用多模倾斜光纤光栅(Tilted fiber Bragg grating,TFBG)实现多模光纤中简并模式群(Degenerated mode group,DMG)的分别探测。多模倾斜光纤光栅将光纤中的导模耦合到辐射光线,用透镜将辐射光线聚焦在焦平面上,使非简并模式群对应的辐射光线的像在焦平面上分离。在焦平面上用空间滤波器使简并模式群选择透过并进行分别探测。用芯径为62.5 μm的标准通信多模光纤进行了简并模式群分别探测实验,用单模光纤激励多模光纤,测量简并模式群功率随激励光纤的横向对准偏差的变化关系。低阶简并模式群的测量结果与仿真计算结果基本一致,高阶简并模式群的测量结果与仿真结果有一些不同。用倾斜光纤光栅进行单简并模式群激励,在单简并模式群条件下测量简并模式群耦合效率的想法还需进一步实验验证。

Separate detection of the degenerate mode groups (DMGs) of multimode fiber was realized by using a multimode tilted fiber Bragg grating (TFBG). With multimode TFBG, the DMGs were coupled into the radiation ray that was focused separately on the focal plane of a lens. The radiation ray corresponding to the nondegenerate mode group is separated on the focal plane, so that the radiation ray of DMGs was selected and detected. The experiment of DMG separate detection was conducted with a standard communication multimode fiber with a core diameter of 62.5 μm. The multimode fiber was excited by a single mode fiber, and optical power of DMGs was measured while changing the lateral misalignment of exciting fiber. The measured results of the lowerorder DMGs were in consistence with the numerical results, but there appeared a few deviations for the higherorder DMGs. Further experiments needed to be conducted to verify the idea of exciting a single DMG in a multimode fiber using a TFBG and then measuring the coupling coefficient under this condition.