Thermally-induced Long-period Grating Mode Converter On Tapered Polymer Waveguide

In recent years,with the rapid development and wide application of all kinds of "Internet +" technologies,the demand of people for bandwidth of optical communication systems has been increasing,such as e-commerce,e-government,online medical care and so on.However,the transmission capacity of single-mode optical fiber communication system based on wavelength division multiplexing(WDM)technology is gradually approaching its theoretical limit.In order to break through this capacity limitation for the rapid growth of capacity demand,mode division multiplexing(MDM)technology based on few-mode optical fibers emerged as the times require.MDM technology introduces multiple modes as independent channels for each multiplexing wavelength,so that the communication capacity can be multiplied(depending on the number of modes used).Considering that the future flexible MDM optical communication system should have the same routing and reconfiguration functions as today's wavelength division multiplexing system,the mode converter,as one of the most important functional devices in the MDM system,should also be broadband and reconfigurable.In this dissertation,a thermallyinduced long period grating mode converter based on tapered waveguide is proposed.The converter uses planar optical waveguide structure which is easy to integrate,and is fabricated by using polymer optical waveguide material with simple fabrication process and high cost performance.The proposed mode converter has simple structure and is easy to fabricate.It just includes only a tapered polymer waveguide and a long period grating electrode heater fabricated on the tapered waveguide.Among these,the tapered waveguide can expand the bandwidth of the device,while the long period grating induced by current heating can realize the reconfiguration function of the device.In this paper,the basic principle,parameter design,fabrication and testing of the proposed mode converter are studied.The main work is as follows:Firstly,the development status of optical communication is introduced,the important role of MDM technology and mode converter in MDM system is explained,the-state-ofthe-art of mode converter is introduced,and the research significance of broadband and reconfigurable mode converter is discussed.Secondly,the theoretical knowledge involved in the design process of thermallyinduced long-period grating mode converter based on tapered polymer waveguide is briefly introduced,including the analysis method of planar waveguide,mode characteristics,coupled-mode theory of long-period grating and thermo-optical characteristics of polymer materials.Subsequently,the paper focuses on the design of the device's structural parameters,focusing on the expansion of the device's bandwidth and the realization of reconfigurable functions,including the design of optical waveguide,long-period grating,and electrodes.After that,the device design is completed on the basis of further simulation and optimization of the design parameters using RSoft software.Finally,the fabrication and testing of devices are systematically studied.It includes process flow design,mask pattern design,device fabrication,test analysis and process optimization.Finally,the mode converter achieved conversion efficiency of more than 90%(10 dB)in the whole C + L band.The related experimental results are analyzed and discussed,and further optimization schemes are proposed.On this basis,the research work is summarized and prospected.