  Ultra-low-noise laser sources are crucial for a variety of applications,
including microwave synthesizers, optical gyroscopes, and the manipulation of
quantum systems. Silicon photonics has emerged as a promising solution for
high-coherence applications due to its ability to reduce system size, weight,
power consumption, and cost (SWaP-C). Semiconductor lasers based on
self-injection locking (SIL) have reached fiber laser coherence, but typically
require a high-Q external cavity to suppress coherence collapse through
frequency-selective feedback. Lasers based on external-cavity locking (ECL) are
a low-cost and turnkey operation option, but their coherence is generally
inferior to SIL lasers. In this work, we demonstrate quantum-dot (QD) lasers
grown directly on Si that achieve SIL laser coherence under turnkey ECL. The
high-performance QD laser offers a scalable and low-cost heteroepitaxial
integration platform. Moreover, the QD laser's chaos-free nature enables a 16
Hz Lorentzian linewidth under ECL using a low-Q external cavity, and improves
the frequency noise by an additional order of magnitude compared to
conventional quantum-well lasers.
