Research On Properties Of Vector Mode Conversion Based On Long Period Fiber Grating In Few-mode Fibers

Long period fiber grating is widely used in optical fiber communication system,which has the advantages of low back reflection,low crosstalk,low insertion loss,small size,compatibility with optical fiber and simple preparation process.Long period fiber grating can realize the coupling of the fundamental mode to the higher-order core modes in the same direction.In recent years,in order to realize the mode conversion in mode-division multiplexing system,the mode conversion method based on long period fiber grating written in few-mode fiber has attracted great attention and has got research progresses.This paper studies the coupling characteristics of fundamental mode HE₁₁ to higher-order core vector modes(TE₀₁,TM₀₁ and HE₂₁)which based on full vector coupled mode theory for fiber gratings.The main research contents are as follows:1.The coupling characteristics of the vector mode of mechanically induced micro-bend long period fiber grating?MBLPFG?written in step-index few-mode fiber are studied,and the influences of grating period,micro-bend amplitude, grating length and coupling coefficient of mechanically induced MBLPFG on vector mode coupling are studied.The results show that coupling coefficient is proportional to the micro-bend amplitude in mechanically induced MBLPFG,the strength of vector modes coupling can be effectively tuned by changing the micro-bend amplitude of the fiber which is applying pressure.2.A vector mode conversion method based on mechanically induced MBLPFG written in few-mode fiber which has inverse-parabolic index is proposed.The results show that mechanically induced MBLPFG written in few-mode fiber which has inverse-parabolic index has similar characteristics as mechanically induced MBLPFG written in step-index few-mode fiber in micro-bend amplitude,grating length and coupling coefficient.Compared with the step-index few-mode fiber,the few-mode fiber which has inverse-parabolic index supports the degenerate separation of the vector modes.So mechanically induced MBLPFG written in few-mode fiber which has inverse-parabolic index can convert fundamental mode HE₁₁ to specific high-order vector modes(TE₀₁,TM₀₁,and HE₂₁)at specific wavelengths,the resonant wavelength interval of the converted high-order vector mode is greater than 60nm,and tuning range of resonant wavelength is 100nm.3.The vector mode coupling characteristics of the tilted long period fiber grating?TLPFG?written ring fiber are studied.and the influences of the tilt angle,amplitude function,grating length,and coupling coefficient on mode conversion are analyzed.The results show that the grating can realize the conversion of the fundamental mode to specific high-order vector modes(TE₀₁,TM₀₁,and HE₂₁)at different wavelengths which interval greater than 150nm.As the length of the grating increases,mode conversion undergoes under coupling,full coupling,and over coupling.As the amplitude function increases,over coupling occurs between modes,the conversion efficiency increases first and then decreases,and the resonance wavelength decreases.Tilt angle plays a key role in mode conversion,the maximum mode conversion efficiency can be obtained when tilt angle is approximately 84°.This paper studies the vector mode conversion characteristics of long period fiber grating written in few-mode fiber,which has important guiding significance for the design and manufacture of mode converter based on long period fiber grating written few-mode fiber.The mechanically induced MBLPFG based on few-mode fiber which has inverse-parabolic index and the TLPFG based on ring fiber proposed in the thesis possess considerable potential for application in vector mode multiplexing,generation of vortex beams and orbital angular momentum?OAM?multiplexing.