Microresonator-Based Optical Frequency Combs for Microwave and Millimeter-Wave Applications

Optical frequency comb is a powerful technology that coherently links the optical frequency and microwave frequency, and it has revolutionized metrology, time keeping, spectroscopy, and ranging. In the past decade, the microresonator-based soliton frequency combs, or soliton microcombs, have been extensively studied due to their small footprint, wide optical span, and high coherence. One major advantage of these microcombs is their high repetition-rate, ranging from GHz to 1 THz, which makes them ideal candidates for a range of applications, including wavelength multiplexing, self-referencing, and photonics-based high-speed RF oscillators. In this thesis, the soliton microcombs are used to build coherent links between optical, mmWave, and microwave frequencies. High-frequency millimeter-Waves are demonstrated with high power and high coherence by directly photodetecting the microcombs on a ultrahigh-speed photodiode. Significantly, the low noise of optical references can be coherently divided down to the generated mmWaves using integrated optical frequency division. On the other hand, the high repetition-rate can also bring challenges in its accurate detection when the frequency is above the bandwidths of photodiodes and electronics. In the later chapter of this thesis, an optical Vernier frequency division approach based on dual-comb coherent sampling is developed to coherently divide down the mmWave frequency to microwave frequency, which is used to count and stabilize the microcomb sub-THz rep-rate using GHz bandwidth optoelectronics. Additionally, the microresonator dual-comb technique also enables the full electrical spectrum control including its amplitudes, phases, and the corresponding temporal waveforms. And an integrated photonics-based microwave arbitrary waveform generator was demonstrated with the potential to achieve high analog bandwidths and large effective number of bits thanks to the high repetition-rates of microcombs.