  Mid-infrared semiconductor lasers in photonic integrated circuits are of
considerable interest for a variety of industrial, environmental, and medical
applications. However, photonic integration technologies in the mid-infrared
lag far behind the near-infrared range. Here we present the monolithic
integration of mid-infrared quantum cascade lasers with low-loss passive
waveguides via butt-coupling. The passive waveguide losses are experimentally
evaluated to be only 1.2 +- 0.3 dB/cm, with negligible butt-coupling losses. We
demonstrate continuous-wave lasing at room temperature of these
active-to-passive waveguide coupled devices. Moreover, we report a frequency
comb operation paving the way toward on-chip, mid-infrared, dual-comb sensors.
