https://doi.org/10.3390/chemosensors11010030
“Laser spectroscopy-based sensors are based upon the direct and indirect detection of laser light interacting with a target object, which inherently allows for trace gas non-invasive measurements with high precision and high accuracy, as well as fast response. To date, a variety of spectroscopic techniques, from absorbance, reflectance, transmission and scattering, to established methods such as direct absorption spectroscopy (DAS), wavelength modulation spectroscopy (WMS) or calibration-free WMS [1,2], photoacoustic spectroscopy (PAS), multi-pass cell-based laser absorption spectroscopy, cavity-enhanced absorption spectroscopy (CEAS) or integral cavity output spectroscopy (ICOS) or cavity ring-down spectroscopy (CRDS), and Raman spectroscopy have been widely used for most atmospheric molecule (including isotope) detection [3], and the sensitivity achieved by these spectroscopy methods can be comparable to that of traditional mass spectrometry [4]. Therefore, laser spectroscopy-based detection techniques and sensors have been attractive and powerful analysis tools for environmental sensing, defense and public security, biomedical, industrial and agricultural applications, etc. Among various laser light sources, quantum cascade lasers (QCLs), initially demonstrated at Bell Labs in 1994, are ideal light sources for spectroscopic applications, especially for external-cavity quantum cascade lasers (ECQCL), which cover almost the entire MIR spectral region (between 2.5 µm and 25 µm), and provide broadly spectral tuning intervals up to several hundred cm−1 by a single ECQCL module; thus, ECQCL-based spectroscopy sensors can be used for simultaneous detection of multiple light gas molecules with narrow-band absorption features or heavy molecules with broadband spectral signatures [5].”