Narrow-linewidth optical coherent oscillators in ultra-low loss silicon nitride

Licentiatavhandling, 2025

In recent years, advancements in technologies such as optical coherent communication, precision measurement, optical detection and ranging, 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 high-Q 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 intrinsic and extrinsic Q factors of the microring resonator, we achieve an on-chip optical parametric oscillator with an output power of 215 mW and an intrinsic linewidth of 220 Hz. In addition, we suppress the frequency noise of both semiconductor lasers and soliton microsombs using an original self-injection locking method. We reduce the intrinsic linewidth of the semiconductor laser from 818 000 Hz to 135 Hz, and compress the intrinsic linewidth of comb lines of the soliton microcomb to below 1 Hz. These results pave the way for on-chip integration of high-power, narrow-linewidth lasers and optical parametric oscillators.