Integrated narrow-linewidth optical coherent oscillators in ultra-low loss silicon nitride

Doctoral thesis, 2026

In recent years, advancements in technologies such as optical coherent communication and precision measurement, have raised the bar for the coherence, power, noise, and other key parameters of light sources. On-chip light sources have emerged as the ideal solution when small size, low weight, low power consumption and cost-effectiveness matter the most. Currently, integrated light sources include semiconductor lasers and chip-based optical parametric oscillators. However, due to their small cavity volume, both types suffer from high quantum noise, poor coherence compared to solid-state or gas lasers.

As a popular integrated photonic platform, silicon nitride has a significant potential for addressing these challenges since it has high nonlinearity, wide transparent window, and good compatibility with other materials. In our previous work, we have realized low-loss long waveguides and microring resonators. In this thesis, we further reduce the propagation loss of dispersion-engineered silicon nitride waveguides by smoothing the sidewall roughness. By periodically modulating the losses of microring resonators, we achieve an on-chip optical parametric oscillator with a record signal power of 215 mW. In addition, we build hybrid integration stage and suppress the intrinsic linewidth of a semiconductor laser to 9 Hz using self-injection locking method. Finally, by implementing external feedback circuits for optical parametric oscillators, we suppress the intrinsic linewidth of the signal to below 1 Hz using an optical fiber loop, and to approximately 10 Hz using an integrated waveguide loop. These results pave the way for on-chip integration of high-power, narrow-linewidth lasers and optical parametric oscillators.