Compact Turnkey Soliton Microcombs at Microwave Rates via Wafer‐Scale Fabrication

ABSTRACT Soliton microcombs generated in nonlinear microresonators facilitate the photonic integration of timing, frequency synthesis, and astronomical calibration functionalities. For these applications, low‐repetition‐rate () soliton microcombs are essential as they establish a coherent link between optical and microwave signals. However, the required pump power typically scales with , and the device footprint scales with , rendering low‐ soliton microcombs challenging to integrate within photonic circuits. This study designs and fabricates microresonators on 4‐inch wafers with highly compact form factors. The resonator geometries are engineered from ring to finger and spiral shapes to enhance integration density while attaining quality factors over . Driven directly by an integrated laser, we demonstrate soliton microcombs with below 10 GHz via turnkey initiation, which is used for synthesizing microwaves with phase noise below –130 at 100 kHz offset frequency. The modules are robust against external perturbations and can be automatically set up, ensuring stable operation over hours. Our work enables high‐density integration and practical deployment of soliton microcombs for chip‐based microwave photonics and spectroscopy applications.