微腔光梳重频调控与光谱应用

Towards microcomb repetition rate control and its application in spectroscopy

利用高品质因子(高Q)微腔中材料的光学非线性可以实现高相干微腔光梳的产生,这为光频梳系统的小型化和片上集成开辟了新的可能性.对于光学频率梳,梳齿间距是光频梳的关键参数,其与产生方式相关,反过来也影响了光梳的应用.芯片化的微腔光梳一般具有高重频的特点,这一特点适用于波分复用光通信、光计算、太赫兹波合成等应用.但对于精密光谱分析等应用,过大的梳齿间隔会带来光谱欠采样等问题,这一问题在中红外波段尤为明显.本文重点介绍了微腔光梳的重频调控,特别是其在光谱测量中的应用;简要分析了不同方式所产生光频梳的重频特点,以及不同应用对光梳重频的要求,特别是对双光梳测量系统;还介绍了微腔光梳与电光梳相结合的间隔光学差频(interleaved difference frequency generation,iDFG)技术,基于此技术可以实现GHz重频的中红外光梳的产生.将iDFG技术与相向传播(counter-propagating,CP)的孤子对相结合,可以实现中红外波段高相干、高分辨率的光谱测量.本文简要介绍了微腔光梳的发展,侧重于微腔光梳在光谱分析上的应用.

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(2–20μm).This band is known as the“molecular fingerprint”region and is of keen interests in spectroscopy,as molecular transitions in the midinfrared 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 highrepetition-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.