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Applications and Fundamentals of Microresonator Frequency Combs

Periodic Reporting for period 2 - MICROCOMB (Applications and Fundamentals of Microresonator Frequency Combs)

Reporting period: 2021-01-01 to 2022-12-31

The project aimed to deliver cutting-edge application-oriented research results and train a group of early-career scientists
to ensure EU competitiveness in optical frequency-comb technology and, more broadly, in photonics R&D across
the academic and industrial sectors. On the research side, the project team has concentrated on addressing the existing
technological limitations of microresonator combs and understanding open fundamental problems.
The training was done at a series of hands-on workshops and research schools engaging international experts
from across the Globe. Secondments between the academic and industrial partners made an important contribution
to providing our researchers with cutting-edge technical skills and exposing them to various environments
to make them confident and competitive in the R&D job market.

The project research area links to many problems important for modern society. Data processing and transmission
applications emerge through the multitude of spectral lines constituting the comb spectrum,
where each of these lines can serve as an independent and simultaneous
with the other communication channel allowing to boost the data transmission and improve precession
of distance measurement by lasers. The spectral sensitivity of combs is such that they are used to record the variation of
the spectra emitted by stars due to invisible exoplanets rotating around them and tiny changes in
the concentration of pollutants in the atmosphere.


Overall research objectives of the project were to develop:
- microcomb technology for signal processing and optical communications
- microcomb based precision spectroscopy tools
- on-chip microcomb based photonic modules and packaged solutions
- microcomb sources based on second-order nonlinearities
- understanding of fundamental physics behind the comb solitons relevant for the applications
The research programme of the consortium embraced four work packages pushing frontiers of the microresonator frequency
comb technology in the areas of (1) signal processing and optical communication, (2) spectroscopy, metrology and astronomy,
(3) hybrid integration and packaged solutions for comb generator modules, and (4) frequency conversion using chi(2) nonlinearity.

In (1), the consortium explored applications associated with RF and THz signal processing based on micro-comb technologies
developed numerical and theoretical models and made integrated devices for these applications. We achieved progress
on microresonator applications for wavelength division multiplexing in optical communications and developed space
division multiplexing for massively parallel transmission. We have achieved several results in generating
ultra-broadband combs and demonstrated novel comb-based optical arbitrary waveform generation and detection schemes.

In (2), we developed comb self-referencing on a chip, realised dual-comb spectroscopy with two microcombs of
slightly different repetition rates and explored comb generation in the mid-infrared for molecular spectroscopy applications.
We developed electrically pumped mode-locked lasers on a silicon platform for an integrated dual-comb spectrometer.
Extending combs to visible and infrared was achieved in LiNbO_3-on-insulator waveguide and resonators with cascaded nonlinearity.

(3) The outside-laboratory applications of frequency combs require developing packaged solutions. We developed
the first such devices using microcombs for astro and optical data processing applications. Our technique uses the
so-called photonic wire bonds to connect the different components. We have also explored a more advanced solution
to the problem: using chip-scale light sources integrated on the same chip with a microresonator using the heterogeneously
integrated mode-locked lasers with III/V gain sections, amplifiers, and saturable absorbers that are
all electrically pumped on a single silicon chip.

In (4), the consortium developed the first microcombs based on cascaded second-order nonlinearities using several
material platforms. We demonstrated the first Turing pattern combs in lithium niobate and explored the thin-film
lithium niobate resonators and waveguides for comb generation. Our work on cadmium-silicon-phosphate allowed
us to generate microcombs in the mid-infrared. To achieve these results, we have fabricated whispering-gallery
microresonators for frequency doubling, providing the highest quality factors to date, 10^8.
We predicted and observed first spectrally staggered combs. We have explored gallium-phosphate
and its mixed chi(2) and chi(3) nonlinear response to develop comb sources in the visible.


The significant elements of the training programme delivered by the project consortium were transferrable skills
workshops, hands-on lab courses, webinars and scientific schools. The consortium delivered four transferrable skills workshops
to boost early-stage researchers' employability. These workshops were:
-Computational methods for nonlinear photonics,
-Taking an idea to a product,
-Entrepreneurial challenges and IP management in small and medium R&D businesses,
-Space for physicists (R&D career in space industry).

Training in specific technical skills in demand in the photonics industry and university labs, and essential for
the success of the consortium research programme, were addressed by a series of courses, which included –
-Modern coherent optical communications;
-Large scale photonic integration (two parts);
-Full-field photonic components and integrated circuits;
-Femtosecond laser-based comb technology.

The consortium has organised three scientific schools collocated with conferences for the early-stage researchers,
which attracted many external participants and world-leading experts in the area.
These events have raised the international profile of the research done by the consortium, facilitated the flow of ideas,
dissemination of results and provided practical training in communication skills and life contact with potential employers
for the consortium researchers. Training materials and research outcomes were regularly uploaded to the consortium
website at https://www.microcomb-eu.org/ and further disseminated using the @microcomb twitter hashtag.
-Exploitation of dispersion engineering to demonstrate coherent microcomb spectra spanning 97% of an octave.

-Theory and experiments on dissipative Kerr solitons in dispersion-modulated microresonators.

-Demonstration of photonic integrated circuits and sub-systems enabling spectral processing of optical frequency combs.

-Demonstration of a fibre-connected comb base spectral processor.

-x40 improvement in data rate with respect to state of the art has been attained thanks to the combination of microcomb technology with multi-core fibre.

-Demonstration of operational dual-comb spectrometers using electrically-pumped III/V-on-silicon near-infrared mode-locked lasers and microresonators.

-Demonstration of visible and infrared microresonator combs via frequency doubling and parametric down-conversion.
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