Microwave frequency synthesis via multifrequency self-injection locking

Microwave signal sources based on photonic integrated circuits offer exceptional compactness and robustness. Self-injection locking enables Kerr frequency comb generation in microring resonators, producing a low-phase-noise beat-note signal; however, the output comb power is typically low. Here, we use multifrequency self-injection locking by matching an integer ratio between the laser and microring mode grids, making the microwave signal power limited only by the available pump power, while the phase noise is still determined by the resonator Q-factor. Simultaneous locking to multiple adjacent microresonator modes separated by a free spectral range of 15 GHz is achieved. Phase noise reduction of the microwave beat-note signal is characterized, and the underlying physical mechanisms are analyzed. Our findings demonstrate significant potential for telecommunications and radio-over-fiber systems.