  Silicon nitride (SiN) waveguides with ultra-low optical loss enable
integrated photonic applications including low noise, narrow linewidth lasers,
chip-scale nonlinear photonics, and microwave photonics. Lasers are key
components to SiN photonic integrated circuits (PICs), but are difficult to
fully integrate with low-index SiN waveguides due to their large mismatch with
the high-index III-V gain materials. The recent demonstration of multilayer
heterogeneous integration provides a practical solution and enabled the
first-generation of lasers fully integrated with SiN waveguides. However a
laser with high device yield and high output power at telecommunication
wavelengths, where photonics applications are clustered, is still missing,
hindered by large mode transition loss, nonoptimized cavity design, and a
complicated fabrication process. Here, we report high-performance lasers on SiN
with tens of milliwatts output through the SiN waveguide and sub-kHz
fundamental linewidth, addressing all of the aforementioned issues. We also
show Hertz-level linewidth lasers are achievable with the developed integration
techniques. These lasers, together with high-$Q$ SiN resonators, mark a
milestone towards a fully-integrated low-noise silicon nitride photonics
platform. This laser should find potential applications in LIDAR, microwave
photonics and coherent optical communications.
