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Structure Design And Property Research Of Hollow-Core Photonic Bandgap Fibers

Posted on:2010-03-20Degree:DoctorType:Dissertation
Country:ChinaCandidate:H ZhangFull Text:PDF
GTID:1118360278465397Subject:Electromagnetic field and microwave technology
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Advent of photonic crystal fibers(PCFs) opens up a new era of development of optical fibers,which exhibit many unique properties that are not realized in conventional optical fibers and hold great potential,and PCFs have become a research hotspot in the field of optical communications.According to the difference of guiding mechanisms,PCFs can be.divided into two types.One is total internal reflection PCFs(TIR-PCFs),the other is photonic bandgap fibers(PBGFs).More researches concentrate on the former one and matured theory has been developed.But PBGFs posses a few unique properties,such as highly effective coupling,low nonlinear and controllable transmission window, and they have attracted a considerable amount of attention from various fields including army.The dissertation focuses on structure designs and properties research of hollow-core photonic bandgap fibers(HC-PBGFs) from theory.Firstly,it presents a brief review of photonic crystal and some fundamental concepts,the development and properties of PCFs,especially the properties of HC-PBGFs are summarized.And then,numerical methods and mathematic tools are illustrated in the following chapter,by which properties of HC-PBGFs are analyzed,involving core design,mode characteristics,dispersion,nonlinear,and loss.The primary works could be described as follows:1.A brief introduction to four types of lattice structure of photonic crystal is presented.By means of plane wave extension method(PWM) to solve the eigenvalue equation of electric field,we come at out of plane bandgap structure diagram with index ratio 1.45:1,and conclude by detailed analysis that more larger air filling fraction is,more larger propagation constants are needed to form photonic bandgap.In addition,from bandgap propagation diagram we can found that guiding light in defect core for HC-PBGFs needs satisfy two conditions:(1) beam frequency lying in range of photonic bandgap,(2) propagation constant of guiding mode abides by the condition ofβ≤kn2.2.A full-vector finite element method(FEM) based on hybrid edge/nodal element and perfectly matched layers boundary conditions are deduced in detail. Furthermore,the FEM model of the magnetic field vector for PCFs is given by this method.And we take advantage of the model to investigate the dispersion of triangular lattice TIR-PCF,and corresponding results are in good agreement with that of the literature published,which verifies the correctness and effectiveness of the model.This chapter provides a theoretical basis for the follow-up chapters.3.Triangular lattice HC-PBGFs to be studied are designed by using the bandgap propagation diagram,the different core radius of the fibers are simulated by full-vector FEM model solver.A brief analysis of the reasons for the formation of surface modes(SMs) is presented,the optimal range of core radius that is free of SMs is R = 0.9-1.0A.And then,we design HC-PBGFs with rounded hexagons air hole arranged by triangular lattice,and simulate the fibers with different outer core radius and different normalized core thickness. By analysis it shows that the introduction of the silica ring can not only inhibit SMs but also induce SMs,whether SMs appear or not depends on the choice of the core ring thickness.The desired range of outer core radius and core ring thickness free of SMs is derived.In the final we investigate thin-wall core ring of the fibers and conclude that the optimal normalized thickness range is T= 0.3-0.6.4.Mode degeneracy of 19-cell HC-PBGFs with C6v symmetry is discussed in detail and 16 models found are classified and named according to the label of traditional step-index fiber.And then we investigate properties of 7-cell HC-PBGFs,including mode characteristics,dispersion,power fraction in silica,mode effective area,nonlinear coefficient and loss,and together with material effect.By analysis we can conclude that material dispersion can be neglected for HC-PBGFs with high air filling fraction,but the contribution of material effect to nonlinear coefficient can not be ignored,and the nonlinear coefficient of HC-PBGFs is roughly 3 orders of magnitude lower than that of conventional optical fibers.Finally,controllable dispersion property of HC-PBGFs is discussed.By simulating we found that theoretically the zero dispersion wavelength can be tailored by respectively changing rounded diameter of air holes,pitch,refractive index,normalized thickness of core rings, and the ratio of hole diameter to pitch.At the same time the tailoring of dispersion slope can also be realized by changing rounded diameter of air holes or pitch or normalized thickness of core rings.5.The modal characteristics of HC-PBGFs based on a square lattice with rounded square air holes are investigated,by using a full-vector FEM.And it was found that there are two advantages for this type fiber,one is operating under broad bandgap and another is single mode.And then,the leakage loss was analyzed completely.Simulations show that the number of cladding rings plays a key role to the leakage loss,while the rounded diameter,the core diameter and the hole pitch have a small influence on leakage loss but for a given wavelength the desired lowest leakage loss could be obtained by tuning them.In addition, the dispersion properties of the fibers are investigated for the first time.By simulation,we can found the core diameter and the number of cladding rings have a small influence on dispersion comparing to rounded diameter,hole pitch and air filling fraction.And dispersion property is found to satisfy scaling law of HC-PBGFs,furthermore the desired dispersion wavelength or desired dispersion slope could be obtained by properly changing the structure of the fibers.
Keywords/Search Tags:Photonic crystal fibers (PCFs), Photonic bandgap fibers (PBGFs), Full-vector finite element method (FEM), Dispersion, Loss, Surface modes (SMs)
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