  We experimentally demonstrate a simple configuration for mid-infrared (MIR)
frequency comb generation in quasi-phase-matched lithium niobate waveguides
using the cascaded-$\chi^{(2)}$ nonlinearity. With nanojoule-scale pulses from
an Er:fiber laser, we observe octave-spanning supercontinuum in the
near-infrared with dispersive-wave generation in the 2.5--3 $\text{\mu}$m
region and intra-pulse difference-frequency generation in the 4--5
$\text{\mu}$m region. By engineering the quasi-phase-matched grating profiles,
tunable, narrow-band MIR and broadband MIR spectra are both observed in this
geometry. Finally, we perform numerical modeling using a nonlinear envelope
equation, which shows good quantitative agreement with the experiment---and can
be used to inform waveguide designs to tailor the MIR frequency combs. Our
results identify a path to a simple single-branch approach to mid-infrared
frequency comb generation in a compact platform using commercial Er:fiber
technology.
