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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]

• 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]

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

Detection of dimethyl sulfide (CH 3 SCH 3 ) is significant for protection of environmental atmosphere and safety of industrial and agricultural production. A tunable laser absorption spectroscopy (TLAS)-based dimethyl sulfide sensor, employing a 5 m hollow waveguide (HWG) as a gas cell and a 3.37 μm, continuous-wave, DFB, room temperature interband cascade laser (ICL), is reported. Wavelength modulation spectroscopy second harmonic normalized by the first harmonic (WMS 2f/1f) technique is used to retrieve the concentration of dimethyl sulfide with high sensitivity. The sensor has a mass of ∼5 kg and consumes ∼50 W of electrical power. The sensor performance evaluation shows that it has a sensitivity of 20 ppb with time resolution of 1s, a rapid rise time and fall time of ∼27 s and ∼25 s and a measurement precision of 21 ppb. Furthermore, an Allan deviation analysis shows that the detection limit can reach 2.8 ppb for a 125 s averaging time. Overall, the reported sensor can be applied to environmental monitoring, industrial production and the biomedicine field for dimethyl sulfide detection. [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]

Selective catalytic reduction (SCR) is an effective technique to reduce nitrogen oxide (NO X ) emission in high-temperature combustion processes. SCR requires strict reaction conditions, including proper temperature and ammonia (NH 3 ) injection volume. To satisfy SCR conditions, we present a low-cost, in situ optical sensor for multiparameter measurement during SCR processes. The sensor simultaneously detected NH 3 concentration, water (H 2 O) concentration, temperature, and pressure by using a single near-infrared diode laser. To ensure the high-quality multiparameter measurement, we realized two novel ideas: (1) wavelength modulation spectroscopy (WMS) with varied modulation amplitude and (2) an optimized multispectral fitting algorithm. We demonstrated the method by theory analysis, model simulation, laboratory verification, and field-testing with a diesel engine. The relative measurement uncertainty of NH 3 concentration, H 2 O concentration, temperature, and pressure were 8.4%, 8.0%, 5.3% and 7.0%, respectively. The results of the field test were consistent with those obtained by thermocouples and electrochemical sensors. The presented sensor is quite suitable for SCR processes and other harsh-environment applications because of its low cost and robustness. The proposed multispectral detection method also can be used for other WMS applications. [ABSTRACT FROM AUTHOR]

• Generation of high-quality OFCs via direct Multi-frequency modulation for a semiconductor laser. • The design process of MFM signal is detailed to generate OFCs with expected number of comb lines. • OFCs composed of 9, 15, and 27comb lines are realized with flatness of 0.58 dB, 1.24 dB, and 2.53 dB. • The direct MFM method is cost-effective and easy to implement with no loss of laser power. Direct modulation of a semiconductor laser is a simple and low-cost method to generate compact optical frequency combs (OFCs). Herein, the use of direct injection current multi-frequency modulation (MFM) on a distributed feedback (DFB) semiconductor laser is proposed to generate OFCs. The number of modulation signals, frequency components, and modulation indices of the MFM signal are calculated according to the desired number of comb lines of the OFCs. The feasibility of this method was demonstrated by a series of experiments. OFCs composed of 9, 15, and 27 comb lines were generated with flatness values of 0.58 dB, 1.24 dB, and 2.53 dB, respectively, using a 1.58-μm DFB laser. The proposed method is easy to implement, it offers high flexibility in the selection of the repetition frequency. The proposed method is inexpensive and can improve the OFC quality with no laser-power loss. [ABSTRACT FROM AUTHOR]

• We investigated the dependence of compact optical frequency comb generated by direct modulation of a laser on the modulation waveforms and different working modes. • The limitation of relaxation oscillation frequency can be avoided by low-frequency pulse modulation with a narrow pulse in gain-switching mode, and have a better performance than that in continuous wave mode. • The OFC performance become better with increase of modulation frequency in gain-switching mode, and the spectral linewidth of OFC is very closed to the laser linewidth in constant operation. • The performance of OFCs is optimized by changing modulation signal waveform and parameters without changing the linewidth and optical power of OFCs, and this method has the advantages of simple operation, flexible settings, and low cost. Optical frequency combs (OFCs) generated by direct injection modulation of semiconductor lasers are attractive for their compactness, simplicity, and low cost. We investigated the dependence of the performance of this type of OFC on the modulation waveform and mode on a distributed feedback laser diode (DFB-LD). OFCs can be achieved with various modulation waveforms (sinusoidal, triangular, sawtooth, and sinc) in continuous wave (CW) mode without the limitation of modulation frequency higher than the relaxation oscillation frequency (ROF). In gain-switching (GS) mode, however, this limitation can be avoided by narrow-pulse modulation with frequency lower than the ROF for high spectral resolution, and have a better performance than that in CW mode. We experimentally demonstrated that OFCs generated by pulse modulation have the best performances over various modulation waveforms mentioned above. Specifically, the OFC parameters improvements of carrier-to-noise ratio (CNR) and spectral width by pulse modulation over common used sinusoidal modulation are 37% and 30% respectively. Further, the OFC performance becomes better with increasing modulation frequency in GS mode. We also evaluated the spectral linewidth of OFC being very closed to that of constant operation, which is the absolute highest limit of spectral resolution of an OFC whether in CW or GS mode. Thus, the requirement for high OFC performance of spectral resolution, CNR, and spectral width could be a simple compromising of modulation waveform, frequency, and working mode on a semiconductor diode laser with our investigation. Optimizing of OFCs performance is achieved by changing the waveform or parameters of the modulating signal during gain-switched laser. This method not only reduces the system cost and complexity, but also improves the flexibility of the system. [ABSTRACT FROM AUTHOR]