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Optique adaptative et non linéaire pour le contrôle spatial de la propagation dans une fibre multimodale

Abstract : Over the last decade, multimodal fibers have received renewed interest in the scientific community, largely due to the possibility of increasing the bandwidth of optical links using new spatial multiplexing techniques. This is not the only area of application ; so many adaptive optics techniques were developed to control the usually speckled beam profile at the output of multimode fibers. These wavefront shaping techniques have demonstrated their efficiency with light radiation propagating in linear regime generally, in particular for fiber-based imaging applications. At the same time, the Kerr beam self-cleaning phenomenon was recently observed in parabolic graded-index multimode fibers (GRIN). It leads to a progressive transfer of energy from the different propagated modes to the fundamental mode along the fiber. The higher the number of guided modes of the fiber, the more complex the nonlinear interactions are. This PhD work is at the crossroads of these two themes by using beam shaping techniques to control themodal propagation in GRIN fibers in the Kerr beam self-cleaning regime. I developed an adaptive optimization device including a deformable mirror to shape the wavefront of the laser beam launched into a 52μm core diameter GRIN fiber and a camera-based system at the output of the multimode fiber providing an objective function to be minimized. First, the parameters of the injection system, the optimization algorithm and the objective function were optimized to ensure a fast convergence of the propagated field profile towards the targeted mode. This device allowed to study the active control of the Kerr beam self-cleaning phenomenon in order to synthetize other modes than the fundamental one. Thus, in this nonlinear regime, I demonstrated the synthesis of six low order modes, besides the fundamental one, at the output of the GRIN fiber. The Kerr effect assists the optimization process to get output intensity pictures closely resembling the pattern of the target mode, which is impossible to obtain in a linear propagation regime under the same conditions. The study of the dynamics of modal synthesis assisted by Kerr beam self-cleaning has highlighted the different power thresholds required for the synthesis of the target modes. The influence of wavefront shaping on the modal population thermalization was numerically studied. This study shows the impact of modal groups in the energy exchange process, which makes modal synthesis robust, especially to the variations of the initial conditions. Finally, it showed that wavefront shaping enables the regulation of the modal population thermalization speed.
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Submitted on : Tuesday, May 25, 2021 - 12:11:20 PM
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Etienne Deliancourt. Optique adaptative et non linéaire pour le contrôle spatial de la propagation dans une fibre multimodale. Optique / photonique. Université de Limoges, 2021. Français. ⟨NNT : 2021LIMO0024⟩. ⟨tel-03234354⟩

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