Study On LPG And SLPG Mode Converter Based On Special Optical Fiber

Mode converter has a role in Mode Division Multiplexing（MDM）system,which mainly realizes the conversion between fundamental mode and higher-order mode.For the moment,the implementation method of mode converter can be divided into two categories:the mode converter based on spatial light and the mode converter based on all-fiber.The all-fiber mode converters based on long period grating has the advantages of small volume,low loss,low cost,easy integration,high stability and high conversion efficiency,which have attract scholars at home and abroad to research and develop.Superimposed long period grating（SLPG）developed on the basis of long period grating（LPG）have unique optical characteristics,which can improve the performance of mode converter.Dual-core fiber（DCF）and photonic crystal fiber（PCF）are special fibers which have been developed in recent decades and own many excellent optical properties.Therefore,the study of mode converter based on dual-core fiber and PCF has theoretical and practical significance.Based on summarizing the research status of mode converters at home and abroad,LP01-LP21 and LP01-LP02 mode converters based on dual-core fiber and an all-fiber ultra-broadband mode converter based on SLPG are proposed in this paper.We have done theoretical and experimental research on the proposed mode converters,which has certain reference value for the future application of mode converter.The main contents are as follows:Firstly,the transmission mode of DCF and PCF are studied by solving the Helmholtz equation.Based on the coupled mode theory,the coupled mode equations of LPG and SLPG are studied.On this basis,the theoretical models of mode converters based on DCF and SLPG mode converter based on PCF are established.Secondly,the transmission mode of DCF with different wavelengths is simulated based on finite element method.Based on the theoretical model of DCF mode converter,the LP01-LP21 and LP01-LP02 mode converters were designed.We did simulation the influence of core distance on the bandwidth and coupling efficiency of the mode converters by using the beam propagation method and software.Thirdly,the transmission mode of PCF with different wavelengths is simulated based on finite element method.Based on the theoretical model of SLPG mode converter of PCF,we proposed the all-fiber ultra-bandwidth LP01-LP11 double SLPG mode converter and all-fiber ultra-bandwidth LP01-LP11 triple SLPG mode converter.Based on the coupled mode equation of SLPG,the influence of SLPG parameters on the performance of mode converter was simulated and analyzed by software.Finally,according to the simulation results of SLPG mode converter based on PCF,the double SLPG mode converter and triple SLPG mode converter were fabricated by CO₂laser.The all-fiber ultra-bandwidth LP01-LP11 mode conversion based on PCF was demonstrated.