Performance evaluation of a 1.6-µm methane DIAL system from ground, aircraft and UAV platforms

High-average-power single-frequency pulse optical parametric oscillator based on pulse-integrated seed-injection automatic locking

Near-infrared nanosecond (ns) single-longitudinal-mode (SLM) pulse light generated from an optical parametric oscillator (OPO) is an important source in nonlinear optics and high-precision spectral analysis. In this Letter, a stable SLM near-infrared ns pulse light source generated from the OPO is presented, which is achieved by developing a seed-injection automatic locking technique based on a pulse-integrated photodetector (PIPD). Depending on the PIPD, the peak power of the pulse light detected by the photodiode is converted to the average power by integrating several pulses. As a result, the detector saturation is thoroughly eliminated, and the interference signal including the resonance point between seed and pulse lights can easily be attained by scanning the resonator length. On this basis, a microcontroller unit (MCU) is employed to realize automatic locking by looking for the minimum value of the interference signal. Finally, a SLM 824 nm pulse light source with an output power of 20.5 W and a linewidth of 51.42 MHz is obtained. The presented method can pave the way to implement a low-cost and compact high-average-power SLM pulse OPO.

High-average-power single-longitudinal-mode pulse laser based on a pulse saturation seed-injection locking OPO

A high-average-power and narrow-linewidth nanosecond (ns) pulse 824 nm laser is a crucial source for the generation of deep-ultraviolet (DUV) 248 nm laser by means of the sum-frequency process with the 354.5 nm laser. To this purpose, in this Letter, we present a seed-injection-locked high-average-power ns pulse single-longitudinal-mode (SLM) 824 nm laser. By developing a novel, to the best of our knowledge, pulse-saturated seed-injection locking method, disturbance of the pulse laser on the locking of the injected seed laser is successfully eliminated. As a result, the output power of 824 nm laser is up to 21.2 W at the incident pump power of 48.1 W, and the pulse width is 15 ns. Especially, the signal-to-noise ratio of the detected modulated sideband signal exceeds 28 dB, which ensures that the achieved linewidth of the 824 nm laser is as narrow as 38.8 MHz. These results demonstrate the potential of the proposed pulse saturation seed-injection locking OPO cavity for high-power and narrow-linewidth laser applications.

Pore Structure and Adsorption Characteristics of Maceral Groups: Insights from Centrifugal Flotation Experiment of Coals

Coal has various types of macerals, which have different pore structures and adsorption properties that change with coal's thermal metamorphism. In-depth study of the characteristics of different coal macerals, especially the pore structure and adsorption properties, can better predict the coal reservoir gas storage capacity and migration ability. In this study, the sub-samples enriched in a specific maceral group with different coal ranks and particle sizes were obtained by centrifugal flotation experiments. Then, experiments containing low-temperature N₂ isotherm adsorption (LT-N₂GA), low-temperature CO₂ isotherm adsorption (LT-CO₂GA), and methane isothermal adsorption were carried out on the sub-samples to quantitatively analyze the evolution characteristics of pore structure and adsorption properties of different maceral groups. The results showed the following: (1) The separation effect of the light maceral groups by centrifugal flotation experiments increased with the decrease of particle sizes, which were treated with the heavy liquid of low and medium densities, while that of the heavy maceral groups had the relatively best separation effect in the particle sizes of 0.1-0.125 mm, which were treated with the heavy liquid of high densities. (2) The vitrinite-enriched samples had more ultra-micropores (mainly within the diameter range of 0.4-0.65 nm), while the inertinite enriched samples had more mesopores and transition pores (mainly within the diameter range of 40-50 nm). (3) For the low-rank coal, inertinite had more potential methane adsorption capacity. However, for the medium- and high-rank coal, vitrinite had more potential methane adsorption capacity. (4) For the low-rank coal, the adsorption potential and adsorption space increased with the increase of the inertinite content, while the adsorption potential, adsorption space, and surface free energy for the medium- and high-rank coal increased with the increase of vitrinite content. It is expected that the results can deepen the understanding about the gas storage capacity and migration ability and be used in the prevention of gas outburst and the reduction of carbon emission.

Optical Methods of Methane Detection

Methane is the most frequently analyzed gas with different concentrations ranging from single ppm or ppb to 100%. There are a wide range of applications for gas sensors including urban uses, industrial uses, rural measurements, and environment monitoring. The most important applications include the measurement of anthropogenic greenhouse gases in the atmosphere and methane leak detection. In this review, we discuss common optical methods used for detecting methane such as non-dispersive infrared (NIR) technology, direct tunable diode spectroscopy (TDLS), cavity ring-down spectroscopy (CRDS), cavity-enhanced absorption spectroscopy (CEAS), lidar techniques, and laser photoacoustic spectroscopy. We also present our own designs of laser methane analyzers for various applications (DIAL, TDLS, NIR).

A Study of a Miniature TDLAS System Onboard Two Unmanned Aircraft to Independently Quantify Methane Emissions from Oil and Gas Production Assets and Other Industrial Emitters

In recent years, industries such as oil and gas production, waste management, and renewable natural gas/biogas have made a concerted effort to limit and offset anthropogenic sources of methane emissions. However, the state of emissions, what is emitting and at what rate, is highly variable and depends strongly on the micro-scale emissions that have large impacts on the macro-scale aggregates. Bottom-up emissions estimates are better verified using additional independent facility-level measurements, which has led to industry-wide efforts such as the Oil and Gas Methane Partnership (OGMP) push for more accurate measurements. Robust measurement techniques are needed to accurately quantify and mitigate these greenhouse gas emissions. Deployed on both fixed-wing and multi-rotor unmanned aerial vehicles (UAVs), a miniature tunable diode laser absorption spectroscopy (TDLAS) sensor has accurately quantified methane emissions from oil and gas assets all over the world since 2017. To compare bottom-up and top-down measurements, it is essential that both values are accompanied with a defensible estimate of measurement uncertainty. In this study, uncertainty has been determined through controlled release experiments as well as statistically using real field data. Two independent deployment methods for quantifying methane emissions utilizing the in situ TDLAS sensor are introduced: fixed-wing and multi-rotor. The fixed-wing, long-endurance UAV method accurately measured emissions with an absolute percentage difference between emitted and mass flux measurement of less than 16% and an average error of 6%, confirming its suitability for offshore applications. For the quadcopter rotary drone surveys, two flight patterns were performed: perimeter polygons and downwind flux planes. Flying perimeter polygons resulted in an absolute error less than 36% difference and average error of 16.2%, and downwind flux planes less than 32% absolute difference and average difference of 24.8% when flying downwind flux planes. This work demonstrates the applicability of ultra-sensitive miniature spectrometers for industrial methane emission quantification at facility level with many potential applications.

Standoff Chemical Detection Using Laser Absorption Spectroscopy: A Review

Remote chemical detection in the atmosphere or some specific space has always been of great interest in many applications for environmental protection and safety. Laser absorption spectroscopy (LAS) is a highly desirable technology, benefiting from high measurement sensitivity, improved spectral selectivity or resolution, fast response and capability of good spatial resolution, multi-species and standoff detection with a non-cooperative target. Numerous LAS-based standoff detection techniques have seen rapid development recently and are reviewed herein, including differential absorption LiDAR, tunable laser absorption spectroscopy, laser photoacoustic spectroscopy, dual comb spectroscopy, laser heterodyne radiometry and active coherent laser absorption spectroscopy. An update of the current status of these various methods is presented, covering their principles, system compositions, features, developments and applications for standoff chemical detection over the last decade. In addition, a performance comparison together with the challenges and opportunities analysis is presented that describes the broad LAS-based techniques within the framework of remote sensing research and their directions of development for meeting potential practical use.

Performance assessment of a coherent DIAL-Doppler fiber lidar at 1645 nm for remote sensing of methane and wind

We report on the performances of a coherent DIAL/Doppler fiber lidar called VEGA, allowing for simultaneous measurements of methane and wind atmospheric profiles. It features a 10µJ, 200 ns, 20 kHz fiber pulsed laser emitter at 1645 nm, and it has been designed to monitor industrial methane leaks and fugitive emissions in the environment. The system performance has been assessed for range-resolved (RR) and integrated-path (IP) methane measurements in natural background conditions (i.e. ambient methane level). For RR measurements, the measured Allan deviation at τ=10 s is in the range of 3-20 ppm, depending of the aerosol load, at a distance of 150 m, with 30 m range resolution, and a beam focused around 150-200 m. For IP measurements, using a natural target at 2.2 km of distance, the Allan deviation at τ=10 s is in the range of 100-200 ppb. In both cases, deviation curves decrease as τ^-1/2, up to 1000 seconds for the longest averaging time. Finally, the lidar ability to monitor an industrial methane leak is demonstrated during a field test.

Characterization of a single-frequency bismuth-doped fiber power amplifier with a continuous wave and modulated seed source at 1687 nm

In this paper, we report the performance of a bismuth-doped fiber amplifier at 1687 nm. This wavelength region is particularly interesting for laser-based spectroscopy and trace gas detection. The active bismuth-doped fiber is pumped at 1550 nm. With less than 10 mW of the seed power, more than 100 mW is obtained at the amplifier's output. We also investigate the signal at the output when a wavelength-modulated seed source is used, and present wavelength modulation spectroscopy of methane transition near 1687 nm. A significant baseline is observed in the spectra recorded when the fiber amplifier is used. The origin of this unwanted background signal is discussed and methods for its suppression are demonstrated.

Optical Energy Variability Induced by Speckle: The Cases of MERLIN and CHARM-F IPDA Lidar

In the context of the FrenchGerman space lidar mission MERLIN (MEthane Remote LIdar missioN) dedicated to the determination of the atmospheric methane content, an end-to-end mission simulator is being developed. In order to check whether the instrument design meets the performance requirements, simulations have to count all the sources of noise on the measurements like the optical energy variability induced by speckle. Speckle is due to interference as the lidar beam is quasi monochromatic. Speckle contribution to the error budget has to be estimated but also simulated. In this paper, the speckle theory is revisited and applied to MERLIN lidar and also to the DLR (Deutsches Zentrum für Luft und Raumfahrt) demonstrator lidar CHARM-F. Results show: on the signal path, speckle noise depends mainly on the size of the illuminated area on ground; on the solar flux, speckle is fully negligible both because of the pixel size and the optical filter spectral width; on the energy monitoring path a decorrelation mechanism is needed to reduce speckle noise on averaged data. Speckle noises for MERLIN and CHARM-F can be simulated by Gaussian noises with only one random draw by shot separately for energy monitoring and signal paths.

Industrial point source CO2 emission strength estimation with aircraft measurements and dispersion modelling

CO₂ remains the greenhouse gas that contributes most to anthropogenic global warming, and the evaluation of its emissions is of major interest to both research and regulatory purposes. Emission inventories generally provide quite reliable estimates of CO₂ emissions. However, because of intrinsic uncertainties associated with these estimates, it is of great importance to validate emission inventories against independent estimates. This paper describes an integrated approach combining aircraft measurements and a puff dispersion modelling framework by considering a CO₂ industrial point source, located in Biganos, France. CO₂ density measurements were obtained by applying the mass balance method, while CO₂ emission estimates were derived by implementing the CALMET/CALPUFF model chain. For the latter, three meteorological initializations were used: (i) WRF-modelled outputs initialized by ECMWF reanalyses; (ii) WRF-modelled outputs initialized by CFSR reanalyses and (iii) local in situ observations. Governmental inventorial data were used as reference for all applications. The strengths and weaknesses of the different approaches and how they affect emission estimation uncertainty were investigated. The mass balance based on aircraft measurements was quite succesful in capturing the point source emission strength (at worst with a 16% bias), while the accuracy of the dispersion modelling, markedly when using ECMWF initialization through the WRF model, was only slightly lower (estimation with an 18% bias). The analysis will help in highlighting some methodological best practices that can be used as guidelines for future experiments.

An Overview of Small Unmanned Aerial Vehicles for Air Quality Measurements: Present Applications and Future Prospectives

Assessment of air quality has been traditionally conducted by ground based monitoring, and more recently by manned aircrafts and satellites. However, performing fast, comprehensive data collection near pollution sources is not always feasible due to the complexity of sites, moving sources or physical barriers. Small Unmanned Aerial Vehicles (UAVs) equipped with different sensors have been introduced for in-situ air quality monitoring, as they can offer new approaches and research opportunities in air pollution and emission monitoring, as well as for studying atmospheric trends, such as climate change, while ensuring urban and industrial air safety. The aims of this review were to: (1) compile information on the use of UAVs for air quality studies; and (2) assess their benefits and range of applications. An extensive literature review was conducted using three bibliographic databases (Scopus, Web of Knowledge, Google Scholar) and a total of 60 papers was found. This relatively small number of papers implies that the field is still in its early stages of development. We concluded that, while the potential of UAVs for air quality research has been established, several challenges still need to be addressed, including: the flight endurance, payload capacity, sensor dimensions/accuracy, and sensitivity. However, the challenges are not simply technological, in fact, policy and regulations, which differ between countries, represent the greatest challenge to facilitating the wider use of UAVs in atmospheric research.

Towards quantitative atmospheric water vapor profiling with differential absorption lidar

Differential Absorption Lidar (DIAL) is a powerful laser-based technique for trace gas profiling of the atmosphere. However, this technique is still under active development requiring precise and accurate wavelength stabilization, as well as accurate spectroscopic parameters of the specific resonance line and the effective absorption cross-section of the system. In this paper we describe a novel master laser system that extends our previous work for robust stabilization to virtually any number of multiple side-line laser wavelengths for the future probing to greater altitudes. In this paper, we also highlight the significance of laser spectral purity on DIAL accuracy, and illustrate a simple re-arrangement of a system for measuring effective absorption cross-section. We present a calibration technique where the laser light is guided to an absorption cell with 33 m path length, and a quantitative number density measurement is then used to obtain the effective absorption cross-section. The same absorption cell is then used for on-line laser stabilization, while microwave beat-frequencies are used to stabilize any number of off-line lasers. We present preliminary results using ∼300 nJ, 1 μs pulses at 3 kHz, with the seed laser operating as a nanojoule transmitter at 822.922 nm, and a receiver consisting of a photomultiplier tube (PMT) coupled to a 356 mm mirror.

Self-calibration and laser energy monitor validations for a double-pulsed 2-μm CO2 integrated path differential absorption lidar application

Double-pulsed 2-μm integrated path differential absorption (IPDA) lidar is well suited for atmospheric CO₂ remote sensing. The IPDA lidar technique relies on wavelength differentiation between strong and weak absorbing features of the gas normalized to the transmitted energy. In the double-pulse case, each shot of the transmitter produces two successive laser pulses separated by a short interval. Calibration of the transmitted pulse energies is required for accurate CO₂ measurement. Design and calibration of a 2-μm double-pulse laser energy monitor is presented. The design is based on an InGaAs pin quantum detector. A high-speed photoelectromagnetic quantum detector was used for laser-pulse profile verification. Both quantum detectors were calibrated using a reference pyroelectric thermal detector. Calibration included comparing the three detection technologies in the single-pulsed mode, then comparing the quantum detectors in the double-pulsed mode. In addition, a self-calibration feature of the 2-μm IPDA lidar is presented. This feature allows one to monitor the transmitted laser energy, through residual scattering, with a single detection channel. This reduces the CO₂ measurement uncertainty. IPDA lidar ground validation for CO₂ measurement is presented for both calibrated energy monitor and self-calibration options. The calibrated energy monitor resulted in a lower CO₂ measurement bias, while self-calibration resulted in a better CO₂ temporal profiling when compared to the in situ sensor.