  Mid-infrared optical frequency combs are of significant interest for
molecular spectroscopy due to the large absorption of molecular vibrational
modes on one hand, and the ability to implement superior comb-based
spectroscopic modalities with increased speed, sensitivity and precision on the
other hand. Substantial advances in mid-infrared frequency comb generation have
been made in recent years based on nonlinear frequency conversion,
microresonator Kerr frequency combs, quantum cascade lasers and mode locking
regimes. Here we demonstrate a simple, yet effective method for the direct
generation of mid-infrared optical frequency combs in the region from
${2.5-4~\mu{\rm m}}$, i.e. ${2500-4000~{\rm cm}^{-1}}$ covering a large
fraction of the functional group region, directly from a conventional and
compact erbium-fiber-based femtosecond laser in the telecommunication band
(i.e. ${1.55~\mu{\rm m}}$). The wavelength conversion is based on dispersive
wave generation within the supercontinuum process in large-cross-section and
dispersion-engineered silicon nitride (${\rm Si_3N_4}$) waveguides. The
long-wavelength dispersive wave, with its position lithographically determined,
performs as a mid-infrared frequency comb, whose coherence is demonstrated via
optical heterodyne measurements. Such a simple and versatile approach to
mid-infrared frequency comb generation is suitable for spectroscopic
applications in the first mid-infrared atmospheric window. Moreover, the
compactness and simplicity of the approach have the potential to realize
compact dual-comb spectrometers. The generated combs have a fine teeth-spacing,
making them also suitable for gas phase analysis.
