Research On The All-fiber Modulator Based On Graphene-coated Microfiber

Optical modulator is a critical optical device in high-speed optical fiber communication systems.How to obtain an optical modulator with good compatibility with optical fiber system,miniaturization,wide bandwidth and low loss is one of the most popular topic in the field of optical communication.Owing to its unique Dirac band structure and the strong inter-band transitions,the graphene film allows broadband light-matter interactions with ultrafast responses and can be readily used with ultrafast optical modulators.The microfiber(MF)with subwavelength diameter can not only enhance the light-matter interaction on the MF surface,but also realize highly efficient optical connection between microstructures and standard optical fibers.In this work,we combined graphene with microfiber together and proposed a broadband electro-optical modulator based on graphene-coated double D-shape microfiber,then the optimized structures and the characteristics of the modulator are also analyzed.The main research contents are listed as follows:(1)The thesis firstly describes the research status of microfiber,graphene and graphene fiber modulators.And it also introduces the single mode propagation conditions,electric field distribution and energy distribution of the microfiber,and introduces the energy band structure,unique optical and electrical properties of graphene.(2)The operating principle of the optical modulator is introduced.According to the surface conductivity formula of graphene,the variation of graphene's chemical potential as a function of the applied voltage is analyzed.The thesis also shows the electrical conductivity,the dielectric constant,the refractive index and parameters of graphene change with its chemical potential.Then the simulation analysis of waveguide with different structures is carried on,and the impact of different microfiber structures,microfiber sizes,graphene layers and graphene chemical potential on waveguide refractive index and mode power loss are understood.By comparison,the double D-shape microfiber with 210nm grinding radius and the graphene layers locate in the fiber abraded surface,the modulator has the strongest ability of light absorption,which means that the structure we get makes the modulator performance optimized.(3)Based on the influence of above factors on the light absorption capacity of the modulator,a broadband electro-optical modulator based on graphene-coated double D-shape microfiber is proposed.In our design,to enhance the interaction between the evanescent fields of microfiber and the graphene,the upper and lower surfaces of the double D-shape microfiber are coated by two-lamellae of graphene which are separated by an Al2O3 spacer.With only a modulation voltage as low as 2.35V,by using the interaction between the graphene and strong evanescent field on the surface of the double D-shape microfiber,the electro-optical modulator can reach the modulation depth as high as 90%within the frequency ranging from 149.896THz to 182.756THz and the 3dB modulation bandwidth as large as 177.36THz within the frequency ranging from 182.756THz to 350THz.The modulator has the advantages of high integration,simple structure and broadband modulation bandwidth.