Research On Mode Converters Based On Tilted Fiber Gratings

Mode Division Multiplexing(MDM)is anew multiplexing technology succeeding the multiplexing technologies such as Time Division Multiplexing and Wavelength Division Multiplexing.It can greatly improve the capacity of communication system by using different modes to transmit information.The conversion between fundamental mode and high-order modes is one of the key factors to realize the mode multiplexing.Based on the above research background,this paper presents a new method based on Tilted Fiber Bragg Grating(TFBG)to achieve the conversion from fundamental mode to high-order vector modes.The full-vector complex coupled-mode theory of TFBG is applied to study the TFBG to achieve the conversion from fundamental mode to high-order vector modes.The researches of the TFBG based on step index optical fiber and inverse-parabolic graded-index fiber(IPGIF)are compared,and the conversion from basic mode(HE11)to high order vector mode(TE01,TM01 and HE21)is achieved in the latter.The influence of modulation direct component ?,grating tilted angle 0,grating amplitude function(depth of modulation)? and grating length L on the spectrum of coupling conversion are analyzed.The research shows that the mode coupling conversion in the TFBG based on step index optical few-mode fiber can be achieved.However,the Degenerate vector modes of the LP11 mode group cannot be effectively separated in that fiber.Forthe TFBG based on the inverse-parabolic graded-index fiber,its structure can break the degeneracy of the vector modes in the LP11 mode group,the effective refractive index difference between the high-order vector modes has reached over 2.0 × 10-4.The coupling conversion of the high-order vector modes could be achieved at different reflective wavelengths.The position of the reflection spectrum is only affected by the direct component ?,as the ?increases,the position of the reflection peak moves to the long wavelength direction.The grating tilted angle ? has different effects on the reflection spectrum of the base mode and the high-order modes,the coupling coefficient and reflectivity exist the optimal extreme points with the changing of the 0.As the ? increases,the reflectivity increases.As the L increases,the reflectivity increases and the reflection spectrum narrows.The proposed TFBG from fundamental mode coupled to high-order vector modes provides a new technical mean to generate the high-order vector modes in fibers.It has an important applicative value in the field of Mode Division Multiplexing technologies.