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Dark-Soliton Engineering in Microresonator Frequency Combs

Periodic Reporting for period 1 - DarkComb (Dark-Soliton Engineering in Microresonator Frequency Combs)

Reporting period: 2018-05-01 to 2019-10-31

We often take it for granted, but every time we send an email, stream videos or use a social network, we make use of a complex network infrastructure of fiber-optic cables connecting remote parts of our planet. Fiber-optic communication systems rely on a technology called wavelength division multiplexing, whereby hundreds of lasers of different frequencies are used to cover the bandwidth available in the fiber link. All these lasers can in principle be replaced by a single light source known as microresonator frequency comb — a chip-scale device that generates multiple evenly-spaced frequencies when pumped by a single-frequency laser. Replacing multiple lasers with a single one is an extremely promising prospect for decreasing energy consumption in data communications while enabling extremely fast data transmission.

However, one practical concern when dealing with a single-laser source for wavelength division multiplexing is the amount of power that can be obtained per channel. A key outstanding issue with microresonator frequency combs is that they display relatively low power conversion efficiency, meaning that only a fraction of the input power of the laser is successfully transferred to the new frequency components. The overall objective of DarkComb is to investigate a novel microresonator comb source called “dark soliton”, which displays unusually high-power conversion efficiency, but has been very little investigated by the research community. This aim requires: (1) to understand the fundamental complex nonlinear interactions of dark solitons in optical microresonators, (2) to develop a suitable arrangement of ultra-low-loss microresonators for generating dark solitons with a performance compatible with fiber communication systems and (3) to realize system-level experiments demonstrating unprecedented communication speeds. If successful, the DarkComb project will mark the start of a new, practical, high-performance and compact technology platform that will enable the next generation of optical communication systems.
In the first 18 months of the project, we have gained a significant physical understanding of the formation dynamics of dark solitons in optical microresonators. We have proved their superior power conversion efficiency and demonstrated numerically that the arrangement of linearly coupled cavities proposed in DarkComb can indeed produce dark solitons on demand — an aspect that has never been demonstrated before. In addition, we have demonstrated an ultra-low-loss platform based on the silicon nitride material. This photonic integration platform will form the basis for the fabrication of the devices and components envisioned in DarkComb that shall be developed in the upcoming periods.
Our model provides unprecedented quantitative prediction of dark soliton dynamics in optical microresonators. One essential aspect of the formation of dark solitons is that they rely on a particular feature of the microresonator known as avoided mode crossing. Our model has been validated with microresonators where these avoided mode crossings are fixed yet uncontrollable. In the upcoming periods, we expect to generate experimentally dark solitons in the arrangement of linearly coupled cavities envisioned in the DoA, where the avoided mode crossings are adequately engineered. This is a crucial aspect to ensure that dark soliton microresonator combs not only offer high performance but can be engineered in a highly reproducible fashion. We expect to demonstrate an unprecedented combination of stability, reproducibility and conversion efficiency in microresonator frequency combs in the next reporting period. Our ultra-low-loss silicon nitride photonic integration platform constitutes an outstanding basis for this accomplishment.
Silicon nitride microresonator fabricated at Chalmers premises. The bar corresponds to 2 micrometers