| The development of increasingly digitized,hyper-connected and globally data driven modern society has led to an exponential growth in global data traffic.The inevitable development path of“Mobile Internet→Internet of Everything→Intelligent Internet of Everything”poses new challenges to existing technologies for communication capacity expansion.So far,different physical dimensions of optical field such as time,frequency（wavelength）,quadrature（amplitude/phase）,and polarization have been exploited to ease the forthcoming capacity crunch,which have almost reached their scalability limits and become saturated.The new technology of data transmission based on orbital angular momentum（OAM）beams provides a new way to further expand optical fiber transmission capacity.In this paper,starting from the basic definition and physical characteristics of OAM beams,we focus on the special optical fiber structure design and the theoretical issues involved in the realization of OAM beam stable transmission.We propose and design the required vortex optical fiber structures from multiple scenarios.From theoretical analysis to actual structure proposal and performance numerical verification and its corresponding communication system concept proposal,the potential performances of OAM beams in various vortex fibers and multiplexing scenarios are systematically studied,which provides a feasible solution for the practical application of OAM beams in future data transmission system.The major contents of this dissertation are summarized as follows:（1）In view of the problem that the capacity of existing polymer fiber is limited to Gigabit,a new type of hollow-ring-core polymer fiber is proposed.The proposed fiber can support stable transmission of 26 OAM modes（central wavelength is 670 nm）with perfect structure.The effective refractive index difference between near-degenerate modes can reach the order of 10-3～10-2,the mode purity is higher than 99.73%,and the dispersion curve within the whole band is flat.The influences of the central-hollow size,core thickness and cladding thickness on the performance of the fiber are analyzed,and the influences of the geometric deformation of the fiber such as ellipse,misalignment and diametrical nonuniformity on the performance of OAM modes propagated in the fiber are studied,the results show that the fiber has a certain manufacturing tolerance.The proposed fiber can be used in OAM mode-division multiplexing（OAM-MDM）communication link with 30-fold increase in transmission capacity compared to traditional polymer fibers（1～2 Gb/s）.（2）A novel microstructured polymer fiber is proposed for terahertz（THz）vortices transmission,focusing on the enhancement of the quantity of modes and bandwidth.On the basis of the vortex fiber,its working frequency band is extended to the THz frequency band to further improve the communication capacity.The proposed fiber can transmit up to 66 THz OAM modes simultaneously,covering an ultra-wide bandwidth of 0.65 THz（0.6～1.25 THz）,with a minimum purity of 97.06%and a maximum intrinsic crosstalk of-15.32 d B.Besides,large mode effective refractive index difference(>2.197×10-3)is realized over 0.6～1.25 THz while maintaining single-mode condition radially.This fiber technology can provide a solution for flexible interconnection between key components in terahertz systems,and can accelerate the development of compact,lightweight terahertz communication systems.（3）The introduction of multi-core design on the basis of THz vortex fiber provides a new idea for the expansion of communication capacity,that is,the use of multi-core THz vortex fiber.Therefore,a new type of air-assisted multi-OAM multi-ring fiber with central hollow,concentric ring cores,and isolated layers consisting of circular air-hole arrays is proposed for THz OAM modes guidance.The design method for structure parameters including ai-hole size,thickness of ring cores,and width of isolated layers is introduced in detail.The final optimized fiber efficiently supports 132 OAM modes in 0.6～1.5 THz,178 OAM modes in 0.7～1.5 THz,with low-level inter-more and inter-core crosstalk,the high-order radial modes are suppressed within the whole frequency range meanwhile.The proposed fiber has the characteristics of near-zero flat dispersion curve,low confinement loss,low bending loss,high mode purity（（29）94.83%）and wide operating frequency band.The conceptual diagram of the proposed fiber base OAM-MDM combined with core-division multiplexing（CDM）and wavelength-division multiplexing（WDM）transmission link and the available information channels in the designed fiber are discussed further.The realized fiber is expected to dramatically extend the transmission capacity and spectral efficiency.This new technology of multi-ring core terahertz vortex fiber is expected to solve the technical problem of flexible and lightweight interconnection in ultra-high-capacity terahertz systems. |
PDF Full Text Request