Topological Photonic Crystal Fibre

As an important medium in the field of modern optical information,every progress of fibre promotes the development of its related fields such as optical communication,optical sensing,nonlinear optics,fibre laser,quantum optics,etc.,which has a profound impact on the change of human society.Photonic crystal fibre(PCF)is an important member of the fibre family.Periodic photonic crystal cladding structure makes it have abundant degrees of design freedom.In order to meet the needs of the development of modern society,there are still some problems to be solved in the field of fibre design,such as the broadband realization of single-polarization single-mode(SPSM)and so on.Topological photonics is a new direction in the field of optics.It is the combination of topological principle and optical system,which provides a new degree of freedom of manipulation different from traditional optical means.In recent years,its development is mainly reflected in the application of topological band theory in photonic crystal system and the extensive study of photonic topological edge state.As a widely used photonic crystal product,photonic crystal fibre has not been designed using topologi-cal non-trivial band topology physics.In this paper,the basic principle of topological photonics is applied to the design of novel optical fibre.The specific contents are as follows:1.For the first time,based on the traditional silica photonic crystal fibre with a nodal line of Dirac point,we introduce the Weyl point in its photonic band structure,by breaking the Dirac degenerate line.Thus,the singular and robust chiral Weyl surface states,which are usually realized in complex topological photonic crystals,are intro-duced into the traditional optical fibres composed of simple structures and materials,which provides a new idea for the design of chiral fibre.2.We have designed a new type of topological PCF,which is called Dirac-vortex fibre.Referring to Jackiw-Rossi model in topological physics,we make modulation with vortex form for the two-dimensional facet of silica PCF with band structure of Dirac points.Mathematically,the system satisfies the Dirac equation with two-dimensional vortex mass terms.After modulation,the band structure of Dirac points is opened to form a bandgap.Meanwhile,the number of guided modes of fibre in the bandgap is equal to the absolute value of winding number of the vortex.Based on this,we have realized the fibre design with arbitrary number of nearly-degenerate guided modes.In addition,we propose a single-vortex fibre design which can realize effective SPSM transmission in a broadband frequency range of more than one octave by using the fact that the dispersion of topological guided mode is mainly located in the middle of the Dirac bandgap.3.In order to be compatible with the mature stack-and-draw process of silica pho-tonic crystal fibre,we propose a discrete version of design of single-vortex Dirac fibre.The fibre is composed of two kinds of photonic crystals similar to benzene ring structure with single and double bond,so that only four kinds of glass capillary tubes with differ-ent wall thickness are needed to construct its preform.We verify the robustness of the vortex design and the topological guided mode by considering the possible structural deformation of the fibre in the drawing process.In addition,we also propose a scheme to make topological guided mode transmit inside a hollow core.In summary,this paper mainly introduces the control means of topological physics in the field of optical fibre,which opens up a whole new way for the design of novel fibre,and also makes PCF as a new platform for studying topological physics.