  Optical frequency combs (OFCs) at Mid-Infrared (MIR) wavelengths are
essential for applications in precise spectroscopy, gas sensing and molecular
fingerprinting, because of its revolutionary precision in both wavelength and
frequency domain. The microresonator-based OFCs make a further step towards
practical applications by including such high precision in a compact and
cost-effective package. However, dispersion engineering is still a challenge
for the conventional chi-3 micro-ring resonators and a MIR pump laser is
required. Here we develop a different platform of a chi-2 optical superlattice
box resonator to generate MIR OFC by optical parametric down conversion. With
near-material-limited quality factor of 2.0*10^7, broadband MIR OFC can be
generated with over 250 nm span around 2060 nm, where only a common
near-infrared laser is necessary as pump. The fine teeth spacing corresponds to
a measurable radio frequency beat note at 1.566 GHz, and also results in a fine
spectroscopy resolution. Its linewidth is measured to be 6.1 kHz, which reveals
a low comb noise that agrees well with the clean temporal waveforms. With high
output power of over 370 mW, such MIR OFC is capable for long distance sensing
and ranging applications.
