Design of on-chip mid-IR frequency comb with ultra-low power pump in near-IR

Broadband mid-infrared frequency combs are of particular interest to mid-infrared spectroscopy due to their ruler-like precise discrete comb teeth. However, the state-of-the-art mid-infrared frequency combs are usually limited to low integration level and high pump power as a result of the conventional way of mid-infrared frequency comb generation--producing a near-infrared frequency comb first and then convert it to mid-infrared regime through a nonlinear process. Here, we theoretically investigate two integrated designs for generating mid-infrared frequency combs with ultra-low power pump based on the lithium-niobate on insulator (LNOI) platform. Utilizing periodically poled lithium-niobate (PPLN) waveguides and microring electro-optic phase modulators, we switch the conventional order of comb generation and nonlinear conversion. This paradigm shift significantly improves the conversion efficiency of mid-infrared frequency comb generation and obviates the need for femtosecond lasers. Our theoretical results predict that a broadband mid-infrared frequency comb around 4.3 µm with nanowatt-power-level comb teeth can be produced from continuous-wave (CW) inputs whose power is lower than 5 mW with an ultra-high conversion efficiency above 1800 %/W. Our designs of mid-infrared frequency comb have high controllability, flexibility and integration level, enabling the miniaturization of mid-infrared spectrometers.