Spatial Beam Self-Cleaning in Bi-Tapered Multimode Fibers

We report the spatial beam self-cleaning in bi-tapered conventional multimode fibers (MMFs) with different tapered lengths. Through introduction of structure MMFs, input poor quality from a high-power fiber laser can be converted to centered, bell-shaped short length, due strengthened nonlinear modes coupling. It is found that MMF longer length at same waist diameter shows better effect and larger spectral broadening. The obtained results offer new method improve low cost. Furthermore, it may interesting for manufacturing MMF-based devices obtain quasi-fundamental mode spatiotemporal mode-locked lasers.