Ultra-low-loss silicon nitride photonic integrated circuits for highly coherent lasers

The development of integrated photonics has played an important role in the proliferation of high-speed telecommunications, and has the potential to impact numerous applications including precision metrology, sensing, navigation, imaging, and computation. For virtually all such optical systems, optical loss represents a key performance metric. Achieving extremely low optical loss often requires a corresponding increase in the device footprint. This work explores a regime in integrated photonics in which optical loss at parity with the world-record-lowest loss in any integrated platform is achieved, but in a form factor that is planar, fabricated using conventional CMOS processes, and an order of magnitude smaller in footprint. While prior works with comparable loss were been limited by device size and integration limitations to single-device demonstrations, the improved silicon-nitride waveguide platform presented herein enables higher photonic integrated circuit (PIC) complexity than previously explored. These properties are used to create several novel integrated devices, including ultra-high Q and record-high finesse integrated optical resonators, a hybrid-integrated laser with fiber-laser coherence properties, a low-noise microcomb source, a single-mode Raman laser, and a twenty-three meter integrated optical delay line.While the first part of this thesis thus explores capabilities intrinsic to integrated silicon nitride waveguides, the latter part of the thesis develops novel processing techniques to enable these high performance PICs to interface with other electrical and optical components. A deuterated silicon dioxide thin-film deposition is developed to enable integration of such silicon nitride PICs with active optical materials, enabling the demonstration of a heterogeneous laser. Piezoelectric tuning of silicon nitride resonators is also explored, which could allow silicon nitride PICs to be reconfigured with dramatically lower power consumption. Finally, a study is presented on extending the benefits and designs of ultra-low loss silicon nitride waveguides to silicon-on-insulator waveguides, which are an attractive platform for many ultra-low loss PICs due to preexisting silicon photonics infrastructure