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A standoff methane (CH4) sensor with actual hard topographic targets (usually called non-cooperative targets) is essential for natural gas pipeline leakage inspection and many other practical applications. To address this requirement, a miniaturized and low-power-consumption gas sensor was developed based on tunable diode laser absorption spectroscopy for standoff CH4 detection with a non-cooperative target. Wavelength modulation spectroscopy with a 1 f normalized 2 f detection method was employed for calibration-free CH4 measurement. A Kalman filter algorithm was used to improve the precision of the detection. The performance of the standoff CH4 sensor was evaluated comprehensively under various conditions, including different incident angles, different hard topographic targets, and different standoff distances. The results show that the measurement precision is 0.107% and the sensitivity is 4.08 parts per million per meter (ppm·m) with a time resolution of 1 s and a standoff distance of 40 m. The detection limit can achieve 1.24 ppm·m at an optimal integration time of 70 s. This sensor can be easily integrated into mobile platforms, which lays the foundation for intelligent leak inspection. Graphical abstract This is a visual representation of the abstract. [ABSTRACT FROM AUTHOR]

The wavelength modulation‐active laser heterodyne spectroscopy (WM‐ALHS), which introduces wavelength modulation technique into active laser heterodyne spectroscopy, is reported to enhance the gas standoff detection capability of weak return optical power. The ALHS gas standoff detection system is developed using a 1.65 μm butterfly packaged distributed feedback laser with tail fiber and an all‐fiber structure, which successfully detects methane concentrations at a distance of 35 cm using an aluminum plate as the reflective surface. Allan variance analysis shows that the system has good stability, and the detection limit at 29.2 s integration time is 0.84 ppm m. Standoff detection experiments with direct absorption‐active laser heterodyne spectroscopy (DA‐ALHS) and wavelength modulation spectroscopy (WMS) are carried out on the basis of the WM‐ALHS system and the detection limits of 4.49 and 1.64 ppm m were achieved within integration times of 18.6 and 29 s for DA‐ALHS and WMS, respectively. The superiority of the WM‐ALHS method compared to these two spectral detection methods is demonstrated. Furthermore, the feasibility of applying WM‐ALHS to gas standoff detection has been verified, laying the foundation for the development of this spectroscopic technology. [ABSTRACT FROM AUTHOR]

Simultaneous measurement of ammonia and ethylene is of great significance for atmospheric environmental monitoring, industrial process controlling and medical respiratory diagnosing. A wavelength modulation spectroscopy based on a 9.56 μm quantum cascade laser (QCL) and a hollow waveguide with a length of 5 m is reported for simultaneous gas sensing of ammonia (NH3) and ethylene (C2H4). The strong absorption lines of NH3 and C2H4 located at 1046.4 and 1045.97 cm−1, respectively, are selected, which can be covered by current tuning of the QCL. Based on the measured frequency modulation characteristics of the QCL, the modulation amplitude is optimized. The sensor performance evaluation shows that it has a sensitivity of 63.2 ppb and 0.775 ppm for NH3 and C2H4 respectively, with time resolution of 1 s. Furthermore, Allan variance analysis shows that the detection limit of NH3 and C2H4 can reach 7 ppb and 45 ppb, respectively, for about 100 s averaging time. The system is stable and robust, thus could be used in real‐time monitoring of atmospheric environment and extended to the field of exhaled breath analysis in the future. [ABSTRACT FROM AUTHOR]

• Simultaneous measurement of CO and H 2 S for fault detection of SF 6 insulated equipment using TDLAS. • Dynamic response, accuracy, sensitivity, and detection limit are evaluated in the laboratory. • Offering a solution for online fault detection of SF 6 insulation equipment. To address the issues of high detection limits, long response times, and susceptibility to cross-interference in the detection of characteristic gases CO and H 2 S produced by SF 6 electrical equipment failures, a dual-species laser sensor prototype for SF 6 decomposition products in electrical equipment was developed based on tunable diode laser absorption spectroscopy (TDLAS). The absorption lines of 2334 nm and 1576 nm were selected for CO and H 2 S, respectively. The concentration inversion was performed using wavelength modulation spectroscopy (WMS) with first harmonic normalization of the second harmonic. The experimental results demonstrate that a rapid response time of ∼30 s is achieved and the sensor has detection accuracies of 0.59 % for CO and 2.09 % for H 2 S, with minimum detection limits of 0.46 ppb and 14.00 ppb, respectively. The dual-species laser sensor allows for convenient, rapid, and precise detection of CO and H 2 S, showing promising application prospects in electrical environment monitoring. [ABSTRACT FROM AUTHOR]

A sensitive carbon disulfide (CS 2 ) sensor based on wavelength modulation spectroscopy (WMS) in the mid-infrared fingerprint region was reported. The strong band spectrum of carbon disulfide's combination band ( ν 1 + ν 3), near 2177.6 cm −1 , was used in the sensor. To enable WMS for wideband spectrum, a parameter of Spectral Discrimination ( SD ) was defined and used as a criterion for optimizing the modulation index, which was finally determined to be around 0.8 instead of 2.2 commonly used for an isolated line spectrum. And the second harmonic signal was processed with empirical mode decomposition (EMD) algorithm to distinguish and remove the optical fringes buried in the detected signal. The sensor performances were tested by implementing a set of CS 2 concentration experiments. The results showed that the sensor response linearity was 0.999 and the relative measuring error was less than 1%. The sensitivity, i. e. 3σ detection limit, was as low as 60 ppb with integration time of 240 s with optical path length of 5 m, corresponding to absorbance of 3 × 10 −5 . Overall, benefiting from strong fingerprint absorption and methods of applying WMS to wideband spectrum, the sensor is promising for both environmental and industrial CS 2 detection. [ABSTRACT FROM AUTHOR]

Purpose This paper aims to provide a sensor for fast, sensitive and selective ethylene (C2H4) concentration measurements.Design/methodology/approach The paper developed a sensor platform based on tunable laser absorption spectroscopy with a 3,266-nm interband cascade laser (ICL) as an optical source and a hollow waveguide (HWG) as a gas cell. The ICL wavelength was scanned across a C2H4 strong fundamental absorption band, and an interference-free C2H4 absorption line located at 3,060.76 cm−1 was selected. Wavelength modulation spectroscopy with the second harmonic detection (WMS-2f) technique was used to improve the sensitivity. Furthermore, the HWG gas cell can achieve a long optical path in a very small volume to improve the time response.Findings The results show excellent linearity of the measured 2f signal and the C2H4 concentration with a correlation coefficient of 0.9997. Also, the response time is as short as about 10 s. The Allan variance analysis indicates that the detection limit can achieve 53 ppb with an integration time of 24 s.Practical implications The ethylene sensor has many meaningful applications in environmental monitoring, industrial production, national security and the biomedicine field.Originality/value The paper provides a novel sensor architecture which can be a versatile sensor platform for fast and sensitive trace-gas detection in the mid-infrared region. [ABSTRACT FROM AUTHOR]

Standoff detection of flammable and toxic gases simultaneously is important for leakage warning and safe production in oil and gas industries. A portable and robust single-ended laser sensor is developed to retrieve path-averaged gas concentrations of methane (CH 4) and hydrogen sulfide (H 2 S) by collecting the backscattered light from a distant non-cooperative target. Time-division multiplexing of two semiconductor lasers with pigtails are employed to cover the absorption lines of CH 4 and H 2 S at 1653.73 nm and 1578.13 nm, respectively. Wavelength modulation spectroscopy with 1 f normalized 2 f detection (WMS-2 f /1 f) method is used for gas measurement to eliminate the change of return light intensity caused by different distances and different non cooperative targets. Kalman filter is developed as a post-processing method to reduce the output fluctuations during standoff gas measurement. The performance of the sensor is assessed, including signal noise ratio under different standoff distances and hard targets, measurement precision, time-response and the detection limit. The reported sensor allows for real-time non-contact standoff detection of CH 4 and H 2 S, with great potential for its application in oil and gas industries. [Display omitted] • Simultaneous standoff gas detection for CH 4 and H 2 S using WMS-2f/1f method. • Adaptive Kalman filter reduces the standard deviation to 30–75% with a response time of 3 s. • SNRs are evaluated under different standoff distances and common topographic hard targets. • It has great potential for multi-species remote sensing applications in the oil and gas industries. [ABSTRACT FROM AUTHOR]

The frequency modulation (FM) efficiency and frequency modulation/intensity modulation (FM/IM) phase shift of mid-infrared interband cascade lasers (ICLs) are studied experimentally. The modulation parameters of 2997 and 3266 nm ICLs are characterized using tunable laser absorption spectroscopy (TLAS) with HO absorption lines located at 2998.8 and 3263.3 nm, respectively. The FM efficiency is determined by the distance between two zero crossings of the measured wavelength modulation spectrum with the second-harmonic (WMS-2 f) detection signal, whereas the FM/IM phase shift is extracted by measuring the time delay between the laser intensity and frequency response, using the HO absorption lines as markers. The results show that the FM efficiency is more than four times larger than that of conventional near-infrared distributed feedback lasers and that it decreases monotonically with increasing modulation frequency. The response of the FM/IM phase shift shows three distinct regions in its response to the increasing modulation frequency. The FM characteristics of ICLs are different from those of both conventional diode lasers and quantum cascade lasers because of the different semiconducting materials and working principles involved. This study can help to optimize wavelength modulation spectroscopy (WMS)-based sensor performance and improve simulation models for WMS. [ABSTRACT FROM AUTHOR]

• An in-situ, portable and robust laser sensor system was developed for measurement of NH 3 , H 2 O and T in the deNOx process. • A calibration-free WMS-VMA was proposed to ensure the high-performance of the tri-spectral line analysis. • An algorithm was presented for simultaneous, rapid and accurate inversion of multiple parameters. • The sensor system has great potential of ammonia injection optimization in deNOx process of CFPPs. Simultaneous measurement of the ammonia, flue gas temperature and water vapor concentration in denitrification (deNOx) processes is of great importance for improving the deNOx efficiency of coal-fired power plants (CFPPs) and maximizing energy conservation and emission reduction. A portable, robust, low-cost and low-power laser sensor was developed based on wavelength modulation spectroscopy (WMS) with a single near infrared diode laser, which covers the optimized NH 3 spectral line at 6612.73 cm−1 and H 2 O line pair at 6612.02 and 6609.85 cm−1. A calibration-free for WMS with varied modulation amplitude (WMS-VMA) was proposed to improve the performance of the tri-spectral line analysis. A probe, based on Herriott cell structure, coupled with deformation resistance, dust-proof, air-tight and anti-seismic, was design to adapt to the harsh environment of CFPPs. Lab-based sensor evaluation showed that the measurement accuracies for the temperature, H 2 O and NH 3 concentration are 2.9 %, 4.8 % and 5.1 %, respectively. Allan variance analysis shows that the sensor has good stability, and the measurement sensitivity at five-second resolution are 0.1 ppm and 0.03 % for NH 3 and H 2 O concentration, respectively. Furthermore, the feasibility of the sensor system was also verified by field tests at both deNOx inlet and outlet in a practical CFPP, denoting great potential of ammonia injection optimization in deNOx process. [ABSTRACT FROM AUTHOR]

• Evaluation of gas flow regimes on spectral line broadening in HWGs. • Equivalent pressure to determine spectral line broadening of flowing gas in HWG. • Improvements of sensitivity, selectivity, and immune to flow rate fluctuation. • Optimized scheme for high-precision HWG gas sensor with no extra cost of hardware or software. Hollow waveguide (HWG) based gas sensors boast fast response time, high-sensitivity, compactness, but less-precision for their intrinsic very small bore-diameter and sampling volume. We investigated the effects of gas flow regimes on the complicated spectral line broadening in HWGs, and analyzed quantitatively by taking atmospheric water vapor spectrum as an example. We derived an equivalent pressure to simplify the calculation of spectral line broadening in nonuniform pressure of HWGs, and proposed to enhance the performance of such kinds of sensors with wavelength modulation spectroscopy by laminar flow regime, pump down mode, and the optimized flow rate. Experimental results showed dramatic improvements with the scheme: sensitivity increase by 18.6%, lower uncertainty, and immune to flow fluctuation. The method is efficient, convenient and with no extra cost. It can promote both HWG and substrate-integrated HWG (iHWG) based gas sensors. [ABSTRACT FROM AUTHOR]

Acetonitrile (CH 3 CN), a toxic volatile organic compound (VOC), has been widely employed as an important tracer for research of atmospheric transport processes and climate changes. In order to continuously monitor or remote sense ultra-low concentration of CH 3 CN in the ambient air, we developed a mid-infrared laser absorption sensor for simultaneous detection of CH 3 CN and ambient H 2 O and CH 4 at the atmospheric window of 3.3 µm. The sensor operated at the fundamental absorption v 5 -band of CH 3 CN using wavelength modulation spectroscopy with a distributed feedback interband cascade laser. A multispectrum nonlinear fitting algorithm was developed to deconvolute CH 3 CN absorption with atmospheric absorption of H 2 O and CH 4. The sensor achieved not only high sensitivity and selectivity detection of CH 3 CN but also detection of H 2 O and CH 4 as well. The detection limits for CH 3 CN is 160 ppt, correspond to absorbance as low as 2.3 × 10−7, which is 3 orders of magnitude lower than atmospheric background absorption (10−4 level). The size and weight of the sensor are as low as 32 × 24 × 20 cm and 2.6 kg, respectively. The sensor has the advantages of low measurement uncertainties of less than 1.8 % and good response linearity of 0.999. The technique is expected to be practically feasible for detection of many other VOCs applications. [Display omitted] • Simultaneous trace detection of atmospheric acetonitrile, water, and methane. • Broadband fundamental absorption sensing by an interband cascade laser. • Acetonitrile detection limit 160 ppt corresponding absorbance as low as 2.3 × 10−7. • Highly sensitive and selective sensing below atmospheric absorption background. [ABSTRACT FROM AUTHOR]