Mitigating the Effects of Concave Damage by Adding a Lining Layer in a Ring-Core Fiber for Improving Propagation of Orbital Angular Momentum Modes

In order to support the transmission of orbital angular momentum (OAM) modes effectively, the ring-core fiber has received a lot of attention due to the similar structure of the electric field distribution with the intensity profile of OAM mode. Both photonic crystal fibers and other micro-structured fibers almost contain a ring-core structure to support the transmission of more OAM modes. However, the middle of the ring-core fiber is usually a large air hole, and such a structure makes it difficult to manufacture the ring-core perfectly. Concave damage may occur in the ring-core. The effects of concave damage on the transmission of OAM modes are analyzed. The concave damage leads to a large effective refractive index difference between the odd and even modes of vector mode, and a method for proper compensation is proposed. The lining is added to the inner side of the ring-core, which is made of the same material as the cladding, and slightly increasing the refractive index of the lining material promotes the performance of the compensation. After numerical simulation, the walk-off distance can be increased by a factor of 2–20 after compensation.