We experimentally demonstrate Kerr beam self-cleaning in 90 m long standard graded-index (GRIN) multimode fiber (MMF) for normal and oblique launching conditions. Sub-nanosecond pump pulses are launched in the normal dispersion regime of the GRIN MMF. We measure the simultaneous evolution of the near field modal profile and the output spectrum for various pump powers under different launching conditions. Experimental findings establish that the nonlinear coupling among the guided modes leads to reshaping the output speckle pattern into a bell shape (${\mathit{LP}}_{01})$ or higher-order (${\mathit{LP}}_{11}$ and ${\mathit{LP}}_{21}$) spatially clean beam profile at very low input pump powers, whereas the spectral domain exhibits generation of modulation instability peaks. Threshold power for the self-cleaning process increases effectively for higher-order modes.

我们在 90 m 长的标准渐变折射率 (GRIN) 多模光纤 (MMF) 中通过实验证明了克尔光束在正常和倾斜发射条件下的自清洁能力。亚纳秒泵浦脉冲在 GRIN MMF 的正常色散范围内发射。我们测量了不同发射条件下各种泵浦功率的近场模态分布和输出频谱的同时演变。实验结果表明，导模之间的非线性耦合导致将输出散斑图案重塑为钟形（${\mathit{LP}}_{01})$ 或更高阶（${\mathit{LP}}_{11}$ 和 ${\mathit{LP}}_{21}$) 在非常低的输入泵浦功率下空间干净的光束轮廓，而谱域表现出调制不稳定性峰值的产生。对于高阶模式，自清洁过程的阈值功率有效增加。