Study On Mode Converter Based On Superimposed Long Period Gratings In Few Mode Fiber

Mode-division multiplexing?MDM?system has attracted wide attention of research-ers all of the world because it breaks through the transmission capacity limitation of sin-gle-mode optical fiber based communication.Few-mode multiplexing system can multiply the transmission capacity of the channel based on the limited modes as independent transmission channel to transmit signals.Mode converter is the key componet in MDM system,which is mainly to realize the conversion from the fundmental mode to high-er-order core mode in few-mode fiber?FMF?.Superimposed long period grating?SLPG?is a new type of passive optical device,which can realize spot shaping,mode conversion and compactness of mode converter.Therefore,the study of mode converter based on SLPG is of theoretical and practical significance.Based on reviewing the research status of mode converters at home and abroad,an all-fibe wide-bandwidth mode converter based on SLPG is proposed in this paper,and its theoretical and experimental research is conducted.The main contents of the thesis are as follows:Firstly,the transmission mode and the eigen equation of transmission mode in step index FMF is studied based on Maxwell's equations and scalar method;two coupled-mode equations based on SLPG are studied with mode coupling principle;and the mathematical model of mode converter based on SLPG is established,which provides the main basis for theoretical and experimental research of mode converter.Secondly,the transmission characteristics of two-mode fiber is analyzed with finite element method,and the dispersion curve of LP₀₁ and LP₁₁ modes are obtained.The mode converter realizing conversion from LP₀₁ mode to LP₁₁1 mode in two-mode fiber is numer-ically analyzed with the mathematical model of mode converter based on SLPG,and the effects of grating period interval,grating length and coupling coefficient on the perfor-mance of mode converter are studied.The performance of mode converters based on sin-gle grating and superimposed grating are analyzed and optimized.Thirdly,the transmission mode of four-mode fiber is analyzed based on the finite element method,and the dispersion curves of LP₀₁,LP₁₁,LP₂₁ and LP₀₂ modes are ob-tained.The mode converters realizing conversion from LP₀₁ mode to LP₁₁,LP₂₁ and LP₀₂mode in four-mode fiber are numerically analyzed based on the mathematical model of mode converter based on SLPG,and the effects of grating period interval,grating length and coupling coefficient on the performance of mode converter are studied.The perfor-mance of mode converters based on single grating and superimposed grating are analyzed and optimized.Finally,single grating and superimposed grating are fabricated under CO₂ laser irra-diation to realize all-fibe mode converter.The effect of grating parameters on the perfor-mance of mode converter is analyzed experimentally;the efficiency and bandwidth of mode converter based on single grating and superimposed grating are analyzed and com-pared;and the temperature response of mode converter is studied.In this paper,the mode converter based on SLPG is studied theoretically and ex-perimentally,which has certain reference value for realizing compact and broadband mode converter.