High-efficiency and broadband coherent optical comb generation in integrated X-cut lithium niobate microresonators

TL;DR

This paper demonstrates high-efficiency, broadband coherent optical microcombs in integrated X-cut lithium niobate microresonators, leveraging ultralow-loss waveguides and innovative design to surpass existing performance and explore new nonlinear states.

Contribution

It introduces a novel approach for generating normal-dispersion Kerr microcombs on X-cut TFLN, including a microcomb driven by both Kerr and Raman effects within a single mode.

Findings

Surpassed state-of-the-art microcomb performance metrics.

Demonstrated a microcomb combining Kerr and Raman nonlinearities.

Achieved extended spectral spans with interleaved frequency combs.

Abstract

The ability to generate efficient and coherent frequency combs using photonic integrated circuits offers tremendous potential for a range of applications. In particular, "microcombs" based on chip-integrated resonators are poised to revolutionize optical communication, computation, and sensing systems, especially when paired with fast electro-optic (EO) devices. X-cut thin-film lithium niobate (TFLN) is a promising platform for developing the next-generation of microcomb-driven integrated photonic systems, providing a diversity of functionalities through combined $\chi^{(3)}$ and EO nonlinearities. In this context, normal-dispersion Kerr microcombs are critically needed because of their standout advantages, yet this dispersion regime remains unexplored for comb generation on X-cut TFLN. Here, we leverage ultralow-loss photonic waveguides, as well as strategic resonator designs that allow us to tailor Raman effects and engineer desired spatial mode interactions, for the robust generation of normal-dispersion Kerr microcombs. Specifically, we show microcombs that substantially surpass state-of-the-art bright cavity soliton and EO microcombs on X-cut TFLN in key performance metrics. Additionally, we demonstrate a novel microcomb whose existence is underpinned by both normal-dispersion Kerr dynamics and stimulated Raman scattering, in a single spatial mode of a microresonator. This microcomb manifests itself as two interleaved frequency combs centered about the pump and Stokes frequencies, resulting in extended spectral spans. Our work will unlock high-speed and low energy consumption photonic circuits for communications, frequency synthesis, and signal processing enabled by a monolithic microcomb technology, while also stimulating further investigations of new nonlinear states that may synergize the strong hybrid nonlinearities unique to X-cut TFLN.