Laser Absorption Spectra Research Articles

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.

A fast closed-form approximate iterative fitting algorithm based on laser absorption spectrum

This paper presents a novel approach—an efficient closed-form approximation iterative fitting algorithm based on laser absorption spectra. Through this closed-form approximation iterative fitting, key parameters such as peak value, spectral line width, and normalized signal area serve as indicators for iteration completion, improving the speed without compromising accuracy. Furthermore, it employs the spectral signal of n cycles as a window for further processing, minimizing external interference. The results show that the proposed method averages 9.75 iterations, while the Levenberg–Marquardt fitting method averages 60.17 iterations. The average iteration time for the proposed method is 588.83 ms, a substantial 81.7% reduction compared to the 3210.5 ms required by the Levenberg–Marquardt fitting. These results decisively demonstrate the efficacy of the proposed method in reducing iteration time and enhancing measurement precision.

Mid-infrared Laser Spectroscopy of Jet-Cooled Formic Acid Trimer: Mode-Dependent Line Broadening in the C-O Stretching Region.

Building on recent progress in the vibrational spectroscopy of the formic acid trimer, we present the first high-resolution measurements of the jet-cooled laser absorption spectrum of (HCOOH)3. The spectra of the lowest- and highest-frequency C-O stretching fundamentals are analyzed whereas the third band is not observed, complicated by monomer and dimer absorptions at 1219 cm-1 (8.2 μm). Vibration-rotation parameters are obtained for the band at 1172.31512(68) cm-1 whereas the C-O stretch at 1246.33(5) cm-1 exhibits a significantly larger breadth, allowing only resolution of the coarse PQR structure. Vibrational predissociation can be ruled out, and intramolecular vibrational redistribution mechanisms are discussed, particularly coupling to the concerted proton exchange within the cyclic dimer subunit. Ultimately, the question remains open. The prospects of high-resolution measurements of other trimer bands or isotope substitution experiments, which might assist in revealing the mode-specificity of the underlying broadening mechanisms, are discussed.

Theoretical and experimental study of inversion of spectral absorption function using experimental data of laser absorption spectrum with slow scanning and fast modulation of wavelength

Based on the tested data of laser absorption spectra, a matrix slicing method is proposed to invert the absorption function of spectral lines by using the two parameters of laser modulation frequency and laser scanning range as well as transmitted wave signal and reference wave signal under the condition of slow uniform scanning wavelength and fast periodic modulation wavelength. When the modulation is single frequency sinusoidal modulation, an accurate contour of the spectral line absorption function can be obtained by using the matrix data consisting of the values of the transmitted wave signal by the reference wave signal through the minimum value of two slice integrals with the interval of half modulation period, and the amplitude of modulation can be estimated. When the fast modulation of the wavelength is distorted to the multi-frequency superposition modulation, the absorption function is also formed by using the complementarity of multiple slices. The method above is utilized for investigating a real absorption function inversion process involving multiple overlapping absorption lines in the range of the scanning wavelengths. Moreover, the scanning range of laser wavelength can be calibrated by the interval parameters of several spectral lines in the scanning wavelength range. The absorption function of CO at 4300.700 cm<sup>–1</sup> and CO<sub>2</sub> at 6336 cm<sup>–1</sup> are successfully obtained by using this matrix slice method for experimental verification.

Time-to-frequency conversion method for tunable diode laser absorption spectrum

The dynamic wavelength tuning characteristics of the laser is one of the most important factors affecting the measurement accuracy of Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology. We propose a time-frequency conversion method to obtain accurate measurement spectrum, which uses experimentally measured direct absorption signal (time domain) of standard methane gases to compare with simulated spectrum (frequency domain) from the HITRAN database. Then, a polynomial model is used to establish the relationship between time domain (sample points) and frequency domain (wavenumber). To verify the feasibility and accuracy of the method, the methane absorption signals of three standard concentrations (5%, 10% and 15%) were time-to-frequency converted by our method and etalon, and the methane concentrations were calculated and compared. Comparing methane concentration results, the maximum relative error between our method and etalon is less than 0.5%. The results showed that it is viable to use this method to achieve dynamic wavelength measurement in fast scanning of distributed feedback lasers (DFB), and the restricted polynomial model can properly describe the dynamic wavelength tuning characteristics of lasers. Furthermore, the method has the advantages of high accuracy and low cost, which is important for improving the accuracy of TDLAS technology applications.

Hyperspectral absorption of water around 2 μm based on a boradband tunable, narrow linewidth Tm-doped fiber laser

The 1.8–2.0 μm waveband contains abundant absorption lines of water, which are much stronger than those in the traditional 1.3–1.5 μm waveband, exhibiting huge potentials for absorption spectrum applications of water. In the hyperspectral absorption spectrum, physical parameters of the target molecule can be derived from lots of absorption lines within a broadband scanning range, achieving the results more robust, accurate and versatile than the results from the conventional tunable diode laser absorption spectrum in which only one or two absorption lines are used. The key to hyperspectral absorption is the development of broadband tunable, narrow linewidth laser sources emitting in the wavelength range of interest. With a tunable fiber FP filter and a fiber saturable absorber, a Tm-doped fiber laser is established, featuring broadband tenability and narrow linewidth. Taking advantage of the re-absorption characteristics of Tm-doped silica fibers, a wavelength tuning range covering 60 nm from 1910–1970 nm is obtained through the appropriately designing of the active fiber length. The measured laser linewidth at steady state is smaller than 0.1 nm, which is suitable for water absorption spectrum. Hyperspectral absorption measurements of water in air and alcohol flame are conducted. In room-temperature air, more than 40 absorption lines are recognized within a tuning range of 1910–1965 nm, while, in alcohol flame, the number of detected lines reaches about 50. Comparison with the HITRAN2016 database gives a laser linewidth of about 0.06 nm which is very close to the static linewidth measured by an OSA. The temperature of the air is derived to be 298 K with a water mole fraction of about 2%, which is consistent with the measurement of the hygrothermograph. And the calculation indicates an alcohol flame temperature of about 1220 K, which is very close to the measurement result of the thermocouple.

TDLAS Tomography System for Online Imaging and Dynamic Process Playback of Temperature and Gas Mole Fraction

Tunable diode laser absorption spectroscopy (TDLAS) tomography enables online diagnosis of dynamic flames by imaging its temperature variations and exhaust gases, owing to its rapid response. Each tomographic image requires huge laser absorption spectrum data along multiple laser paths. For a high temporal resolution, the data sampling rate needs to be as high as possible which challenges the hardware implementation of parallel data acquisition of multiple channels. An online TDLAS tomography system was designed on field programmable gate array (FPGA) and PC for temperature and gas mole fraction distributions measurement. An on-chip parameter extraction module based on digital lock-in amplifier (DLIA) extracts absorption spectrum parameters at 1000 frames per second (FPS), and the framerate of real-time reconstruction on host PC can reach up to 302 FPS. A high-speed buffer on FPGA termed as transient spectrum recorder continuously records the latest five seconds of raw data. Both the playback of absorption spectrum parameters and raw data provide a way for detailed and in-depth understanding of the dynamic combustion process. Experiments were carried out by using an alcohol burner to evaluate the online monitoring and offline playback abilities of the system.

New line positions and effective line intensities of the ν2 band cis-HONO near 1661 cm−1 from quantum cascade laser absorption spectroscopy

High-resolution quantum cascade laser absorption spectrum of cis-HONO of the v2 band has been recorded and analyzed, for the first time, in the mid-infrared region of 1659.2–1662.2 cm−1 (R-branch of the v2 band, N=O stretch). Positions and effective line intensities were determined for about 60 new lines. The absolute absorption frequencies were directly measured during spectral scan using a wavelength meter with an accuracy of ∼0.0017 cm−1. The corresponding simulation spectrum was calculated using the PGOPHER code. The calculated line positions are in a good agreement with the experimentally measured frequencies within the wavelength meter uncertainty. Effective line intensities were determined, with uncertainties of about 17%, by scaling the measured HONO absorption intensities to the previously reported line intensity of a cis-HONO line located in the same spectral region near 1659.85 cm−1.

Noise Immune TDLAS Temperature Measurement Through Spectrum Shifting by Using a Mach–Zehnder Interferometer

A noise immune temperature measurement method is proposed by using tunable diode laser absorption spectroscopy (TDLAS). Noise immunity is achieved through spectrum shifting by a Mach–Zehnder interferometer. The driving current of a distributed feedback (DFB) laser is tuned for linear wavenumber scanning and followed by a saw-tooth climbing laser signal. The laser signal travels through the interferometer, and a high-frequency carrier signal of megahertz level modulates the saw-tooth signal away from background noises. When the output laser of the interferometer passes through the target gas, the absorption spectrum is imposed on the intensity profile. In this way, the absorption spectrum is shifted to a high-frequency spectral band from a low-frequency band where both the absorption spectrum and the background noises appear. A part of the output laser of the interferometer is split to provide a reference for baseline extraction and hence absorption spectrum extraction. For the dual-color temperature extraction method, laser absorption spectra of the target gas centered at two different wavenumbers are achieved. Numerical simulations and actual experiments at different temperatures, e.g., using a heating device and Bunsen burner verified the noise immunity of the proposed method. Performance comparisons with the direct absorption spectroscopy (DAS) showed that the proposed method generated more precise temperature values in a noisy background.

Research on method for detection of harmful gases in livestock based on open path laser absorption spectrum

The breath and feces of livestock and poultry will produce harmful gases such as ammonia, hydrogen sulfide and carbon dioxide. When the concentration of these gases exceeds the standard, they will seriously harm the health of livestock and poultry and even keepers. Fast and accurate monitoring of these harmful gases is of great significance in improving the health status of animals and keepers. Laser spectroscopy technology is an effective means of gas detection. Because of its high sensitivity and selectivity, it is widely used in gas monitoring. However, the currently adopted method of pumping sample gas detection based on the long optical path gas absorption cell is susceptible to the influence of the high ash and high humidity environment of the livestock and poultry house, causing detection errors. Open path laser absorption spectroscopy technology does not require a gas absorption cell to detect the gas concentration in free space. Based on this principle, this paper develops out the research on the open path laser absorption spectroscopy detection method for harmful gases in livestock and poultry houses, focusing on the design of laser collimation system, laser reflection system, laser focusing system, and laser detection system. The ammonia gas was used as the detection object to verify the feasibility of detecting harmful gas in livestock and poultry houses with open path laser absorption spectroscopy.

Research on Laser Absorption Spectrum Detection Technology for CO2 and NH3 Regional Emission

High-sensitivity emission monitoring of CO2 and NH3 in farmland region is very important for analyzing environment and climate change. The on-line detection technology was studied based on open-path tunable diode laser absorption spectroscopy technology (OP-TDLAS). The detection spectrum of open path was extracted to realize accurate concentration inversion by designing the multi-line fitting algorithm. The system detection stability was evaluated by Allan variance that the detection limit of NH3 and CO2 were about 0.048 L/L and 4.31 L/L respectively. The detection experiment was carried out in farmland of north Anhui to prove that both of the fertilization and straw returning to the field were the emission source of CO2 and NH3 from soil. The growth effect of the former was slower but higher than that of fertilization. This stable spectrum detection method can obtain large-scale concentration results and clear emission rules, which provide technical support for the environment-friendly agriculture

Research on method for high sensitive detection of harmful gases in livestock houses based on laser absorption spectrum

Harmful gases such as ammonia and hydrogen sulfide in livestock and poultry houses can seriously damage the health of livestock and poultry as well as animal keepers, so it is great significant to detect these harmful gases rapidly and accurately for the improvement of the welfare of animals and the health of animal keepers. Laser absorption spectroscopy is a gas detection method with the advantages of high sensitivity and selectivity, and is widely used in industrial gas detection. However, it needs further exploring to verify whether laser absorption spectroscopy is useful in detecting low concentration harmful gases in livestock and poultry houses. This paper researches on the method for high-sensitivity detection of harmful gases in livestock and poultry houses based on laser absorption spectroscopy by detecting the absorption signals of ammonia with a self-designed system including a tunable laser wavelength scanning system, a photoelectric detecting system and a long light path gas absorption well, and verifies that laser absorption spectroscopy can be used for detecting harmful gases in livestock and poultry houses.

A Lagrangian interpolation-assisted direct laser absorption spectrum analyzer based on digital signal processor for methane detection

A novel Lagrangian interpolation-based direct laser absorption spectroscopy (LI-DLAS) technique was presented to suppress noise in infrared gas detection by incorporating Lagrangian interpolation and nonlinear least-square fitting (NLLSF). An LI-DLAS analyzer was reported for methane (CH4) detection using a 1654 nm distributed feedback (DFB) laser, a compact digital signal processor (DSP), and a multi-pass gas cell (MPGC) with a 16 m optical path length. The performance of the developed LI-DLAS CH4 analyzer was evaluated by means of laboratory experiments. Compared with the traditional DLAS-based sensor without Lagrangian interpolation, the detection sensitivity was improved from 6 ppmv to 2 ppmv, and the detection stability was enhanced as the Allan–Werle deviation was dropped from 1.514 to 0.531 ppmv for a 1 s averaging time. Compared with a DLAS analyzer based on LabVIEW platform, the DSP-based CH4 analyzer shows the merits of compact size and low cost with potential filed-deployable applications in industrial monitoring and control.

Multi-gas detection in power transformer oil based on tunable diode laser absorption spectrum

Online dissolved gas analysis (DGA) is an effective technique to obtain the health status information of insulation oil in power transformers. The typical hydrocarbon gases (methane, ethyne, ethene, ethane) decomposed from insulation oil, are the key indicators to diagnose the fault type and severity. To realize contactless and field measurement, a multi-gas detection system based on tunable diode laser absorption spectrum (TDLAS) is proposed in this paper. Critical technique problems on the light source, long path gas cell, and the topology for multi-gas detection are investigated. Individual central wavelengths of hydrocarbon gases are set at as 1653.72 nm for methane, 1530.37 nm for ethyne, 1620.04 nm for ethene and 1679.06 nm for ethane in the near infrared band. A 10.13 m long multi-pass gas cell is developed with the advantage of anti-vibration design. To share the optical absorption path, an optical switch is adopted to merge the multi-gas detection system. The high sensitivity of the TDLAS multi-gas detection system was demonstrated in the laboratory calibrations, with ethyne detection reaching sub-parts per million level, and the other hydrocarbon gases achieving ppm level. Furthermore, the comparison detection in the real 220 kV power transformer gave evidence to the effective monitoring of DGA, proving it an alternative approach to online detection of multiple gases in power transformer oil.

Research on SF6 Field Detection Technology Based on Optical Method

SF6 gas is widely used in electrical equipment, but it is inevitable that failure will occur. However, the current diagnostic methods have the disadvantages of slow response speed, high requirements for the detection environment, and detection of exhaust gas pollution to the atmosphere. Based on this, this paper studies the SF6 decomposition product detection technology based on optical method, including SF6 purity detection based on infrared absorption, SO2 detection based on ultraviolet fluorescence and H2S and CO detection technology based on TDLAS (Tunable Diode Laser Absorption Spectrum). Furthermore, the zero emission detection device was developed to perform calibration, temperature and flow compensation, and various gas detection tests. The results showed that the accuracy of SF6 purity detection data was within 0.04%, and the detection accuracy of H2S and SO2 was within 2%. The detection accuracy of CO is within 1%, which complies with relevant testing standards. . The developed equipment has the characteristics of high detection accuracy, short response time, safety and environmental protection.

Two-stage velocity distribution measurement from multiple projections by tunable diode laser absorption spectrum

A novel approach to using tunable diode laser absorption spectrum (TDLAS) is developed for nonuniform velocity distribution measurement by Doppler effect. An analysis of the energy in direct absorption spectrum at low frequencies is made by Fourier transform, because the TDLAS method offers the advantages in using Beer law to deal with coupling relations between velocity distribution and corresponding length of velocity region. By comparing with traditional TDLAS-Doppler velocity measurement, advantages of this approach to the more exact solution of core flow velocity by signal process without using extra lasers and detectors are explored. Following the published theory, between velocity regions at multiple projections the absorbance about average in frequency offsets and the absorbance about difference in frequency offsets are incorporated into an improved fitting model. A solution to obtaining changes of absorbance energy at low frequencies by Fourier transform is used to demonstrate the ability to recover minor change in absorbance under different conditions, inferring a better method to realize the simultaneous measurement of velocity distribution. The influences of these parameters, such as projection angles and noise during absorption, are investigated by the multiple projection simulations at rovibrational transitions of H<sub>2</sub>O near 7185.6 cm<sup>–1</sup> from three projections. This approach is validated in a two-stage velocity distribution model, demonstrating the ability to exactly measure core flow, with a precision of 0.9% RMS (root mean square). The high velocity in the core flow is less influenced by the random noise in absorption due to nearly linear relationship between the difference in frequency offsets and the ratio of length of velocity region. Some satisfied results can be obtained when larger angles of projection are arranged. The combination of 0°, 30°, and 60° will be a reasonable optic design considering the limitation of spatial resolution. In conclusion, the novel approach to velocity distribution measurement based on TDLAS-Doppler from multiple projections has great potential applications in engine diagnosis and gas dynamic research.

TDLAS-Based Detection of Dissolved Methane in Power Transformer Oil and Field Application

Methane is one of the indicative gases in power transformer oil, and the detection of methane dissolved in oil with high accuracy is of great importance for dissolved gases analysis and fault diagnosis inside power transformers. Based on the Beer–Lambert spectral absorption law, dissolved methane detection with tunable diode laser absorption spectrum (TDLAS) is proposed in this paper for the advantages of high sensitivity and resolution. On the basis of wavelength modulation spectroscopy, a specialized TDLAS system was established. To meet the actual needs of field testing, the anti-vibration design of an integrated Herriott cell and a gas pressure (P)/temperature (T) setting are worked out. Photodetector, collimator, and Herriott cell are integrated into one component to reduce the effects of vibration. It is investigated that the temperature has little effect on the second harmonic amplitude in the range of 30 °C ~ 50 °C, and the vacuum pressure is reasonably set at about 1 kPa. Experimental results showed that the resolution of sensitivity could be reached as 6.8 mV/( $\mu$ L/L), the maximum deviation was less than $\pm 4~\mu$ L/L, and the response time is less than 5 min. In the end, field application was also carried out, proving it is a prospective online sensing technique to serve oil-immersed power transformers better.

TDLAS直接吸收法和波长调制法在线测量CO2的比较

According to the statistics, 39.65 billion tons of coal has been consumed up from China in 2015. Obviously, CO2 emission from power generation and thermal supply industry segment contributes a large part to global warming. Therefore, it is of great importance for online monitoring of CO2 emissions from coal-fired power plants. The CO2 concentration was measured by tunable semiconductor laser absorption spectrum(TDLAS), which had the benefits of high accuracy, rapid response, non-contact, etc. In order to prevent interference from absorption of other gases, a distributed feedback laser with a center wavelength of 1 580 nm was chosen as the light source. Simulated flue gas was made referring to the concentration of CO2 in the flue gas of the power plant. The direct absorption(DA) and wavelength modulation(WM) methods were used to measure the CO2 concentration. The results show that the relative standard deviation(RSD) for DA and WM are 0.94% and 0.22%, while the maximum relative errors and detection limits for DA and WM are 2.64% and 1.65%, 0.013 6% and 0.001 4%. It is found that WM is better than DA in measurement qualities. But the WM method is influenced by the calibration method in field application and the line width is interfered by the fluctuations of the parameters such as pressure and temperature, resulting in reduced long-term stability. In the case of the DA method, the concentration can be calculated directly by measuring the absorbance, line strength and pressure without calibration. It has a high signal-to-noise ratio for the measurement of gas with high concentration. Meanwhile, the measurement accuracy is high enough to meet the requirement for CO2 online measurement in power plants. Therefore, the DA method is a better choice for the CO2 measurement in the flue gas of power plant boiler.

Laser absorption spectroscopy for measurement of He metastable atoms of a microhollow cathode plasma

We generated a 0.3-mm-diameter DC, hollow-cathode helium discharge in a gas pressure range of 10–80 kPa. In discharge plasmas, we measured position-dependent laser absorption spectra for helium 23S1–23P0 transition with a spatial resolution of 55 µm. From the results of the analysis of the measured spectra using Voigt functions and including both the Doppler and collision broadening, we produced two-dimensional maps of the metastable 23S1 atomic densities and gas temperatures of the plasmas. We found that, at all pressures, the gas temperatures were approximately uniform in space with values in the range of 400–1500 K and the 23S1 atomic densities were ∼1019 m−3. We also found that the two-dimensional density distribution profiles became ring-shaped at high gas pressures, which is qualitatively consistent with the two-dimensional fluid simulation results.

Tracing Acetylene Dissolved in Transformer Oil by Tunable Diode Laser Absorption Spectrum

Dissolved gas analysis (DGA) is widely used in monitoring and diagnosing of power transformer, since the insulation material in the power transformer decomposes gases under abnormal operation condition. Among the gases, acetylene, as a symbol of low energy spark discharge and high energy electrical faults (arc discharge) of power transformer, is an important monitoring parameter. The current gas detection method used by the online DGA equipment suffers from problems such as cross sensitivity, electromagnetic compatibility and reliability. In this paper, an optical gas detection system based on TDLAS technology is proposed to detect acetylene dissolved in transformer oil. We selected a 1530.370 nm laser in the near infrared wavelength range to correspond to the absorption peak of acetylene, while using the wavelength modulation strategy and Herriott cell to improve the detection precision. Results show that the limit of detection reaches 0.49 ppm. The detection system responds quickly to changes of gas concentration and is easily to maintenance while has no electromagnetic interference, cross-sensitivity, or carrier gas. In addition, a complete detection process of the system takes only 8 minutes, implying a practical prospect of online monitoring technology.