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

Mid-infrared optical frequency comb generation from a chi-2 optical superlattice box resonator