Quantum cascade lasers (QCLs) are semiconductor lasers based on intersubband transitions between energy states created by quantum confinement in the conduction band of semiconductor multilayers. The laser core is composed of hundreds of layers of quantum wells and barrier materials (typically InGaAs/InAlAs) that together define most of the laser properties such as wavelength, bias voltage, and output power, to a degree that is unprecedented among semiconductor devices and that led the inventors of this type of lasers to talk about "materials by design" and "bandgap engineering." The great advantage of QCLs over other laser types in the mid-infrared region of the electromagnetic spectrum (referred to in this paper as the region between 4 and 12 μm) is their ability to use direct electronic transitions for light generation, thus being one of the few, if any, options for direct generation of coherent mid-infrared radiation by electrical pumping with no optical excitation, no second-order processes, no complex alignment procedures, and with the added flexibility of being able to tailor the active material properties to match the desired emission wavelength and characteristics. This powerful tool for spectroscopic analysis can be tuned to match the fingerprint absorption characteristics of several molecules in the mid-infrared, as well as serve many other industrial, security, medical, and environmental applications needing high-power beams at these wavelengths. Powers in the watts range have been demonstrated in several laser configurations and designs and at different wavelengths. In this paper we will focus on the high-power emission from industrial-grade devices fabricated to achieve watt-level powers in the midwave IR (MWIR) and longwave IR (LWIR) regions of the spectrum, i.e., the 3-5 μm and 8-12 μm regions, respectively. In addition, we show how these lasers can be fabricated in order to match low-power-consumption requirements of some single-mode applications, and, finally, how the high power from single emitters can be scaled up in an array configuration.
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