Mid-infrared optical frequency combs are of significant interest for molecular spectroscopy due to the large absorption of molecular vibrational modes on the one hand, and the ability to implement superior comb-based spectroscopic modalities with increased speed, sensitivity and precision on the other hand. Here, we demonstrate a simple, yet effective, method for the direct generation of mid-infrared optical frequency combs in the region from 2.5 to 4.0 μm (that is, 2,500–4,000 cm⁻¹), covering a large fraction of the functional group region, from a conventional and compact erbium-fibre-based femtosecond laser in the telecommunication band (that is, 1.55 μm). The wavelength conversion is based on dispersive wave generation within the supercontinuum process in an unprecedented large-cross-section silicon nitride (Si₃N₄) waveguide with the dispersion lithographically engineered. The long-wavelength dispersive wave can perform as a mid-infrared frequency comb, whose coherence is demonstrated via optical heterodyne measurements. Such an approach can be considered as an alternative option to mid-infrared frequency comb generation. Moreover, it has the potential to realize compact dual-comb spectrometers. The generated combs also have a fine teeth-spacing, making them suitable for gas-phase analysis.

由于分子振动模式的大量吸收，并且另一方面能够以更高的速度，灵敏度和精度实现基于梳子的光谱模式，因此中红外光学频率梳对分子光谱具有重大意义。在这里，我们演示了一种简单而有效的方法，用于直接生成2.5至4.0μm（即2,500–4,000 cm ^{-1）范围内}的中红外光学频率梳），覆盖了电信频段（即1.55μm）中传统的紧凑型基于纤维的飞秒激光器所覆盖的大部分官能团区域。波长转换基于超连续谱过程中在史无前例的大截面氮化硅（Si ₃ N ₄）波导，采用光刻技术对色散进行光刻设计。长波长色散波可以充当中红外频率梳，其相干性通过光学外差测量得到证明。可以将这种方法视为中红外频率梳生成的替代选项。而且，它有可能实现紧凑的双梳状光谱仪。产生的梳齿还具有良好的齿距，使其适合气相分析。