Endlessly self-injection-locked photonic integrated lasers

Abstract Narrow-linewidth integrated lasers are essential tools for optical atomic clocks, fibre sensing and other technologies. Recent advances in silicon nitride low-loss photonic integrated circuits allowed compact self-injection-locked semiconductor lasers to achieve exceptional coherence. Yet this architecture remains limited by sensitivity to detuning and feedback phase: narrow-linewidth operation occurs only at specific operating points, and maintaining the locked state requires active control, limiting robustness and hindering practical deployment. Here we demonstrate a photonic integrated turnkey self-injection-locked laser that eliminates these constraints, guaranteeing self-injection locking to a narrow-linewidth state at any drive current. Our approach exploits the feedback-phase dispersion, set by the spatial arrangement of resonator couplers—which governs the evolution of locking regimes across consecutive cavity modes. By engineering the feedback-phase dispersion and overlapping locking ranges in a high- Q silicon nitride photonic integrated microresonator, we constrain the laser to remain on narrow-linewidth branches during tuning, eliminating transitions into the free-running state. The photonic integrated laser exhibits persistent locking with intrinsic linewidths below 10 Hz at all drive currents, as well as during mode-hop-free 1.5-GHz frequency chirps enabled by monolithically integrated piezoelectric actuators. The architecture transforms the sensitive self-injection locking of low-loss photonic integrated circuits into a robust and scalable approach to ultralow-noise lasers.