Towards Octave-Spanning Low-Power Microresonator Frequency Combs: Optical Characteriazation of Silicon Nitride Chips

Abstract: Optical microresonators with small modal volumes and high quality factors (Q) are ideal platforms for the research on nonlinear optics and quantum opto-mechanics. Among the multitude of materials used for microresonators, silicon nitride (SiN) with its high Kerr nonlinearity, low mechanical loss, wide transparency window from the visible to the mid-IR, absence of nonlinear (e.g. two or three photon) absorption and access with CMOS-compatible fabrication techniques, becomes one of the most promising materials for this research. Benefiting from such advantages, SiN microresonators can be used for microresonator frequency comb generation through four-wave mixing via Kerr nonlinearity. Up-to-date, fabricated SiN photonic chip-based microresonators can generate broadband microresonator frequency combs with 100 GHz – 1 THz mode spacings to spectrally cover more than the telecommunication bands of 1270 – 1640 nm from a single input laser wavelength, opening a new pathway to highly compatible optical communication systems.

However, currently the performance of SiN photonic chip-based microresonators is still limited by their high power requirement, determined by the low input power coupling efficiency from optical fibers to SiN waveguides and the low quality factor Q of the microresonators. The coupling efficiency is typically around 15% and improving it requires deliberate designs and fabrication

techniques. The Q of SiN microresonators is typically around 10^6, and improving the Q is presently hindered by the absorption and scattering induced by the material interfaces and the gases trapped in the material during the deposition. The improvements of the coupling efficiency and Q will reduce the power requirement for microresonator frequency comb generation and enables packaging SiN chip-scale frequency combs into integrated systems.

Anomalous group velocity dispersion is required for the microresonator frequency comb generation and the bright Kerr dissipative soliton formation. In addition, a broadband frequency comb can be achieved with a proper dispersion profile. Recently an important advancement to achieve broadband coherent frequency combs is the observation of the dispersive wave emission in SiN microresonators. To generate such dispersive waves in microresonators, the higher order dispersion needs to be precisely tailored and experimentally characterized.

This master thesis addresses the issues mentioned above. It aims to achieve broadband microresonator frequency combs with

low power requirement. To realize such a goal, three milestones have been achieved: (1) the optimization of the input power coupling efficiency from optical fibers to SiN waveguides is investigated with simulations, and >40% coupling efficiency is achieved experimentally; (2) the waveguide-ring resonator coupling and the coupling ideality describing the practical resonator Q are investigated with theoretical models, simulations and experimental characterization; (3) an experimental setup to characterize the microresonator dispersion up to the fourth order is established. With the three milestones mentioned above, a standard routine combining theoretical models, versatile simulation tools and novel experimental characterization platforms to achieve broadband microresonator frequency combs with low power requirement is demonstrated.