The advent of optical frequency combs revolutionized many research fields from metrology to high precision spectroscopy. It was recently demonstrated that broadband quantum cascade lasers can operate as frequency combs. As such, they operate under direct electrical pumping at both mid-infrared and terahertz frequencies, reaching powers in the watt range with multi-terahertz bandwidths. As their key application field, they unlock the advantages in speed and accuracy of the dual-comb spectroscopy technique in a frequency range where molecules have their fundamental vibrational and rotational bands. In this Chapter we review the design and basic functioning principles of these devices, the characterization of their coherence properties as well as few example applications.

The state-of-the-art of mid-IR laser absorption spectroscopy is reviewed to take advantage of the stronger absorption lines. The properties of mid-IR diode lasers are discussed, including quantum well, inter-band cascade and quantum cascade lasers for gas sensing at wavelengths beyond two microns. As an alternative to diode lasers, mid-IR laser sources based on down-conversion from the near-IR are reviewed using either difference frequency generation or optical parametric oscillation and examples are given of their design as tuneable mid-IR CW sources or as mid-IR frequency combs. Examples of compact mid-IR laser combs formed from micro-resonators in silicon are also discussed. The important spectroscopic techniques of wavelength modulation spectroscopy, cavity-enhanced, evanescent-wave and dual-comb spectroscopy are all discussed in the context of the mid-IR with examples of the performance that can be attained. The performance and limitations of the most common mid-IR transmitting fibres and mid-IR detectors are also reviewed. Finally a comparison is given of the relative merits of gas absorption spectroscopy in the near-IR and mid-IR and where each has an important role to play.