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 ...

We experimentally demonstrate that Kerr spatial self-cleaning of a pulsed beam can be obtained in an amplifying multimode optical fiber. An input peak power of 500 W only was sufficient to produce a quasi-single-mode emission from the double-clad ytterbium doped multimode fiber (YMMF) with non-parabolic refractive index profile. We compare the self-cleaning behavior observed in the same fiber with loss and with gain. Laser gain introduces new opportunities to achieve spatial self-cleaning of light in multimode fibers at a relatively low power threshold.

We study a coupled cavity laser configuration where a passively Q-switched Nd:YAG microchip laser is combined with an extended cavity, including a doped multimode fiber. For appropriate coupling levels with the extended cavity, we observed that beam self-cleaning was induced in the multimode fiber thanks to nonlinear modal coupling, leading to a quasi-single mode laser output. In the regime of beam self-cleaning, laser pulse duration was reduced from 525 to 225 ps. We also observed a Q-switched mode-locked operation, where spatial self-cleaning was accompanied by far-detuned nonlinear frequency conversion in the active multimode fiber.

The reshaping of multimode waves in optical fibers is a process where the spatial and spectral degrees of freedom are inherently coupled. Our experiments demonstrate that pumping a graded-index multimode fiber with sub-ns pulses from a microchip Nd:YAG laser leads to supercontinuum generation with a uniform bell-shaped spatial beam profile.