Title: Multi-colour frequency combs in the Kerr microresonators
Author: Hai-Zhong Weng, Vikash Kumar, Huilan Tu, Qiaoyin Lu, Weihua Guo, Dmitry Skryabin, John Donegan
Conference: 2024 24th International Conference on Transparent Optical Networks (ICTON)
Link to full paper: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10647246
What is this paper all about?
The paper explores the generation of multi-colour frequency combs using a single Kerr microresonator, with a specific focus on silicon nitride (Si3N4) microring resonator devices. A frequency comb is a special type of light that consists of many evenly spaced “colors” (or frequencies). If you imagine the colors of light as teeth on a comb, the gaps between them are equal, like the teeth of a real comb. Kerr microresonators are compact optical cavities that utilize the Kerr nonlinearity to produce a spectrum of discrete optical frequencies, forming a frequency comb (fig. b). Two key effects influence the generation of these combs:
- Nonlinearity of Si3N4Si3N4: This effect works to broaden the frequency comb.
- Dispersion: This counteracts the nonlinear effect, limiting the comb’s width.
When the effects of nonlinearity and dispersion balance each other, the system reaches a low-noise, stable state known as the soliton state. By carefully engineering the dispersion properties, we demonstrated the ability to overcome these competing effects, enabling the generation of more than one group of frequency combs using a single continuous pump laser (laser that’s wavelength is centered at resonance condition of our resonator) and a single microresonator. The study highlights how precise tuning of dispersion and coupling rates in microresonators can achieve robust and efficient multi-colour frequency comb generation.
What have you discovered?
We have discovered that by designing a highly over-coupled Silicon Nitride microresonator with an appropriate dispersion coefficient, we can generate two optical parametric oscillation (OPO) sidebands (frequencies that are generated far from the pump frequency) that are separated by more than 100 THz. One of these sidebands, located around 245 THz, can act as a new pump wave (as a new laser source), triggering Kerr frequency comb generation due to anomalous dispersion, as shown in the results section. Additionally, cross-phase modulation (XPM, when these generated frequencies effect other frequencies in the frequency comb) between the signal-based OPO comb and the original pump (or idler wave) leads to the creation of multiple microcombs, as seen in the final image. We also investigated a case with two closely spaced modes for the TM mode (Transverse Magnetic mode) and found that a soliton can be generated from the TM₁₀ mode, with the TM₀₀ mode remaining as the primary comb.
So what
This study enables the generation of multi-colour frequency combs, providing compact and versatile light sources for applications in photonic communication, metrology (precise measurement of time like atomic clock), and spectroscopy (study of the interaction between matter and electromagnetic radiation this can be used for many things like gas sensing and breathalyzer). It offers insights into designing efficient microresonators, addressing challenges in generating stable, multi-functional combs, and extending frequency combs into new wavelength ranges by overcoming dispersion and laser limitations. The coexistence of multiple microcombs can reshape the spectral profile and enhance the link between optical and microwave systems.
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