  Low-noise lasers are of central importance in a wide variety of applications,
including high-spectral-efficiency coherent communication protocols,
distributed fiber sensing and long-distance coherent LiDAR. In addition to low
phase noise, frequency agility, that is the ability to achieve high-bandwidth
actuation of the laser frequency, is imperative for carrier recovery in
coherent communications, triangular chirping in frequency-modulated
continuous-wave (FMCW) based ranging or any optical phase locking as routinely
used in metrology. While integrated silicon-based lasers have experienced major
advances, and are now employed on a commercial scale in data centers integrated
lasers with lowest phase noise are based on optical feedback from photonic
circuits that lack frequency agility. Here, we demonstrate a
wafer-scale-manufacturing-compatible hybrid photonic integrated laser that
exhibits ultralow intrinsic linewidth of < 40 Hz, while offering unsurpassed
megahertz actuation bandwidth, with a tuning range larger than 1 GHz. Our
approach uses ultralow-loss (< 1 dB/m) Si3N4 photonic microresonators, combined
with aluminium nitride (AlN) microelectromechanical systems (MEMS) based
stress-optic actuation. Electrically driven low-phase-noise lasing is attained
by self-injection locking of an Indium Phosphide (InP) laser chip, and only
limited by fundamental thermorefractive noise. By utilizing difference-drive
and apodization of the photonic chip, at actuation response up to 10 MHz is
achieved. We leverage this capability to demonstrate a compact coherent LiDAR
engine that can generate up to 800 kHz FMCW triangular optical chirp signals,
requiring neither any active linearization, nor predistortion compensation, and
perform a 10 m optical ranging experiment, with a resolution of 12 cm.
