of a DFB laser by the use of an etalon confirms that it consists of two parts: a 1st harmonic component with an amplitude that is linear with the sweep and a nonlinear 2nd harmonic component with a constant amplitude. Simulations show that, among the various factors that affect the line shape of a background-subtracted peak-normalized 2f signal, such as concentration, phase shifts between intensity modulation and frequency modulation, and WMFR, only the last factor has a decisive impact. Based on this and to avoid the impractical use of an etalon, a novel method to pre-determine the parameters of the WMFR by fitting to a background-subtracted peak-normalized 2f signal has been developed. The accuracy of the new scheme to determine the WMFR is demonstrated and compared with that of conventional methods in CF-WMS by detection of trace acetylene. The results show that the new method provides a four times smaller fitting error than the conventional methods and retrieves concentration more accurately.

In situ H2O and temperature detection close to burning biomass pellets using calibration-free wavelength modulation spectroscopy

The design and application of an H2O/temperature sensor based on scanned calibration-free wavelength modulation spectroscopy (CF-WMS) and a single tunable diode laser at 1.4 μm is presented. The se ...

Tomographic laser absorption spectroscopy using Tikhonov regularization.

The application of tunable diode laser absorption spectroscopy (TDLAS) to flames with nonhomogeneous temperature and concentration fields is an area where only few studies exist. Experimental work explores the performance of tomographic reconstructions of species concentration and temperature profiles from wavelength-modulated TDLAS measurements within the plume of an axisymmetric McKenna burner. Water vapor transitions at 1391.67 and 1442.67nm are probed using calibration-free wavelength modulation spectroscopy with second harmonic detection (WMS-2f). A single collimated laser beam is swept parallel to the burner surface, where scans yield pairs of line-of-sight (LOS) data at multiple radial locations. Radial profiles of absorption data are reconstructed using Tikhonov regularized Abel inversion, which suppresses the amplification of experimental noise that is typically observed for reconstructions with high spatial resolution. Based on spectral data reconstructions, temperatures and mole fractions are calculated point-by-point. Here, a least-squares approach addresses difficulties due to modulation depths that cannot be universally optimized due to a nonuniform domain. Experimental results show successful reconstructions of temperature and mole fraction profiles based on two-transition, nonoptimally modulated WMS-2f and Tikhonov regularized Abel inversion, and thus validate the technique as a viable diagnostic tool for flame measurements.