Design of a microcomb based optical oscillator with a low RF repetition rate phase noise

Abstract

Advanced wireless communication systems operating at E-band frequencies (60– 90 GHz) and beyond require oscillators with extremely low phase noise, a demand that is increasingly difficult to meet with conventional electronic approaches. Photonic signal generation via two-point optical frequency division(OFD) addresses this challenge by transferring the inherent frequency stability of optical resonators to the microwave domain. Silicon nitride (Si3N4) microresonator frequency combs, in particular the photonic molecule architecture, comprising two evanescently coupled microrings, offer a chip-scale route to integrated, low phase noise local oscillators suitable for next-generation communication links. This thesis presents the design, implementation, and experimental characterization of a Si3N4 photonic molecule microcomb operated as an RF oscillator through two-point OFD. Multiple dissipative Kerr soliton (DKS) states were accessed and characterized using a single pump laser. Two-point optical stabilization via optical injection locking (OIL) of an auxiliary laser was investigated across all accessible soliton numbers. The repetition rate at ≈49.921 GHz was characterized using a high-speed photodiode and a Rohde & Schwarz phase noise analyzer, with additional measurements at 12.48 GHz using a ÷4 RF divider. Contrary to the original intent, OIL with the auxiliary laser consistently degraded the repetition-rate phase noise relative to the free-running configuration. Two unexpected results were observed. First, higher-order soliton states (two-soliton and above) exhibited lower phase noise than the single-soliton state, attributed to increased carrier power and a stabilizing role of the avoided mode crossing (AMX). Second, the measured phase noise fell marginally below the theoretical two-point OFD limit, interpreted as evidence of passive pump-noise filtering by the coupled resonator. A preliminary system-level evaluation on an Ericsson 5G testbed yielded a signal to noise and interference ratio (SNIR) of 26 dB, demonstrating a feasible but not yet communication-grade photonic local oscillator. These results motivate further study of coupled-resonator geometries as passive noise-suppression mechanisms for integrated microwave photonic sources.