Reaserch On Vector Mode Coupling In Few-Mode Fiber Bragg Grating

Mode Division Multiplexing(MDM)-a new type of multiplexing technology could greatly increase the capacity of optical fiber communication system.Many current reaserches show that there may be a variety of methods to realize MDM,and obtainng high-order vector modes of some methods is a key factor to be solved.This paper presents a new method based on Fiber Bragg Grating to realize coupling from fundamental mode to high-order vector modes.By using full-vector complex-mode theory of titled FBG,UV-side-illumination FBG based on two few-mode fiber structure(step index optical fiber and ring core fiber)are investigated.The conversion from fundamental mode to high-order vector modes(including TE01,TM01 and HE21 modes)is achieved.The influence of attenuation factor a of the UV-side-illumination,grating amplitude function ? and grating length on the coupling characterization are analyzed.For FBG based on step few-mode fiber,the numerical results show that the vector-mode coupling could be achieved by adjusting the parameters of the grating properly.But the degeneracy of the LP11 mode group vector mode can't be eliminated.For FBG based on ring few-mode fiber,the numerical results show that the degeneracy of the vector mode in LP11 mode group could be eliminated,thanks to the effective refractive index difference 1.8×10-4 between high-order vector modes.The efficient coupling could be achieved at different reflection wavelengths.The attenuation factor a of UV-side-illumination could be optimized for the purpose of getting higher coupling coefficient and the reflectivity.As the grating amplitude function ? increases,the reflectivity increases and the position of the reflection peak moves to the long wavelength direction,and the reflection spectrum becomes wider.In the process of increasing the value of ?,TE01 mode and HE21 s mode will interact,resulting in decreased reflectivity.Meanwhile,the relative position of the two reflections will interchange.The proposed method for high-order vector mode generation in this paper will be of high value for related research in MDM system.