Abstract: Light waves with orbital angular momentum (OAM) can be applied in the mode division multiplexing technology, which offers a new way to enlarge the communication capacity from another dimension. The generation of the OAM is the key of this technology. Based on the effective medium theory, we designed a special structure at the end of the optical fiber: vortex structures which make the effective refractive index increases clockwise or counter-clockwise. When the incident light goes through this structure, the different refractive index will cause the different optical path and the corresponding phase delay alone the azimuthal angle, i.e. the OAM light. The above OAM generation process is simulated using an FDTD software.