  Mid-wave infrared (MIR, 3--5 $\mu$m) optical frequency combs (OFC) are of
critical importance for spectroscopy of fundamental molecular absorption
features in space and terrestrial applications. Although in this band OFCs can
be obtained via supercontinuum or difference frequency generation using optical
pumping schemes, unprecedented source miniaturization and monolithic design are
unique to electrically-pumped semiconductor laser structures. To date,
high-brightness OFC generation in this region has been demonstrated in quantum-
and interband cascade lasers (QCL/ICL), yet with sub-optimal spectral
properties. Here, we show a MIR quantum well diode laser (QWDL) OFC, whose
excellent spectral uniformity, narrow optical linewidths, and milliwatt optical
power are obtained at a fraction of a watt of power consumption. The
continuously tunable source offers ~1 THz of optical span centered at 3.04
$\mu$m, and a repetition rate of 10 GHz. As a proof-of-principle, we show a
directly-battery-operated MIR dual-comb source with almost 0.5 THz of optical
coverage accessible in the electrical domain in microseconds. These results
indicate the high suitability of QWDL OFCs for future chip-based real-time
sensing systems in the mid-infrared.
