Abstract
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.
Highlights
Standoff detection of chemical species, where both the personnel and the detection system are at some distance from the object being measured to realize chemical detection, is a highly sought-after capability for a wide range of applications [1]
As reviewed in Reference [30], there are other similar methods for calibration-free measurement, including the recovery of the absorption profile based on high order harmonic signals and direct absorption spectroscopy (DAS) calibrated wavelength modulation spectroscopy (WMS), which can be employed to eliminate the effects of light intensity variation
Laser absorption spectroscopy (LAS)-based standoff detection technology has been a powerful tool for remote chemical analysis with applications spanning from environmental monitoring, through industrial emission monitoring and process control, to leak detection for health and safety, as well as defense and security
Summary
Standoff detection of chemical species (e.g., some atmospheric constituents, explosive, toxic, or hazardous gases), where both the personnel and the detection system are at some distance from the object being measured to realize chemical detection, is a highly sought-after capability for a wide range of applications [1]. Compared with conventional absorption spectroscopy using broadband incoherent radiation sources, LAS-based chemical sensing offers a highly desirable combination of high-sensitivity and high-speed detection, and the collimated laser source with high brightness allows beam propagation over large distances It can be configured with a cooperative target (e.g., corner-cube, retroreflector) for open-path detection, but more importantly can be used for true standoff sensing with non-cooperative targets (e.g., diffusely scattering topographic) for more flexible and diverse applications. 2020, 12, 2771 solution of standoff LAS systems is to arrange a laser transmitter and receiver face to face or both at the same end and with a retroreflector at the distal end The latter is more common in remote detection applications because of its more practical characteristics. In view of the urging demands in both the use and development of standoff chemical detection technology, a goal of this manuscript is to provide a comprehensive understanding and an overview of the related laser spectroscopic techniques and a reference for future research and development in this field
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