Study On Structural Design And Characteristics Of Orbital Angular Momentum Photonic Crystal Fibers

With the rapid development of optical communication,communication capacity has become the key to constrain the development of information technology.At present,space division multiplexing technology has become the best way to make the optical fiber communication capacity close to the Shannon limit.In the space division multiplexing method,the orbital angular momentum（OAM）multiplexing technology can effectively improve the channel capacity using the orthogonality of the OAM beams,and how to design the fiber structure to realize the OAM low-loss transmission is key problem.Photonic crystal fibers（PCFs）are considered as excellent carriers of OAM modes due to their flexibility in structural design,enabling low-loss transmission of OAM modes.However,in the design process,the effective refractive index differences between different vector modes should be bigger than10^-4 to ensure the stable transmission of OAM.In this paper,different PCF structures are designed,and the finite element method is used for modeling analysis in order to achieve low confinement loss,low nonlinear coefficient,large effective mode field area and flat dispersion during OAM mode transmission.Firstly,a photonic crystal OAM fiber based on a six-fold quasi-crystal structure is proposed.The structure uses pure Si O₂ as the background material,and by designing the structure of the cladding air holes the effective refractive index differences between different vector modes are bigger than 10^-4.The fiber can support 18 OAM modes within the wavelength range of 1.00-1.85μm,most of whose confinement loss is controlled within the range of10^-9-10^-10d B/m.At the wavelength of 1.55μm the nonlinear coefficient can be as low as3.16 W^-1/km at the minimum and only 3.77 W^-1/km at the maximum.In order to increase the number of transmission modes,an OAM fiber based on a twelve-fold photonic quasi-crystal structure is designed,which uses a high-refractive-index material tellurite glass as its ring core,and its refractive index at the wavelength of 1.55μm is 2.027811,effectively increasing the refractive index difference between the annular core and the cladding,thus increasing the number of OAM modes supported in the fiber.The fiber supports transmission of 110 OAM modes in the wavelength range of 1.46-1.60μm,with the effective mode-field area of most modes bigger than 100μm²,which has the advantages of small confinement loss and relatively flat dispersion.Finally,a photonic crystal OAM fiber with a large mode-field area is designed.The fiber uses Si O₂ as the background material and circular air holes in the cladding,which reduces the difficulty of fiber fabrication and exhibits excellent optical property.It can support 134 OAM modes in a wide range of 1200-2000 nm,the effective mode-field area reaches259.97-423.9μm²,the confinement losses of all eigenmodes are as low as 10^-9-10^-10 d B/m,and the nonlinear coefficient is lower than 0.47 W^-1/km.The proposed fiber has good application potential.