Towards microcomb repetition rate control and its application in spectroscopy

Leveraging the optical nonlinearities in high-Q microresonators, coherent microcombs can be generated in chip-integrated devices, which paves a way towards miniaturized optical frequency comb systems. Line spacing or repetition rate is a critical quantity for optical frequency combs, which is relevant to the comb generation approach and impacts the comb applications in return. Chip-scale microcombs generally have a large line spacing. This feature makes them well-suited for applications including wavelength-multiplexed communications, optical computing, and THz-wave synthesis. However, for high resolution spectroscopy, large line spacing can cause undersampling of the absorption signature. Thus, low repetition rate microcombs are needed, but their generation is a significant challenge due to the enlarged mode volume and reduced pump efficiency. In particular, this challenge is more significant for the mid-infrared band (2-20 mu m). This band is known as the "molecular fingerprint" region and is of keen interests in spectroscopy, as molecular transitions in the mid-infrared have orders of magnitude higher than that in the visible or near-infrared bands. Realizing chip-integrated optical frequency comb with appropriate line spacing in this band, especially combined with the dual-comb spectroscopy (DCS) technique, can greatly advance the development of molecular spectroscopy and trace gas detection. Here, we review comb line spacing control and its application in spectroscopy. We first introduce the features on comb line spacing for different comb generation approaches including femtosecond laser combs, electro-optic combs, and microcombs. Then, we discuss the requirements on comb line spacing for different applications such as astrocombs, ultra-low phase noise microwave synthesis, and dual-comb measurements. Considering the trade-off between measurement speed and spectral resolution, GHz line spacing is a suitable choice for molecular gas DCS in the ambient environment. To realize such integratable comb in the mid-infrared based on microcombs, a technique called iDFG was introduced. By using the repetition rate signal of micro combs to obtain the specific electro-optic comb driving signal, and then combining the two beams to perform optical difference frequency, the iDFG generated mid-infrared combs enjoy flexibility in comb line spacing tuning. For DCS, excellent relative frequency stability between two combs (i.e., high mutual coherence) is required. Hence, a dual-comb system combining iDFG and CP solitons is also developed. In addition to the inherent advantages of miniaturization, the CP solitons derive high mutual coherence passively via the Rayleigh backscattering induced soliton interactions. By combining iDFG with CP solitons, it becomes possible to perform highly mid-infrared DCS with GHz resolution. Generally, the spectral bandwidth of low-repetition-rate optical frequency combs is relatively small, but iDFG uses a high-repetition-rate microcomb to generate a mid-infrared optical comb. When using a difference frequency waveguide with large phase matching bandwidth, it can obtain a larger spectral bandwidth. Therefore, iDFG-DCS still has its advantages. The review can serve as a general introduction of microcombs with an emphasize on spectroscopy applications.