Microresonator Frequency Combs Exploiting Quadratic and Cubic Optical Nonlinearities

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QuadCOMB aims at exploring novel concepts for microresonator-based frequency comb sources that combine second-order (‘quadratic’) and third-order (‘cubic’) optical nonlinearities in a chip-scale device. Over the previous years, so-called Kerr combs, generated in high-Q microresonators with cubic nonlinearities, have raised significant interest both from a theoretical and a technological perspective. Specifically, these devices offer broadband spectra with uniform line powers and free spectral ranges of tens or hundreds of gigahertz, while being fully amenable to chip-scale integration. Kerr frequency combs thus open the potential to disrupt a series of highly relevant applications, ranging from massively parallel wavelength-division multiplexing (WDM) in optical communications to high-precision optical ranging and high-resolution spectroscopy. However, despite this potential, widespread deployment of Kerr combs is still hindered by the high pump power levels and by the low power conversion efficiency via the rather weak cubic nonlinearities. In addition, efficient schemes for controlling the carrier-envelope offset frequency of Kerr combs are still lacking, thereby impeding the application of Kerr combs in high-precision optical metrology and optical frequency standards. Within QuadCOMB, we will explore, implement, and experimentally demonstrate novel approaches for frequency comb generation in photonic integrated circuits (PIC) that offer improved power conversion efficiency along with stabilized carrier-envelope offset frequency by combining Kerr-type cubic with quadratic optical nonlinearities. We consider hybrid device concepts, where cubic and quadratic optical nonlinearities are first realized on distinct integration platforms and then merged on a package-level by 3D-printed chip-chip connections, as well as monolithically integrated devices, built on a single integration platform that simultaneously features cubic and quadratic nonlinearities. As a starting point, we will develop a theoretical framework that accounts for the simultaneous influence of cubic and quadratic nonlinearities and that will allow us to study the dynamics of comb formation and to predict the characteristics of the resulting combs. Based on this, we will design, fabricate and characterize hybrid and monolithic comb sources, exploiting silicon-nitride (Si3N4) PIC with cubic nonlinearities and AlGaAs-on-Insulator PIC offering both quadratic and cubic nonlinearities. Based on successful implementation of the comb sources, we shall demonstrate their viability in proof-of-concept experiments related to ultra-broadband signal processing and high-resolution spectroscopy. Bringing together leading researchers from the participating German and French groups and including a renowned scientist as a Mercator Fellow, QuadCOMB can rely on an internationally unique combination of skills in the field of photonic integration, nonlinear optics and chip-scale frequency comb sources.