BEGIN:VCALENDAR VERSION:2.0 PRODID:-//Talks.cam//talks.cam.ac.uk// X-WR-CALNAME:Talks.cam BEGIN:VEVENT SUMMARY:Complex nonlinear wave dynamics in ultrafast fibre lasers and thei r intelligent control - Sonia Boscolo (Aston University) DTSTART:20220718T110000Z DTEND:20220718T113000Z UID:TALK176081@talks.cam.ac.uk DESCRIPTION:Ultrafast fibre lasers represent interesting realisations of d issipative nonlinear systems with dynamics driven by a complex interplay a mong the effects of nonlinearity\, dispersion and energy exchange\, thus p roviding an ideal platform for the fundamental exploration of complex nonl inear wave phenomena. Machine-learning approaches and the use of genetic a nd evolutionary algorithms (EAs) have recently led to several dramatic imp rovements in dealing with the multivariable optimisation problem associate d with achieving desired operating regimes of fibre lasers [1]\, which is otherwise a laborious task if addressed with a trial and error procedure. In this talk\, we will provide a snapshot of our recent progress in the co ntrol of non-stationary nonlinear dynamics in fibre lasers by the use of E As.\nBreathing solitons exhibiting periodic oscillatory behaviour form an important part of many different classes of nonlinear wave systems. Recent ly\, thanks to the development of real-time detection techniques\, they ha ve also emerged as a ubiquitous mode-locked regime of ultrafast fibre lase rs [2\,3]. The excitation of breather oscillations in a laser naturally tr iggers a second characteristic frequency in the system\, which therefore s hows competition between the cavity repetition frequency and the breathing frequency. Nonlinear systems with two competing frequencies show frequenc y locking\, in which the system locks into a resonant periodic response fe aturing a rational frequency ratio\, and quasi-periodicity following the h ierarchy of the Farey tree and the structure of the devil&rsquo\;s stairca se [4]. Whilst frequency-locking phenomena have been extensively studied t heoretically and experimentally in many physical systems\, all the investi gations so far relate to systems where an external\, accurately controllab le modulation adds a new characteristic frequency to the system. In [5]\, we introduced an approach based on an EA for the generation of breather dy namics in fibre lasers with controlled characteristics\, which relies on s pecific features of the radio-frequency spectrum of the breather laser out put to optimise the intra-cavity nonlinear transfer function steered by el ectronically driven polarisation control. In this talk\, benefiting from t his approach and further developing it to directly pinpoint frequency-lock ed breathers\, we demonstrate that a breather mode-locked fibre laser is a passive system showing frequency locking at Farey fractions [6]. The freq uency-locked states occur in the sequence they appear in the Farey tree an d within a pump-power interval given by the width of the corresponding ste p in the devil&rsquo\;s staircase. The breather laser may therefore serve as a simple model system to explore universal synchronisation dynamics of nonlinear systems.\nFirst introduced in the context of oceanic waves\, the concept of extreme events or rogue waves (RWs)\, i.e.\, statistically-rar e giant-amplitude waves\, has been transferred to other natural environmen ts such as the atmosphere\, as well as to the solid grounds of research la boratories [7]. As RWs appear from nowhere and disappear without a trace\, their emergence is unpredictable and non-repetitive\, which make them par ticularly challenging to control. Here\, we extend the use of EAs to the a ctive control of extreme events in a fibre laser cavity [8]. Feeding real- time spectral measurements into an EA controlling the electronics to optim ise the cavity parameters\, we are able to trigger wave events in the cavi ty that have the typical statistics of RWs in the frequency domain and on- demand intensity. This accurate control enables the generation of the stro ngest optical RWs observed so far with a spectral peak 32.8 times higher t han the significant intensity threshold. The extreme spectral events obser ved correlate with extreme variations of the pulse energy\, thus qualifyin g as temporal RWs as well. Importantly\, significant frequency up- or down -shifting of the optical spectrum is also associated with the emergence of these waves. Given the generality of our control strategy\, which relies on the statistical defining characteristics of RWs independent of the part icular physical model\, it is reasonable to expect the machine-learning me thod used in this work to be applicable to the control of RWs in many diff erent systems.\nReferences\; [1] Meng F\, Dudley JM. Toward a self-driving ultrafast fiber laser. Light: Science & Applications 2020\, 9: 26. [2] Pe ng J\, Boscolo S\, Zhao Z\, Zeng H. Breathing dissipative solitons in mode -locked fiber lasers. Science Advances 2019\, 5(11): eaax1110. [3] Peng J\ , Zhao Z\, Boscolo S\, Finot C\, Sugavanam S\, Churkin DV\, Zeng H. Breath er molecular complexes in a passively mode-locked fibre laser. Laser & Pho tonics Reviews 2021\, 15(7): 2000132. [4] Jensen MH\, Bak P\, Bohr T. Comp lete devil's staircase\, fractal dimension\, and universality of mode-lock ing structure in the circle map. Physical Review Letters 1983\, 50(21): 16 37. [5] Wu X\, Peng J\, Boscolo S\, Zhang Y\, Finot C\, Zeng H. Intelligen t breathing soliton generation in ultrafast fibre lasers. Laser & Photonic s Reviews 2022\, 16(2): 2100191. [6] Wu X\, Zhang Y\, Peng J\, Boscolo S\, Finot C\, Zeng H. Farey tree and devil's staircase of frequency-locked br eathers in ultrafast lasers. 2022\, Submitted. [7] Dudley JM\, Genty G\, M ussot A\, Chabchoub A\, Dias\, F. Rogue waves and analogies in optics and oceanography. Nature Reviews Physics 2019\, 1: 675--689. [8] Wu X\, Zhang Y\, Peng J\, Boscolo S\, Finot C\, Zeng H. Control of spectral extreme eve nts in ultrafast fibre lasers by an evolutionary algorithm. 2022\, Submitt ed.\n \; LOCATION:Seminar Room 1\, Newton Institute END:VEVENT END:VCALENDAR