Overcoming the thermo-refractive noise limit for a full-spectrum Hertz-linewidth microcomb

Chip-scale microcombs combining high repetition rates and Hertz-level linewidths are critical for precision measurements and parallel coherent communications. Self-injection locking (SIL) has been widely used but faces a fundamental limitation: the microresonator must simultaneously generate the comb and suppress noise, creating a coherence ceiling set by thermo-refractive noise (TRN). This imposes a fundamental trade-off between repetition rate and linewidth. Here, we transcend this barrier with a cascaded architecture that decouples these two functions. First, SIL to a microresonator produces a soliton comb, defining the repetition rate while releasing the intracavity pump. Second, optical injection locking targets this pump via a fiber delay, extending the system's effective coherence time to overcome the TRN-imposed linewidth ceiling. This functional decoupling suppresses noise by ~70 dB, yielding a 100-GHz microcomb with uniform 3-15 Hz Lorentzian linewidth across all comb teeth. Our modular design enables scalable and coherent microcombs for next-generation metrology, sensing, and communications.