  Low-noise lasers are of central importance in a wide variety of applications,
including high spectral-efficiency coherent communication protocols,
distributed fibre 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 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 sub-100 Hz-level intrinsic linewidth 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 25 Hz while
offering unsurpassed megahertz actuation bandwidth, with a tuning range larger
than 1 GHz. Our approach uses ultralow-loss (1 dB/m) Si$_3$N$_4$ photonic
microresonators, combined with aluminium nitride (AlN) or lead zirconium
titanate (PZT) 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 thermo-refractive noise. By utilizing difference drive and
apodization of the photonic chip, a flat 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 12.5 cm.
