Abstract
Broadband-tunable external-cavity quantum cascade lasers (EC-QCLs) are established as attractive light sources for mid-infrared (MIR) 'finger print' molecular spectroscopy, e.g., for detection and identification of chemical compounds. In this chapter we report on two prototypical examples of the use of EC-QCLs for the spectroscopic detection of hazardous substances: standoff detection of explosives and sensing of hazardous chemicals in water. In both application scenarios, a multidisciplinary approach is applied. Our standoff system allows the contactless identification of traces of various common explosives over distances of several meters. Besides laser power and tuning range, sophisticated hyperspectral image analysis is essential for providing high sensitivity at a low false-alarm rate. For this purpose, we also address the question of eye safety. Furthermore, because of the high spectral irradiance, the QCL enables measurement on systems with extremely low light transmission. A QCL-based infrared spectroscopic system allows extension of the optical path length in water by a factor of ten. This feature enables an autonomous measurement mode in a real-world environment using a simple transmission cell for continuous water flow, avoiding the need for manual sampling.
QCLs are unipolar mid- to far-infrared light sources based on electrons making intersubband transitions within the conduction band. In the past few years, QCLs have experienced tremendous progress due to the improvement in the design of both the active regions and waveguide structures. Advances are also reported in the field of fabrication technology as well as in mounting and encapsulating technology. The advances include room-temperature continuous-wave (CW) operation, high output power, high wall-plug efficiency, and wide spectral coverage. Exploiting the benefits of mature, established materials, the emission wavelengths of QCLs can be engineered from 3.3 μm to about 12 μm using InGaAs/AlInAs semiconductor heterostructures, grown by lattice matching or strain compensation on InP substrates. The high versatility and robustness of QCLs makes them ideal radiation sources for real-world applications.
The MIR-wavelength range is of utmost importance for spectroscopic applications, as the fundamental rotational-vibrational transitions of most organic molecules are found in this energy range. Fourier transform infrared spectroscopy (FTIR) has long been a standard method in analytical chemistry for material analysis and identification. For certain types of applications, spectroscopic methods based on laser sources offer superior possibilities in comparison to the use of an FTIR setup. This is especially the case for highresolution spectroscopy in the gas phase, as the linewidth of a CW laser source is much smaller than the resolution achievable even for FTIRs with very long mirror paths. Other aspects include the much higher spectral power density offered by a laser and the possibility of guiding a collimated laser beam over large distances, a feature that is crucial for standoff measurement schemes. Furthermore, efficient coupling of light into optical fibers is possible only when using a laser light source.
In this chapter, we demonstrate the advantages offered by EC-QCLs for field applications using two prototypical examples, both aimed at the detection of hazardous substances. The presented techniques might also prove to be useful in a much broader area of material identification.
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