Sub-hertz-linewidth hybrid-integrated laser for isolator-free silicon photonics

This work presents a hybrid-integrated external-cavity laser with a 0.45 Hz Lorentzian linewidth and exceptional feedback tolerance of up to −8.1 dB, enabling isolator-free photonic integrated circuits. Unlike current high-Q locking configurations, our approach leverages a broadband external cavity, effectively mitigating thermorefractive noise and mode-hopping while ensuring stable operation. Through a detailed analysis of noise suppression in low-Q locking configurations, we demonstrate that extending the external cavity to increase the effective photon lifetime achieves noise suppression comparable to high-Q locking. This relaxes the stringent Q-factor requirements for self-injection locking, enabling chip-scale lasers with coherence surpassing bulky fiber lasers while maintaining compatibility with broadband external cavities. The demonstrated ultra-high coherence and strong feedback insensitivity make this laser an ideal candidate for large-scale heterogeneous photonic integration, with transformative applications in optical and quantum computing, precision sensing, and metrology.