The Research Of Decreasing Differential Mode Group Delay Utilizing Strong Mode Coupling In Few Mode Fiber

The modal dispersion characteristics of few-mode fibers is one of the most important features of few-mode fibers in mode-division multiplexing system.The mode dispersion in a space-division multiplexing system will directly determine the computational complexity at the receiving end.Thus,we want the system to have a modal dispersion as low as possible.Since the value of the mode dispersion mainly based on the fiber structure and the mode coupling degree,these two aspects will be the main approaches to reduce the modal dispersion in a few-mode fibers.The theoretical and numerical studies in this thesis will mainly include the following aspects: 1.Analysis of the mode dispersion representation based on phase constant.We compared its position in the overall dispersion effects in few-mode fibers.Also,by comparing the examples of polarization mode coupling and coupling of the principle mode demonstrates the feasibility of utilizing strong coupling effects for differential mode delay reduction.2.Analysis of the different types of mode coupling model.By introducing the field coupling model,power coupling model and transfer matrix model,we evaluated their own characteristics and different applications.Under the general condition of the waveguide,we set up the mathematical model for optical signal transmission.3.Using the field coupling model to calculate situations like constant refractive index perturbation and the phase matching process in periodic disturbance in optical fiber.The entire fiber transmission group delay characteristics were calculated and analyzed by the transmission matrix model.By utilizing the transmission matrix model we obtained the relationship between the differencial mode delay reduction and the number of coupling nodes in a fiber link as well as the coupling efficiency.4.Two types of low delay few-mode fibers are presented.For the graded-index fiber with inner cladding trench,the effects of refractive index and the inner cladding trench to the mode field and differencial mode group delay are analyzed and verified through simulation using finite-difference method.Then for three-core super-mode fiber.by adjusting the normalized operating frequency,we got the result of its low DMGD feature by finite-element method and analyzed the results.5.We used mode scrambers to achieve strong mode coupling for DMD reduction.We studied fiber mode scrambers based on macro-bend,micro-bend and tilted long-period fiber grating.We evaluated their feasibilities and adopted tilted long-period fiber gratings as the final choice.6.For tilted long-period fiber grating.We set them as coupling nodes in typical step index fibers and muti-core super-mode fibers.The coupling coefficients and DMD charactors are simulated.And then we studied tilted long period grating written in the ring core fiber to further analize its mode coupling and the influence of mode field character.