We experimentally demonstrate that pumping a gradedindex multimode fiber with sub-ns pulses from a microchip Nd:YAG laser leads to spectrally flat supercontinuum generation with a uniform bell-shaped spatial beam profile extending from the visible to the mid-infrared at 2500 nm. We study the development of the supercontinuum along the multimode fiber by the cut-back method, which permits us to analyze the competition between the Kerr-induced geometric parametric instability and stimulated Raman scattering. We also performed a spectrally resolved temporal analysis of the supercontinuum emission. The strong modal confinement and the versatile dispersion engineering of single-mode fibers (SMFs) have permitted to demonstrate efficient and spatially coherent supercontinuum (SC) sources spanning from the ultra-violet to the mid-infrared (MIR) [1]. However, the small mode area of SMFs limits the accepted energy to relatively low values (less than 20 μJ for subns pulses). For this reason, SC sources based on SMFs cannot be used for applications where high pulse energies are required. Although multimode fibers (MMFs), such as graded-index (GRIN) fibers, permit the propagation of high energy pulses, these are subject to mode beating and mixing, owing to the difference of modal propagation constants and linear mode coupling. Modal interference brings a speckled intensity pattern at the MMF output, which prevents the use of MMFs whenever the preservation of spatial beam quality is required [2].Recent experiments by Krupa et al. [3] led to the unexpected discovery that Kerr nonlinearity of glass fibers above a certain threshold pulse power may lead to the generation of a selfsustained bell-shaped nonlinear beam in a highly multimode GRIN fiber. This means that linear mode mixing can be effectively washed out by means of the Kerr effect, so that a cleaned multimode light beam remains effectively selfpreserved. Kerr self-cleaning (KSC) stems from nonlinear coupling among the fundamental mode and higher-order modes [3,4]. KSC occurs at power levels at least one order of magnitude lower than the critical power of catastrophic light self-focusing in a GRIN MMF [5]. Because it is a conservative process, KSC is fundamentally different from the well-known Raman beam cleanup that is observed at the Stokes wavelength [6]. In addition, KSC occurs before a substantial pump spectral broadening has occurred [3]. For powers above the KSC threshold, nonlinear spectral broadening in MMFs results from a complex interplay between the spatial and temporal degrees of freedom [7][8][9]. Because of the self-imaging of the multimode beams in a GRIN MMF, the Kerr effect leads to a long-period intensity grating which induces mode conversion [4] and quasi-phase-matched (QPM) four-wave mixing (FWM) [10]. For temporal multimode femtosecond solitons in the anomalous dispersion regime [11], the nonlinear index grating produces an effective periodic nonlinearity which, in turn, induces a series of dispersive wave sidebands [12,13]. On the other hand, for ...
Propagation of light in multimode optical fibers usually gives a spatial and temporal randomization of the transmitted field similar to the propagation through scattering media. Randomization still applies when scattering or multimode propagation occurs in gain media. We demonstrate that appropriate structuration of the input beam wavefront can shape the light amplified by a rare-earth-doped multimode fiber. Profiling of the wavefront was achieved by a deformable mirror in combination with an iterative optimization process. We present experimental results and simulations showing the shaping of a single sharp spot at different places in the output cross-section of an ytterbium-doped fiber amplifier. Cleaning and narrowing of the amplifier far-field pattern was realized as well. Tailoring the wavefront to shape the amplified light can also serve to improve the effective gain. The shaping approach still works under gain saturation, showing the robustness of the method. Modeling and experiments attest that the shaping is effective even with a highly multimode fiber amplifier carrying up to 127 modes.
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