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Since their first demonstration in 1994, quantum cascade lasers (QCLs) have shown remarkable improvements in terms of technology and overall performance. The first devices were operated in pulsed mode only, at cryogenic temperatures, with a multimode emission and a tiny average output power. In less than one decade, advancements in the design, fabrication, and processing of QCLs have led to important achievements, such as pulsed operation at room temperature, strong reduction in the threshold current and dissipated electrical power, single-mode continuously tunable emission with distributed-feedback (DFB) gratings, continuous-wave (CW) operation at cryogenic temperature in a first step and at room temperature currently in several spectral ranges, as well as a high output power (up to several hundreds of milliwatts in single-mode operation). All of these milestones made QCLs very versatile laser sources in the mid-infrared spectral region, leading to applications in various fields covering, e.g., defense (directional infrared countermeasure systems for civil and military aircrafts) or free-space optical communications. However, their broad spectral coverage over the important mid-infrared fingerprint region has so far resulted in the most widespread use of QCLs in the field of high-precision and high-resolution spectroscopy and trace gas sensing. Owing to their nice spectral properties as well as their fast tuning and direct-modulation capabilities via their injection current, QCLs have been implemented in many sensitive spectroscopy techniques, such as long path length, balanced detection, wavelength-modulation spectroscopy, frequency-modulation spectroscopy, photoacoustic or quartz-enhanced photoacoustic spectroscopy, and cavity ring-down spectroscopy.
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