The design and operating principles of quantum cascade lasers (QCLs) are reviewed along with recent developments in high-power cw and broadband devices. Cw power levels of several watts at room temperature have been achieved at 4.6-μm wavelength; broadband single-mode tuning (≈400 cm−1) has been achieved using an external-cavity QCL with a grating as a tuning element. An alternative approach, consisting of a monolithically integrated array of single-mode QCLs individually current-driven by a microcontroller, has led to broadband single-mode tuning over a range of 200 cm−1 without requiring the use of moving parts. This spectrometer on a chip holds promise for high-brightness compact trace-gas sensors capable of operating in harsh environments.
