Nonlinear Photonics and Fibre Lasers

About

Though nonlinear physics has a rather long history, beginning with the works of Newton and Huygens, science and technologies of the 19th and most of the 20th century had been dominated by linear mathematical models and linear physical phenomena. 

Over the last decades there has been growing recognition of the importance of physical systems in which nonlinearity introduces a rich variety of fundamentally new properties - properties that can never be observed in linear models or implemented in linear devices. 

The understanding and mastering of nonlinear physical systems has the potential to enable a new generation of engineering concepts. Our research is at the interface between fundamental nonlinear science and practical applications across a range of disciplines.

AIPT research in the field of neuromorphic photonics is supported by the recently awarded European Training Network POSTDIGITAL grant coordinated by AIPT (with 13 partners including industry such as IBM, Thales and SMEs LightOn, VLC Photonics and IniLabs.

Impactful Research

Our research focuses on practical applications, for example, advancing and unlocking new capabilities within existing technologies like LIDAR. In addition to LIDAR's conventional usefulness for measuring distances, we added the remarkable ability for LIDAR to recognise textures. We achieved this using mode-locked fibre lasers with dynamic states of polarisation.

Patent applications from AiPT include, among others, novel functionalities for LIDAR and radar in the field of self-driving cars. UK companies such as RDM Group and Highways England have already expressed interest in the joint development of the next generation of LIDAR technology.

This technology will be developed further in partnership with H2020 Innovative Training Networks, MOCCA and MEFISTA alongside industrial partners such as Thales, III-V Labs, AMO, RDM and NKT Photonics.

Highlights of Our Research in 2019-2020

Breathing dissipative solitons in an ultrafast fibre laser

Breathing dissipative solitons form an important part of numerous nonlinear systems, manifesting themselves as nonlinear waves in which energy concentrates in a localised and oscillatory fashion. Yet, the observation of dissipative breathers has been mainly confined to optical micro-resonators.

This collaborative research with the East China Normal University in Shanghai experimentally demonstrates breathing solitons in a mode-locked fibre laser.

The direct disclosure of the fast evolutionary behaviour of breathers is made possible for the first time in experiments by a combination of real-time dispersive Fourier transform and spatio-temporal intensity measurements, and dispersion engineering. In the normal-dispersion regime of the laser cavity breathers are excited in the laser under the pump threshold for stationary mode locking.

Our work first establishes a general, deterministic route to induce soliton breathing in normal-dispersion fibre cavities in that the breather generation regime is accessible by solely changing the pump strength. Besides, for the first time in experiments with mode-locked fibre lasers, breather-pair molecules are also generated in the cavity.

Our experimental findings are corroborated by numerical simulation of the laser model, and will trigger parallel investigations in other physical systems.

Peng J., Boscolo S., Zhao Z. & Zheng H. Breathing dissipative solitons in mode-locked fiber lasers. Science Advances 5, eaax1110 (2019).

Analysis of laser radiation using the Nonlinear Fourier Transform

Modern high-power lasers exhibit a rich diversity of nonlinear dynamics, often featuring nontrivial co-existence of linear dispersive waves and coherent structures. While the classical Fourier method adequately describes extended dispersive waves, the analysis of time-localised and/or non-stationary signals call for more nuanced approaches. Yet, mathematical methods for simultaneous characterisation of localized and extended fields are not yet well developed.

In our work published in Nature Communications, we demonstrated how the Nonlinear Fourier transform (NFT) based on the Zakharov-Shabat spectral problem can be applied as a signal processing tool for representation and analysis of coherent structures embedded into dispersive radiation. We used full-field, real-time experimental measurements of mode-locked pulses to compute the nonlinear pulse spectra. For the classification of lasing regimes, we presented the concept of eigenvalue probability distributions. In addition, we presented two field normalisation approaches, and showed the NFT can yield an effective model of the laser radiation under appropriate signal normalisation conditions.

The full-field information made available by the experimental methodology, together with the information about the nonlinear content provided by the NFT can help understand better the underlying dynamics in a wide range of lasers, and nonlinear systems in general.

Sugavanam S, Kopae MK, Peng J, Prilepsky JE, Turitsyn SK. Analysis of laser radiation using the Nonlinear Fourier transform. Nature Communications. 2019 Dec 11;10(1):1-0.

Bidirectional ultrafast lasers for gyroscopic measurements

Active ring laser gyroscopes (RLG) operating on the principle of the optical Sagnac effect are used in a range of applications, such as inertial guidance systems, seismology, and geodesy, which require both high bias stability and high angular velocity resolutions.

Operating at such accuracy levels demands special precautions like dithering or multi-mode operation to eliminate frequency lock-in or similar effects introduced due to synchronisation of counter-propagating channels. In our work, we showed how bidirectional ultrafast fibre lasers can be used to circumvent the limitations of continuous wave RLGs.

Instead of using traditional averaged approaches of measurement, we used real-time intensity and spectral measurement techniques like spatio-temporal dynamics and the Dispersive Fourier Transform, transforming the bidirectional ultrafast laser to an ultrafast gyroscope with acquisition rates of the order of the laser repetition rate. This is at least two orders of magnitude faster than commercially deployed versions.

Our results reveal the high potential of application of novel methods of optical Sagnac effect measurements, allowing enhancement of rotation sensitivity and resolution by several orders of magnitude.

Chernysheva M, Sugavanam S, Turitsyn S. Real-time observation of the optical Sagnac effect in ultrafast bidirectional fibre lasers. APL Photonics. 2020 Jan 1;5(1):016104

Visualisation of extreme value events in optical communications

Fluctuations of temporal signal propagating along long-haul transoceanic scale fiber links can be visualised in the spatio-temporal domain drawing visual analogy with ocean waves. Substantial overlapping of infоrmation symbols or use of multi-frequency signals lead to strong statistical deviations of local peak power from average signal power level.

We considered here a long-haul optical communication systems from this unusual angle, treating them as physical systems with a huge number of random statistical events, including extreme value fluctuations, that potentially might impact quality of data transmission.

We applied the well established concepts of adaptive wave front shaping used in imaging through turbid medium to detect the detrimental phase modulated sequences in optical communications that can cause extreme power outages (rare optical waves of ultra-high amplitude) during propagation down the ultra-long fiber line.

We illustrated the concept by a theoretical analysis of rare events of high-intensity fluctuations – optical freak waves, taking as an example an increasingly popular optical frequency division multiplexing data format where the problem of high peak to average power ratio is the most acute.

We have also shown how such short-lived and extreme value spikes in the optical data streams are affected by nonlinearity and demonstrate negative impact of such events on the system performance.

Stanislav Derevyanko, Alexey Redyuk, Sergey Vergeles, and Sergei K. Turitsyn, Visualisation of extreme value events in optical communications, APL Photonics, 3, 060801 (2018).