He Jinze, Li Yang
Opt Express. 2020 Oct 12;28(21):30771-30783. doi: 10.1364/OE.401881.
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.
宽带中红外频率梳由于其类似尺子的精确离散梳齿，在中红外光谱学中备受关注。然而，由于传统的中红外频率梳产生方式——先产生近红外频率梳，然后通过非线性过程将其转换到中红外波段，目前最先进的中红外频率梳通常局限于低集成度和高泵浦功率。在此，我们基于绝缘体上铌酸锂（LNOI）平台，从理论上研究了两种用于产生超低功率泵浦的中红外频率梳的集成设计。利用周期性极化铌酸锂（PPLN）波导和微环电光相位调制器，我们改变了梳状生成和非线性转换的传统顺序。这种范式转变显著提高了中红外频率梳产生的转换效率，并且无需飞秒激光器。我们的理论结果预测，通过功率低于5 mW的连续波（CW）输入，以高于1800%/W的超高转换效率，可以产生中心波长约为4.3 µm、梳齿功率为纳瓦级的宽带中红外频率梳。我们的中红外频率梳设计具有高可控性、灵活性和集成度，能够实现中红外光谱仪的小型化。
