Advances in resonator-based Kerr frequency combs with high conversion efficiencies

Recent developments in resonator-based Kerr frequency combs promise excellent applications in a wide range of fields such as biosensing, spectroscopy, optical communications, light detection and ranging (LiDAR), frequency synthesis, astronomical detection, and quantum optics. A key figure of merit (FOM) for Kerr frequency combs is the pump-to-comb conversion efficiency, which is critical for applications requiring sufficient comb power and low power consumption. In this review, we first discuss the limited conversion efficiency of dissipative Kerr soliton in an anomalous dispersion microresonator based on its underlying physical characteristics. And then, we summarize the recent advances in Kerr frequency combs with high conversion efficiencies in both anomalous and normal dispersion regimes. We classify them according to various soliton states, excitation methods as well as novel material platforms. The final section of the paper presents an overview of current progress and glances at potential directions for future research.