基于环状光纤的OAM-PMD动态方程和固定双折射级联模型，推导了OAM模式的一阶OAM-PMD系数方程，并仿真了环状光纤具有一定椭圆度时，几何双折射对OAM模式对应的本征奇-偶模的有效折射率差的影响，以及一阶OAM-PMD系数随角频率的变化。结果表明，OAM模式的一阶OAM-PMD系数不仅与OAM模式对应的奇-偶模的有效折射率差有关，还会随角频率变化而变化，并且环状光纤椭圆度的增大会导致OAM模式的一阶OAM-PMD系数增大，且对相对低阶的OAM模式的一阶OAM-PMD系数影响更明显，从而严重影响OAM模式复用系统的性能和传输距离。

Gapless linear energy dispersion of graphene endows it with unique nonlinear optical properties, including broadband nonlinear absorption and giant nonlinear refractive index. Herein, we experimentally observed that fewlayers graphene has obvious nonlinear absorption and large nonlinear refraction, as investigated by the Z-scan technique in the mid-infrared (mid-IR) regime. Our study may not only, for the first time to our knowledge, verify the giant nonlinear refractive index of graphene (～10^?7 cm²∕W) at the mid-IR, which is 7 orders of magnitude larger than other conventional bulk materials, but also provide some new insights for graphene-based mid-IR photonics, potentially leading to the emergence of several new conceptual mid-IR optoelectronics devices.