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Dark-Soliton Engineering in Microresonator Frequency Combs

Periodic Reporting for period 2 - DarkComb (Dark-Soliton Engineering in Microresonator Frequency Combs)

Reporting period: 2019-11-01 to 2021-04-30

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 part of the project, we gained a significant physical understanding of the formation dynamics of dark solitons in optical microresonators. We have demonstrated a photonic platform with ultra-low-loss based on silicon nitride material. This photonic integration platform has formed the basis for the fabrication of an arrangement of linearly coupled cavities, named photonic molecules. This arrangement has allowed us to obtain coherent microcombs with record-high conversion efficiency and reproducibility. This achievement constitutes the core idea of DarkComb, and it delivers a performance significantly beyond the state of the art whose results have been published in Nature Photonics.

With our microcombs, and in collaboration with the Technical University of Denmark, we have demonstrated that microcombs offer the performance required to power the next-generation transmission systems. To be more concrete, we have demonstrated that a single microcomb has the performance to transmit data beyond one petabit per second (results under peer review). This results from the unique combination of microcomb technology with multi-core optical fiber.

The next frontier consists of unraveling the phase noise dynamics of this dual-comb cavity arrangement. This is of crucial relevance for the realization of the envisioned applications in the field of optical spectroscopy and advanced multi-dimensional communication systems.
We have demonstrated an integrated photonic platform based on dispersion-engineered silicon nitride with a loss performance significantly beyond what is commercially available and at the forefront of the research landscape.
Based on this platform we have demonstrated, numerically and experimentally, an arrangement of linearly coupled cavities that results in microcombs with record-high conversion efficiency and reproducibility.
We have demonstrated a novel system-level architecture that dramatically simplifies the receiver scheme in fiber-optic communications, that utilizes an inherent physical property of frequency combs.
Finally, we have for the first time, demonstrated a fiber-optic communication system in the petabit per second regime using a single microcomb.

In the second half of the project, we aim at:
1. Unraveling the phase coherence and correlations of our dark soliton microcombs
2. Realizing record-breaking demonstrations in fiber-optic communications based on our microcombs, with regards to transmission speed and software simplicity.
3. Exploring new wavelength regions, in particular in the 2um wavelength range.
4. Investigate alternative uses of our technology in the context of high-resolution comb-based spectroscopy.
Silicon nitride microresonator fabricated at Chalmers premises. The bar corresponds to 2 micrometers