  Recent advances in the development of ultra-low loss silicon nitride
(Si3N4)-based photonic integrated circuits have allowed integrated lasers to
achieve a coherence exceeding those of fiber lasers and enabled unprecedentedly
fast (Megahertz bandwidth) tuning using monolithically integrated
piezoelectrical actuators. While this marks the first time that fiber laser
coherence is achieved using photonic integrated circuits, in conjunction with
frequency agility that exceeds those of legacy bulk lasers, the approach is
presently compounded by the high cost of manufacturing DFB, as required for
self-injection locking, as well as the precise control over the laser current
and temperature to sustain a low noise locked operation. Reflective
semiconductor optical amplifiers (RSOA) provide a cost-effective alternative
solution but have not yet achieved similar performance in coherence or
frequency agility, as required for frequency modulated continuous wave (FMCW)
LiDAR, laser locking in frequency metrology or wavelength modulation
spectroscopy for gas sensing. Here, we overcome this challenge and demonstrate
an RSOA-based and frequency agile integrated laser tuned with high speed, good
linearity, high optical output power, and turn-key operability while
maintaining a small footprint. This is achieved using a tunable extended
distributed Bragg reflector (E-DBR) in an ultra-low loss 200 nm thin Si3N4
platform with monolithically integrated piezoelectric actuators. We
co-integrate the DBR with a compact ultra-low loss spiral resonator to further
reduce the intrinsic optical linewidth of the laser to the Hertz level -- on
par with the noise of a fiber laser -- via self-injection locking.
