Photonic integrated circuits (PICs) have revolutionized optical communication and precision metrology by enabling compact, scalable, and high-performance optical systems. However, conventional integrated laser sources suffer from limitations such as poor coherence, limited frequency agility, and thermal constraints that hinder their broader adoption in high-performance applications. This thesis reports on the development of a low-loss 200 nm thick silicon nitride (SiN) platform with monolithically integrated aluminum nitride (AlN) piezoactuators. By leveraging these advancements, this work explores the development of low-noise, frequency-agile photonic integrated lasers based on SiN PICs hybrid-integrated with either distributed feedback (DFB) or Fabry-Pérot (FP) lasers for self-injection locking, as well as with reflective semiconductor optical amplifiers (RSOAs) to realize extended distributed Bragg reflector (DBR) lasers.

A key innovation in this research is the monolithic integration of piezoelectric actuators onto SiN chips, enabling frequency tunability via the stress-optic effect. Unlike conventional thermal tuning, which is limited in speed and efficiency, piezoelectric actuation offers rapid tuning rates exceeding 100 times those of bulk lasers, dramatically improving the agility and responsiveness of the integrated lasers. Additionally, this technology has been extended to the visible domain at 460 nm, paving the way for applications in quantum technologies, atomic clocks, and bio-imaging. Beyond SiN, this thesis also explores lithium niobate as a platform for photonic integrated lasers, demonstrating low-noise and highspeed tuning capabilities in this material system. Furthermore, these lasers have been utilized to demonstrate frequency-modulated continuous-wave (FMCW) LiDAR, showcasing their potential for high-resolution ranging and imaging applications. The results presented in this thesis mark a significant advancement in the field of integrated photonics, bridging the gap between high-performance bulk lasers and compact, scalable photonic integrated solutions. The demonstrated performance improvements in coherence, tunability, and integration pave the way for next-generation photonic systems that combine the best attributes of traditional bulk lasers with the scalability and robustness of integrated photonics.