微腔光梳重频调控与光谱应用
摘要
<p indent="0mm">Leveraging the optical nonlinearities in high-<italic>Q</italic> 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 <sc>(2–20 μm).</sc> 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 mol