Mini symposium on «Field dynamics in Microresonators and active cavities»

The goal of this Mini-Symposium is to cover the latest progress in studies of the field dynamics in microresonators and active cavities. Photonic crystal, microresonator and fiber laser-based sources of optical frequency combs will be discussed. The physical mechanisms underlying intracavity frequency conversion mechanisms based on either quadratic or cubic nonlinearities will be analyzed by means of different analytical and numerical techniques, and compared with the experiments. Novel cavity architectures for enhancing the frequency conversion efficiency or reducing the power thresholds will be introduced, paving the way to the widespread use of optical frequency combs in a variety of technological applications. Excellent researchers will present their results and will be available to discuss and exchange ideas with the students.

Jose Chavez Boggio, Leibniz Institut fur Astrophysik
Tobias Hansson, Linköping University, Sweden
Iolanda RiccardiCNR-Istituto Nazionale di Ottica (INO), Italy
Alfredo De Rossi, Thales, SA
Pedro Parra, Université libre de Bruxelles (ULB), Belgium
Sonia Boscolo, Aston University, UK
Philippe Grelu, Université de Bourgogne, France

9:00 – 9:15 Welcome by Prof. Stefan Wabnitz 
9:15 – 10:00 Jose Chavez Boggio “Efficient Kerr soliton comb generation in micro-resonator with interferometric back coupling”
10:00– 10:45 Tobias Hansson “Multiple comb generation in 𝜒(3) microresonators” 

Coffee break 30 min 

11:15 – 12:00 Iolanda Riccardi, “Optical frequency combs in quadratic resonators” 
12:00 – 12:45 Alfredo De Rossi, “Photonic crystal parametric sources” 

Lunch break 90 min 

14:15 – 15:00 Pedro Parra “Dissipative localized states in dispersive quadratic cavities: Bifurcation structure and stability”
15:00 – 15:45 Sonia Boscolo “Intelligent control of breather dynamics in ultrafast fibre lasers” 
15:45 – 16:30 Philippe Grelu “Smart lasers tame complex ultrafast laser dynamics” 

Mini-Symposium Location: Reading Room of DIET (2nd floor), Sapienza University of Rome, Via Eudossiana 18, 00184 Rome, Italy 

Iolanda Ricciardi is a Research Scientist at CNR-Istituto Nazionale di Ottica (INO). She received Physics Degree cum laude in 2002 and PhD in 2007 at Università di Napoli “Federico II”. During the PhD activity, she participated to the Virgo Project for gravitational wave detection (associate scientist at Istituto Nazionale di Fisica Nucleare). In 2007, she joined INO as a post-doc fellow and in 2010 she became a research scientist. She works in the Nonlinear and Quantum Optics Laboratory. Her main research is focused on both the development of innovative laser sources, through interactions in quadratic nonlinear media, and their applications in quantum optics and high resolution spectroscopy. Current activity is devoted to frequency comb formation in cavity-enhanced nonlinear processes; generation of nonclassical states of light, precision spectroscopy with cold stable molecules. Iolanda Ricciardi participated to several national and international projects. She is co-author of more than 80 publications on ISI-journals and book chapters (more than 2000 citations).

 

Tobias Hansson received the M.Sc. degree in engineering physics and the Ph.D. degree in electrical engineering from Chalmers University of Technology, Sweden, in 2007 and 2011, respectively. Following his Ph.D., he has been a postdoctoral fellow working on theory and modelling of optical frequency combs and mode-locked lasers in both Italy (University of Brescia) and Canada (INRS-EMT) from 2012-2018. Since 2018 he is an assistant professor in the Theoretical Physics division at the Department of Physics, Chemistry and Biology (IFM) at Linköping University. He has a track record of theoretical research in nonlinear optics and photonics and has authored and co-authored a total of more than 80 peer-reviewed journal and conference contributions. His research interests are primarily focused on the modelling of optical frequency combs, mode-locked lasers and microresonator devices.

Philippe Grelu has been Professor of Physics at Université de Bourgogne, in Dijon, France, since 2005. After receiving his PhD at University of Paris-Orsay in quantum optics (1996), his interest moved to ultrafast nonlinear optics and mode-locked fiber lasers. He developed a key expertise in nonlinear optical cavity dynamics, with major contributions in the fast-developing field of dissipative solitons. He mainly develops experimental research at laboratory ICB, co-operated between CNRS and the University. He was instrumental in the setting up of the Photonics Department at ICB in 2015, which hosts about 60 researchers, and has been chairing the Department since. With his team from ICB, he initiated the field of smart lasers in 2015. He has authored over 150 scientific publications.

 

Sonia Boscolo received the BSc and MSc degrees in Physics from Université de Bourgogne (Dijon, France) and a PhD degree in Engineering and Applied Science from Aston University (Birmingham, UK) in 1998 and 2002, respectively. Since 2002, she has been working with Aston Institute of Photonic Technologies at Aston University, where she is a Senior Research Fellow and the Programme Director for the Erasmus Mundus Joint Master’s Degree programme EMIMEO funded by the European Commission. She has broad theoretical and modelling expertise in nonlinear optics and photonics, specialised in bridging mathematical methods with their application in the context of optical fibre communications and laser systems as well as in the design and modelling of novel nonlinear photonic systems and devices. She has published over 200 refereed journal and conference papers, 5 book chapters and 3 patents, and co-edited a book on ‘Shaping Light in Nonlinear Optical Fibers’ published by Wiley. She has been the Principal Investigator in 3 research projects sponsored by the EPSRC and Leverhulme Trust (UK) and 2 British Council-sponsored collaborative research projects with European institutions, and the UK Leader of 4 European projects (Erasmus+ Mobility, Erasmus Mundus Partnership NANOPHI, Marie S.-Curie Actions, EMIMEO).

 

Jose Chavez Boggio, Leibniz Institut fur Astrophysik
“Efficient Kerr soliton comb generation in micro-resonator with interferometric back coupling”
Abstract:
Nonlinear Kerr micro-resonators have enabled fundamental breakthroughs in the understanding of dissipative solitons, as well as in their application to optical frequency comb generation. However, the conversion efficiency of the pump power into a soliton frequency comb typically remains below a few percent. In this talk we describe a new architecture to generate soliton frequency combs, namely a hybrid Mach-Zehnder ring resonator geometry, consisting of a micro-ring resonator embedded in an additional cavity with twice the optical path length of the ring. The resulting interferometric back coupling enables to achieve an unprecedented control of the pump depletion: pump-to-frequency comb conversion efficiencies of up to 98\% of the usable power are experimentally described with a soliton crystal comb. The robustness of the proposed on-chip geometry is discussed and the generation of a large variety of dissipative Kerr soliton combs is described. Those Kerr solitons require a lower amount of pump power to be accessed, when compared with an isolated micro-ring resonator with identical parameters. Finally, we show that micro-resonators with feedback enable accessing new regimes of coherent soliton comb generation, and are well suited for comb applications in astronomy, spectroscopy and telecommunications.

___________________

Tobias Hansson, Linköping University, Sweden
Multiple comb generation in 𝜒(3) microresonators”
Abstract:
The generation of optical frequency combs in microresonator devices has the potential for enabling widespread applications in a broad range of fields such as frequency metrology, optical communications and spectroscopy. However, a disadvantage of conventional Kerr comb synthesizers is that they only emit radiation in a spectral range centered around the pump laser frequency. In this talk we consider ways of overcoming this limitation by simultaneous comb generation and frequency conversion of the pump laser field in resonators engineered to phase-match the third-harmonic generation process. We present recent advances in modeling the nonlinear dynamics of multiple combs in different spectral regions of dispersive microresonators that feature cubic nonlinearities. We introduce a theoretical formalism, based on cavity-averaged equations, and discuss their solution by numerical methods. We particularly consider the importance of the stability properties of the homogeneous solution in generating various types of multi-frequency combs, including coupled cavity solitons, and discuss the influence of walk-off on the comb formation process.

___________________

Iolanda RiccardiCNR-Istituto Nazionale di Ottica (INO), Italy
“Optical frequency combs in quadratic resonators”
Abstract:
Optical frequency combs (OFCs), consisting of thousands of equally spaced sharp laser frequencies, originally revolutionized the field of absolute frequency metrology, and nowadays represent a fast-growing research field, as they are routinely used in a wide range of scientific and technological applications. An alternative approach to the traditional techniques for OFC generation based on femtosecond mode-locked lasers focused on comb formation via third-order χ(3)nonlinear interactions, paving the way to monolithic OFCs, thanks to a considerable reduction in size, complexity and power consumption. More recently, however, OFCs have also been demonstrated in continuously driven cavities with χ(2) nonlinearities. They represent a new promising paradigm of comb generation, exploiting the inherently higher efficiency of quadratic nonlinear interactions, with the distinctive feature of a simultaneous emission of OFCs both around the pump of the nonlinear process and in spectral regions octave distant from the pump.

The talk will provide an overview of our experimental results about OFC generation in different quadratic nonlinear processes, as second harmonic generation (SHG), in singly or doubly resonant configuration, and optical parametric oscillation. Besides experiments, theoretical models have been derived: first, a frequency-domain modal expansion approach, and successively time-domain mean-field equations including the effects of cavity dispersion, able to model the full temporal and spectral dynamics of the different fields. Some important aspects of these models will be presented, too.

In the end, we explored the potential of quadratic combs as valuable sources of nonclassical light. Our latest experimental results on quantum correlation observation between bright twin beams, generated in a doubly resonant cavity SHG, will be shown.

___________________

Alfredo De Rossi, Thales, SA
“Photonic crystal parametric sources” 
Abstract:
Photonic Crystal Parametric Sources are a new object, where a very efficient parametric interaction takes place in a physical volume on the order of a cubic wavelength. This tiny optical cavity contains a few modes and only three of them are allowed to interact by controlling their spectral spacing. Parametric oscillations appear at a power threshold in the 50uW order. Below, threshold, the parametric interaction leads to the spontaneous emission of correlated photon pairs. I will discuss preliminary results about the properties of stimulated emission and discuss the agreement with theory.

___________________

Pedro Parra, Université libre de Bruxelles (ULB), Belgium
“Dissipative localized states in dispersive quadratic cavities: Bifurcation structure and stability”
Abstract:
Localized dissipative structures (LSs) appear in a large variety of natural domains, such as population dynamics, plasma physics, solid mechanics, and nonlinear optics. Their formation is in general related with the coexistence of different stable extended states within a parameter range of the system, and the locking of front waves connecting them.

In optics, these states may arise in externally driven nonlinear cavities, where light can be trapped and interact continuously with a nonlinear medium. Light localization in dissipative systems is hold by a pairwise equilibrium where nonlinearity compensates dispersion and/or diffraction, while energy dissipation is balanced through external energy driving.

My main interest focuses on the characterization of the bifurcation structure and stability of these states in different passive cavities. In Kerr nonlinear cavities, for example, LSs organize in two main bifurcation structures. In the presence of uniform-bistability, i.e., when two uniform states of the system coexist, LSs form due to the locking of plane fronts, and lead to a collapsed homoclinic snaking structure. In contrast, when a Turing periodic pattern and a uniform state coexist (Turing-bistability), LSs form through the locking of patterned fronts, and organize differently, leading to a standard homoclinic snaking.

In the past years my research has focused on the study of quadratic cavities. The results of my work show that, in these cavities, LSs appear due to the same previous mechanism and lead to different realizations of the standard and collapsed homoclinic snaking structures. In degenerate optical parametric oscillators, the dynamics and stability of LSs has been analyzed in doubly and singly resonant configurations, where static and dynamic LSs of different morphologies have been characterized. In cavity enhanced second harmonic generation, a bifurcation analysis has been also carried out in a doubly resonant configuration yielding similar results.

___________________

Sonia Boscolo, Aston University, UK
“Intelligent control of breather dynamics in ultrafast fibre lasers” 
Abstract:
In addition to their growing use as sources of ultrashort pulses for many applications, mode-locked fibre lasers constitute an ideal platform for the fundamental exploration of complex nonlinear wave dynamics. However, reaching a desired operating regime in a fibre laser generally depends on precisely adjusting multiple parameters in a high-dimensional space, in connection with a wide range of accessible pulse dynamics. Machine-learning strategies and the use of evolutionary and genetic algorithms have recently led to a number of dramatic improvements in dealing with such a multivariable optimisation problem [1,2]. Yet, existing machine-learning tools are mostly designed to target regimes of parameter-invariant, stationary pulse generation, while the intelligent excitation of evolving pulse patterns in a laser remains largely unexplored. 

Breathing solitons form an important part of many different classes of nonlinear wave systems, manifesting themselves as localised temporal/spatial structures that exhibit periodic 

oscillatory behaviour. Recently, they have also emerged as ubiquitous mode-locked regime of ultrafast fibre lasers [3,4]. These nonlinear waves are attracting significant research interest in optics in virtue of their connection with a range of important nonlinear dynamics, including rogue wave formation [5], the Fermi-Pasta-Ulam recurrence, turbulence, chimera states, chaos and modulation instability phenomena. Breathers also bear interesting possibilities for practical applications, such as in dual-comb spectroscopy or the direct generation of high-amplitude ultrashort pulses from a laser cavity. 

In this talk, I will review our recent demonstration of an evolutionary algorithm for the self-optimisation of the breather regime in a mode-locked fibre laser, based on the optimal four-parameter tuning of the intracavity nonlinear transfer function through electronically driven polarisation control [7]. We have defined compound merit functions relying on the characteristic features of the radiofrequency spectrum of the laser output, which are capable to locate various self-starting breather regimes in the laser, including single breathers with controllable breathing ratio and period, and breather molecular complexes with a controllable number of elementary constituents. Our work opens novel opportunities for the exploration of highly dynamic, non-stationary operating regimes of ultrafast lasers, such as soliton explosions, non-repetitive rare events and intermittent nonlinear regimes.

___________________

Philippe Grelu, Université de Bourgogne, France
“Smart lasers tame complex ultrafast laser dynamics” 
Abstract:
Commercial mode-locked fiber lasers generally offer a limited pulse output versatility, routinely reduced to a single on/off firing switch. This contrasts with the large range of ultrafast pulse regimes obtained in photonics laboratories. However, shifting from one pulse regime to another usually involves a tedious manual adjustment of the laser cavity parameters. This is rooted into the complexity of nonlinear laser dynamics, which most often precludes the establishment of analytical relationships between the output pulse parameters and the laser cavity parameters.

In recent years, significant efforts led to the self-generation of laser regimes combining a computer control of accessible cavity parameters with the use of a feedback loop fed from measured laser output features. In 2015, we achieved an important step involving artificial intelligence, with the implementation of evolutionary algorithms to control the cavity’s degrees of freedom according to the user’s pre-defined objective. I will present the principles of such ultrafast lasers driven by genetic or evolutionary algorithms, with a description of some of the subsequent progress made since their inception.