In our information society, the synthesis, distribution, and processing of radio and microwave signals are ubiquitous in wireless networks, telecommunications, and radars. The current tendency is to use carriers in higher frequency bands, especially with looming bandwidth bottlenecks due to demands for, for example, 5G and the “Internet of Things.” ‘Microwave photonics,’ a combination of microwave engineering and optoelectronics, might offer a solution.
A key building block of microwave photonics is optical frequency combs, which provide hundreds of equidistant and mutually coherent laser lines. They are ultrashort optical pulses emitted with a stable repetition rate that corresponds precisely to the frequency spacing of comb lines. The photodetection of the pulses produces a microwave carrier.
In recent years there has been significant progress on chip-scale frequency combs generated from nonlinear microresonators driven by continuous-wave lasers. These frequency combs rely on the formation of dissipative Kerr solitons, which are ultrashort coherent light pulses circulating inside optical microresonators. Because of this, these frequency combs are commonly called ‘soliton microcombs.