  Mid-infrared laser frequency combs are compelling sources for precise and
sensitive metrology with applications in molecular spectroscopy and
spectro-imaging. The infrared atmospheric window between 3-5.5 $\mu$m in
particular provides vital information regarding molecular composition. Using a
robust, fiber-optic source of few-cycle pulses in the near-infrared, we
experimentally demonstrate ultra-broad bandwidth nonlinear phenomena including
harmonic and difference frequency generation in a single pass through
periodically poled lithium niobate (PPLN). These $\chi^{(2)}$ nonlinear optical
processes result in the generation of frequency combs across the mid-infrared
atmospheric window which we employ for dual-comb spectroscopy of acetone and
carbonyl sulfide with resolution as high as 0.003 cm$^{-1}$. Moreover, cascaded
$\chi^{(2)}$ nonlinearities in the same PPLN directly provide the
carrier-envelope offset frequency of the near-infrared driving pulse train in a
compact geometry.
