| Summary: | The performances of mid-infrared (IR) quantum cascade lasers (QCLs) are now reaching a maturity level that enables a variety of applications which require compact laser sources capable of watt-range output powers with high beam quality. We review the fundamental design issues and current performance limitations, focusing on InGaAs/AlInAs/InP QCLs with emission in the 3-6 μm wavelength range. Metamorphic materials broaden the available compositions for accessing short emission wavelengths (λ<inline-formula><tex-math notation="LaTeX">$ \leq $</tex-math></inline-formula>3.5 μm) or for integration with GaAs- and Si-photonics platforms. Conduction-band engineering through the use of varying compositions throughout the active-region structure has been utilized to achieve the highest performance levels to date. Interface roughness scattering plays a dominant role in determining both the lower-laser-level lifetime as well as the carrier-leakage current. Numerous approaches have been implemented in attempts to control, scale, and stabilize the spatial mode to high output powers. Of all approaches photonic-crystal structures with high built-in index contrast, thus capable of maintaining modal properties under strong self-heating, are the most promising device configuration for achieving single-spatial-mode, single-lobe reliable CW operation to multiwatt-range power levels. Such devices have demonstrated to date >5W front-facet output powers with diffraction-limited beams in short-pulse operation.
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