Investigation and optimization of photonic molecule microcombs with low repetition rates

Abstract

Frequency combs are a key technology for many applications, such as optical clocks, precision spectroscopy, and wavelength division multiplexing (WDM) in optical communications. Integrated frequency combs leveraging the Kerr nonlinearity can operate with milliwatt-level pump powers, though their conversion efficiency (CE) typically remains below 10%. In recent years, photonic molecules have been introduced to overcome the problem of generally low CE observed in microcombs. Reported efficiencies have exceeded 50% by transitioning from single cavities to photonic molecule configurations. To date, these levels of CE have only been demonstrated for microresonators with an FSR of 100 GHz. For low FSR photonic molecules, this improvement is challenging due to higher intrinsic losses and a power distribution across a larger cavity volume. In this thesis, we used Ikeda map-based simulations to identify critical design parameters for achieving higher CE in low FSR configurations. We characterized existing chips and compared measured comb spectra with simulations to extract key parameters and quantify the currently achievable CE for different FSRs. This data was used to analyze the influence of multiple parameters on the CE, including coupling factors, input power, and comb detuning. Based on these investigations, we developed improved parameter sets for microresonators with repetition rates of 25 GHz and 50 GHz. The simulated CE increased from 25% (simulation of existing devices) to over 45% for the 25 GHz design. For the 50 GHz design, we present a parameter set that achieves a CE of over 65%. These CE values can be achieved over a range of commonly used input powers in the milliwatt regime. Our results demonstrate the potential to design photonic molecules with tailored FSRs, enabling greater flexibility across applications.