Recent advancements in ultra-low-loss silicon nitride (Si3N4)-based photonic integrated circuits have surpassed fiber lasers in coherence and frequency agility. However, high manufacturing costs of DFB and precise control requirements, as required for self-injection locking, hinder widespread adoption. Reflective semiconductor optical amplifiers (RSOAs) 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 RSOAbased and frequency-agile fully hybrid integrated extended distributed Bragg reflector (E-DBR) laser with high-speed tuning, good linearity, high optical output power, and turn-key operability. It outperforms Vernier and self-injection locked lasers, which require up to five precise operating parameters and have limitations in continuous tuning and actuation bandwidth. We maintain a small footprint by utilizing 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. The photonic integrated E-DBR lasers operate at 1550 nm and feature up to 25 mW fiber-coupled output power in the free-running and up to 10.5 mW output power in the self-injection locked state. The intrinsic linewidth is 2.5 kHz in the free-running state and as low as 3.8 Hz in the self-injection locked state. In addition, we demonstrate the suitability for FMCW LiDAR by showing laser frequency tuning over 1.0 GHz at up to 100 kHz triangular chirp rate with a nonlinearity of less than 0.6% without linearization by modulating a Bragg grating using monolithically integrated aluminum nitride (AlN) piezoactuators. (c) 2024 Optica Publishing Group under the terms of the