Low-noise radiofrequency and microwave signals are considered of pivotal importance to a wide range of fundamental research and industrial applications including telecommunication networks, radar and lidar systems, long baseline interferometry and tests of fundamental constants. Conventionally, low-noise microwave signals were provided by bulky and expensive microwave oscillators. Due to the increasing need for compact low-noise microwave sources that are suitable for out-of-door navigation, timekeeping and high-speed communication, various alternative methods have been proposed. Among these proposals, optical-frequency-comb based low-noise microwave generation has shown the promise of generating microwaves with extremely low phase noise level that is only limited by fundamental quantum shot noises.
SOLISYNTH aimed to demonstrate a coherent and compact optical frequency comb based on the platform of optical microresonators with high quality factors. The innovative approach was to use the unique nonlinear dynamics of dissipative cavity solitons to attenuate noises that are induced by various mechanisms. Once the frequency comb in the optical domain is stabilized, the stability can be transferred to the radiofrequency/microwave domain, thus producing low-noise signals at cavity soliton repetition rates with low power consumption and small footprint. Moreover, because dissipative cavity solitons are formed with delicate physics, they exhibit unique characteristics such as relaxation oscillations. To date, these characteristics have not been fully understood, and a careful investigation on this subject is not only of fundamental importance but also critical to the practical applications based on cavity solitons. In this work we have investigated the complex dissipative soliton dynamics with novel excitation approaches and electrooptic comb based ultrafast examination, and we also have studied how the soliton dynamics introduce noises in the generated microwaves. The project consists of two primary phases: (phase I) soliton comb generation with dispersion-engineered microresonators and investigation of the transient soliton dynamics; and (phase II) spectral purification and generation of low-noise microwaves.
Overall, the goal of SOLISYNTH has been fully achieved. Several major objectives, including the generation of broadband and mid-infrared frequency combs based on nano/micro-fabricated resonators and waveguides, the seeding and switching of complex comb states, and the generation of low-noise microwave signals with soliton microcombs, have all been demonstrated. The research results achieved by this project have been published or are currently under review for high-impact journals.
