Reaserch On Vector Mode Coupling In Few-mode Fiber Long Period Grating

With the development of the internet,the high-speed information services have led to an increasing demand for the capacity of optical fiber communication.Module division multiplexing(MDM)is a new multiplexing technique that can improve communication capacity.There are many ways to implement this technology,within which the Orbital angular momentum(OAM)is used in MDM system.Efficient generation of higher-order vector modes is a key factor in some MDM technologies.This paper presents the implementation of the coupling between fundamental mode and high order vector modes in UV single-illumination LPFG and tilted LPFG.It can provide a new technical means for generating high order vector mode in fiber grating.This thesis has important application value in the field of mode division multiplexing technology research.In this thesis,by using coupled-mode theory of tilted grating,the coupling between the fundamental mode and high order vector modes(TE01,TM01 and HE21)in UV single-side illumination LPFG and tilted LPFG based on the few-modes step-index,annual-index,inverse-parabolic index fiber are investigated.The research indicates that the LPFG based on step-index fiber can realize efficient coupling between fundamental mode and high vector modes,but the step-index fiber structure is not suitable for OAM transmission.The LPFG based on annual index fiber and inverse-parabolic index fiber can also achieve the coupling between the fundamental mode and high order vector modes,and the transmission peaks of high-order modes are distributed in different wavelength bands.The annular-index fiber and inverse-parabolic index fiber can effectively break the degenerate of vector mode in LP11 mode group.The maximum coupling efficiency can be obtained by optimizing the parameters of LPFG.For fiber grating structure under the UV unilateral exposure,UV exponential attenuation factor of the optimal value is about 0.2～0.5,while for the tilted grating,optimum tilt Angle is about 88°.As the grating amplitude function increases,the transmissivity increases,and the position of transmission peak moves to the direction of short wavelength.As the length of the grating increases,the transmissivity increases and the position of transmission peak is not affected.