Modal Analysis Of Chiral Step-profile And Bragg Fibers

Optical fiber communication (OFC) technology has been advancing rapidly for sev-eral decades, supporting the increasingly information-driven society and economy. Tomaintain this trend, on the physical ground of OFC, we must continue developing newtransmission media for optical signals and new devices for optical information process-ing. Fiber is the basic component of OFC system, the innovation of which thus will beof substantial significance. New types of fibers may be developed by considering twoaspects: changing the geometry structure of fiber; using novel media as constituting ma-terials. For the former, the representative achievement should be the much developedtechnology of photonic crystal fibers; while for the latter, such as considering using chi-ral media or negatively refractive media as constituting materials, research is still on itsfundamental stage. Our research object is the chiral fibers with finite number of layers,of which we mainly investigate the two most special types: chiral step-profile fibers (thesimplest chiral fibers) and chiral Bragg fibers (the simplest chiral photonic crystal fibers),by means of modal analysis.We first study the general modal properties of chiral fibers with finite number oflayers. Considering the more general electromagnetic parameters of media (with arbitrarychiral parameter; with permitting and permeability having the same sign for the real part,and the absolute value of real part larger than that of imaginary part), we develop themodal theory for chiral fibers with finite number of layers. From the viewpoint of mirrorsymmetry of modal theory, we provide the physical insights into the modal bifurcationof chiral fibers with finite number of layers: no matter to which layer of an achiral fiberwith finite number of layers the chirality is introduced (mirror symmetry breaking of thestructure), induced will be the bifurcation of the originally two-fold degenerate and mirrorimage modes (mirror symmetry breaking of the states supported by the structure).For the chiral step-profile fibers without negative refraction, we derive the cutoffconditions of the guided modes and study the modal bifurcation. Under the impedance-matching condition, we show the exact path of modal bifurcation. Under the impedance-mismatching condition, we study the different contributions of core chirality and claddingchirality to the modal bifurcation. Additionally, we point out the single polarized modeguidancepropertyoftheimpedance-mismatchedstep-profilefiberswithachiralcladding. For the chiral step-profile fibers with negative refraction, we study the guidance oftransverse electromagnetic (TEM) modes. We derive the guidance conditions for TEMmodes and point out the counter-intuitive property of TEM modes in fibers—arbitrarinessofmodalfield—themodalfielddistributionisintermsofarbitrarycomplexanalyticfunc-tions. Based on the novel spacial distribution property of modal field of TEM modes, wepropose conceptually a new type of space-division multiplexing.The light confinement in one-dimensional (1D) photonic crystal fibers could be wellunderstood on the basis of the energy-band theory of1D photonic crystals (PCs). As theprecondition for investigating chiral Bragg fibers, we organize the energy-band theory ofachiral1D PCs, and develop the energy-band theory of chiral1D PCs. We give the exactproof of the inevitable existence of Brewster angle on an interface separating differentachiral media, which explains why the unit cell must be composed of at least three layersfor achiral1D PCs to have three-dimensional (3D) complete band gaps. For the chiral1DPCs, we show that if the lattice structure is designed subtly,3D complete band gaps arepossible to be achieved even when the unit cell contains only two layers.We investigate the modal properties of Bragg fibers with only the core being chiraland hollow-core Bragg fibers with a complete band gap chiral cladding. For the former,we point out the conditions for the fibers to support circular polarized modes, and demon-strate that the introduction of weak chirality into the core of the Bragg fiber that is underTM operation will make the fiber possessing wavelength selectivity and circular polar-ization selectivity simultaneously. For the latter, we show that the complete band gapmay remarkably suppress the contribution of material absorption of cladding media to themodal loss.