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Applications and Fundamentals of Microresonator Frequency Combs

Periodic Reporting for period 1 - MICROCOMB (Applications and Fundamentals of Microresonator Frequency Combs)

Reporting period: 2019-01-01 to 2020-12-31

The consortium has established the cross-sectorial training and research programme bringing together leading European academic and industrial centers working in the area of optical frequency combs in microresonators (microcombs).

The programme combines and shares some of the world-leading experience and expertise in the microcombs and trains a new generation of scientists in this actively developing area bordering physics and photonic engineering and having pronounced applied and fundamental dimensions. The programme will have a lasting impact increasing European innovation capacity through expanding the knowledge base, new IP, trained
personnel, better-equipped laboratories and future collaborations leading to product development.

Early-stage researchers involved in the proposed network have been receiving the broad and well-balanced training in device characterisation and fabrication, experimental techniques and theory and modelling in the best academic, research centre and industrial environments.
"In the first reporting period from 01/01/2019 to 31/12/2020 the Microcomb ITN delivered on the following commitments:

WP1 Ethics Requirements collected from all project partners by Coordinator

WP2 Management:

• Consortium Agreement was signed by all partners in March 2019
• Creation of the project’s website, which has been used as the main point of information for the project
• Data Management Plan for the project established
• Project Boards have met regularly (Supervisory Board, Training Board and Research and Exploitation Board). All Board meeting minutes are shared confidentially on the project’s website
• Recruitment of 15 ESRs
• ESR Personal Development Plans collected by the Coordinator

WP3 Dissemination and Outreach:

• Despite the obstacles caused by the Covid-19 the Consortium participated in (mainly online) outreach and dissemination events throughout the first reporting period. These events are available for viewing on the Microcomb website.
• The MICROOMB website Twitter (@microcomb) and a Linkedin portal for the Microcomb ITN were created to help partners and ESRs with networking inside and out of the consortium.

WP4; WP5; WP6 Training and conferences:

• Delivery of Transferable skills workshops 1 (EPFL) and 2 (IBM)
• Delivery of Intensive technical and lab courses 2 (UGENT) and 4 (UPV)
• Plans firmed up for the Intensive technical and lab courses 1 (KIT&CUT) and 5 (MENLO)
• Series of webinars were put in place of the Scientific School and Network Conference 1 (BATH) (adjustments made due to the Covid-19 outbreak)
• Plans made for the Scientific Schools and Network Conference 2 (EPFL)
• Preparations for an additional network conference planned for mid-April 2021 are in the final stage – this conference is to complement secondments, which are currently postponed due to the Covid-19 pandemic

WP7-10 Research

The research is continuing steadily considering a negative impact caused by Covid-19.

WP7. ""Signal processing & optical communications"".

Bath has developed and applied the model for counterrotating soliton-combs in micro-resonators with high repetition rates. This setup is implemented in MPL and EPFL.
CUT and EPFL have observed solitons and dispersive waves in the microresonator-dimer system. Bath has predicted the soliton-blockade effect. EPFL is developing a model for the synthetic frequency dimensions.
EPFL has successfully generated low-noise dissipative Kerr solitons in Si3N4 in the microwave K- and X-band in 2018 for the first time.
Within the ongoing effort to improve the performance of the integrated microwave oscillators in the framework of the Microcomb ETN program, EPFL has conducted a series of phase noise measurements using a packaged silicon nitride device.
CUT has taken a leading role in the development of the optical frequency domain reflectometry (swept-wavelength interferometry) method for the assessment of integrated photonic devices. In the context of microresonators, this technique allows for disentangling the coupling rate from the intrinsic loss contributions in the assessment of the loaded quality factors over the telecommunications C and L bands.

WP8: ""Microcomb technologies for spectroscopy, metrology and astronomy"".

EPFL performed SD-OCT imaging on an ex-vivo fixed mouse brain tissue using the soliton microcomb, alongside an SLD for comparison, and demonstrated the principle viability of soliton based SD-OCT. Moreover, EPFL demonstrated the potential for circular ranging, i.e. optical sub-sampling, due to the high coherence and temporal periodicity of the soliton state. Most, importantly, EPFL demonstrated that the residual intensity noise (RIN) of the soliton microcomb is not limited by incoherent amplified spontaneous emission noise, but photon shot noise and that the excess RIN contributions of all teeth of the microcomb are highly correlated, which means that they do not contribute to the SD-OCT signal.
GENT has designed a low-noise III-V-on-silicon comb generator on a photonic chip, that emits a flat-top spectrum of 1400 lines at a repetition frequency of 1.0 GHz, a feature never approached by other ultra-miniaturized comb synthesizers.

WP9 ""Hybrid integration and packaged solutions for comb generator modules"".

GENT has demonstrated the first results of the operational hybrid integrated mode-locked laser.
MENLO were able to successfully package a silicon nitride microresonator chip with a 20 GHz free spectral range for operation at 1550 nm. The combination of thermoelectric cooler and thermistor has allowed to accurately stabilize the temperature at setpoints between 20°C and 35°C, which was used to tune the chip resonances relative to the laser wavelength.
Lensed fibres at the in- and output provided a total through-coupling efficiency of 8 %.
Although MENLO expects that significantly higher efficiency should be possible, this was enough to generate a comb spectrum with an external amplified laser.

WP10 Frequency conversion in microresonators with chi(2) nonlinearity

FRB has fabricated millimetre-sized microresonators made of AgGaSe2 and CdSiP2 and demonstrated
optical parametric oscillation tunable from 2.8 to 3.5 µm wavelength. FRB has also experimentally demonstrated the basic concept of cascading second-order nonlinearities for comb generation in LiNbO3 microresonators for the first time.
BATH has developed a theoretical model and numerical codes for frequency combs LiNbO3 microresonators. Bath demonstrated sparse combs and developed the theory allowing them to predict their power thresholds."

Observation of solitons and dispersive waves in the microresonator-dimer system.
Prediction of the soliton-blockade effect.
Prediction of the synthetic frequency dimensions.


Generation of photonic chip-based resonant supercontinuum.
Demonstration of high-resolution spectroscopy with III-V-on-silicon modelocked laser.
Demonstration of hybrid SiN-LiNbO integrated platform for electro-optic conversion.


Demonstration of a butt-coupling setup for hybrid-integration of a III/V source and SiN comb generator.


Demonstration of high-Q non-oxyde crystal microresonators for chi-2 photonics.
Demonstration of first frequency combs via second-harmonic generation in LiNbO microresonators.