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Nonlinear propagation in multimode optical fiber amplifiers

Abstract : The current long-haul transmission and worldwide complex interconnection of network infrastructure is based on single mode optical fiber which transports information in the form of light pulses. Single mode fibers have low attenuation and dispersion rate as all energy is confined to a single mode and no energy is lost into the cladding. As a result, single mode fibre is suitable for long distance transmission whereas multimode fibre, is mainly used for short distance communication. In addition, single mode fiber delivers diffraction limited high beam quality, guides only Gaussian like field distributions and allows high coupling efficiency. However, due to its core diameter size single mode fiber could not satisfy the future huge communication bandwidth demand, and it is not suitable for very high power fiber amplifiers and lasers. Multimode optical fibers are designed to carry several modes, due to their large core radius. In principle, graded index active multimode fibers could be used for high power fiber amplifiers and fiber lasers. In addition, ytterbium doped multimode fibers are used to overcome the current bandwidth deficit of single mode fibers, through the techniques of spatial mode division multiplexing. Nowadays, high beam quality fiber amplifiers and lasers are required for different applications. However, multimode interferences that result in degradation of beam quality are the main problem for developing high power fiber lasers and fiber amplifiers. So, the question is how to overcome these constraints and use multimode fibers (step-index or graded-index) for these and other applications. The most feasible approach is to convert the Kerr nonlinearity from being a problem to an opportunity for beam cleaning. We performed a numerical study by solving coupled mode equations to investigate this phenomena further. By increasing the input signal power, we observed the transfer of energy from the high order modes to the fundamental mode, due to nonlinear and non-uniform gain coupling along the course of propagation. As a result, the fundamental mode experienced higher gain due to the exchange of power with the other high order modes, and beam cleaning is demonstrated. Furthermore, to explore the nonlinear propagation in doped multimode fibers, we implemented a 3D NLSE in the presence of saturated gain, Kerr nonlinearity and disorder. Our numerical simulations reveal that by increasing the input signal power, the speckled beam reshapes itself after a certain fiber distance, into a clean beam close to single mode operation due to the laser gain and Kerr nonlinearity. The spatial beam self-cleaning phenomenon is further confirmed by examining the beam width at the end of the fiber length, which has reduced diameter than at the beginning of the fiber. Analysis of our results show that numerical simulations agree well with preliminary experimental results.
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Submitted on : Wednesday, October 27, 2021 - 10:34:12 AM
Last modification on : Thursday, September 8, 2022 - 3:59:03 AM
Long-term archiving on: : Friday, January 28, 2022 - 6:30:36 PM


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  • HAL Id : tel-03405240, version 1



Mesay Addisu Jima. Nonlinear propagation in multimode optical fiber amplifiers. Optics / Photonic. Université de Limoges; Università degli studi (Brescia, Italie). Facoltà di ingegneria, 2021. English. ⟨NNT : 2021LIMO0036⟩. ⟨tel-03405240⟩



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