| As the communication capacity of traditional single-mode optical fiber communication links is gradually approaching the theoretical limit,space division multiplexing technology based on the spatial dimension of electromagnetic waves has become an important means for modern optical fiber communication links to expand communication capacity.Among the communication fiber types for the space division multiplexing system,few-mode fiber has been widely studied in the fields such as large-capacity information transmission system setup,multimode-manipulation optical device fabrication and optical microscopy imaging,due to its mode controllability,manufacturing feasibility and structural compatibility,etc.The complete transmission of optical signals in few-mode fibers is easily affected by factors such as inter-mode crosstalk and differential mode delay.In view of this situation,weakly-coupled few-mode fibers and low differential-mode-delay few-mode fibers can be designed accordingly to suppress them.In this thesis,a series of structural design,characteristic analysis and theoretical derivation are carried out around these two important types of few-mode fibers—weakly-coupled few-mode fibers and low differential-mode-delay few-mode fibers,which have provided the numerical and theoretical references for related few-mode fiber researches such as design,manufacturing and experimentation.The main work can be summarized as follows:(1)A design of cross-arranged side-hole-assisted weakly-coupled ring-core few-mode fiber is proposed,which solves the problem mutual restriction between spatially-degenerated mode transmission and mode crosstalk among these modes in the weakly-coupled fewmode fiber with symmetric core.Based on a weakly-coupled ring-core few-mode fiber supporting LP modes,a design of cross-arranged air holes and high-index side holes is proposed around the circular ring core.With the help of the assisted structure,the mode fields of spatially-degenerated modes(LP11a and LP11b,LP21a and LP21b,LP31a and LP31b)leak in the direction of air holes,and are diffused in the direction of high-index side holes,realizing the change the original mode field distributions in the ring core and achieving the purpose of improving the crosstalk between spatially-degenerated modes.Not only the low crosstalk characteristics between the spatially-degenerated modes are realized,but also the low crosstalk characteristics between the LP modes can be kept.In addition,the nonlinear characteristics,fiber loss,bending loss and dispersion in this fiber are analyzed by numerical calculation method,and the feasibility of the cross-arranged side-hole-assisted weakly-coupled ring-core few-mode fiber is finally determined for the transmission systems.(2)Influence differences and principle analysis of core asymmetry on weakly-coupled few-mode fiber performances are discussed;a mode normalized space density(M-NSD)calculation is proposed,based on the mode number.The research content has a certain numerical and theoretical reference for realizing the comprehensive analysis of spacedivision-multiplexing fibers based on different asymmetric cores.Three common asymmetric core shapes(ellipse,rectangle and diamond)and three common core structures(solid core structure,ring core structure and strongly-coupled multi-core structure)are combined to form nine asymmetric core structures,respectively,and all asymmetric core structures have the same refractive index,size and aspect ratio,realizing that asymmetric core structure is the only variable.Based on the different asymmetric cores,the values and changing trends of the some important fiber performance parameters are compared and analyzed.Then,the influence mechanism of core asymmetry on the fiber performance parameters is explored.In addition,a normalized space density calculation based on the mode number is proposed,which can comprehensively evaluate the nonlinearity and compatibility of optical fibers,and avoid the problem of inaccurate evaluation of space density coefficient due to the large numbers of modes but the small effective modal areas of the modes.(3)Theory of realizing low differential-mode delay-few-mode fiber determination in a certain wavelength range are explored,based on the wavelength dependence of the effective index.This method based on the wavelength dependence of the mode effective refractive index is simple and fast,and solves the problem of slow evaluation of the differential mode delay using the conventional theoretical calculation formula.Under the condition of achieving low differential mode delay between modes,the relationship between the effective refractive index/effective refractive index difference between modes and wavelength,i.g.,its wavelength dependence,is theoretically derived.Then,a comparative analysis of the correlation coefficients about the wavelength dependence of the effective refractive index/effective refractive index difference among the low-DMD few-mode fibers and the common few-mode fibers are carried out.Finally,using the wavelength dependence of the effective refractive index in the low differential-mode-delay few-mode fiber,the differential mode delay of a graded-index core cladding-trench few-mode fiber supporting linearly-polarized modes is optimized.Compared with the initial reproduced results(1520-1620 nm:81.08 ps/km,and 1550 nm:65.21 ps/km),the maximum differential mode delay in the wavelength range from 1520 to 1620 nm decreases by 40.35%,and the differential mode delay at 1550 nm decreases by 44.72%.In summary,this thesis contains two parts:weakly-coupled few-mode fiber and low differential-mode-delay few-mode fiber.A novel weaklycoupled ring-core few-mode fiber design supporting spatially-degenerated modes is proposed,the influence differences and influence mechanisms of the asymmetric core on performance parameters in weakly-coupled fewmode fiber are explored,and the theory of determining the low differentialmode-delay few-mode fiber in a certain wavelength range is proposed and verified based on the wavelength dependence of effective refractive index.The research results in this thesis provide a certain theoretical reference value for the structural design and characteristic analysis of novel fewmode fibers. |
