Mode Properties Research Of Multi-layer Structure Orbital Angular Momentum Fiber

Light beams carrying orbital angular momentum?OAM?have many exotic features,for example,the helical phase structure and the annular “ring” intensity profile.Base on those features,light beams carrying OAM has great values in many fields.In the optical communication field,the orbital angular momentum of light is considered to be a new degree of free,in addition to amplitude,phase,wavelength and polarization.The OAM is expected to provide a new latitude for fiber channel multiplexing.On the basis of combination with the performance study of many transmission waveguides for OAM modes,we design a multi-layer optical fiber for OAM modes.And we study the stability and walk off length of OAM modes in this fiber by using the finite element software COMSOL.The main works and results of this study are as follows:Firstly,by researching refractive index difference,2? walk off length and 10 ps walk off length of the multi-layer fiber,we find that the fiber can support many OAM modes,at least 60 OAM modes,with topological charges from 1 to 15.Simultaneously,the 10 ps walk off length of some modes can reach more than 10 km.Secondly,by researching the relationship between the thickness and refractive index of the fourth layer and the refractive index difference,2? walk off length and 10 ps walk off length,we find that the number of OAM modes is increased when the thickness of the fourth layer of the fiber is increased.The number of OAM modes and refractive index difference are increased when the refractive index of the fourth layer of the fiber is increased,the stability of OAM modes is improved.But the 2? walk off length and 10 ps walk off length are decreased for most of modes with reduced thickness or reduced refractive index of the fourth layer of the fiber.Finally,by researching a directional coupler based on the multi-layer optical fiber,we find that the coupling length of HE_10,1,EH_8,1,HE_11,1 and EH_9,1 are less than 6mm when the space between the fourth layer of the two fibers is 1?m.And the coupling length is increased when the space is increased.Generally speaking,this paper study a novel multi-layer optical fiber for OAM mode transmission.We find a 5-layer fiber can support 60 OAM modes and the modes have good stability.Compared to the 3-layer fiber reported by other literatures,this 5-layer fiber supports more than 24 OAM modes.This indicated that the 5-layer fiber is more useful for OAM modes multiplexing.