Research

Our research activities are mainly based on nonlinear effects in optical fibers. Our aim is to develop breakthrough scenario dealing with light-by-light optical processing and provide optical functionalities for future optical networks. More specifically, our research interests deal with modulational instabilities, polarization control, nonlinear pulse shaping, signal regeneration, chaos dynamics, temporal compression, optical cloaking, supercontinuum generation, space division multiplexing as well as fiber cavities.
 
Light-by-light polarization control

Among the three independent features that characterize a light beam propagating in a monomode optical fiber, namely, frequency, energy and state-of-polarization (SOP), the SOP remains the most elusive variable which is still difficult to predict and control. Indeed, the residual random birefringence associated with mechanical stress, bending, squeezing, vibrations or temperature variations make the SOP of a light beam totally unpredictable after only several tens of meters of propagation. My ERC PETAL project proposes a radically new approach to polarization control issue. The breakthrough idea we investigate, is the possibility for light to self-condensate its SOP into a locked state whatever its initial condition and surroundings without resorting to an opto-electronic feedback. The principle of operation of this device, called Omnipolarizer, is based on a counter-propagating cross-polarization interaction between an arbitrary polarized incident signal and its backward replica generated at the fiber end by means of an amplified reflective loop. We have successfully exploited the device in order to self-repolarize a 40-Gbit/s signal. The bistability of the Omnipolarizer has been exploited to design an all-optical flip-flop memory and an all-optical scrambler. Finally, a fascinating physical aspect of the Omnipolarizer has also been exploited in order to demonstrate a polarization-based temporal tunneling effect as well as a polarization-based temporal cloaking of optical data.

Nonlinear pulse shaping

Optical sources emitting picosecond pulses at very high repetition rates are now widely exploited in numerous scientific applications such as waveform measurement, ultra-high capacity telecommunication systems, clock generation, metrology or component testing. Among all the reported techniques for the generation of high repetition-rate pulse trains, those based on the nonlinear reshaping in optical fiber of an initial beat-signal into a train of well-separated pulses have been proved to be attractive and efficient. Indeed, since those methods are cavity-less, they are free from the typical constraints inherent to cavity‐based mode-locked fiber lasers. They combine wavelength tunability, low jitter and can be designed in a wide range of repetition-rates. In this context, we studied and exploited the method based on a multiple Four-Wave Mixing process taking place in an anomalous dispersion fiber. We have successfully implemented this principle for the generation of high quality pulses with repetition rates ranging from 20 GHz up to 2 THz for telecom applications. Granted by an ANR research program SO FAST, we have developed the following prototype at 40 GHz. Our know-how on pulse reshaping has also be exploited in more fundamental studies among which, optical undular bores and rogues wave prototype generation such as Peregrine and Ma solitons.

Space-division multiplexing

In order to face the non-stop growing demand of data-traffic in nowadays optical networks and to prevent a capacity crunch, a new milestone in the landscape of optical communications has emerged in the last few years, i.e. space-division multiplexing (SDM). In fact, in a SDM platform, each fiber core or mode represents an independent channel, which in principle allows to largely overcome the capacity limit imposed on traditional single-mode systems. In this context, the spatial-dimension offers new horizons for all-optical nonlinear processing. One could provide alternative approaches to multiple-input multiple-output techniques, few-mode amplifiers, spatial demultiplexing, or at least, relax their constraints and provide large granularity for future SDM networks. Granted by the ANR research program APOFIS, we have exploited a multiple Inter-modal FWM phenomenon occurring in a 1.8-km long graded-index 6-LP-mode fiber from Prysmian supporting 4 mode-groups and have successfully demonstrated a simultaneous threefold modal and wavelength conversion process of a 10-Gbit/s On/Off Keying signal in the C-band. The conversion process is based on a parallel Bragg-scattering phase-matched IFWM process occurring between the LP01, LP11, LP02 and LP31 spatial modes. We have chosen this particular configuration of modes to illustrate the proof-of-principle experiment but note that similar results could be achieved with any other set of higher-order modes. The conversion efficiency for each IFWM process has been characterized as a function of the initial signal wavelength and resulting converted signals in higher-order modes show well-opened eye-diagrams and error-free processing.

Polarization Domain Walls

A domain wall (DW) is a type of topological defect that connects two spatial stable states of a physical system. DWs are known to form as a result of a spontaneous symmetry breaking phase transition in a variety of systems, among which the most popular are magnetism, condensed matter, spinor Bose-Einstein condensates. Domain wall structures have been widely studied in ferroelectric materials in which they are known to bind regions in which all spins or magnetic dipoles are aligned in different directions. In this project, we study their equivalent in optics. Originally, optical domain walls refer to vectorial structures that have been predicted theoretically by M. Haelterman in the defocusing regime of an isotropic single-mode fiber more than 20 years ago. The domain wall corresponds to a localized structure of the kink type that separates two regions of space with different polarizations. In this framework, the fast polarization knots leads to two anticorrelated coupled twin-waves for which the strong binding force imposed by cross-phase interaction can compensate for linear and nonlinear impairments. The polarization distribution is then locked along the propagation within well-defined and robust temporal regions interconnected by polarization domain walls (PDWs). In the Lab, we presently study the possibility to transmit optical data using polarization domain walls. The information are then coded on the polarization rather than on the intensity profile of the wave.

Nonlinear effects in soft-glass fibers

Because of its combined properties of high coherence, large bandwidth, brightness and potential compactness, supercontinuum generation has been a topic of high interest in nonlinear optics. Broadband light sources have found many applications in the field of spectroscopy, metrology, telecommunication or biology. To this aim, numerous efforts have been first dedicated to investigate supercontinuum generation in fused silica fibers with record brightness and spectral expansion ranging from ultraviolet (UV) to mid-infrared (MIR). However, since intrinsic transmission window of fused silica makes supercontinuum expansion a very hard task above 2.2 µm, the recent trend is to progressively study alternative materials so as to spread further in the MIR region. In this context, given their remarkable optical and chemical properties, chalcogenide and Tellurite-based materials have been found to be promising candidates. Indeed, depending on their chemical composition, the infrared transparency can exceed 10 µm, whereas the Kerr nonlinearity can be three orders of magnitude higher than standard fused silica. This combination of both properties makes them particularly attractive for broadband mid-infrared SC generation and nonlinear processing above 2 µm. In this area of research, we characterize and exploit soft-glass fibers designed in our Lab by the group of Prof. Smektala thanks to their dedicated drawing tower facilities, to study SC generation and develop new nonlinear processing functions for Telecom applications, in particular towards the MIR.