Resonant photonic oscillators and regenerative frequency dividers

Optical Kerr frequency combs can be generated when a nonlinear optical microcavity is pumped with coherent light. The frequency combs demodulated on a fact photodiode can produce low noise microwave signals. A photonic system involving a laser, a nonlinear optical cavity and a fast photodiode, as well as auxiliary parts, forms a microwave photonic oscillator. The microwave photonic oscillators involving optical cavities experienced fast development over last decade. The devices based on Kerr frequency combs demonstrated signals with carrier frequencies exceeding 10 GHz and phase noise below -130 dBc/Hz at 10 kHz and -150 dBc/Hz at >1MHz frequency offsets. Packaging the devices in less than 1 cm3 form factors and achieving exceptional environmental insensitivity was demonstrated. Improvement of the spectral purity and stability of the comb oscillators as well as observation of curious optical phenomena, such as formation of discrete time crystals, was realized when a coherent dichromatic optical pump was involved in the comb generation. We here highlight a few recent achievements in realization and study of the microwave photonic oscillators based on Kerr optical frequency comb generated in ultra-high Q whispering gallery mode (WGM) resonators. We also would like to attract attention to the feasibility of generation of low noise microwave signals using lasers self-injection locked to monolithic Fabry-Perot resonators.