Hertz-linewidth and frequency-agile photonic integrated extended-DBR lasers

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.