Report Title: Normal-Dispersion Combs in LiNbO3 Microresonators
Speaker: Zongda Li
Host: Professor Wei Xiaoming
Report time: 10:00 am on Wednesday, December 31st, 2025
Report location: Academic Lecture Hall, Room 213, 2nd Floor, Physics Building (Building 18)
All teachers and students are welcome to participate!
School of Physics and Optoelectronics
December 30, 2025
Report Summary
The proliferation of fibre optic networks is one of the greatest advancements of the 21st century. These networks form the fundamental infrastructure of the internet that is woven into the very fabric of our everyday lives and powers the enormous digital economy that, in 2024, outperforms most other sectors with an average growth rate of 7.6% [1]. Our demand for telecom bandwidth is expected to rise exponentially in the coming years. To meet this challenge and prevent an economically damaging bandwidth bottleneck, a next-generation multi-frequency optical source is needed to expand our network capacities. In this collaborative work with researchers at Harvard University, we demonstrate a promising candidate: normal-dispersion optical frequency combs induced by engineered avoided mode-crossing in commercial-ready Lithium Niobate (LiNbO3) optical microresonators [2].
We experimentally observed two dynamically distinct forms of normal dispersion combs manifesting in LiNbO3 microresonators with free-spectral ranges of 100 GHz and 25 GHz. Importantly, both microresonator designs incorporate a compact Euler bend [Fig. 1(a)] that weakly couples the fundamental and first-order transverse electric modes of the resonator, leading to an avoided-mode crossing near the driving frequency.
In the 100 GHz microresonators, we observed a normal dispersion comb spanning 200 nm [Fig. 1(b)]. The measured comb has a pump-to-comb energy conversion ratio of 40%, which is significantly higher than the typical 10% conversion ratio of the more widely studied temporal cavity soliton [3]. Our numerical simulations revealed that this structure exhibits self-forming and self-arranging capabilities, where pulses not only emerge spontaneously from an empty cavity under appropriate conditions, but also rearrange themselves into equally spaced soliton crystals' without any external influence.
On the other hand, in the 25 GHz microresonators, we leveraged the strong stimulated Raman scattering (SRS) of LiNbO3 and generated stably coexisting normal dispersion Kerr-Stoke combs [Fig. 1(c)]. Similar to the 100 GHz microresonators, we observed a stable normal dispersion comb centred around the driving wavelength of 1590 nm. However, the strong SRS facilitated the formation of a separate but coherent Raman Stoke comb at the longer wavelength side, effectively expanding the spectral bandwidth of the frequency comb by an additional 100 nm.
References
[1] OECD, OECD Digital Economy Outlook 2024 (Volume 1): Embracing the Technology Frontier. in OECD Digital Economy Outlook. OECD, 2024.
[2] Y. Song, Z. Li, X. Zhu, N. Lippok, M. Erkintalo, and M. Loncar, ‘High-efficiency and broadband coherent optical comb generation in integrated X-cut lithium niobate microresonators’, 2025, arXiv.
[3] P.-H. Wang et al., ‘Intracavity characterization of micro-comb generation in the single-soliton regime’, Opt. Express, vol. 24, no. 10, p. 10890, May 2016.
Speaker's Biography
Zongda Li is currently a research fellow at the University of Auckland nonlinear optics group, from which he also obtained his PhD in 2024. His works focus on nonlinear optics, temporal cavity soliton (CS) physics, and other Kerr-induced temporally localised structures. Some ongoing research involves CSs in the presence of higher-order dispersion, novel localised structures in normal dispersion regimes and coherent phase-locking between soliton and injected beams.
