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Electrical and electronic engineering
Optical fibersplay important roles in telecommunication, imaging, sensing, lasers and amplifiers. The rapid development of fiberopticsover the past decades has beenunderpinnedbyvarious designs of the optical fibers and bythe rapid improvement of the understanding of the waveguiding mechanismsand associated models. According to the cross-section refractive index distribution, optical fibers can be generally classified into four basic types: two-layer step-index fiber, three-layer step-index fiber, three-layer depressed-core fiber and hollow-core fiber.Among these four basic fibertypes,the modeproperties and the applications of the three-layer step-index and depressed-core fibers have notbeensufficiently investigated. This thesis presents a detailedmode analysis in three-layer step-index and depressed-core fibers and their applications.A complete dispersion diagram includingthe core and cladding modes in athree-layer step-index optical fiber has beendevelopedfor the first time,using both analyticalmethodandfull-vector finite element method. Modetransition from the cladding-typeto core-type modes as a function of the core radius wasstudiedas a contribution to deepening the knowledge ofconventional step-index optical fibers.Based on the developed complete dispersion diagram for thethree-layer step-index optical fiber, it wasfound that asmall-core fiber with a nano/micro-sized core supportsonly cladding-typemodes. The self-imaging phenomenon ofthe pure cladding modes in thesmall-core fiber has beenstudied, and itscomparison tothe behaviourof the core-typemodes in the conventional multimode fibershas Vbeen carried out. The discrete nature and the exponential growth behaviourof the self-imagingof the cladding-typemodeswas establishedfor the first time.The resultsprovide new insights and design rules fora number of multimode interference devices such as optical couplers, optical modulators, multimode fiber lasers and space-division multiplexing devices. The depressed-core fiber, consisting of a low-index solid coreanda high-index cladding surrounded by air, is in effect a bridge between the conventional step-indexfiber and the tube-type hollow-core fiber from the pointof view of the index profile. In theresearch, a complete dispersion diagram of thedepressed-core fiber has beenobtained for the first time by solving the full-vector eigenvalue equations.The waveguiding in the depressed-core fiber wasanalyzed using thetheory of anti-resonant and the inhibited coupling guiding mechanisms.An asymmetric planar anti-resonant reflecting optical waveguide model (asymmetric planar ARROWmodel)wasproposed for the depressed-core fiber.A high-index polymer-coated no-core fiber as an example of the depressed-core fiberhas beenstudiedtheoretically and experimentally. The appearance of the periodic transmission loss dipsin the spectrum of a long or bent polymer-coated no-core fibersamples reflectsthe anti-resonance nature of the depressed-core fiber. Theexperiments show that the overall change inspectrallossis greater than 31dB at the dip positionaround 1550 nm and the average sensitivity is up to 14.77 dB/m-1, as the bend radius changes from∞ (straight) to47.48 cm.Theresults indicate thatthe polymer-coated no-core fibershave the potential to be used in many devices including curvature sensors and tunable loss filters.VIWhile the superposition of the spectra of multiple modes led to broad dips, it has beenfound that anindividual mode can cause sharp dips in the transmission spectra of apolymer-coated no-core fiber. As a result of this phenomenon and the large thermo-optical and thermal expansion coefficients of the polymer coating, a compact(length < 10 mm), high sensitivity and linear response temperature sensor with the sensitivity as high as -3.784 nm/Chas been demonstrated experiment
Lian, X. (2020) Mode Analysis and Applications of Three-Layer Step-Index and Depressed-Core Optical Fibers, Doctoral Thesis, Technological University Dublin. DOI:10.21427/9R2K-R557
A thesissuccessfully submitted for the degree of Doctor of Philosophy.