Raman-enhanced Beam Self-cleaning with Femtosecond Pulse Pumping at 1.56 μm

Graded-index (GRIN) multimode fibers (MMF) have attracted much attention in recent years. Due to its large core size, high power transmission and capacity communication are possible by utilizing multiple modes. The MMFs have been widely used in mode-locked lasers [1], supercontinuum generation [2], beam self-cleaning (BSC) [3] and so on. Among these applications, BSC is an unique phenomenon as it is quite different from the well-known spatial soliton balanced by the diffraction and nonlinearity. The BSC can be roughly classified into two types: Kerr-induced and Raman-induced BSC. Previous report show that the BSC in the anomalous dispersion region of GRIN MMF is weaker than that in normal dispersion region [3]. While the reason behind it is still controversial, we found that the Raman-induced frequency shift (RIFS) could enhance the BSC compared to the pure Kerr nonlinearity when pumping at the anomalous dispersion region.