  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 THz frequencies, making them very attractive
for dual-comb spectroscopy. Performance levels are continuously improving, with
average powers over 100 mW and frequency coverage of 100 cm$^{-1}$ in the
mid-infrared. In the THz range, 10 mW of average power and 600 GHz of frequency
coverage are reported. As a result of the very short upper state lifetime of
the gain medium, the mode proliferation in these sources arises from four wave
mixing rather than saturable absorption. As a result, their optical output is
characterized by the tendency of small intensity modulation of the output
power, and the relative phases of the modes to be similar to the ones of a
frequency modulated laser. Recent results include the proof of comb operation
down to a metrological level, the observation of a Schawlow-Townes broadened
linewidth, as well as the first dual-comb spectroscopy measurements. The
capability of the structure to integrate monolithically non-linear optical
element as well as to operate as a detector show great promise for future chip
integration of dual-comb systems.
